1. CHAPTER 5
SPECIAL MOTOR AND SINGLE-PHASE INDUCTION MOTOR
5.1 STEPPER MOTOR
A stepper motor (or step motor) is a brushless, synchronous electric motor that can
divide a full rotation into a large number of steps. The motor's position can be controlled
precisely, without any feedback mechanism. Stepper motors are similar to switched
reluctance motors (which are very large stepping motors with a reduced pole count, and
generally are closed-loop commutated.)
Fundamentals of Operation
Stepper motors operate differently from DC brush motors, which rotate when voltage is
applied to their terminals. Stepper motors, on the other hand, effectively have multiple
"toothed" electromagnets arranged around a central gear-shaped piece of iron. The
electromagnets are energized by an external control circuit, such as a microcontroller. To
make the motor shaft turn, first one electromagnet is given power, which makes the gear's
teeth magnetically attracted to the electromagnet's teeth. When the gear's teeth are thus
aligned to the first electromagnet, they are slightly offset from the next electromagnet. So
when the next electromagnet is turned on and the first is turned off, the gear rotates
slightly to align with the next one, and from there the process is repeated. Each of those
slight rotations is called a "step," with an integer number of steps making a full rotation.
In that way, the motor can be turned by a precise angle.
Stepper motor characteristics
1. Stepper motors are constant power devices.
2. As motor speed increases, torque decreases.
3. The torque curve may be extended by using current limiting drivers and increasing the
driving voltage.
4. Steppers exhibit more vibration than other motor types, as the discrete step tends to
snap the rotor from one position to another.
5. This vibration can become very bad at some speeds and can cause the motor to lose
torque.
6. The effect can be mitigated by accelerating quickly through the problem speeds range,
physically damping the system, or using a micro-stepping driver.
114

2. 7. Motors with a greater number of phases also exhibit smoother operation than those
with fewer phases.
Open-loop versus closed-loop commutation
Steppers are generally commutated open loop, ie. the driver has no feedback on where the
rotor actually is. Stepper motor systems must thus generally be over engineered,
especially if the load inertia is high, or there is widely varying load, so that there is no
possibility that the motor will lose steps. This has often caused the system designer to
consider the trade-offs between a closely sized but expensive servomechanism system
and an oversized but relatively cheap stepper.
A new development in stepper control is to incorporate a rotor position feedback (eg. an
encoder or resolver), so that the commutation can be made optimal for torque generation
according to actual rotor position. This turns the stepper motor into a high pole count
brushless servo motor, with exceptional low speed torque and position resolution. An
advance on this technique is to normally run the motor in open loop mode, and only enter
closed loop mode if the rotor position error becomes too large -- this will allow the
system to avoid hunting or oscillating, a common servo problem.
Types
There are three main types of stepper motors.
1. Permanent Magnet Stepper
2. Hybrid Synchronous Stepper
3. Variable Reluctance Stepper
Permanent magnet motors use a permanent magnet (PM) in the rotor and operate on the
attraction or repulsion between the rotor PM and the stator electromagnets. Variable
reluctance (VR) motors have a plain iron rotor and operate based on the principle of that
minimum reluctance occurs with minimum gap, hence the rotor points are attracted
toward the stator magnet poles. Hybrid stepper motors are named because they use use a
combination of PM and VR techniques to achieve maximum power in a small package
5.2 UNIVERSAL MOTOR
A wound field DC motor with the field and armature windings connected in series is
called either a "series-wound motor" or a "universal motor," because of its ability to
operate on AC or DC power. The ability to operate on AC or DC power is because the
current in both the field winding and the armature (and hence the resultant magnetic
fields) will alternate (reverse polarity) at the same time, and hence the mechanical force
generated is always in the same direction.
The torque of a series-wound or universal motor declines slowly with speed. Although
this can be advantageous for some applications, it also means that, unloaded, the motor
115

3. may "run away" and speed up to the point of mechanical failure. However factors such as
external load and internal mechanical resistance may adequately limit the speed.
Operating at normal power line frequencies, universal motors are very rarely larger than
one kilowatt (about 1.3 horsepower). Universal motors also form the basis of the
traditional railway traction motor in electric railways. In this application, to keep their
electrical efficiency high, they were operated from very low frequency AC supplies, with
25 and 16.7 hertz (Hz) operation being common. Because they are universal motors,
locomotives using this design were also commonly capable of operating from a third rail
powered by DC.
An advantage of the universal motor is that AC supplies may be used on motors which
have some characteristics more common in DC motors, specifically high starting torque
and very compact design if high running speeds are used. The negative aspect is the
maintenance and short life problems caused by the commutator. As a result such motors
are usually used in AC devices such as food mixers and power tools which are used only
intermittently, and often have high starting-torque demands. Continuous speed control of
a universal motor running on AC is easily obtained by use of a thyristor circuit, while
(imprecise) stepped speed control can be accomplished using multiple taps on the field
coil. Household blenders that advertise many speeds frequently combine a field coil with
several taps and a diode that can be inserted in series with the motor (causing the motor
to run on half-wave rectified AC).
Universal motors generally run at high speeds, making them useful for appliances such as
blenders, vacuum cleaners, and hair dryers where high RPM operation is desirable.
Motor damage may occur due to overspeeding (running at an RPM in excess of design
limits) if the unit is operated with no significant load. On larger motors, sudden loss of
load is to be avoided, and the possibility of such an occurrence is incorporated into the
motor's protection and control schemes. In some smaller applications, a fan blade
attached to the shaft often acts as an artificial load to limit the motor speed to a safe
value, as well as a means to circulate cooling airflow over the armature and field
windings.
"Universal" or "Series-wound" motors generally operate better with DC current, but they
have the ability to operate with AC current as well, making them very versatile for a
broad range of applications. However, there is little to no means to control the motor's
speed accurately. Unlike induction motors, the "goal" of this motor is to run a load at the
highest speed possible, which has specific advantages for appliances such as vacuum
cleaners and blenders and such. Many automotive starter motors are either series-wound
or compound-wound motors because of the high starting torque.
116

4. 5.3 SERVO MOTOR
A servo motor is a dc, ac, or brushless dc motor combined with a position sensing
device(e.g. a digital decoder). In this section, our discussion will be focused on the three-
wire DC servo motors that are often used for controlling surfaces on model airplanes. A
three-wire DC servo motor incorporates a DC motor, a geartrain, limit stops beyond
which the shaft cannot turn, a potentiometer for position feedback, and an integrated
circuit for position control.
Servos are extremely useful in robotics. The motors are small and are extremely powerful
for thier size. A standard servo such as the Futaba S-148 has 42 oz/inches of torque,
which is pretty strong for its size. It also draws power proportional to the mechanical
load. A lightly loaded servo, therefore, doesn't consume much energy. The guts of a servo
motor are shown in the picture below. You can see the control circuitry, the motor, a set
of gears, and the case. You can also see the 3 wires that connect to the outside world. One
is for power (+5volts), ground, and the white wire is the control wire.
Operation
The servo motor has some control circuits and a potentiometer (a variable resistor, aka
pot) that is connected to the output shaft. The potentiometer allows the control circuitry
to monitor the current angle of the servo motor. If the shaft is at the correct angle, then
the motor shuts off. If the circuit finds that the angle is not correct, it will turn the motor
the correct direction until the angle is correct. The output shaft of the servo is capable of
travelling somewhere around 180 degrees. Usually, its somewhere in the 210 degree
range, but it varies by manufacturer. A normal servo is used to control an angular motion
of between 0 and 180 degrees. A normal servo is mechanically not capable of turning any
farther due to a mechanical stop built on to the main output gear. The amount of power
applied to the motor is proportional to the distance it needs to travel. So, if the shaft needs
to turn a large distance, the motor will run at full speed. If it needs to turn only a small
amount, the motor will run at a slower speed. This is called proportional control. How do
you communicate the angle at which the servo should turn? The control wire is used to
communicate the angle. The angle is determined by the duration of a pulse that is applied
to the control wire. This is called Pulse Coded Modulation. The servo expects to see a
pulse every 20 milliseconds (.02 seconds). The length of the pulse will determine how far
the motor turns. A 1.5 millisecond pulse, for example, will make the motor turn to the 90
degree position (often called the neutral position). If the pulse is shorter than 1.5 ms, then
the motor will turn the shaft to closer to 0 degress. If the pulse is longer than 1.5ms, the
shaft turns closer to 180 degrees.
5.4 RELUCTANCE MOTOR
A reluctance motor is a type of synchronous electric motor that induces non-permanent
magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon
of magnetic reluctance.
117

5. A reluctance motor, in its various incarnations, may be known as a:
• Synchronous reluctance motor
• Variable reluctance motor
• Switched Reluctance Motor
• Variable reluctance stepping motor
Reluctance motors can have very high power density at low-cost, making them ideal for
many applications. Disadvantages are high torque ripple when operated at low speed, and
noise caused by torque ripple. Until recently, their use has been limited by the complexity
inherent in both designing the motors and controlling them. These challenges are being
overcome by advances in the theory, by the use of sophisticated computer design tools,
and by the use of low-cost embedded systems for motor control. These control systems
are typically based on microcontrollers using control algorithms and real-time computing
to tailor drive waveforms according to rotor position and current or voltage feedback.
Design and operating fundamentals
The stator consists of multiple salient (ie. projecting) electromagnet poles, similar to a
wound field brushed DC motor. The rotor consists of soft magnetic material, such as
laminated silicon steel, which has multiple projections acting as salient magnetic poles
through magnetic reluctance. The number of rotor poles is typically less than the number
of stator poles, which minimizes torque ripple and prevents the poles from all aligning
simultaneously—a position which can not generate torque.
When a rotor pole is equidistant from the two adjacent stator poles, the rotor pole is said
to be in the "fully unaligned position". This is the position of maximum magnetic
reluctance for the rotor pole. In the "aligned position", two (or more) rotor poles are fully
aligned with two (or more) stator poles, (which means the rotor poles completely face the
stator poles) and is a position of minimum reluctance.
When a stator pole is energized, the rotor torque is in the direction that will reduce
reluctance. Thus the nearest rotor pole is pulled from the unaligned position into
alignment with the stator field (a position of less reluctance). (This is the same effect used
by a solenoid, or when picking up ferromagnetic metal with a magnet.) In order to sustain
rotation, the stator field must rotate in advance of the rotor poles, thus constantly
"pulling" the rotor along. Some motor variants will run on 3-phase AC power (see the
synchronous reluctance variant below). Most modern designs are of the switched
reluctance type, because electronic commutation gives significant control advantages for
motor starting, speed control, and smooth operation (low torque ripple).
6.5 CAPACITOR START-INDUCTION RUN MOTOR
A capacitor start induction run motor is similar in some ways to the split phase type. The
main difference is that a capacitor is placed in series with the start winding. The capacitor
start motor produces considerably more starting torque than the split phase motor. After
118

6. the start winding is disconnected from the circuit, the performance is nearly identical with
the split phase motor. Logically, capacitor start motors should be used where the load
acceleration requirements exceed the capacity of a split phase motor.
A motor capacitor,such as a start capacitor or run capacitor, including a dual run
capacitor, is an electrical capacitor that boosts the current to an electric motor, such as in
air conditioners, hot tub/jacuzzi spa pumps, or forced air heat furnaces. A round dual run
capacitor (described below) is used in some air conditioner compressor units, to boost
both the fan and compressor motors.
Run capacitors
Run capacitors are designed for continuous duty, and they are energized the entire time
the motor is running. Run capacitors are rated in a range of 3–70 microfarads (µF), with
voltage classifications of 370 V or 440 V. Single phase electric motors need a capacitor
to energize a second-phase winding. If the wrong run capacitor is installed, the motor will
not have an even magnetic field, and this will cause the rotor to hesitate at those spots
that are uneven. This hesitation can cause the motor to become noisy, increase energy
consumption, cause performance to drop, and cause the motor to overheat. However, a
motor will not be ruined just because a run capacitor is faulty.
Start capacitors
Start capacitors briefly increase motor starting torque and allow a motor to be cycled on
and off rapidly. Start capacitors have ratings above 70 microfarads (µF), with three major
voltage classifications: 125 V, 250 V, and 330 V. A start capacitor stays energized long
enough to rapidly bring the motor to 3/4 of full speed and is then taken out of the circuit,
such as by a centrifugal switch that releases when rotating at or around that speed.
Examples of motor capacitors are: a 35 µF, at 370 V, run capacitor, or an 88–108 µF at
250 V start capacitor
6.6 SHADED POLE MOTOR
A shaded-pole motor is a type of AC single-phase induction motor. As in other
induction motors the rotating part is a squirrel-cage rotor. All single-phase motors require
a means of producing a rotating magnetic field for starting. In the shaded-pole type, a part
of the face of each field pole carries a copper ring called a shading coil. Currents in this
coil delay the phase of magnetic flux in that part of the pole enough to provide a rotating
field. The effect produces only a low starting torque compared to other classes of single-
phase motors.
119

7. These motors have only one winding, no capacitor nor starting switch, making them
economical and reliable. Because their starting torque is low they are best suited to
driving fans or other loads that are easily started. Moreover, they are compatible with
TRIAC-based variable-speed controls, which often are used with fans. They are built in
power sizes up to about 1/6 hp or 125 watts output. For larger motors, other designs offer
better characteristics.
The first photo is of a common C-frame motor. With the shading coils positioned as
shown, this motor will start in a clockwise direction as viewed from the long shaft end.
The second photo shows detail of the shading coils
Construction Of Shaded-Pole Motors
The three types of shaded-pole motor are salient-pole, skeleton and distributed-winding.
Salient-pole construction has many main-winding coils. The number of poles is the same
as the number of coils. In most cases there are four coils. Some of the older refrigerator
fans are silent-pole, shaded-pole motors. They used a felt soaked in oil for lubricating the
rotary part.
The skeleton type is used for horsepower from 0.00025 to 0.03. This type of motor uses
triple shading. Three shading coils of different throw for each pole are used. In this type
bearings are self-aligning iolite. Wick-type oilers are used to spread the lubrication so
that it covers the whole rotating shaft. A squirrel-cage rotor to be slightly offset from the
pole, and it easily compressed as the coil is energized. The sucking effect of the coil
causes the rotor to be pulled back inside the hole in laminated core. This type of motor is
used on can openers, small knife sharpeners, clocks and timers. It has an advantage over
the synchronous type originally used in clocks, as it is self-starting and will start again if
the power fails. The old clock-type synchronous motors had to be started by hand.
The third type of construction of shaded-pole motors is the distributed-winding. The
stator laminations are similar to those used for single-phase or polyphase induction
motors. The main winding is also similar to the start winding except that it is short-
circuited upon itself. It is displaced from the main winding by less than the 90º usually in
induction motors.
Performance Of Shaded-Pole Motors
If this type of motor of motor is properly lubricated in most cases the skeleton type is
sealed it will last (continuously operated) for over 25 years. Most clock motors and fan
motors are limited only by the physical abuse they receive. If they are kept plugged in (in
the case of a clock) and operating continuously as timers or similar devices, without
overloading, there is no reason why they cannot last indefinitely. There is only one coil in
the skeleton-type motor. If the voltage is stable and the temperature is normal, there is no
reason why it will not continue to operate without maintenance of any kind. Various
types of physical and electrical abuse can cause them to fail, however.
120

8. If the timer motor has been used to power the timing mechanism of an oven, then it will
be only a matter of eight to ten years of sporadic use that will cause it to lose its
lubrication. The heat from the oven can cause it to lose its oil and the seals to break
down. This shade pole motor has smooth running, high efficiency, low noise and long
life.
Shaded pole motors have low starting torque and are available only in fractional and
subfractional horsepower sizes. Slip is about 10%, or more at rated load. A time varying
flux is induced in the poles by the main winding. When the pole flux varies, it induces a
voltage and a current in the shading coil which opposes the original change in flux. This
opposition retards the flux changes under the shaded portions of the coils and therefore
produces a slight imbalance between the two opposite rotating stator magnetics fields.
The net rotation is in the direction from the unshaded to the shaded portion of the pole
face.
These motors are often used to drive electric clocks and, occasionally, phonograph
turntables. In these applications, the speed of the motor is as accurate as the frequency of
the mains power applied to the motor.
Even by the standards of shaded pole motors, the power output of these motors is usually
very low. Because there is often no explicit starting mechanism, the rotor must be very
light so that it is capable of reaching running speed within one cycle of the mains
frequency. Alternatively, the rotor may be provided with a squirrel cage, so that the
motor starts like an induction motor, once the rotor is pulled into synchronism with its
magnet, the squirrel cage has no current induced in it and so plays no further part in the
operation. A further development dispenses with the shading rings altogether. The
application of power giving the magnetised rotor enough of a 'flick' to move it fast
enough to establish synchronism. A mechanical means prevents the rotor from starting in
the wrong direction. This design will only work satisfactorily if the standstill load is near
to zero and has very little inertia.
121