2. Induction and Synchronous Motor Fundamentals (1 PDH)
PDHengineer.com
Course No. E-1005
Introduction
Motors are electromagnetic devices that are used to convert electrical energy into
mechanical work. There are three classes of AC motors – synchronous motors, induction
motors and series wound motors. The most common motor classes are synchronous and
induction.
NEMA MG 1-2003 has the following definitions:
An induction machine is an asynchronous machine that has a magnetic circuit interlinked
with two electric circuits, or sets of circuits, rotating with respect to each other. Power is
transferred from one circuit to another by electromagnetic induction.
A synchronous machine is an alternating-current machine in which the average speed of
normal operation is exactly proportional to the frequency of the system to which it is
connected.
Synchronous motors
A synchronous motor is a synchronous machine used for a motor. A synchronous motor
cannot start without being driven. They need a separate starting means.
There are several types of synchronous motors. These include direct current excited
synchronous motor (field poles are excited by direct current), a permanent magnet
synchronous motor (field excitation is provided by permanent magnets) and a reluctance
synchronous motor (starts as an induction motor, is normally provided with a squirrel-
cage winding, but operates at synchronous speed).
Synchronous motors have fixed stator windings electrically connected to the AC supply
with a separate source of excitation connected to a field winding on the rotating shaft. A
three-phase stator is similar to that of an induction motor. The rotating field has the same
number of poles as the stator, and is supplied by an external source of DC. Magnetic flux
links the rotor and stator windings causing the motor to operate at synchronous speed. A
synchronous motor starts as an induction motor, until the rotor speed is near synchronous
speed where it is locked in step with the stator by application of a field excitation. When
the synchronous motor is operating at synchronous speed, it is possible to alter the power
factor by varying the excitation supplied to the motor field.
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3. An important advantage of a synchronous motor is that the motor power factor can be
controlled by adjusting the excitation of the rotating DC field. Unlike AC induction
motors which run at a lagging power factor, a synchronous motor can run at unity or even
at a leading power factor. This will improve the overall electrical system power factor,
voltage drop and also improve the voltage drop at the terminals of the motor.
Synchronous motors can supply reactive power to counteract lagging power factor cause
by inductive loads. As the DC field excitation is increased, the power factor (as
measured at the motor terminals) becomes more leading. If the excitation is decreased,
the power factor of the motor becomes more lagging.
Refer to the above graph. The curves on the graph show the effect of excitation (field
amps) on the stator and on the system power factor. There are separate V curves for No-
Load and Full Load cases. A manufacturer may also have curves for other percentages of
full load (25%, 50%, 75%). From this particular curve, to determine the field excitation
that will produce a unity power factor at full load: Go up the Y-axis to unity power factor
(100%). Come across the X-axis to the peak of the Power Factor V curve for full load
operation. Come back down the Y-axis from that point to determine the field amps. In
this case, the field amps is just over 10 amps. Notice that at unity power factor, the stator
full load amps is at the minimum value. As the field amps increases above what is
required for unity power factor, the motor becomes more leading. As the amps decrease
below what is required for unity power factor, the motor becomes more lagging. In either
case, the stator amps increases above that required for unity power factor.
Synchronous motors can be classified as brush excitation or brushless excitation. Brush
excitation consists of cast-brass brushholders mounted on insulated steel rods and
supported from the bearing pedestal. The number of brushes for a particular size and
rating depends on the field current. Sufficient brushes are supplied to limit the current
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4. density to a low value. The output of a separate DC exciter is applied to the slip rings of
the rotor. A brushless excitation system utilizes an integral exciter and rotating rectifier
assembly that eliminates the need for brushes and slip rings.
Synchronous motors are started using several reduced voltage methods. The most
common is starting across the line with full AC voltage to the windings. As the motor
speed increases, the discharge resistor provides the torque required for the motor to reach
synchronous speed. Once synchronous speed is reached, the starting resistor is switched
out of the field circuit and excitation can be applied to lock the stator and field poles in
sync. The DC excitation system is used to apply current to the field winding creating a
rotating electromagnet field that couples the rotor field to the rotating AC field in the
armature winding when the motor is operating at synchronous speed. If the North and
South poles of the rotor and stator are aligned, the rotor will lock in step with the stator
and the motor will synchronize. If the rotor poles are 180 degrees out of phase with the
stator poles, but the motor is accelerating, it is likely that accelerating torque along with
magnetic attraction will combine to pull the rotor rapidly into pole alignment with the
stator.
Other starting methods include reduced voltage starting, such as using an
autotransformer. Another starting approach is to switch out the starting resistor and apply
DC excitation based upon the time after the motor AC supply power is applied. In this
approach, the acceleration time of the motor needs to be known and the motor must be
able to reach nearly synchronous speed without excitation.
Some applications use a speed signal to apply DC excitation when the motor has
accelerated to 90 – 95% of rated speed. The timing for switching out the starting resistor
and applying DC excitation is monitored by electronics on the rotating field. Application
of the field can be accomplished using solid-state devices instead of mechanical breakers
or contactors.
Once the motor’s field poles are in step with the stator frequency, two factors determine
the synchronous speed of the motor. The first is the frequency of the applied voltage, and
the second is the number of poles in the motor.
Speed = Freq x 120
Poles
Synchronous motor efficiencies are higher than those of induction motors. Their inrush
currents are low. They can be designed with torque characteristics to meet the
requirements of the driven load and available power supply. A synchronous motor’s
speed/torque characteristics are ideally suited for direct drive of large horsepower, low
rpm loads such as reciprocating compressors. Their precise speed regulation makes them
an ideal choice for certain industrial processes.
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5. Synchronous motors are used in the pulp, paper processing, water processing treatment,
petrochemical and mining industries, to name a few. They are used for chippers,
crushers, pumps, and compressor drives to name a few applications.
Synchronous motors are designed to meet NEMA MG1-21.21. Noise tests are performed
per IEEE 85 and performance tests per IEEE 115 for machine efficiency, temperature
rises, starting characteristics and other parameters.
Induction motors
Induction motors are simple and rugged and relatively cheap to construct. They consist
of a wound stator and a rotor assembly. They have fixed stator windings that are
electrically connected to an AC power source. Current is induced in the rotor circuit.
The resulting magnetic field interacts with the stator field for the “induction” to occur.
No separate power source is required to provide the rotor field. An induction motor can
be started and accelerated to steady state running conditions simply by applying AC
power to the fixed stator windings of the motor. They do not rely on brushes like a DC
motor does. Induction motors have a longer life than synchronous motors and are
common for applications above 1 kW.
There are a couple of types of induction motors – a squirrel-cage motor and a wound-
rotor motor. A squirrel-cage motor is one where the secondary circuit consists of a
number of conducting bars that have their end pieces connected by metal rings or plates
at each end. A wound-rotor motor in one where the secondary circuit has a polyphase
winding or coils whose terminals are either short circuited or closed through suitable
circuits.
The rotor assembly of an induction motor, when looked at from the end, resembles a
squirrel cage (or a hamster exerciser). Thus the name squirrel-cage motor refers to an
induction motor. The most common rotor type has cast aluminum conductors (bars) and
short-circuiting end rings. The position of the bars in relation to the surface of the rotor,
the shape, cross sectional area and material of the bars determine the rotor characteristics.
A bar with a large cross sectional area will exhibit a low resistance. A copper bar will
have a low resistance compared to a brass bar of equal proportions. The rotor design will
determine the starting characteristics of the motor. The rotor turns when the moving
magnetic field induces a current in the shorted conductors.
The stator of an induction motor is the outer body of the motor. This houses driven
windings on an iron core. The standard stator has three windings for a three-phase
design. A single-phase motor typically has two windings. The core of the stator is made
up of a stack of round pre-punched laminations pressed into a frame that is made of
aluminum or cast iron. Laminations are round with a round hole where the rotor is
positioned. The inner surface of the stator has slots or grooves where the windings are
positioned. The arrangement of the windings determines the number of poles that a
motor has. A stator is like an electromagnet and has poles (north and south) in multiples
of two (2-pole, 4-pole, etc.). The voltage rating of the motor is determined by the number
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6. of turns on the stator. The power rating of the motor is determined by the losses. These
include copper loss, iron loss and the ability of the motor to dissipate the heat generated
by the losses. The design of the stator determines the rated speed of the motor as well as
the full load/full speed characteristics.
The synchronous speed of the motor is the speed where the magnetic field rotates. It is
determined by the number of poles in the stator and the frequency of the power supply. It
is the absolute upper limit of motor speed. There is no difference between the rotor speed
and rotating field speed. This means no voltage is induced in the rotor bars and therefore
no torque is developed. When running, the rotor must rotate slower than the magnetic
field, to cause the proper amount of rotor current to flow so that the torque that develops
is able to overcome the winding and friction losses and therefore drive the load. This
speed difference is called slip.
Most motors use the squirrel cage design. An alternate design, wound rotor, is used when
variable speed is desired. Compared to squirrel cage rotors, wound rotors are expensive
and require more maintenance. A wound rotor motor has controllable speed and torque.
Single-phase AC induction motors are typically used in devices requiring low torque like
fans and other household appliances. A split-phase induction motor is used in larger
household appliances such as washers and dryers. These are designed to provide greater
starting torque.
Common terms
Locked rotor torque – the minimum torque that the motor develops at rest for all angular
positions of the rotor at rated voltage and frequency.
Locked rotor current – the steady state current from the line at rated voltage and
frequency with the rotor locked.
Breakdown torque – the maximum torque that the motor develops at rated voltage and
frequency without an abrupt drop in speed.
Pull up torque – the minimum torque developed during the period of acceleration from
rest to the speed that breakdown torque occurs.
In order to perform useful work, the induction motor must be started from rest and both
the motor and load accelerated up to full speed. As the motor accelerates, the torque and
the current will alter with rotor speed if a constant voltage is maintained. The starting
current of a motor, with a fixed voltage will drop slowly as the motor accelerates and will
begin to fall significantly when the motor has reached between 80% and full speed. The
general curve for an induction motor indicates a high current until the motor has almost
reached full speed. The locked rotor current of a motor typically falls between 550% and
750% of the full load current. Refer to the typical curve below.
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7. An induction motor operates due to the torque developed by the interaction of the stator
field and the rotor field. These fields are due to currents which are resistive and reactive.
The torque developed is dependent on the resistive current and is related to the I2R of the
rotor.
Once the motor is up to speed, it operates at low slip at a speed determined by the number
of stator poles. The synchronous speed of a 4-pole machine operating at 60 hertz is 1800
rpm.
Enclosure types (protection) and cooling for induction and synchronous
motors
There are several types of protection and cooling for motors. Only a few are listed
below.
Open Machine
An open machine means a machine that has no restriction to ventilation other than the
mechanical construction of the machine. There are several types of open machines,
including the following:
• A drip proof machine is a machine that is protected from drops of liquid or solid
particles that could strike or enter the enclosure at any angle from 0 to 15 degrees
downward from the vertical.
• A splash proof machine is a machine that is protected from drops of liquid or
solid particles that could strike or enter the enclosure at any angle not greater than
60 degrees downward from the vertical.
• A guarded machine is a machine where all openings that have direct access to
live metal or rotating parts are limited in size by the structural parts or by screens,
baffles or other means to prevent accidental contact with hazardous parts.
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8. • A semi-guarded machine is a machine in which part of the ventilating openings
in the machine (usually the top half) are guarded, but others are left open.
Weather Protected Machine
There are two types of weather protected machines: a Type 1 which is guarded with its
ventilation constructed to minimize the entry of rain, snow and air-borne particles, or
Type 2 which is constructed similar to a Type 1, but its ventilation is constructed at the
intake and discharge such that storms or high winds cannot blow directly into the electric
parts of the machine itself.
Totally Enclosed Machine
A totally enclosed machine is enclosed to prevent the free exchange of air between the
inside and outside of the case, but not airtight or dust tight. A totally enclosed
nonventilated machine is a machine cooled by free convection. A totally enclosed fan-
cooled machine is equipped for cooling by fans integral to the machine but external to the
enclosing parts. A totally enclosed water cooled machine is a machine cooled by
circulating water in contact with the machine parts. A water proof machine is generally
constructed so that a stream of water from a hose will not enter the machine.
Explosion Proof Machine
An explosion proof machine is a machine where the enclosure is designed to withstand
the explosion of a specified gas or vapor which may occur within it, and to prevent the
ignition of the gas or vapor surrounding the machine due to sparks, flashes or explosions
that may occur within the machine casing.
Insulation systems
There are different insulating components used in the process of building a motor, such
as the enamel coating on the magnet wire and the insulation on the leads in the motor
box. Another important component is the dipping varnish which is used to seal scratches
that may have occurred and binds the winding together so that it does not vibrate or chafe
when subjected to the magnetic force that exists in the motor. Insulation systems are
divided into classes based on the thermal aging and failure. Four classes are commonly
used in motors – A, B, F, and H. Refer also to IEEE Std. 1. The temperature classes are
separated by 25 degree C increments. The temperature capability of each class is defined
as the maximum temperature at which the insulation can be operated to yield an average
life of 20,000 hours.
Class A - Rated 105 degrees C
Class B - Rated 130 degrees C
Class F - Rated 155 degrees C
Class H - Rated 180 degrees C
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9. The temperature rise of a motor is the change in temperature when it is being operated at
full load. For example, if a motor is located outside and the temperature is 80 degrees F,
and is then started and operated at full load, the winding temperature would rise from 80
degrees F to a higher temperature. This difference is the motor’s temperature rise.
Nearly all electric motors are rated based on a starting temperature of 40 degrees C
ambient temperature (104 degrees F). The temperature rise is added to the ambient
temperature.
Example: Suppose a motor is designed with Class A insulation and a maximum
temperature rise of 55 degrees C. When operated in a 40 degree C ambient temperature,
the total average winding temperature would be 95 degrees C (40 + 55 degrees C). Class
A insulation is rated for 105 degrees C temperature, and this difference of 10 degrees is
used to handle “hot spots”. (The winding temperature is an average change of the entire
winding. Some of these spots may be hotter than others – called hot spots.)
Suppose the same motor now is designed with Class B insulation. Class B insulation is
rated for 130 degrees C. The temperature difference is now 25 degrees C.
Designing a motor with higher rated insulation allows for extra thermal capability to
handle higher than normal ambient temperatures, overloads, or extend motor life because
the motor will be able to handle overheating. Overheating may be due to frequent starts,
high or low voltages or voltage imbalances. By changing the insulation class, it is
usually possible to increase the service factor of a motor from 1.0 to something higher,
such as 1.15. The same change could also make the motor more suitable for operation in
high elevations where the air is thinner and has less cooling effect. The service factor is
the ability to continuously run a motor at or above its nameplate full load horsepower
rating. A 1.15 service factor on a 100 hp motor allows it to be run continuously at 115%
of its nameplate rating (or 115 hp).
As a general rule, insulation life doubles for each 10 degrees of unused insulation
temperature capability. For instance, for a motor designed for 110 degrees C (including
the ambient temperature, rise and “hot spot” allowance) with Class B insulation, there
would be a used capacity of 20 degrees C. This difference would raise the motor
insulation life from 20,000 to 80,000 hours. (20,000 hours = normal life; doubled for 10
degrees = 40,000; doubled for another 10 degrees = 80,000 hours). This also applies if a
motor isn’t loaded to its full capacity or if it’s operated at a lower than 40 degree C
ambient temperature. Similarly, if motors operate above their rated temperature, then the
insulation life is halved for each 10 degrees of over temperature.
Insulation life can also be affected, aside from temperature, by moisture, chemicals, oil
and vibration to name a few other factors. Also, the classification of an insulation system
is based on the temperature rating of the lowest rated component used in the system.
Therefore if one Class B component is used with Class F components, the entire system
must be classified as Class B.
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10. There are several NEMA specifications and IEEE specifications devoted to motor design,
specification and installation. Refer to these for further information.
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