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1. WIREROD MILL ROLLER TABLE
DESCRIPTION OF WIREROD MILL ROLLER TABLE
The roller table has 3 sections each section driven by a separate DC mill duty motor. A common thyristor
converter has been provided for the 3 DC motors. Reversible converter has been provided for regenerative
braking of the table motors and for reverse inclining operation of the roller table. Wire rod mill is at HRM
i.e. Hot Rolling Mills. Wire rod mill having 7 stands.
Provision has been made for running any one or two or all three sections at a time. While the roller table is
running any one section motor can be switched off independent of other sections, however once the roller
table is running any section motor which is off or any section motor which might have stopped due to fault
conditions cannot restarted as it is undesirable to apply full armature voltage step to the motor armature.
Production of wire rod from quality and stainless steel is subjected to constant change.The growing
demands on the quality of the finished products and on theflexibility and cost effectiveness of the plants
necessitate new concepts and strategies from the plant manufacturer. this supplies not only to machine
engineering, but also to new innovative processes and technologies. The holistic consideration of the
complete process is fundamental for meeting the customer‘s expectations.
In a wire rod mill, the performance in term of rolled output plays a vital role in the bottom line of the
organisation. In the rolling system, raw material (in block form) is fed one after one, after adequate
heating. Then the heated material is drawn by a series of roller and the output is collected through a
conveyor, in a coil form. The whole system runs continuously. In order to have trouble free working, the
entire coil should be perfect round coil. A concave deformation at a small portion of the ring is called
KINK and when it forms, the wire rod refuses to pass through the conveyor. It was observed that the
KINK formed in the last few coils only.
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Description of roller table control equipment
There are three sections of the roller table each section driven by a DC motor rated 0/7.5/15 kW,
All the line equipment on the armature side via protective relays negative line isolator, line contactors,
voltmeters, ammeter with shunt and dynamic braking contactor and on the field side viz. field failure relay
and voltmeter are mounted in the line and control cubicle.
The control circuit input knife switch and fuses, transformer/rectifier unit, all the inter locking relays and
timers are mounted in the line and control cubicle.
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Description of power and control scheme for roller table drive thyristor controller
From incoming supply available available(415,3-phase), a 75 KVA Dry type transformer steps up the
voltage to 440V. The transformer has an electrically operated MCCB in the primary. The DC output has
two shunts after which DC is taken by cables from the converter cubicles to line and control
cubicles.Thethyristor bridge consists of twelve nos. of thyristors and six fuses in a reversible six pulses.
Circulating current free connections.The roller table consists of 3 DC motors
Dc motor of Wirerod Mill Roller Table
Name Plate Details of DC Motor for Wirerod mill Roller Table
Frame MDX 3/31
Separate excitation 230V
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Rating and Service Conditions for Roller Table Drive Thyristor Converter for Wire Rod Mill
Voltage variations ±10%
Frequency variations ±3%
Fault level 25MVA
Voltage 0 to 460V DC
Current 1)110 A continuous
2)275 A for 15 sec.
Voltage Full field 230V DC
Current Full field 4.5A
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2. DC MOTORS
Electric motors are broadly classified into two different categories: DC (Direct Current) and AC
(Alternating Current). Within these categories are numerous types, each offering unique abilities that suit
them well for specific applications. In most cases, regardless of type, electric motors consist of a stator
(stationary field) and a rotor (the rotating field or armature) and operate through the interaction of
magnetic flux and electric current to produce rotational speed and torque. DC motors are distinguished
by their ability to operate from direct current.
A direct current (DC) motor is a fairly simple electric motor that uses electricity and a magnetic field to
produce torque, which causes it to turn. At its most simple, it requires two magnets of opposite polarity
and an electric coil, which acts as an electromagnet. The repellent and attractive electromagnetic forces of
the magnets provide the torque that causes the motor to turn.
Anyone who has ever played with magnets knows that they are polarized, with a positive and a negative
side. The attraction between opposite poles and the repulsion of similar poles can easily be felt, even with
relatively weak magnets. A DC motor uses these properties to convert electricity into motion. As the
magnets within the motor attract and repel one another, the motor turns.
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Electromechanical Energy Conversion
An electromechanical energy conversion device is essentially a medium of transfer between an
input side and an output side. Three electrical machines (DC, induction and synchronous) are used
extensively for electromechanical energy conversion. Electromechanical energy conversion occurs when
there is a change inmagnetic flux linking a coil, associated with mechanical motion.
The input is electrical energy (from the supply source), and the output is mechanical energy (to the load).
Energy source Energy load
The Input is mechanical energy (from the prime mover), and the output is electrical energy.
energy conversion device
energy conversion device
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DC motors consist of one set of coils, called armature winding, inside another set of coils or a set of
permanent magnets, called the stator. Applying a voltage to the coils produces a torque in the armature,
resulting in motion.
• The stator is the stationary outside part of a motor.
• The stator of a permanent magnet dc motor is composed of two or more permanent magnet pole
• The magnetic field can alternatively be created by an electromagnet. In this case, a DC coil (field
winding) is wound around a magnetic material that forms part of the stator.
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• The rotor is the inner part which rotates.
• The rotor is composed of windings (called armature windings) which are connected to the external
circuit through a mechanical commutator. Both stator and rotor are made of ferromagnetic
materials. The two are separated by air-gap.
A winding is made up of series or parallel connection of coils.
• Armature winding - The winding through which the voltage is applied or induced.
• Field winding - The winding through which a current is passed to produce flux (for the
electromagnet) Windings are usually made of copper.
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DC Motor Basic Principles
If electrical energy is supplied to a conductor lying perpendicular to a magnetic field, the interaction
of current flowing in the conductor and the magnetic field will produce mechanical force (and therefore,
Value of Mechanical Force
There are two conditions which are necessary to produce a force on the conductor.The conductor
must be carrying current, and must be within a magnetic field.When these two conditions exist, a force will
be applied to the conductor, which will attempt tomove the conductor in a direction perpendicular to the
magnetic field. This is the basic theoryby which all DC motors operate.
The force exerted upon the conductor can be expressed as follows.
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F = B i l Newton
where B is the density of the magnetic field, l is the length of conductor, and i the value of current flowing
in the conductor. The direction of motion can be found using Fleming‘s Left Hand Rule.
Fleming‘s Left Hand Rule
The first finger points in the direction of the magnetic field (first - field), which goes from the North pole
to the South pole. The second finger points in the direction of the current in the wire (second - current).
The thumb then points in the direction the wire is thrust or pushed while in the magnetic field (thumb -
torque or thrust).
Principle of operation
Consider a coil in a magnetic field of flux density B. When the two ends of the coil are connected across
a DC voltage source, current I flows through it. A force is exerted on the coil as a result of the
interaction of magnetic field and electric current. The force on the two sides of the coil is such that the
coil starts to move in the direction of force.
Torque production in a DC motor
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In an actual DC motor, several such coils are wound on the rotor, all of which experience force,
resulting in rotation. The greater the current in the wire, or the greater the magnetic field, the faster the
wire moves because of the greater force created.
At the same time this torque is being produced, the conductors are moving in a magnetic field. At
different positions, the flux linked with it changes, which causes an emf to be induced (e = di/dt) as
shown in fig. This voltage is in opposition to the voltage that causes current flow through the conductor
and is referred to as a counter-voltage or back emf.
Induced voltage in the armature winding of DC motor
The value of current flowing through the armature is dependent upon the difference between the
applied voltage and this counter-voltage. The current due to this counter-voltage tends to oppose the very
cause for its production according to Lenz‘s law. It results in the rotor slowing down. Eventually, the
rotor slows just enough so that the force created by the magnetic field (F = Bil) equals the load force
applied on the shaft. Then the system moves at constant velocity.
Reversability of a DC machine
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Induced Counter-voltage (Back emf):
Due to the rotation of this coil in the magnetic field, the flux linked with it changes at different
positions, which causes an emf to be induced.
The induced emf in a single coil, e = dᛰc/dt
Since the flux linking the coil, ᛰc =ᛰSin⍵t
Induced voltage :e = ᛰ⍵Cos⍵t
Note that equation gives the emf induced in one coil. As there are several coils wound all around
the rotor, each with a different emf depending on the amount of flux change through it, the total emf can be
obtained by summing up the individual emfs.
The total emf induced in the motor by several such coils wound on the rotor can be obtained by integrating
equation , and expressed as:
Eb= Kᛰ⍵ m
whereK is an armature constant, and is related to the geometry and magnetic properties of the motor, and
⍵ m is the speed of rotation.
The electrical power generated by the machine is given by:
Pdev = EbIa = K ᛰ⍵mIa
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DC Machine Classification
DC Machines can be classified according to the electrical connections of the armature winding and
the field windings. The different ways in which these windings are connected lead to machines operating
with different characteristics. The field winding can be either self-excited or separately-excited, that is,
the terminals of the winding can be connected across the input voltage terminals or fed from a separate
voltage source (as in the previous section). Further, in self-excited motors, the field winding can be
connected either in series or in parallel with the armature winding. These different types of connections
give rise to very different types of machines, as we will study in this section.
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Separately excited machines
The armature and field windings are electrically separate from each other. The field winding is excited by
separate DC source.
Separately exited DC motor
The voltage and power equations for this machine are same as those derived in the previous section.
Note that the total input power = VfIf +VTIa
In these machines, instead of a separate voltage source, the field winding is connected across the main
• The armature and field winding are connected in parallel.
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• The armature voltage and field voltage are the same.
Total current drawn from the supply, IL = If + Ia
Total input power = VT IL
Series DC machine
• The field winding and armature winding are connected in series.
• The field winding carries the same current as the armature winding.
A series wound motor is also called a universal motor. It is universal in the sense that it will run equally
well using either an ac or a dc voltage source.
Reversing the polarity of both the stator and the rotor cancel out. Thus the motor will always rotate the
same direction irregardless of the voltage polarity.
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Compound DC machine
If both series and shunt field windings are used, the motor is said to be compounded. In a compound
machine, the series field winding is connected in series with the armature, and the shunt field winding is
connected in parallel. Two types of arrangements are possible in compound motors:
Cumulative compounding - If the magnetic fluxes produced by both series and shunt field windings are in
the same direction (i.e., additive), the machine is called cumulative compound.
Series DC Motor
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Differential compounding - If the two fluxes are in opposition, the machine is differential compound.
In order to effectively use a D.C. motor for an application, it is necessary to understand its
characteristic curves. For every motor, there is a specific Torque/Speed curve and Power curve. The
relation between torque and speed is important in choosing a DC motor for a particular application.
Torque-speed characteristics of separately-excited DC motor
Tdev Stall torque
Normal operating range
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Speed Control of DC Motors
The speed of a motor is given by the relation
Ra= armature circuit resistance
Obvious that the speed can be controlled by varying
(i) Flux/pole, Φ (Flux Control)
(ii) Resistance Ra of armature circuit (Rheostat Control) and (iii) Applied voltage V (Voltage
Speed Control of Shunt motor:
(i) Variation of Flux or Flux Control Method: By decreasing the flux, the speed can be increased and
vice versa. The flux of a dc motor can be changed by changing Ish with help of a shunt field rheostat.
Since Ishis relatively small, shunt field rheostat has to carry only a small current, which means I2
loss is small, so that rheostat is small in size.
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Flux control method
(ii) Armature or Rheostatic Control Method: This method is used when speeds below the no-load speed
are required. As the supply voltage is normally constant, the voltage across the armature is varied by
inserting a variable rheostat in series with the armature circuit. As controller resistance is increased,
voltage across the armature is decreased, thereby decreasing the armature speed. For a load constant
torque, speed is approximately proportional to the voltage across the armature. From the
speed/armature current characteristic, it is seen that greater the resistance in the armature circuit,
greater is the fall in the speed.
Armature control method
(iii) Voltage Control Method:
(a) Multiple Voltage Control: In this method, the shunt field of the motor is connected permanently to a
fixed exciting voltage, but the armature is supplied with different voltages by connecting it across one of
the several different voltages by means of suitable switchgear. The armature speed will be approximately
proportional to these different voltages. The intermediate speeds can be obtained by adjusting the shunt
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(b) Ward-Leonard System: This system is used where an unusually wide and very sensitive speed control
is required as for colliery winders, electric excavators, elevators and the main drives in steel mills and
blooming and paper mills. M1 is the main motor whose speed control is required. The field of this motor
is permanently connected across the dc supply lines. By applying a variable voltage across its armature,
any desired speed can be obtained. This variable voltage is supplied by a motor-generator set which
consists of either a dc or an ac motor M2 directly coupled to generator G. The motor M2 runs at an
approximately constant speed. The output voltage of G is directly fed to the main motor M1. The voltage
of the generator can be varied from zero up to its maximum value by means of its field regulator. By
reversing the direction of the field current of G by means of the reversing switch RS, generated voltage
can be reversed and hence the direction of rotation of M1. It should be remembered that motor generator
set always runs in the same direction.
Voltage control method
Speed Control of Series Motors
1. Flux Control Method: Variations in the flux of a series motor can be brought about in any one of the
(a) Field Diverters: The series winding are shunted by a variable resistance known as field diverter. Any
desired amount of current can be passed through the diverter by adjusting its resistance. Hence the flux
can be decreased and consequently, the speed of the motor increased.
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(b) Armature Diverter: A diverter across the armature can be used for giving speeds lower than the normal
speed. For a given constant load torque, if Iais reduced due to armature diverter, the ᛰmust increase
(∵TaαᛰIa) This results in an increase in current taken from the supply
(which increases the flux and a fall in speed (NαI/ᛰ). The variation in speed can be controlled by varying
the diverter resistance.
Series motor speed control
(c) Trapped Field Control Field: This method is often used in electric traction. The number of series
filed turns in the circuit can be changed. With full field, the motor runs at its minimum speed which can be
raised in steps by cutting out some of the series turns.
(d) Paralleling Field coils: this method used for fan motors, several speeds can be obtained by
regrouping the field coils. It is seen that for a 4-pole motor, three speeds can be obtained easily.
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2. Variable Resistance in Series with Motor: By increasing the resistance in series with the armature the
voltage applied across the armature terminals can be decreased. With reduced voltage across the armature,
the speed is reduced. However, it will be noted that since full motor current passes through this resistance,
there is a considerable loss of power in it.
A motor and its load may be brought to rest quickly by using either (i) Friction Braking or (ii) Electric
Braking. Mechanical brake has one drawback: it is difficult to achieve a smooth stop because it depends
on the condition of the braking surface as well as on the skill of the operator. The excellent electric braking
methods are available which eliminate the need of brake lining levers and other mechanical gadgets.
Electric braking, both for shunt and series motors, is of the following three types:
(i) Rheostatic or dynamic braking
(ii) Plugging i.e., reversal of torque so that armature tends to rotate in the opposite direction.
(iii) Regenerative braking.
Obviously, friction brake is necessary for holding the motor even after it has been brought to rest.
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Electric Braking of Shunt Motors
(a) Rheostat or Dynamic Braking: In this method, the armature of the shunt motor is disconnected
from the supply and is connected across a variable resistance R. The field winding is left
connected across the supply. The braking effect is controlled by varying the series resistance R.
Obviously, this method makes use of generator action in a motor to bring it to rest.
(b) Plugging or Reverse Current Braking: This method is commonly used in controlling elevators,
rolling mills, printing presses and machine tools etc. In this method, connections to the armature
terminals are reversed so that motor tends to run in the opposite direction. Due to the reversal of
armature connections, applied voltage V and E start acting in the same direction around the
circuit. In order to limit the armature current to a reasonable value, it is necessary to insert a
resistor in the circuit while reversing armature connections.
(c) Regenerative Braking: This method is used when the load on the motor has over-hauling
characteristic as in the lowering of the cage of a hoist or the downgrade motion of an electric train.
Regeneration takes place when Eb becomes grater than V. This happens when the overhauling
load acts as a prime mover and so drives the machines as a generator. Consequently, direction of
Ia and hence of armature torque is reversed and speed falls until E becomes lower than V. It is
obvious that during the slowing down of the motor, power is returned to the line which may be
used for supplying another train on an upgrade, thereby relieving the powerhouse of part of its
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Electric Braking of Series Motor
(a) Rheostat (or dynamic) Braking: The motor is disconnected from the supply, the field connections
are reversed and the motor is connected in series with a variable resistance R. Obviously, now, the
machine is running as a generator. The field connections are reversed to make sure that current
through field winding flows in the same direction as before (i.e., from M to N ) in order to assist
residual magnetism. In practice, the variable resistance employed for starting purpose is itself used
for braking purposes.
(b) Plugging or Reverse Current Braking: As in the case of shunt motors, in this case also the
connections of the armature are reversed and a variable resistance R is put in series with the
(c) Regenerative Braking: This type of braking of a series motor is not possible without modification
because reversal of Ia would also mean reversal of the field and hence of Eb. However, this
method is sometimes used with traction motors, special arrangements being necessary for the
Disadvantages of DC motors
High initial cost
Increased operation and maintenance cost due to presence of commutator and brush gear
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Cannot operate in explosive and hazard conditions due to sparking occur at brush ( risk in
Not suitable in very clean environment
Vulnerable to dust which decreases performance
Care required to maintain the mechanical interface used to get current to rotating field.
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3. AC MOTORS
AC motor is electric motor driven by an alternating current (AC). It commonly consists of two basic parts,
an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic
field, and an inside rotor attached to the output shaft that is given a torque by the rotating field.
There are two main types of AC motors, depending on the type of rotor used. The first type is
the induction motor or asynchronous motor; this type relies on a small difference in speed between the
rotating magnetic field and the rotor toinduce rotor current. The second type is the synchronous motor,
which does not rely on induction and as a result, can rotate exactly at the supply frequency or a sub-
multiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered
through slip rings or by a permanent magnet. Other types of motors include eddy current motors, and also
AC/DC mechanically commutated machines in which speed is dependent on voltage and winding
The synchronous motorhas the special property of maintaining a constantrunning speed under all
conditions of load up to full load. This constant running speed can be maintained even under variable line
voltage conditions. It is, therefore, a useful motor in applications where the running speed must be
accurately known and unvarying. It should be noted that, if a synchronous motor is severely overloaded,
its operation (speed) will suddenly lose its synchronous properties and the motor will come to a halt. The
synchronous speed of the motor used in this experiment is 1800 rpm.
The synchronous motor gets its name from the term synchronous speed, which is the natural speed of the
rotating magnetic field of the stator. As you have learned, this natural speed of rotation is controlled
strictly by the number of pole pairs and the frequency of the applied power.
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Like the induction motor, the synchronous motor makes use of the rotating magnetic field. Unlike the
induction motor, however, the torque developed does not depend on the induction currents in the rotor.
Briefly, the principle of operation of the synchronous motor is as follows: a multi phase source of AC is
applied to the stator windings and a rotating magnetic field is produced. A direct current is applied to the
rotor windings and a fixed magnetic field is produced. The motor is constructed such that these two
magnetic fields react upon each other causing the rotor to rotate at the same speedas the rotating magnetic
field. If a load is applied to the rotor shaft, the rotor will momentarily fall behind the rotating field but will
continue to rotate at the same synchronous speed.
The falling behind is analogous to the rotor being tied to the rotating field with a rubber band.
Heavier loads will cause stretching of the band so the rotor position lags the stator field but the rotor
continues at the same speed. If the load is made too large, the rotor will pull out of synchronism with the
rotating field and, as a result, will no longer rotate at the same speed. The motor is then said to be
overloaded. The synchronous motor is not a self-starting motor. The rotor is heavy and, from a dead stop,
it is not possible to bring the rotor into magnetic lock with the rotating magnetic field. For this reason, all
synchronous motors have some kind of starting device. A simple starter is another motor which brings the
rotor up to approximately 90 percentof its synchronous speed. The starting motor is then disconnected and
the rotor locks in step with the rotating field. The more commonly used starting method is to have the rotor
include a squirrel cage induction winding. This induction winding brings the rotor almost to its
synchronous speed as an induction motor
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Construction of Synchronous Motor
The exterior frame, made of steel, either cast or a weldment, supports the laminated stator core and has
feet, or flanges, for mounting to the foundation. Frame vibration from core magnetic forcing or rotor
unbalance is minimized by resilient mounting the core and=or by designing to avoid frame resonance with
forcing frequencies. If bracket type bearings are employed, the frame must support the bearings, oil seals,
and gas seals when cooled with hydrogen or gas other than air. The frame also provides protection from
the elements and channels cooling air, or gas, into and out of the core, stator windings, and rotor.
When the unit is cooled by gas contained within the frame, heat from losses is removed by coolers having
water circulating through finned pipes of a heat exchanger mounted within the frame. Where cooling water
is unavailable and outside air cannot circulate through the frame because of its dirty or toxic condition,
large air-to-air heat exchangers are employed, the outside air being forced through the cooler by an
externally shaft-mounted blower.
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Stator Core Assembly
The stator core assembly of a synchronous machine is almost identical to that of an induction motor. A
major component of the stator core assembly is the core itself, providing a high permeability path for
magnetism. The stator core is comprised of thin silicon steel laminations and insulated by a surface coating
minimizing eddy currentand hysteresis losses generated by alternating magnetism. The laminations are
stacked as full rings or segments, in accurate alignment, either in a fixture or in the stator frame, having
ventilation spacers inserted periodically along the core length. The completed coreis compressed and
clamped axially to about 10 kg/cm2 using end fingers and heavy clamping plates. Core end heating from
stray magnetism is minimized, especially on larger machines, by using non-magnetic materials at the core
end or by installing a flux shield of either tapered laminations or copper shielding.
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A second major component is the stator winding made up of insulated coils placed in axial slots of the
stator core inside diameter. The coil make-up, pitch, and connections are designed to produce rotating
stator electromagnetic poles in synchronism with the rotor magnetic poles. The stator coils are retained
into the slots by slot wedges driven into grooves in the top of the stator slots. Coil end windings are
boundtogether and to core-end support brackets.
The rotor of a synchronous motoris a highly engineered unitized assembly capable of rotating satisfactorily
at synchronous speed continuously according to standards or as necessary for the application.The central
element is the shaft, having journals to support the rotor assembly in bearings. Located at the rotor
assembly axial mid-section is the rotor core embodying magnetic poles. When the rotor is round it is called
‗‗non salient pole‘‘, or turbine generator type construction and when the rotor has protruding pole
assemblies, it is called ‗‗salient pole‘‘ construction. The non-salient pole construction, used mainly on
turbine generators(and also as wind tunnel fan drive motors), has two or four magnetic poles created by
direct current in coils located in slots at the rotor outside diameter. Coils are restrained in the slots by slot
wedges and at the ends by retaining rings on large high-speed rotors, and fiberglass tape on other units
where stresses permit. This construction is not suited for use on a motor requiring self-starting as the rotor
surface, wedges, and retaining rings overheat and melt from high currents of self-starting. A single piece
forging is sometimes used on salient pole machines, usually with four or six poles. Salient poles can also
be integral with the rotor lamination and can be mounted directly to the shaft or fastened to an intermediate
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Rotor of synchronous motor
Each distinct pole has an exciting coil around it carrying excitation current or else it employs permanent
magnets. In a generator, a moderate cage winding in the face of therotor poles, usually with pole-to-pole
connections, is employed to
dampen shaft torsional oscillation and to suppress harmonic variation in the magnetic waveform.
In a motor, heavy bars and end connections are required in the pole face to minimize and withstand the
high heat of starting duty. Direct current excites the rotor windings of salient, and non-salient pole motors
and generators, except when permanent magnets are employed. The excitation current is supplied to the
rotor from either an external DC supply through collector rings or a shaft-mounted brushless exciter.
Positive and negative polarity bus bars or cables pass along and through the shaft as required to supply
excitation current to the windings of the field poles. When supplied through collector rings, the DC current
could come from a shaft-driven DC or AC exciter rectified output, from an AC-DC motor generator set, or
from plant power. DC current supplied by a shaft mounted AC generator is rectified by a shaft-mounted
The induction motor is the most commonly used type of ac motor. Its simple, rugged construction costs
relatively little to manufacture. The induction motor has a rotor that is not connected to an external source
of voltage. The induction motor derives its name from the fact that ac voltages are induced in the rotor
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circuit by the rotating magnetic field of the stator. In many ways, induction in this motor is similar to the
induction between the primary and secondary windings of a transformer.
The three-phase induction motors are the most widely used electric motors in industry. They run at
essentially constant speed from no-load to full-load. However, the speed is frequency dependent and
consequently these motors are not easily adapted to speed control. We usually prefer d.c. motors when
large speed variations are required. Nevertheless, the 3-phase induction motors are simple, rugged, low-
priced, easy to maintain and can be manufactured with characteristics to suit most industrial requirements.
In this chapter, we shall focus our attention on the general principles of 3-phase induction motors.
Like any electric motor, a 3-phase induction motor has a stator and a rotor. The stator carries a 3-phase
winding (called stator winding) while the rotor carries a short-circuited winding (called rotor winding).
Only the stator winding is fed from 3-phase supply. The rotor winding derives its voltage and power from
the externally energized stator winding through electromagnetic induction and hence the name. The
induction motor may be considered to be a transformer with a rotating secondary and it can, therefore, be
described as a ―transformertype‖a.c. machine in which electrical energy is converted into mechanical
A 3-phase induction motor has two main parts (i) stator and (ii) rotor. The rotor is separated from the stator
by a small air-gap which ranges from 0.4 mm to 4 mm, depending on the power of the motor.
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It consists of a steel frame which encloses a hollow, cylindrical core made up of thin
laminations of silicon steel to reduce hysteresis and eddy current losses. A number
of evenly spaced slots are provided on the inner periphery of the laminations. The insulated connected to
form a balanced 3-phase star or delta connected circuit. The 3-phase stator winding is wound for a definite
number of poles as per requirement of speed. Greater the number of poles, lesser is the speed of the motor
and vice-versa. When 3-phase supply is given to the stator winding, a rotating magnetic field (See Sec.
8.3) of constant magnitude is produced. This rotating field induces currents in the rotor by electromagnetic
The rotor, mounted on a shaft, is a hollow laminated core having slots on its outer periphery. The winding
placed in these slots (called rotor winding) may be one of the following two types:
Squirrel cage type and Wound type
Squirrel cage rotor
It consists of a laminated cylindrical core having parallel slots on its outer periphery. One copper or
aluminum bar is placed in each slot. All these bars are joined at each end by metal rings called end
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rings.This forms a permanently short circuited winding which is indestructible. The entire construction
(bars and end rings) resembles a squirrel cage and hence the name. The rotor is not connected electrically
to the supply but has current induced in it by transformer action from the stator.
Those induction motors which employ squirrel cage rotor are called squirrel cage induction motors. Most
of 3-phase induction motors use squirrel cage rotor as it has a remarkably simple and robust construction
enabling it to operate in the most adverse circumstances. However, it
suffers from the disadvantage of a low starting torque. It is because the rotor bars are permanently short-
circuited and it is not possible to add any external resistance to the rotor circuit to have a large starting
It consists of a laminated cylindrical core and carries a 3-phase winding, similar to the one on the stator
[See Fig. The rotor winding is uniformly distributed in the slots and is usually star-connected. The open
ends of the rotor winding are brought out and joined to three insulated slip rings mounted on the rotor shaft
with one brush resting on each slip ring. The three brushes are connected to a 3-phase star-connected
rheostat as shown in Fig. (8.4). At starting, the external resistances are included in the rotor circuit to give
a large starting torque. These resistances are gradually reduced to zero as the motor runs up to speed.
36. Page 36
Rotating Magnetic Field Due to 3-Phase Currents
When a 3-phase winding is energized from a 3-phase supply, a rotating magnetic field is produced. This
field is such that its poles do not remain in a fixed position on the stator but go on shifting their positions
around the stator. For this reason, it is called a rotating field. It can be shown that magnitude of this
rotating field is constant and is equal to 1.5 ᛰwhereᛰis the maximum flux due to any phase.
Principle of Operation
37. Page 37
When 3-phase stator winding is energized from a 3-phase supply, a rotating magnetic field is set up which
rotates round the stator at synchronous speed Ns = 120 f/P.
The rotating field passes through the air gap and cuts the rotor conductors, which as yet, are
stationary. Due to the relative speed between the rotating flux and the stationary rotor, e.m.f.sare induced
in the rotor conductors. Since the rotor circuit is short-circuited, currents start flowing in the rotor
The current-carrying rotor conductors are placed in the magnetic field produced by the stator.
Consequently, mechanical force acts on the rotor conductors. The sum of the mechanical forces on all the
rotor conductors produces a torque which tends to move the rotor in the same direction asthe rotating field.
The fact that rotor is urged to follow the stator field (i.e., rotor moves in the direction of stator field) can be
explained by Lenz‘s law. According to this law, the direction of rotor currents will be such that they tend
to oppose the cause producing them. Now, the cause producing the rotor
currents is the relative speed between the rotating field and the stationary rotor conductors. Hence to
reduce this relative speed, the rotor starts running in the same direction as that of stator field and tries to
We have seen above that rotor rapidly accelerates in the direction of rotating field. In practice, the rotor
can never reach the speed of stator flux. If it did, there would be no relative speed between the stator field
and rotor conductors, no induced rotor currents and, therefore, no torque to drive the rotor. The friction
38. Page 38
and windage would immediately cause the rotor to slow down. Hence, the rotor speed (N) is always less
than the suitor field speed (Ns). This difference
in speed depends upon load on the motor. The difference between the synchronous speed Ns of the
rotating stator field and the actual rotor speed N is called slip. It is usually expressed as a percentage of
synchronous speed i.e.
% age slip, s = Ns – N/Ns
(i) The quantity Ns - N is sometimes called slip speed.
(ii) When the rotor is stationary (i.e., N = 0), slip, s = 1 or 100 %.
(iii) In an induction motor, the change in slip from no-load to full-load is hardly 0.1% to 3% so that it is
essentially a constant-speed motor.
Starting Torque of 3-Phase Induction Motors
The rotor circuit of an induction motor has low resistance and high inductance. At starting, the rotor
frequency is equal to the stator frequency (i.e., 50 Hz) so that rotor reactance is large compared with rotor
resistance. Therefore, rotor current lags the rotor e.m.f. by a large angle, the power factor is low and
consequently the starting torque is small. When resistance is added to the rotor circuit, the rotor power
factor is improved which results in improved starting torque. This, of course, increases the rotor
impedance and, therefore, decreases the value of rotor current but the effect of improved power factor
predominates and the starting torque is increased.
Squirrel-cage motors. Since the rotor bars are permanently short circuited, it is not possible to add any
external resistance in the rotor circuit at starting. Consequently, the stalling torque of such motors is low.
Squirrelcage motors have starting torque of 1.5 to 2 times the full-load value with starting current of 5 to 9
times the full-load current.
Wound rotor motors. The resistance of the rotor circuit of such motors can be increased through the
addition of external resistance. By inserting the proper value of external resistance (so that R2 = X2),
maximum starting torque can be obtained. As the motor accelerates, the external resistance is gradually cut
out until the rotor circuit is short-circuited on itself for
39. Page 39
Classification Of Induction Motors
Squirrel cage Induction Motor
Among the a.c motors, squirrel cage induction motor is the most popular one. It is quite cheap, robust and
efficient. This motor also has good voltage regulation, high startingtorque and also requires less
maintenance. There are two types of three phase inductionmotors based on the rotor construction. They
are Squirrel cage induction motor and Wound / Slip ring induction motor. Almost 90-95% of the 3
phase induction motors are squirrel cage type.
As the name suggests, the rotor of this kind of motors are looks like the wheelcage of squirrel. Compared
to dc motor and synchronous motor, three phase induction motor (IM) is simple in construction. Let us
now discuss the construction of this kind of motor. As we all know that a three phase induction motor
essentially consists of a stator and a rotor.
Construction of Squirrel cage Induction Motor
Stator is similar for both kinds of three phase induction motors. Stator is the stationary part which is made
up of high grade alloy steel. Stator core carry alternating flux which produces hysteresis and eddy current.
In order to reduce these, stator core consists of highgrade, silicon steel punching. The thickness of
punching varies from 0.35 mm to 0.65 mm. The punching are insulated from each other by oxide /varnish
coating. The insulated stator conductors are placed in the slots. The statorconductors are connected to
form three phase windings. These windings are place on stator core slots. The three phase windings may
be either connected in star or in delta.
40. Page 40
The rotor core is a cylindrical laminated iron core , with slots around core carrying rotor conductors. Steel
laminations are provided on the rotor core. Each slot contains an uninsulated bar conductors of aluminium
or copper. At the end of each rotor, the rotor bar conductors are short circuited by heavy end rings. These
two components – conductors and end rings form the structure of a squirrel cage , that‘s why the rotor is
named so.In motors upto 100kW rating the squirrel cage structure is made up of cast aluminium. The
rotor conductor bars are not exactly parallel to the shaft but they are slightly skewed.
Squirrel cage induction motor rotor
41. Page 41
In case of dc motor, the current is drawn from the supply and conducted into armature conductor through
the brushes and commutator. Though in an induction motor, there is no electrical connection to the rotor,
but currents are induced in the rotor circuit .So same condition prevails as in case of dc motor i.e the
rotor conductors carry current in stator magnetic field and thereby have a force exerted upon them tending
to move them at right angles to the field. When the stator / primary winding of a 3 phase induction motor
is connected to a 3 phase a.c supply, a rotating magnetic field is established which rotates at synchronous
speed. The direction of revolution of this field will depend upon the phase sequence of the primary current
i.e the order of connection of primary terminals to the supply. The direction of this rotating field can be
reversed by interchanging the connection to the supply of any two leads of 3phase induction motor. The
speed at which the produced field will revolve is called the synchronous speed of the motor and is given
by the expression Ns = 120f / P where f and p is supply frequency and number of stator poles respectively.
The direction of rotating magnetic field will be clockwise and rotor is stationary at starting. The relative
motion of rotor with respect to stator field is anticlockwise. By Faraday‘s law of
electromagnetic induction, a voltage / emf will be induced in the conductor of rotor. As the rotor circuit is
complete this induced voltage causes a current to flow in rotor conductors.By applying Right hand rule ,
direction of induced emf or current in rotor conductor is found outwards. Direction of flux due to rotor
current alone is anticlockwise. We know that when a conductor carrying current is put in a magnetic field
42. Page 42
,a force is produced in it. Thus rotor conductors experiences the force whose direction is governed by Left
hand rule. Torques are produced on all rotor conductors which rotates the rotor in same direction as that of
rotating magnetic field . Three phase induction motor is self-starting and the motor is named so since its
operation depends upon induced voltage in the rotor conductors.
Slip ring Induction Motor
The stator windings of a wound-rotor motor are identical to those of a standard squirrel-cage
motor. A wound-rotor motor does fall into the category of induction motors, however, the conductor bars
of the squirrel-cage rotor are replaced with three phase windings wound with the same amount of poles as
the stator. The wound rotor windings terminate in slip rings mounted on the rotor shaft. Carbon brushes
ride on the slip rings, and during start-up they are externally connected in series to a three-phase resistor
bank. (One resistor for each phase, wye connected). Each set of external resistors are shorted out
simultaneously in one or more steps as the motor comes up to speed. Wound-rotor motors were one of the
first types of motors to allow variable-speed operation. By placing high wattage variable resistors in series
with the rotor windings, you could effectively control the speed of the motor. Wound-rotor motors are
especially useful because they are able to deliver high starting torque without overloading the electrical
43. Page 43
The stator of a wound-rotor motor is the same as the stator of a squirrel-cage induction motor. The
windings are placed in the slots of the stator 120 electrical degrees apart. The windings can be wound in
either a wye or a delta configuration. This motor can also be wound to run on a single voltage, or dual
44. Page 44
The rotor of a wound-rotor is made from laminations stacked together in the same manner as the
rotor of a squirrel-cage induction motor, with an oxide or varnish coating between each lamination.
However, the rotor of a wound-rotor motor is constructed by placing insulated coils of wire in the slots
instead of the solid conductor bars. A wound-rotor motor can be distinguished from a squirrel-cage
induction motor by the presence of the coils of wire in the windings instead of the solid conductor bars, by
the presence of the three slip rings on the shaft, and by the presence of an external resistance bank.
Rotor of slip ring induction motor
As with a squirrel-cage induction motor, three phase power is applied to the stator through the three motor
leads, T1, T2 and T3. This establishes the rotating magnetic field in the stator. The coils in the rotor have
current induced in them in a similar manner to a squirrel-cage induction motor. A wound-rotor motor is
normally started with full resistance in the circuit. As the motor accelerates, resistance is gradually
switched out of the rotor circuit. When the motor reaches full speed, all the resistance is switched out and
the rotor windings are shorted. The rotor windings themselves have only slightly more resistance than the
bars in a squirrel-cage rotor. This low resistance results in the same basic characteristics as a 3-phase
squirrel-cage induction motor, but with slightly more slip and slightly lower efficiency.
Wound rotor motors are usually not started with the slip rings shunted, as the rotor resistance is too low,
and starting currents would be much too high. By inserting resistance in the ring circuit, starting currents
are decreased and starting torque is increased.
45. Page 45
Starting and Torque
The maximum torque is produced when the maximum resistance is connected to the rotor and the induced
frequency is at its highest. A high-resistance rotor develops a high starting torque at low starting line
current. When no resistance is applied to the rotor circuit, the motor as the same basic starting torque
characteristics as an induction motor. The starting torque is about 125% of the full-load torque. When
maximum resistance is applied to the rotor, the motor‘s starting torque is almost equal to the motor‘s
breakdown torque, or about 200%. Once the motor starts, the speed of the rotor increases and the induced
frequency decreases, decreasing the induction and induced current in the rotor. The torque also decreases
as the induced current decreases. As the torque decreases toward the minimum torque needed, the
resistance can be removed
from the rotor circuit.Whenresistance is switched out of the circuit, the current increases. This increases
the torque, and allows the motor to continue to accelerate. At this point, there is no resistance, except for
the small amount in the coil conductors, and the motor operates in a manner similar to a standard squirrel-
cage induction motor.
46. Page 46
4.AC Replacement for DC Mill-Duty Motors
For many years, DC mill auxiliary motors for steel mill applications such as screwdowns, shears, tables
and sideguides were specified as ―mill duty.‖ They are of an extremely rugged design based on AIST
dimensional and rating standards, and are therefore interchangeable between motor manufacturers. Many
thousands of mill-duty motors remain in operation today, although many have been replaced with less-
rugged DC or AC motors.
The standard covers Totally Enclosed Non-Ventilated (TENV) and Totally Enclosed Forced Ventilated
(TEFV) designs. Motor manufacturers added their own designations, such as ―separately ventilated‖ and
―blower ventilated‖ to distinguish the type of forced ventilation. In the AC world, there are many
additional designations. The equivalent international standard for Totally Enclosed is IP44, protection
against 1-mm solid objects and splashing water. Other standards include IP54, protection against dust and
splashing water, and IP55, protection against dust and water jets. An increased enclosure protection level
generally involves increased cost.
The key mounting and shaft height dimensions are reproduced in Table.
Leadswere to be brought out on the left side facing the commutator end, also known as the non-drive end.
Terminal (conduit) boxes are an option. In the case of AC motors, there is no commutator, so the non-
drive end designation is more useful.
The standard specifies a horizontal frame split so that the armature can be lifted straight up and out. This
requirement is practical in a DC machine, which has few connections crossing the split. The armature
includes the commutator, the most technically complicated part of the motor, the part most prone to failure
and the part that requires periodic machining. In an AC machine, many leads would cross the split,
decreasing reliability while increasing the time to disassemble. There is also no commutator requiring peri-
odic maintenance. In most cases, a split-frame AC machine is not considered practical.
47. Page 47
Key Mounting and Shaft Height Dimensions From AIST Standard No. 1
Frame no D E F XC Definition of
802 7.625 6.250 8.250 32.875 D bottom of foot to
803 8.500 7.000 9.000 37.000 E width: center of
foot bolt to center
of motor frame
804 9.000 7.500 9.500 39.000 F length: center of
foot bolt to center
of motor frame
806 10.000 8.250 10.500 42.250 XC length from
shaft end to shaft
The shafts shall be replaceable and keyed according to the standard. Runout table motors generally had a
straight shaft, while motors for other applications were often tapered in addition to having the key.
Couplings are more convenient to field-replace when the shaft is tapered. However, since the AC motor
does not have a split frame, the entire motor would be replaced and repairs done in a motor shop.
The rating table introduces some additional terms. Totally Enclosed 1 Hour is the short-time TENV rating,
while Force Ventilated Continuous is the TEFV continuous ratings. The larger 620, 622 and 624 frames
use the term ―self-ventilated,‖ which is taken to mean TEFV from referencing motor manufacturer
It is important that the user understand the application rating of the DC motor being considered for
replacement with a new motor.
• All ratings are at 230 VDC.
48. Page 48
• Field excitation for series, compound and shunt windings are included, with the shunt wound
ratings being the most applicable to AC induction motors.
• The standard does not address the 820, 822 or 824 frames.
Table 2 gives the standard horsepower ratings by frame size, while Table 3 provides the torque ratings by
AIST Standard No. 1 specifies both starting and running torques, with the starting torques higher than the
equivalent running torques. This takes into consideration that the field and armature currents, and therefore
motor torque, can be higher than rated during the short time required for starting. However, the rms current
values must be at or below the rated value during normal operation.
It is useful to remember that motor currents were routinely measured but torque was not. In order to
determine the motor torque field current, armature current and motor design curves were required to
calculate the estimated torque. This occasionally led to heated discussions speculating on the motor‘s
ability to produce torque versus the actual and specified load requirements.
Motor frame size generally determined the current and torque capabilities, while the insulation level and
available applied voltage determined the horsepower and speed capabilities. This will be discussed in more
detail later when comparing the AC mill-duty motor speed and torque capabilities against the standard and
DC mill-duty motors supplied by motor manufacturers.
Horsepower and rpm Ratings of Frame 800 and 600 Motors in AIST Standard No. 1
Totally Enclosed 1 Hour or Force Ventilated Continuous
Frame no. HP Series Compound Shunt Adjustable Speed
802A 5 900 1,025 1,025 1,025/2,050
802B 7.5 800 900 900 900/1,800
802C 10 800 900 900 900/1,800
803 15 725 800 800 800/2,000
804 20 650 725 725 725/1,800
806 30 575 650 650 650/1,950
49. Page 49
Totally Enclosed 1 Hour or Self-Ventilated Continuous at 75°C rise
Frame no HP Series compound Shunt Adjustable
620 275 370 390 390 390/975
622 375 340 360 360 360/1,080
624 500 320 340 340 340/1,020
The standard calls for 230 VDC ratings, but suitable for operation at 500 VDC and reduced torque. This
ambiguous language was added in 1968 when DC rectifiers were beginning to be applied and the effects of
harmonic currents, called ―ripple‖ in the standard, on motor commutation and heating were not fully
understood. Motor manufacturers produced designs with a variety of voltage and torque ratings. Since
these were DC motors, raising the applied voltage from 230 to 500 roughly doubles the base speed and
horsepower ratings, while the nameplate may have the standard 230 VDC ratings. The point here is that
the user needs to understand how the motor is applied before simply changing it out with an AC motor
with the same nameplate horsepower and speed.
The standard specifies a 75°C rise by thermometer method or 110°C by resistance method, but not the
insulation class. In his 1969 article, Sherman observes that when the ―skeleton‖ motor frame became
obsolete around 1939, so did measurement by thermometer. Around 1968, motor manufacturers were
already providing mill-duty motors with Class H insulation systems good for a total temperature of 180°C.
This translates to a margin of error, commonly known as a hot spot allowance, of 30° over a 40°C ambient
(40 + 110 + 30 = 180). The equivalent is a 20°C hot spot allowance over a 50°C ambient, a more common
allowance at the time. The exception was runout table service motors, which were Class F.
50. Page 50
Class F insulation allows for a 155°C total temperature, or a 5°C hot spot allowance according to the
previous calculation. The required allowance for heating errors using modern motor modeling techniques
can be assumed to be much less than it was 40 years ago when the standard was issued. Also, the
application of runout table motors was based on torque, and the rms loading was generally less than the
100% rating, providing additional margin. In any case, the 110°C rise over a 50°C ambient is not allowed
with Class F insulation, since the total of the two is 160°C. An increased ambient temperature level gen-
erally involves increased cost.
It should be noted that Kelvin (K) and Centigrade (C) are used interchangeably for purposes of specifying
temperature rise. The standard uses Centigrade.
Torque Ratings of Frame 800 and 600 Motors in AIST Standard No. 1
Maximum starting torque (lb-ft) Maximum running torque on 230V (lb-ft)
Frame no Series compound Shunt Series compound Shunt
802A 145 115 92 116 90 75
802B 245 198 158 196 154 130
802C 330 263 175 262 205 160
803 545 445 275 440 345 265
804 810 650 435 650 505 390
806 1,370 1,100 725 1,100 855 650
Maximum starting torque (lb-ft) Maximum running torque on 230V (lb-ft)
Frame no Series compound Shunt series compound Shunt
620 19,500 16,650 13,320 15,600 12,950 11,100
622 29,000 24,600 19,660 23,200 19,110 16,380
624 41,100 34,740 27,790 32,800 27,020 23,160
51. Page 51
Adjustable speed motors were to slow down (droop) no more than 15% at rated load. This referred to the
technology of the day, where voltage or CEMF regulators were used to regulate speed without speed
feedback devices. The equivalent AC motor would have volts-per-hertz control, which should hold speed
within a few percent, i.e., have better speed regulation than the DC motor they replace.
Variations in Speed Due to Heating
DC motor flux levels are controlled by field current and motor characteristics. The standard allows for
15–20% speed variation from ambient to rated operating temperature. This corresponds roughly to the
change in the resistance of copper over that temperature range. Existing applications may use field control
by voltage and will experience this type of speed variation, while field control by current applications will
not have this speed variation. The equivalent AC motor should have little speed variation due to heating.
AIST maximum armature inertia
Frame no WK2(lb-ft2) GD2(kg-m2)
802 6 1.0
803 12 2.0
804 60 5.1
806 50 8.4
Variation From Rated Speed
Under normal operating temperature and rated conditions, a speed variation of 7.5% is allowed. The
equivalent AC motor should not see this effect.
The standard does not specify inertia for frames 620–624. Table 4 shows inertias given as WK2 from the
standard and also converted to GD2.
52. Page 52
AC 800 Series Mill-Duty Motors
Over the years, various motor manufacturers have offered AC mill-duty motors. These generally met
various aspects of AIST Standard No. 1, including the dimensional standard, but were not always designed
for the overload duty. The General Electric version, known as the KD, duplicated their DC-type MD
design, including the rolled steel frame, and was designed for a specific AC drive type.
The AIST standard was generally recognized around the world for steel mill auxiliary drives. Toshiba
Mitsubishi Electric Industrial Systems Corp. (TMEIC) recently introduced an AC mill-duty motor
intended to replace existing DC mill-duty motors both dimensionally and functionally. The motors are 3-
phase induction type designed for operation on pulse width modulated (PWM) inverters. AIST Standard
No. 1 uses English units. The new AC mill-duty motor was designed using the metric system and
converted to English units. The following is a review of the features of the frame 806–818 designs
Three types of enclosures are available. These are Totally Enclosed Non-Ventilated (TENV), Drip Proof
Separately Ventilated (DPSV) and Totally Enclosed Separately Ventilated (TESV) types. The Separately
Ventilated (SV) description specifies a separate source of cooling air as contrasted with the more general
Forced Ventilation (FV) designation. Some DC mill-duty motors were force ventilated with ambient air
using motor-mounted fans, which meant they were totally enclosed but subject to mill air.
Key motor mounting dimensions are given in Table 6. The metric (mm) units are from the motor outline
drawings, and the English (inches) units are from the standard. Conversion from the metric system using
25.4 mm/inch gives a maximum deviation of 0.07 inch.
Due to differences in nomenclature standards, a word description and the AIST/JIC is used in Table 6; for
example, ―Bottom of foot to shaft center (D/C),‖ where D is from AIST and C is shown in Figure 1.
The general AC mill-duty motor outline drawing is shown in Figure 1. The end view on the left is from the
non-drive end or opposite side of the motor facing the driven equipment. The terminal box is shown on the
left side for this example. Details by frame size are shown in the Leads section immediately following.
53. Page 53
Terminal boxes for termination of the 3-phase leads are a standard feature, as pictured in Figure 2. DC
mill-duty motors required two armature leads and two field leads, more if the fields were designed to be
reconnected in series or parallel. Replacement of the DC motors will require replacement of the DC leads
and with a new 3-phase cable terminated in the terminal box. The dimensions shown in Figure 2 are in
AC mill-duty motor outline drawing with double shaft extension
Frame 806–812 top-mounted terminal box.
54. Page 54
The standard terminal box location for the smaller frames is on the top of the motor, allowing for ease of
cable routing and termination. Left-side or right-side mounting of the terminal box is optional for the
smaller frame sizes and standard for the larger frame sizes.
Terminal Box Locations by Frame Size
Location of terminal box when facing the non-drive end
Frame no Top Left side Right side
806 Standard Option Option
808 Standard Option Option
810 Standard Option Option
DC mill-duty motors had rolled steel split frames. The AC mill-duty motor frames are rolled steel but not
split, for the reasons noted earlier. They also feature a robust cast aluminum and copper bar squirrel cage
rotor and form wound stator construction. The rotor construction process allows pull-out torques about
300%. Modern design programs allow verification of mechanical strength and electrical flux distribution.
Figure 4 shows the stator winding configuration of an AC motor. The picture shows why the split frame is
not considered practical for the AC mill-duty motor.
Some DC mill-duty motor shafts were tapered for quick field replacement of the coupling on the
replacement rotor shaft. Rotor replacement was possible without removing the entire motor because the
frame was split. Tapered shafts were not generally used for table motor applications, although the frame
may have been split. Permanent magnet table motors did not have the split frame feature. The AC mill-
duty motor shafts are keyed without taper or threaded locking nut. Special provisions for adapting the
coupling must be made in cases where the original motor has these features. Since the AC motor frame is
not split, the motor is replaced as a unit and the tapered shaft feature becomes less important.
55. Page 55
AIST Standard No. 1 provides ratings for series, compound and shunt DC motor excitation with 230 volts
applied from a DC generator. While many DC mill-duty motors were applied at 230 VDC, many were also
applied at 460 VDC, 500 VDC and various other ratings. At this point, it is important to remember that the
motor nameplate does not necessarily correspond to the motor application. The user must know the
application rating, including the rms and overload values, before attempting to apply the same frame size
The standard and general practice used the unit of pounds, with force assumed, in defining torque and
inertia. Note that Table 8 uses the more explicit term ―pounds of force‖ (lbf).
In the case of DC motors, the horsepower (HP) and speed (RPM) are generally assumed to be directly
proportional to the DC voltage applied to the armature, with the torque remaining unchanged. Therefore,
doubling the applied voltage doubles the HP and RPM for the same rated torque. Although this may not be
strictly correct due to motor characteristics, it is close enough for application purposes. Only the variable
speed shunt applications will be considered in comparing the DC and AC mill-duty motors
Ratings for 230 VDC and 500 VDC Applications
Ratings for 500 VDC Ratings for 230 VDC
Frame no HP Base RPM Top RPM Torque HP Base RPM
804 20 725 1,850 188 40 1,440
806 30 650 1,950 242 65 1,413
808 50 575 1,725 457 109 1,250
56. Page 56
AC mill-duty stator before inserting in frame
Useful Conversion Factors
kW HP x 0.746
Torque HP x 5,252 / RPMlbf-ft
0.1686 x WK2
In DC applications, torque was not usually measured, but calculated from currents and design curves, as
discussed earlier. Frames 802–818 generally did not have compensating windings necessary to make the
torque and armature current values proportional over the operating range. That is, 200% current was less
than 200% torque. This should be taken into consideration when specifying the equivalent AC motor.
57. Page 57
The 500 VDC rating in Table 9 was chosen not only because it was in common usage by North American
suppliers, but also to emphasize the need to understand the user‘s application rating. If your application is
at the commonly used 460 VDC, the AC motor ratings would match within ±1 HP when converted from
the design kW rating. The replacement of any existing drive system involves understanding the actual
application ratings, which can be quite different than the motor nameplate.
AC Mill-Duty Ratings Compared to DC Mill-Duty at 500 VDC
Totally Enclosed 1 Hour (TENV) or Force Ventilated Continuous (TEFV, DPSV or TESV)
500 VDC ratings At design VAC ratings
Frame no HP Base
kW HP Base
804 20 1,420 188 15 19 1,250 1,850 172
806 65 1,413 242 44 59 1,300 1,950 238
Variable speed operation includes constant torque from zero to rated base speed, the point at which motor
HP and kW are determined. Operation above base speed is at constant HP and kW; that is, torque
decreases proportionally to the speed increase. NEMA MG-1 decreases the overload requirement as the
top/base speed range increases, capping the decrease at 140% of the base speed overload at a 3-to-1 speed
Frame 818 speed-torque (lb-ft) characteristic from base to top speed.
58. Page 58
The frame 818 speed-torque curve is shown in Figure 5 for a 200% overload application at base speed and the
NEMA MG-1 193% overload at top speed. DC motors being replaced probably adhered to this standard. Since no
equivalent standard exists for AC motors, new applications sometimes specify the same overload throughout the
speed range. Induction motors inherently lose torque capability proportional to speed squared, and therefore may
have no overload capability at top speed unless one is specified.
The AC mill-duty motor is designed for operation from a PWM inverter and operation over the speed
ranges listed above.
The AC mill-duty motor incorporates Class F insulation and is rated based on Class F rise assuming a
40°C maximum ambient.
The speed torque characteristic for the frame 818 motor shows a slip, or speed loss due to load, that is
nearly linear over a wide range of loads. This means that, when used as a table motor with volts-per-hertz
control, applied loads will slow the motor from its no-load speed perhaps 1 or 2%, certainly much less
than the 15% allowed by the standard from no-load to rated load. Provided there is not excessive cable line
drop, overloads of 250% should be possible up to base speed with basic volts-per-hertz control.
Operation above base speed requires speed feedback control using the standard motor. Requirements for
higher overloads are possible but may require a larger standard frame or modified design.
The AC mill-duty motor inertias are less than the maximum values allowed by AIST Standard No. 1, but
higher than those of the equivalent DC motor. One of the commonly advertised features of AC versus DC
is that the AC motor inertia is lower. In this particular case, the rugged, solid rotor construction results in a
higher inertia than the equivalent DC motor manufactured by TMEIC.
Note that the DC motor inertias from other motor manufacturers will be different and may be given with
and without fans. It should also be noted that the response of modern PWM inverters, with speed feedback
device or without, is usually faster than the DC drive they replace. The net result is that the overall perfor-
mance of the new equipment should be at least as good as the replaced drive and motor.
59. Page 59
Additional Features of AC 800 Series Mill-Duty Motors
There are some additional features of the new AC mill-duty motor line that should be
One big advantage of AC motors over DC is their improved efficiency. When replacement of a
DC generator with an inverter is considered, the savings are more than doubled from those
shown. Motor efficiency is a function of design and operating conditions and will vary. The
numbers given in Table 12 are for reference only, comparing AC and DC mill-duty motors for
the same conditions.
AC 818 mill-duty motor characteristic curve
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AC Mill-Duty Inertia Compared to AIST and DC
TMEIC DC mill-duty
Frame no WK2
804 30 5.6 14 2.1-2.2 28 5.3
806 50 8.4 25-26 4.2-4.3 47 7.9
Comparison of AC and DC Motor Efficiencies Under Same Conditions
TMEIC DC mill-duty
TMEIC AC mill-duty
Frame no Kw RPM Efficiency Efficiency Improvement
804 15 725 85.7% 90.1% 2.9%
806 22 650 87.6% 91.5% 3.5%
The efficiencies listed in Table 12 are for rated conditions according to the standard. DC
motors operating at half and quarter speed will have significantly lower efficiencies because they
are basically the same motor operated at lower voltages. The AC mill-duty motors are always
higher efficiency than the equivalent DC mill-duty motor.
Motor Design Ratings
Each motor frame has designs corresponding to the equivalent 460 VDC, 230 VDC and 115
VDC ratings. They are labeled Standard, Medium and Low speed in Table 13. The rotor is the
same design, the stator winding connections are modified for approximately the same VAC and
therefore high efficiency.
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Responsive induction motor control requires that the motor voltage goes higher than the
nameplate rating to force the torque-producing component of stator current to rapidly pick up
load. This is especially true in the constant torque range of Figure 5. Some motor manufacturers
may list the maximum voltage on their nameplates, but in general this statement is true.
Therefore to get the maximum rating and performance, all 21 of the ratings in Table 13 would be
driven by an inverter capable of 420–460 VAC output at 70 hertz.
It is also worth noting that these motors are designed for variable speed applications and that
none of the frames happens to fall on either the 50 or 60 Hz line frequencies. The following
formula is for 3-phase motor synchronous speed:
RPM = (120 x Hertz) / Number of Poles
There is an important distinction between synchronous speed and rated speed in the application
of induction motors. Using Equation 1 and Table 13, one can determine that frame 818 is a 6-
pole design (6 = 120 x 44/880). Referring back to Table 10, we find a frame 818 rating of 870
rpm. This 10-rpm difference corresponds to the induction motor slip frequency required to
produce torque. One can therefore determine that the slip frequency at rating conditions of 100%
load requires approximately 1.4% slip (10/880). The motor nameplate will reflect the 870-rpm
rated condition at 100% load.
AC Mill-Duty Design Ratings
Frame no Speed kW RPM VAC Amps Hertz
804 Standard 15 1,614 370 68 78.4
806 Standard 44 1,314 370 96 65.7
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Roller bearings are used for the drive end, and ball bearings for the non-drive end. The non-drive
end bearing includes a grounding brush for additional protection against unwanted shaft currents
caused by inverter switching. In addition, frames 808 and above use insulated brackets for both
bearings. A special labyrinth seal is also optionally available, as shown in Figure.
Standard bearing arrangements: (a) non-drive end, (b) drive end.
2. Antifriction bearing
3. Standard seal
4. Inner bearing cover
5. Bearing nut
6. Bearing washer
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3. Labyrinth seal
4. Labyrinth cover
5. Inner Bearing cover
6. Bearing nut
7. Bearing washer
8. Bearing insulation brush
9. Outer bearing cover
10. Grease inlet
11. Grease outlet
An AC mill-duty motor in frames 806–818 is now available to replace existing DC mill-duty
motors conforming to AIST Standard No. 1. They are ideally suited to replace table motors and
may be used in other variable speed applications as well. The user must take care to understand
the application of the existing DC motor, as the nameplate may not reflect actual usage.
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5. Speed Control of AC Motors
An important factor in industrial progress during the past five decades has been the increasing
sophistication of factory automation which has improved productivity many fold. Manufacturing
lines typically involve a variety of variable speed motor drives which serve to power conveyor
belts, robot arms, overhead cranes, steel process lines, paper mills, and plastic and fiber
processing lines to name only a few. Prior to the 1950s all such applications required the use of a
DC motor drive since AC motors were not capable of smoothly varying speed since they
inherently operated synchronously or nearly synchronously with the frequency of electrical
input. To a large extent, these applications are now serviced by what can be called general-
purpose AC drives. In general, such AC drives often feature a cost advantage over their DC
counterparts and, in addition, offer lower maintenance, smaller motor size, and improved
reliability. However, the control flexibility available with these drives is limited and their
application is, in the main, restricted to fan, pump, and compressor types of applications where
the speed need be regulated only roughly and where transient response and low-speed
performance are not critical.
Unlike D.C. Motors, A.C. Induction Motors are not suitable for variable speeds. Their speed
control and regulation is comparatively difficult when compared with D.C. Motors.
Speed Control of Slip Ring Induction Motors
A slip ring motor or a phase wound motor is an induction motor which can be started with full
line voltage, applied across its stator terminals. The value of starting current is adjusted by
adding up external resistance to its rotor circuit.
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Slip ring induction motor
Slip Ring Motor Characteristics
A slip ring motor or a phase wound motor is an induction motor which can be started with full
line voltage, applied across its stator terminals. The value of starting current is adjusted by
adding up external resistance to its rotor circuit. Incase of a squirrel cage induction motor, the
value of rotor resistance is very low, which leads to heavy starting current requirement. But in
case of slip ring motors, the rotor resistance is increased by the addition of external resistance.
This external resistance makes the rotor circuit current low and thus the stator current drawn is
also low with a high starting torque. The point to be noted is the ―slip necessary to generate
maximum torque is directly proportional to the rotor resistance.‖ So it is evident that the slip
increases with increase in external resistance. With the above statements, let us discuss the
different methods of speed control of slip ring induction motors.
Speed control from stator side
Changing applied voltage
By varying the supply voltage (V) i,e the voltage supplied to the stator we can vary the speed of
an induction motor. This is a slip control method with constant frequency variable supply
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voltage. From the torque eq. we come to know that the torque developed by theinduction
motor is proportional to the square of the supply voltage. It is also known that slip at maximum
torque is independent of supply voltage. Here speed control is obtained by varying supply
voltage until the torque required by theload is developed at desired speed. This method of speed
control is suitable where load torque decreases with speed eg: fan load.
We have already discussed earlier that torque developed by the induction motor is proportional
to the square of the supply voltage and current proportional to voltage. So when we reduce the
voltage to reduce speed of the motor for same current value, the torque definitely reduces. The
stator voltage control is most suitable where intermittent drive operation is required. This method
is most used for fan / pump loads.
Speed control in single ph IM (domestic fan) is obtained by triac controller. Speed
control is done by varying firing angle of the triac.
Speed control of three ph IM is done by connecting three pairs of back to back connected
thyristors. Speed control is obtained by varying the conduction period of thyristors.
For speed control of small ac motors (single phase supply) we use two thyristors (back to
back) in anti-parallel.
This method is most easiest and cheapest. In this method speed of the motor is controlled by
changing the applied voltage across the motor terminals. Decreasing applied voltage will
decrease the speed of the motor and increasing voltage will increase the speed. But this method
is not used widely for following two reasons
(i) Large change in voltage is required for relatively small change in motor speed
(ii) This large change in voltage may disturb the magnetic conditions of the motor, as it changes
the flux density.
Changing the applied frequency
This method provides wide range of speed control with gradual variation of speed throughout
this range. The major difficulty with this method is how to get the variable frequency supply.
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The auxillary equipment required for this purpose results in a high cost, increased maintenance
and lowering of overall efficiency. That is why this method is not generally used but there are
certain applications where this method is very suitable. The synchronous speed of an induction
motor is given by Ns = 120f /P. The synchronous speed and therefore the speed of the motor can
be controlled by varying supply frequency. The emf induced in the stator of the induction
motor is given by E1 = 4.44 k f φ T1. From the emf equation it can be understood that a change
of frequency will result in a change of flux level unless the induced emf is changed in the same
ratio. An imbalance will result in an excessive flux and saturation or reduced flux and reduced
torque per ampere of current. Excessive flux will cause increase in iron losses. In order to avoid
saturation and to minimize losses, motor is operated at rated air gap flux by varying terminal
voltagewith frequency so as to maintain constant (V/f) ratio. This control method is known as
constant volts per hertz. The variable frequency supply is obtained by the following devices :-
1. Inverter (converts fixed voltage dc to fixed/variable voltage ac of variable frequency) –
voltage source inverter (VSI) and current source inverter (CSI).
2. Cycloconverter (converts fixed voltage, frequency ac to variable voltage
and frequency ac)
Thus speed can be varied by changing supply frequency. As changing in the supply frequency is
a difficult task, this method is used where motor is directly powered from a generator. We can
change the supply frequency by generator by changing speed of prime mover of the generator.
As we increase supply frequency, speed of the motor also increases and vice versa. This method
is being used to some extent in electrically driven ships.
Changing the number of stator poles
As said above, Ns = 120f/p. Thus by changing in number of stator poles we can change thespeed
of induction motor. This method is easily applicable for squirrel cage type induction motors, as
rotor of these motors adopts itself for any number of poles. To use this method, stator is wound
for two or more different winding with different poles. Only one winding will be in circuit at a
time other being disconnected. E.g. stator can be wound with two different windings having no.
of poles 2 and 4 respectively. In this case if supplied frequency is 50 Hz, (i) Ns = 120 * 50 / 2 =
69. Page 69
3000 rpm (for p = 2) and (ii) Ns = 120 * 50 / 4 = 1500 rpm (for p = 4). this method is being used
in elevator and traction motors.
Control from rotor side
Injecting emf in rotor circuit
An emf of same frequency as that of slip of the motor is injected in rotor circuit via slip rings.
When we insert voltage which is in phase with induced rotor emf, it is equivalent to decreasing
resistance of rotor. Whereas when we insert voltage which is opposite in phase with induced emf
in rotor, its like increasing resistance of rotor circuit. Hence by injecting emf in rotor circuit we
can control the speed of a induction motor.
Rotor Rheostat Control
The external rheostat which is used for the starting purpose of these slip ring motors can be used
for its speed control too. But the point to look into is the starting rheostat must be rated for
―continuous‖ operation. We have already discussed about the rheostat starting of slip ring
motors. With the same rheostat added to the rotor circuit, it is possible to regulate the speed of
slip ring motors. The resistance is engaged maximum during starting and slowly cut-off to
increase the speed of the motor. When running at full speed, if the need arises to reduce the
speed, the resistance is slowly added up and thus speed reduces. To understand the speed control,
let us look into the torque-slip relation given below.
Torque T = S/R.
Where S – is the slip of the motor,
And R – is the Rotor resistance
It is evident from the above relation that as the rotor resistance increases, the torque decreases.
But for a given load demand, the motor and thus the rotor has to supply the same torque without
any decrease. So in order to maintain the torque constant, as the rotor resistance increases the slip
also increases. This increase in slip is nothing but decrease in motor speed.
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But there are some disadvantages in this method of speed control. As the rotor resistance is
increased, the ‖ I2
R‖ losses also increases which in turn decreases the operating efficiency of the
motor. It can be interpreted as the loss is directly proportional to reduction in speed. Since the
losses are more, this method of speed reduction is used only for short period only.
This method has two motors mounted on same shaft called in tandem or cascade operation. The
motor ―A‖ which is connected to the mains is called as the main or the master motor. This motor
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has slip rings mounted on its rotor shaft from which the motor ―B‖ gets its supply from is called
as auxiliary or the slave motor. It is to be noted that both the motors are mounted on same shaft.
Thus it is evident that either the motors must run at same speed or it may have some gear
The main motor is necessarily a slip ring induction motor but the auxiliary motor can be slip ring
or squirrel cage induction motor. For satisfactory operation, motor ―A‖ must be phase wound/
slip ring type with the stator to rotor winding ratio of 1:1, so that in addition to cascade
operation, they can also run from supply mains separately. Since the supply for the slave motor is
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from the slip rings of the master motor, and it is forming a chain of sequential operation, the
system is called as ―Tandem or Cascade or Concatenation‖ operation.
Three or four different combinations are possible for attaining different speeds.
1. Main motor may be alone on the mains, where Na = 120f/Pa, where Pa is the number of poles
in motor ―A.‖
2. Auxiliary or the slave motor running alone on the mains, where Nb = 120f/Pb, where Pb is the
number of poles in motor ―B.‖
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3. The combination may be in cascade operation. In this operation, the important point is that the
phase rotation of the stator fields of the motors ―A‖ and ―B‖ must be in same direction. Thus the
synchronous speed of this cascaded motor set is given by Nc = 120f/ (Pa + Pb).
In my next article, we will discuss on the cascade set starting phenomena and speed control by
injecting e.m.f in the rotor circuit.
Speed Control of Squirrel Cage Induction Motor
Among the ac motors, squirrel cage induction motor is the most popular one. It is quite cheap,
robust and efficient. This motor also has good voltage regulation, high startingtorque and also
requires less maintenance. There are two types of three phase induction motors based on the
rotor construction. They are Squirrel cage induction motor and Wound / Slip ring induction
motor. Almost 90-95% of the 3 phase induction motors are squirrel cage type.
Squirrel cage induction motor
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Unlike D.C. Motors, A.C. Induction Motors are not suitable for variable speeds. Their speed
control and regulation is comparatively difficult when compared with D.C. Motors. These are
some of the methods which are commonly used for the speed control of squirrel cage induction
1. Changing Applied Voltage
2. Changing Applied Frequency
3. Changing Number Of Stator Poles
The above three methods are most commonly used for the speed control of squirrel cage
Changing Applied Voltage
This method, even though easiest, it is rarely used. The reasons are (a) for a small change in
speed, there must be a large variation in voltage. (b) This large change in voltage will result in
large change in flux density, thereby seriously disturbing the magnetic distribution/condition of
Changing Applied Frequency
We all know that the synchronous speed of the induction motor is given by Ns = 120f/P. So from
this relation, it is evident that the synchronous speed and thus the speed of the induction motor
can by varied by the supply frequency. This method has its own limitations. The motor speed can
be reduced by reducing the frequency, if the induction motor happens to be the only load on the
generators. Even then the range over which the speed can be varied is very less. This method is
famous in some electrically driven ships although not common in shore.
Changing The Number Of Stator Poles
As we know the relation between the synchronous speed and the number of poles, i.e. Ns =
120f/P. So the number of poles is inversely proportional to the speed of the motor. This change
of number of poles can be achieved by having two or more entirely independent stator windings
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in the same slots. Each winding gives a different number of poles and hence different
synchronous speed. For example, for the same motor, if no. of poles = 2 , 4 or 6, which can be
changed as per speed requirement, and lets say the supply frequency f = 50 Hz,
No. of Poles P = 2, then Ns = 120 * 50/2: So Ns = 3000 r.p.m
No. of Poles P = 4, then Ns = 120 * 50/4: So Ns = 1500 r.p.m
No. of Poles P = 6, then Ns = 120 * 50/6: So Ns = 1000 r.p.m.
Thus the speed control of squirrel cage induction motor can be done easily, but as steps of
reduced speed. This method is used for elevator motors, traction motors and also for machine
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6. Variable Frequency Drive
A Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by
varying the frequency and voltage supplied to the electric motor. Other names for a VFD
are variable speed drive,adjustable speed drive, adjustable frequency drive, AC
drive, microdrive, and inverter.
Frequency (or hertz) is directly related to the motor‘s speed (RPMs). In other words, the faster
the frequency, the faster the RPMs go. If an application does not require an electric motor to run
at full speed, the VFD can be used to ramp down the frequency and voltage to meet the
requirements of the electric motor‘s load. As the application‘s motor speed requirements change,
the VFD can simply turn up or down the motor speed to meet the speed requirement.
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Common VFD Terms
Variable Frequency Drive (VFD)
This device uses power electronics to vary the frequencyof input power to the motor, thereby
Variable Speed Drive (VSD)
This more generic term applies to devices that controlthe speed of either the motor or the
equipmentdriven by the motor (fan, pump, compressor, etc.).This device can be either electronic
Adjustable Speed Drive (ASD)
Again, a more generic term applying to bothmechanical and electrical means of controllingspeed
Working of Variable Frequency Drive
Induction motors, the workhorses of industry, rotate at a fixed speed that is determined by the
frequency of the supply voltage. Alternating current applied to the stator windings produces a
magnetic field that rotates at synchronous speed. This speed may be calculated by dividing line
frequency by the number of magnetic pole pairs in the motor winding. A four-pole motor, for
example, has two pole pairs, and therefore the magnetic field will rotate 60 Hz / 2 = 30
revolutions per second, or 1800 rpm. The rotor of an induction motor will attempt to follow this
rotating magnetic field, and, under load, the rotor speed "slips" slightly behind the rotating field.
This small slip speed generates an induced current, and the resulting magnetic field in the rotor
A VFD converts 60 Hz power, for example, to a new frequency in two stages: the rectifier stage
and the inverter stage. The conversion process incorporates three functions:
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A full-wave, solid-state rectifier converts three-phase 60 Hz power from a standard 208, 460,
575 or higher utility supply to either fixed or adjustable DC voltage. The system may include
transformers if higher supply voltages are used.
Electronic switches - power transistors or thyristors switch the rectified DC on and off, and
produce a current or voltage waveform at the desired new frequency. The amount of distortion
depends on the design of the inverter and filter.
An electronic circuit receives feedback information from the driven motor and adjusts the output
voltage or frequency to the selected values. Usually the output voltage is regulated to produce a
constant ratio of voltage to frequency (V/Hz). Controllers may incorporate many complex
Converting DC to variable frequency AC is accomplished using an inverter. Most currently
available inverters use pulse width modulation (PWM) because the output current waveform
closely approximates a sine wave. Power semiconductors switch DC voltage at high speed,
producing a series of short-duration pulses of constant amplitude. Output voltage is varied by
changing the width and polarity of the switched pulses. Output frequency is adjusted by
changing the switching cycle.
The first stage of a Variable Frequency AC Drive, or VFD, is the Converter. The converter is
comprised of six diodes, which are similar to check valves used in plumbing systems. They
allow current to flow in only one direction; the direction shown by the arrow in the diode
symbol. For example, whenever A-phase voltage (voltage is similar to pressure in plumbing
systems) is more positive than B or C phase voltages, then that diode will open and allow current
to flow. When B-phase becomes more positive than A-phase, then the B-phase diode will open
and the A-phase diode will close. The same is true for the 3 diodes on the negative side of the
bus. Thus, we get six current ―pulses‖ as each diode opens and closes. This is called a ―six-pulse
VFD‖, which is the standard configuration for current Variable Frequency Drives.
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Let us assume that the drive is operating on a 480V power system. The 480V rating is ―rms‖ or
root-mean-squared. The peaks on a 480V system are 679V. As you can see, the VFD dc bus has
a dc voltage with an AC ripple. The voltage runs between approximately 580V and 680V.
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We can get rid of the AC ripple on the DC bus by adding a capacitor. A capacitor operates in a
similar fashion to a reservoir or accumulator in a plumbing system. This capacitor absorbs the ac
ripple and delivers a smooth dc voltage. The AC ripple on the DC bus is typically less than 3
Volts. Thus, the voltage on the DC bus becomes ―approximately‖ 650VDC. The actual voltage
will depend on the voltage level of the AC line feeding the drive, the level of voltage unbalance
on the power system, the motor load, the impedance of the power system, and any reactors or
harmonic filters on the drive.
The diode bridge converter that converts AC-to-DC, is sometimes just referred to as a converter.
The converter that converts the dc back to ac is also a converter, but to distinguish it from the
diode converter, it is usually referred to as an ―inverter‖. It has become common in the industry
to refer to any DC-to-AC converter as an inverter.
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When we close one of the top switches in the inverter, that phase of the motor is connected to the
positive dc bus and the voltage on that phase becomes positive. When we close one of the bottom
switches in the converter, that phase is connected to the negative dc bus and becomes negative.
Thus, we can make any phase on the motor become positive or negative at will and can thus
generate any frequency that we want. So, we can make any phase be positive, negative, or zero.
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The blue sine-wave is shown for comparison purposes only. The drive does not
generate this sine wave.
Notice that the output from the VFD is a ―rectangular‖ wave form. VFD‘s do not produce a
sinusoidal output. This rectangular waveform would not be a good choice for a general purpose
distribution system, but is perfectly adequate for a motor.
If we want to reduce the motor frequency to 30 Hz, then we simply switch the inverter output
transistors more slowly. But, if we reduce the frequency to 30Hz, then we must also reduce the
voltage to 240V in order to maintain the V/Hz ratio (see the VFD Motor Theory presentation for
more on this). How are we going to reduce the voltage if the only voltage we have is 650VDC?
This is called Pulse Width Modulation or PWM. Imagine that we could control the pressure in a
water line by turning the valve on and off at a high rate of speed. While this would not be
practical for plumbing systems, it works very well for VFD‘s. Notice that during the first half
cycle, the voltage is ON half the time and OFF half the time. Thus, the average voltage is half of
480V or 240V. By pulsing the output, we can achieve any average voltage on the output of the
83. Page 83
VFDs Can Control Multiple Motors
Many applications use one or more motors operating in parallel at the same desired speed. Using
one Variable frequency drive (VFD) to control these multiple motors provides a host of
advantages as summarized below.
1. Saves money
2. Cuts cabinet size, complexity and design costs
3. Can reduce footprint of the motors and driven loads
4. Cuts maintenance time and cost
5. Reduces inventory stockingrequirements
6. Reduces control system complexity.
Money is saved because one high horsepower rated VFD is less expensive than multiple low
horsepower VFDs. Each VFD requires its own circuit protection, so using one VFD reduces cost
in this area as well.
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The VFD enclosure can be smaller because one large VFD requires less cabinet space than
multiple smaller units. This saves space and money. Design costs are also cut because it‘s easier
to engineer an enclosure to house one relatively large VFD as opposed to multiple smaller VFDs.
One large VFD also produces less heat than multiple smaller units, further simplifying enclosure
design and saving energy.
In terms of the motors and connected loads, the footprint can also often be reduced. For example,
it may be possible to fit multiple small fans of a smaller size into a confined duct space as
opposed to one large fan.
Maintenance time and costs are cut because only one VFD has to be serviced as opposed to
multiple smaller VFDs, often of varying sizes. This also reduces inventory stocking
requirements. Each motor will be smaller and usually available as on off-the-shelf standard
product. Because many of the motors will be identical, spares can be stocked and replaced
quickly in a failure.
The overall control system also becomes much simpler. Instead of connecting many VFDs to the
main controller, usually a PLC, and synchronizing their operation; only one connection is
required. When programming the PLC, only one VFD speed control loop needs to be configured,
instead of multiple instances.
Multiple motor system
Taken in total, the above benefits may justify use of a VFD in applications where using one VFD
per motor is cost prohibitive. When this is the case, the application benefits from all of the