Download unit 3 ac machines

Electrical engineering & Control Systems(15EE232)
UNIT 3
AC MACHINES
3.1 Single Phase Induction Motor-Double field revolving theory
For lightning and general purposes in homes, offices, shops, small factories single phase system is widely used
as compared to three phase system as the single phase system is more economical and the power requirement in
most of the houses, shops, offices are small, which can be easily met by single phase system. The single phase
motors are simple in construction, cheap in cost, reliable and easy to repair and maintain. Due to all these
advantages the single phase motor finds its application in vacuum cleaner, fans, washing machine, centrifugal
pump, blowers, washing machine, small toys etc.
The single phase ac motors are further classified as:
1.
2.
3.
Single phase induction motors or asynchronous motors.
Single phase synchronous motors.
Commutator motors.
Electrical engineering & Control Systems(15EE232)
Construction of Single Phase Induction Motor
Like any other electrical motor asynchronous motor also have two main parts namely rotor and stator.
Stator: As its name indicates stator is a stationary part of induction motor. A single phase ac supply is given to
the stator of single phase induction motor.
Rotor: The rotor is a rotating part of induction motor. The rotor is connected to the mechanical load through the
shaft. The rotor in single phase induction motor is of squirrel cage rotor type.
The construction of single phase induction motor is almost similar to the squirrel cage three phase motor
except that in case of asynchronous motor the stator have two windings instead of one as compare to the single
stator winding in three phase induction motor.
Stator of Single Phase Induction Motor
The stator of the single phase induction motor has laminated stamping to reduce eddy current losses on its
periphery. The slots are provided on its stamping to carry stator or main winding. In order to reduce the
hysteresis losses, stamping are made up of silicon steel. When the stator winding is given a single phase ac
supply, the magnetic field is produced and the motor rotates at a speed slightly less than the synchronous speed
Ns which is given by
The construction of the stator of asynchronous motor is similar to that of three phase induction motor except
there are two dissimilarity in the winding part of the single phase induction motor.
1.
2.
Firstly the single phase induction motors are mostly provided with concentric coils. As the number of
turns per coil can be easily adjusted with the help of concentric coils, the mmf distribution is almost
sinusoidal.
Except for shaded pole motor, the asynchronous motor has two stator windings namely the main
winding and the auxiliary winding. These two windings are placed in space quadrature with respect to
each other.
Rotor of Single Phase Induction Motor
The construction of the rotor of the single phase induction motor is similar to the squirrel cage three
phase induction motor. The rotor is cylindrical in shape and has slots all over its periphery. The slots
are not made parallel to each other but are bit skewed as the skewing prevents magnetic locking of
stator and rotor teeth and makes the working of induction motor more smooth and quieter. The squirrel
cage rotor consists of aluminium, brass or copper bars. These aluminium or copper bars are called rotor
conductors and are placed in the slots on the periphery of the rotor. The rotor conductors are
permanently shorted by the copper or aluminium rings called the end rings. In order to provide
mechanical strength these rotor conductor are braced to the end ring and hence form a complete closed
circuit resembling like a cage and hence got its name as “squirrel cage induction motor”. As the bars
are permanently shorted by end rings, the rotor electrical resistance is very small and it is not possible
to add external resistance as the bars are permanently shorted. The absence of slip ring and brushes
make the construction of single phase induction motor very simple and robust.
Electrical engineering & Control Systems(15EE232)
Working Principle of Single Phase Induction Motor
NOTE: We know that for the working of any electrical motor whether its ac or dc motor, we require
two fluxes as, the interact of these two fluxes produced the required torque, which is desired parameter
for any motor to rotate.
When single phase ac supply is given to the stator winding of single phase induction motor, the
alternating current starts flowing through the stator or main winding. This alternating current produces
an alternating flux called main flux. This main flux also links with the rotor conductors and hence cut
the rotor conductors. According to the Faraday’s law of electromagnetic induction, emf gets induced in
the rotor. As the rotor circuit is closed one so, the current starts flowing in the rotor. This current is
called the rotor current. This rotor current produces its own flux called rotor flux. Since this flux is
produced due to induction principle so, the motor working on this principle got its name as induction
motor. Now there are two fluxes one is main flux and another is called rotor flux. These two fluxes
produce the desired torque which is required by the motor to rotate.
Double field theory
According to this theory, any alternating quantity can be resolved into two rotating components which
rotate in opposite directions and each having magnitude as half of the maximum magnitude of the
alternating quantity.
In case of single phase induction motors, the stator winding produces an alternating magnetic field
having maximum magnitude of Φ1m.
According to double revolving field theory, consider the two components of the stator flux, each
having magnitude half of maximum magnitude of stator flux i.e. (Φ 1m/2). Both these components are
rotating in opposite directions at the synchronous speed N s which is dependent on frequency and stator
poles.
Let Φf is forward component rotating in anticlockwise direction while Φ b is the backward
component rotating in clockwise direction. The resultant of these two components at any instant gives
the instantaneous value of the stator flux at the instant. So resultant of these two is the original stator
flux.
Fig. Stator flux and its two components
The Fig. shows the stator flux and its two components Φ f and Φb. At start both the components are
shown opposite to each other in the Fig.(a). Thus the resultant Φ R = 0. This is nothing but the
instantaneous value of the stator flux at start. After 90 o , as shown in the Fig. (b), the two components
are rotated in such a way that both are pointing in the same direction. Hence the resultant Φ R is the
algebraic sum of the magnitudes of the two components. So ΦR = (Φ1m/2) + (Φ1m/2) =Φ1m. This is
nothing but the instantaneous value of the stator flux at θ = 90 o as shown in the Fig (c). Thus
continuous rotation of the two components gives the original alternating stator flux.
Both the components are rotating and hence get cut by the motor conductors. Due to cutting of
flux, e.m.f. gets induced in rotor which circulates rotor current. The rotor current produces rotor flux.
This flux interacts with forward component Φf to produce a torque in one particular direction say
anticlockwise direction. While rotor flux interacts with backward component Φ b to produce a torque in
the clockwise direction. So if anticlockwise torque is positive then clockwise torque is negative.
Electrical engineering & Control Systems(15EE232)
At start these two torque are equal in magnitude but opposite in direction. Each torque tries to
rotate the rotor in its own direction. Thus net torque experienced by the rotor is zero at start. And hence
the single phase induction motors are not self starting.
3.2 Constructional Details of Three Phase Induction Motor
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 “transformer- type” a.c. machine in which electrical energy is converted into mechanical
energy.
Advantages
(i)
(ii)
(iii)
(iv)
(v)
It has simple and rugged construction.
It is relatively cheap.
It requires little maintenance.
It has high efficiency and reasonably good power factor.
It has self starting torque.
Disadvantages
(i)
(ii)
It is essentially a constant speed motor and its speed cannot be changed easily.
Its starting torque is inferior to d.c. shunt motor.
Construction
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.
1.
Stator
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 of constant
magnitude is produced. This rotating field induces currents in the rotor by electromagnetic induction.
Electrical engineering & Control Systems(15EE232)
2. Rotor
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:
(i)
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 rings [See Fig. (1.2)]. This forms a permanently shortcircuited 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 torque.
Advantages of squirrel cage induction rotor-


Its construction is very simple and rugged.
As there are no brushes and slip ring, these motors requires less maintenance.
Applications:
Squirrel cage induction motor is used in lathes, drilling machine, fan, blower printing
Squirrel Cage Rotor
(i)
Wound Rotor
Wound rotor. 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 below. 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.
Electrical engineering & Control Systems(15EE232)
The external resistances are used during starting period only. When the motor attains normal speed, the
three brushes are short-circuited so that the wound rotor runs like a squirrel cage rotor.
Advantages of slip ring induction motor  It has high starting torque and low starting current.
 Possibility of adding additional resistance to control speed.
Application:
Slip ring induction motor are used where high starting torque is required i.e in hoists, cranes, elevator etc.
Reasons for Having Skewed Rotor
1. It helps in reduction of magnetic hum, thus keeping the motor quiet,
2. It also helps to avoid “Cogging”, i.e. locking tendency of the rotor. The tendency of rotor teeth remaining under
the stator teeth due to the direct magnetic attraction between the two,
3. Increase in effective ratio of transformation between stator & rotor,
4. Increased rotor resistance due to comparatively lengthier rotor conductor bars, &Increased slip for a given torque
Electrical engineering & Control Systems(15EE232)
Difference between Cage motors and Slip ring Motors
Electrical engineering & Control Systems(15EE232)
3.3 Principle of single phase transformer-EMF equation
Electrical power transformer is a static device which transforms electrical energy from one circuit to another
without any direct electrical connection and with the help of mutual induction between two windings. It
transforms power from one circuit to another without changing its frequency but may be in different voltage
level.
A single phase voltage transformer basically consists of two electrical coils of wire, one called the “Primary
Winding” and another called the “Secondary Winding”. For this tutorial we will define the “primary” side of the
transformer as the side that usually takes power, and the “secondary” as the side that usually delivers power. In a
single-phase voltage transformer the primary is usually the side with the higher voltage.
These two coils are not in electrical contact with each other but are instead wrapped together around a common
closed magnetic iron circuit called the “core”. This soft iron core is not solid but made up of individual
laminations connected together to help reduce the core’s losses.
The two coil windings are electrically isolated from each other but are magnetically linked through the common
core allowing electrical power to be transferred from one coil to the other. When an electric current passed
through the primary winding, a magnetic field is developed which induces a voltage into the secondary winding
as shown.
Single Phase Voltage Transformer
In other words, for a transformer there is no direct electrical connection between the two coil windings, thereby
giving it the name also of an Isolation Transformer. Generally, the primary winding of a transformer is
connected to the input voltage supply and converts or transforms the electrical power into a magnetic field.
While the job of the secondary winding is to convert this alternating magnetic field into electrical power
producing the required output voltage as shown.
Transformer Construction (single-phase)
Electrical engineering & Control Systems(15EE232)
Where:

VP - is the Primary Voltage

VS - is the Secondary Voltage

NP - is the Number of Primary Windings

NS - is the Number of Secondary Windings

Φ (phi) - is the Flux Linkage
Notice that the two coil windings are not electrically connected but are only linked magnetically. A single-phase
transformer can operate to either increase or decrease the voltage applied to the primary winding. When a
transformer is used to “increase” the voltage on its secondary winding with respect to the primary, it is called
a Step-up transformer. When it is used to “decrease” the voltage on the secondary winding with respect to the
primary it is called a Step-down transformer.
However, a third condition exists in which a transformer produces the same voltage on its secondary as is
applied to its primary winding. In other words, its output is identical with respect to voltage, current and power
transferred. This type of transformer is called an “Impedance Transformer” and is mainly used for impedance
matching or the isolation of adjoining electrical circuits.
The difference in voltage between the primary and the secondary windings is achieved by changing the number
of coil turns in the primary winding ( NP ) compared to the number of coil turns on the secondary winding
( NS ).
As the transformer is basically a linear device, a ratio now exists between the number of turns of the primary
coil divided by the number of turns of the secondary coil. This ratio, called the ratio of transformation, more
commonly known as a transformers “turns ratio”, ( TR ). This turns ratio value dictates the operation of the
transformer and the corresponding voltage available on the secondary winding.
It is necessary to know the ratio of the number of turns of wire on the primary winding compared to the
secondary winding. The turns ratio, which has no units, compares the two windings in order and is written with
a colon, such as 3:1 (3-to-1). This means in this example, that if there are 3 volts on the primary winding there
will be 1 volt on the secondary winding, 3 volts-to-1 volt. Then we can see that if the ratio between the number
of turns changes the resulting voltages must also change by the same ratio, and this is true.
Transformers are all about “ratios”. The ratio of the primary to the secondary, the ratio of the input to the output,
and the turns ratio of any given transformer will be the same as its voltage ratio. In other words for a
transformer: “turns ratio = voltage ratio”. The actual number of turns of wire on any winding is generally not
important, just the turns ratio and this relationship is given as:
A Transformers Turns Ratio
Assuming an ideal transformer and the phase angles: ΦP ≡ ΦS
Note that the order of the numbers when expressing a transformers turns ratio value is very important as the
turns ratio 3:1 expresses a very different transformer relationship and output voltage than one in which the turns
ratio is given as: 1:3.
Tramsformer Example No1
A voltage transformer has 1500 turns of wire on its primary coil and 500 turns of wire for its secondary coil.
What will be the turns ratio (TR) of the transformer.
Electrical engineering & Control Systems(15EE232)
This ratio of 3:1 (3-to-1) simply means that there are three primary windings for every one secondary winding.
As the ratio moves from a larger number on the left to a smaller number on the right, the primary voltage is
therefore stepped down in value as shown.
Transformer Action
We have seen that the number of coil turns on the secondary winding compared to the primary winding, the
turns ratio, affects the amount of voltage available from the secondary coil. But if the two windings are
electrically isolated from each other, how is this secondary voltage produced?
We have said previously that a transformer basically consists of two coils wound around a common soft iron
core. When an alternating voltage ( VP ) is applied to the primary coil, current flows through the coil which in
turn sets up a magnetic field around itself, called mutual inductance, by this current flow according to Faraday’s
Law of electromagnetic induction. The strength of the magnetic field builds up as the current flow rises from
zero to its maximum value which is given as dΦ/dt.
As the magnetic lines of force setup by this electromagnet expand outward from the coil the soft iron core forms
a path for and concentrates the magnetic flux. This magnetic flux links the turns of both windings as it increases
and decreases in opposite directions under the influence of the AC supply.
However, the strength of the magnetic field induced into the soft iron core depends upon the amount of current
and the number of turns in the winding. When current is reduced, the magnetic field strength reduces.
When the magnetic lines of flux flow around the core, they pass through the turns of the secondary winding,
causing a voltage to be induced into the secondary coil. The amount of voltage induced will be determined
by: N.dΦ/dt (Faraday’s Law), where N is the number of coil turns. Also this induced voltage has the same
frequency as the primary winding voltage.
Then we can see that the same voltage is induced in each coil turn of both windings because the same magnetic
flux links the turns of both the windings together. As a result, the total induced voltage in each winding is
directly proportional to the number of turns in that winding. However, the peak amplitude of the output voltage
available on the secondary winding will be reduced if the magnetic losses of the core are high.
If we want the primary coil to produce a stronger magnetic field to overcome the cores magnetic losses, we can
either send a larger current through the coil, or keep the same current flowing, and instead increase the number
of coil turns ( NP ) of the winding. The product of amperes times turns is called the “ampere-turns”, which
determines the magnetising force of the coil.
So assuming we have a transformer with a single turn in the primary, and only one turn in the secondary. If one
volt is applied to the one turn of the primary coil, assuming no losses, enough current must flow and enough
magnetic flux generated to induce one volt in the single turn of the secondary. That is, each winding supports
the same number of volts per turn.
As the magnetic flux varies sinusoidally, Φ = Φmax sinωt, then the basic relationship between induced emf, ( E )
in a coil winding of N turns is given by:
emf = turns x rate of change
Electrical engineering & Control Systems(15EE232)
Where:

ƒ - is the flux frequency in Hertz, = ω/2π

Ν - is the number of coil windings.

Φ - is the flux density in webers
This is known as the Transformer EMF Equation. For the primary winding emf, N will be the number of
primary turns, ( NP ) and for the secondary winding emf, N will be the number of secondary turns, ( NS ).
Also please note that as transformers require an alternating magnetic flux to operate correctly, transformers
cannot therefore be used to transform or supply DC voltages or currents, since the magnetic field must be
changing to induce a voltage in the secondary winding. In other words, transformers DO NOT operate on
steady state DC voltages, only alternating or pulsating voltages.
If a transformers primary winding was connected to a DC supply, the inductive reactance of the winding would
be zero as DC has no frequency, so the effective impedance of the winding will therefore be very low and equal
only to the resistance of the copper used. Thus the winding will draw a very high current from the DC supply
causing it to overheat and eventually burn out, because as we know I = V/R.
Transformer Example No2
A single phase transformer has 480 turns on the primary winding and 90 turns on the secondary winding. The
maximum value of the magnetic flux density is 1.1T when 2200 volts, 50Hz is applied to the transformer
primary winding. Calculate:
a). The maximum flux in the core.
Electrical engineering & Control Systems(15EE232)
b). The cross-sectional area of the core.
c). The secondary induced emf.
3.4 Servomotors-Stepper Motors
Before understanding the working principle of servo motor we should understand first the basic of
servomechanism.
Servomechanism
A servo system mainly consists of three basic components - a controlled device, a output sensor, a feedback
system. This is an automatic closed loop control system. Here instead of controlling a device by applying the
variable input signal, the device is controlled by a feedback signal generated by comparing output signal and
reference input signal. When reference input signal or command signal is applied to the system, it is compared
with output reference signal of the system produced by output sensor, and a third signal produced by a feedback
system. This third signal acts as an input signal of controlled device.
This input signal to the device presents as long as there is a logical difference between reference input signal and
the output signal of the system. After the device achieves its desired output, there will be no longer the logical
difference between reference input signal and reference output signal of the system. Then, the third signal
Electrical engineering & Control Systems(15EE232)
produced by comparing theses above said signals will not remain enough to operate the device further and to
produce a further output of the system until the next reference input signal or command signal is applied to the
system. Hence, the primary task of a servomechanism is to maintain the output of a system at the desired value
in the presence of disturbances.
Working Principle of Servo Motor
A servo motor is basically a DC motor(in some special cases it is AC motor) along with some other special
purpose components that make a DC motor a servo. In a servo unit, you will find a small DC motor, a
potentiometer, gear arrangement and an intelligent circuitry. The intelligent circuitry along with the
potentiometer makes the servo to rotate according to our wishes. As we know, a small DC motor will rotate with
high speed but the torque generated by its rotation will not be enough to move even a light load. This is where
the gear system inside a servomechanism comes into the picture. The gear mechanism will take high input speed
of the motor (fast) and at the output, we will get an output speed which is slower than original input speed but
more practical and widely applicable.
Say at initial position of servo motor shaft, the position of the potentiometer knob is such that there is no
electrical signal generated at the output port of the potentiometer. This output port of the potentiometer is
connected with one of the input terminals of the error detector amplifier. Now an electrical signal is given to
another input terminal of the error detector amplifier. Now difference between these two signals, one comes
from potentiometer and another comes from external source, will be amplified in the error detector amplifier and
feeds the DC motor. This amplified error signal acts as the input power of the DC motor and the motor starts
rotating in desired direction. As the motor shaft progresses the potentiometer knob also rotates as it is coupled
with motor shaft with help of gear arrangement. As the position of the potentiometer knob changes there will be
an electrical signal produced at the potentiometer port. As the angular position of the potentiometer knob
progresses the output or feedback signal increases. After desired angular position of motor shaft the
potentiometer knob is reaches at such position the electrical signal generated in the potentiometer becomes same
as of external electrical signal given to amplifier. At this condition, there will be no output signal from the
amplifier to the motor input as there is no difference between external applied signal and the signal generated at
potentiometer. As the input signal to the motor is nil at that position, the motor stops rotating. This is how a
simple conceptual servo motor works.
Electrical engineering & Control Systems(15EE232)
Stepper (Or Stepping ) Motors
The stepper or stepping motor has a rotor movement in discrete steps. The angular rotation is determined by the
number of pulses fed into the control circuit. Each input pulse initiates the drive circuit which produces one step
of angular movement. Hence, the device may be considered as a digital-to-analogue converter. There are three
most popular types of rotor arrangements:
1. Variable reluctance (VR) type
2. Permanent magnet (PM) type
3. Hybrid type, a combination of VR and PM.
Step Angle
The angle by which the rotor of a stepper motor moves when one pulse is applied to the stator (input) is called
step angle. This is expressed in degrees. The resolution of positioning of stepper motor is decided by the step
angle. Smaller the step angle the higher is the resolution of positioning of the motor. The step number or
resolution of a motor is the number of steps it makes in one revolution of the rotor.
Higher the resolution, grater is the accuracy of positioning of objects by the motor. Stepper motor are realizable
for very small step angles. Some precision motors can make 1000 steps in one revolution with a step angle of
0.36 A standard motor will have a step angle of 1.8 with 200 steps per revolution.
Variable Reluctance (VR) Stepper Motor
A variable reluctance (VR) stepper motor can be of single- stack type or the multi-stack type.
i- Single-Stack Variable Reluctance Motor
A Variable reluctance stepper motor has salient-pole (or tooth) stator. The stator has concentrated winding
places over the stator poles (teeth). The number of phases of the stator depends upon the connection of stator
poles. Usually three or four phases winding are used. The rotor is slotted structure made from ferromagnetic
material and carries no winding. Both the stator and rotor are made up of high quality magnetic materials having
very high permeability so that the exciting current required is very small. When the stator phases are excited in a
proper sequence from dc source with the help of semiconductor switch, a magnetic field is produced. The
ferromagnetic rotor occupies the position which presents minimum reluctance to the stator field. That is, the
rotor axis aligns itself to the stator field axis.
Elementary operation of variable reluctance motor can be explained through the diagram of fig.
Electrical engineering & Control Systems(15EE232)
It is a four-phase, 4/2-pole (4 poles in the stator and 2 in the rotor), single-stack, variable reluctance stepper
motor. Four phases are A, B, C and D are connected to dc source with the help of semiconductor switches SA,
SB, SC and SD respectively. The phase winding of the stator are energised in the sequence A, B, C, D, A. When
winding A is excited, the rotor aligns with the axis of phase A. The rotor is stable in this position and cannot
move until phase A is de-energised. Next, phase B is excited and A is disconnected. The rotor moves through 90
in clockwise direction to align with the resultant air gap field which now lies along the axis of phase B. Further,
phase C is excited and B is disconnected, the rotor moves through a further step of 90 in the clockwise direction.
In this position, the rotor aligns with the resultant air gap field which now lies along the axis of phase C. Thus,
as the phases are excited in the sequence A, B, C, D, A the rotor moves through a step of 90 at each transition in
clockwise direction. The rotor completes one revolution through four steps. The direction of rotation can be
reversed by reversing the sequence of switching the winding, that is A, D, C, B, A. it is seen that the direction of
rotation depends only on the sequence of switching the phases and is independent of the direction of currents
through the phases.
The magnitude of step angle for any VR and PM stepper motor is given by
Electrical engineering & Control Systems(15EE232)
The step angle can be reduced from 90 to 45 by exciting phases in the sequence A, A+B , B , B+ C , C, C+ D ,
D, D + A, A. Here (A+B) means that phase winding A and B are excited together and the resultant stator field
will be midway between the poles carrying phase winding A and B. That is, the resultant field axis makes an
angle of 45 with the axis of pole A in the clockwise direction. Therefore when phase A is excited, the rotor
aligns with the axis of phase A. when phases A and B are excited together, the rotor moves by 45 in the
clockwise direction. Thus, It is seen if the windings are excited in the sequence A, A+ B, B, B+ C, C, C+ D, D,
D+A, A the rotor rotates in steps of 45 in the clockwise direction. The rotor can be rotated in steps of 45 in the
anticlockwise direction by exciting the phase in the sequence A, A + D, D, D + C, C, C + B, B, B + A, A. this
method of gradually shifting excitation from one phase to another (for example, from A to B with an
intermediate step of A+ B) is known as micro stepping. It is used to realize smaller steps. Lower values of step
angle can be obtained by using a stepping motor with more number of poles on stator and teeth on rotor.
Consider a four-phase, 8/6 pole, single stack variable reluctance motor shown in fig 2.
The coils wound around diametrically opposite poles are connected in series and four circuits (phase) are found.
These phases are energised from a dc source through electronic switching device. The rotor has six poles (teeth).
For the sake of simplicity, only phase A winding is shown in fig (2-a). when phase A (coil A-A`) is excited,
rotor teeth numbered 1 and 4 aligned along the axis of phase A winding. Next phase winding A is de-energised
and phase winding B is excited. Rotor teeth numbered 3 and 6 get aligned along the axis of phase B and the
rotor moves through a step angle of 15 in the clockwise direction. Further clockwise rotation of 15 is obtained
by de- energising phase winding B and exciting phase winding C. with the sequence A, B, C, D, A, four steps of
rotation are completed and the rotor moves through 60 in clockwise direction. For one complete revolution of
the rotor, 24 steps are required. For anticlockwise rotation of rotor through each step of 15, the phase windings
are excited in reverse sequence of A, D, C, B, A.
ii- Multi-Stack Variable Reluctance Stepper Motor
A multi-stack (or m-stack) variable reluctance stepper motor can be considered to be made up of m identical
single- stack variable reluctance motors with their rotors mounted on a single shaft. The stators and rotors have
the same number of poles (or teeth) and, therefore, same poles pitch. For m-stack motor, the stator poles in all m
stacks are aligned, but the rotor poles are displaced by 1/m of the pole pitch angle from one another. All the
stator pole windings in given stack are excited simultaneously and, therefore, the stator winding of each stack
forms one phase. Thus, the motor has the same number of phases as the number of stacks.
Electrical engineering & Control Systems(15EE232)
Fig.3 shows the cross-section of a three-stack (three-phase) motor parallel to the shift. In each stack, stators and
rotors have 12 poles. For a 12-pole rotor, the pole pitch is 30, and therefore the rotor poles are displaced from
each other by one-third of the pole pitch or 10 . The stator teeth in each stack are aligned. When the phase
winding A is excited rotor teeth of stack A are aligned with the stator teeth as shown in fig.
When phase A is de-energized and phase B is excited, rotor teeth of stack B are aligned with stator teeth. This
new alignment is made by the rotor movement of 10 in the anticlockwise direction. Thus the motor moves one
step (equal to 1/3rd pole pitch) due to change of excitation from stack A to B .
Next phase B is de-energized and phase C is excited. The rotor moves by another step of one-their of pole pitch
in the anticlockwise direction. Another change of excitation from stack C to stack A will once more align the
stator and rotor teeth in stack A. however, during this process the rotor has move one rotor tooth pitch.
Let Nr be the number of rotor teeth and m the number of stacks or phases.
Then
Multi-stack variable reluctance stepper motors are widely used to obtain smaller step size, typically in the range
of 2 to 15 degrees. The variable reluctance motors, both single and multi-stack type, have high torque to inertia
ratio. The reduced inertia enables the VR motor to accelerate the load faster.
Permanent Magnet (PM) Stepper Motor
Permanent-magnet (PM) stepper motor has a stator construction similar to that of the single-stack variable
reluctance motor. The rotor is cylindrical and consists of permanent-magnet poles made of high retentivity steel.
Figure 5 shows a 4/2-pole PM stepper motor. The concentrated windings on diametrically opposite poles are
connected in series to form 2-phase winding on the stator.
Electrical engineering & Control Systems(15EE232)
The rotor poles align with the stator teeth depending on the excitation of the winding. The two coils A-A`
connected in series form phase A winding. Similarly, the two coils BB` connected in series form phase B
winding.
Fig a shows the condition when phase A winding is excited with current iA+. here south pole of rotor is
attracted by the stator phase A pole so that the magnetic axes of the stator and rotor coincide and = 0
In fig. b, phase A winding does not carry any current and the phase B winding is excited by iB+. stator produced
poles now attract the rotor pole and the rotor moves by a step of 90 in the clockwise direction, that is, = 90
In fig. c, phase A winding is excited by iA- and phase B winding is de-energized. The rotor moves through a
further step of 90 in clockwise direction so that = 180.
In fig. d, phase B winding is excited by iB- and phase A winding carries no current. The rotor again moves
further by a step of 90 in clockwise direction so that = 270.
For further 90 clockwise rotation of rotor so that = 360 , phase winding B is de-energized and phase A winding
is excited by current iA+ . Thus, four steps complete one revolution of the rotor.
It is seen that in a permanent-magnet stepper motor, the direction of rotation depends on polarity of phase
current. For clockwise rotor movement the sequence of exciting the stator phase windings is A+, B+, A-, B-,
A+. for anticlockwise rotation the sequence of switching the phase windings should be reversed to A+, B-, A-,
B+, A+.
Electrical engineering & Control Systems(15EE232)
It is difficult to make small PM rotor with large numbers of poles and , therefore stepper motor of this type are
restricted to larger step size in the range of 30 to 90. However disc type PM stepper motors are available to give
small step size and low inertia.
Permanent magnet stepper motors have higher inertia and , therefore lower acceleration than VR stepper motors.
The maximum step rate for PM stepper motors is 300 pulses per second, whereas it can be as high as 1200
pulses per second for VR stepper motors. The PM stepper motors produces more toque per ampere stator current
than VR motor.
Detent Torque or Restraining Torque
The residual magnetism in the permanent magnet material produced a detent torque on the rotor when the stator
coils are not energized. This torque prevent the motor from drifting when the machine supply is turned off.
In case the motor is unexcited, the permanent magnet and hybrid stepping motor are able to develop a detent
torque restricting the rotor rotation. The detent torque is defined as the maximum load torque that can be applied
to the shaft of unexcited motor without causing continuous rotation.
Hybrid Stepper Motor
A hybrid stepper motor combines the features reluctance and permanent magnet stepper motors.
The main advantages of hybrid stepper motors compared with variable reluctance stepper motors are as follows:
1. Small step length.
2. Greater torque per unit volume.
3. Provides detent torque with windings de-energized.
4. Less tendency to resonate.
5. High efficiency at lower speed and lower stepping rates.
Disadvantages of hybrid stepper motors
1. High inertia and weight due to presence of rotor magnet.
2. Performance affected by change in magnetic strength.
3. More costly than variable reluctance stepper motor.
Example (1) Calculate the stepping angle for a 3- stack, 16 – teeth variable reluctance motor.
Electrical engineering & Control Systems(15EE232)
Example (2) Calculate the stepping angle for a 3- phase , 24 – pole permanent magnet stepper motor.
3.5 Universal Motors-Applications
A universal motor is a special type of motor which is designed to run on either DC or single phase AC supply.
These motors are generally series wound (armature and field winding are in series), and hence produce high
starting torque . That is why, universal motors generally comes built into the device they are meant to drive.
Most of the universal motors are designed to operate at higher speeds, exceeding 3500 RPM. They run at lower
speed on AC supply than they run on DC supply of same voltage, due to the reactance voltage drop which is
present in AC and not in DC. There are two basic types of universal motor : (i)compensated type and (ii)
uncompensated type
Construction of Universal Motor
Construction of a universal motor is very similar to the construction of a DC machine. It consists of a stator on
which
field
poles
are
mounted.
Field
coils
are
wound
on
the
field
poles.
However, the whole magnetic path (stator field circuit and also armature) is laminated. Lamination is necessary
to
minimize
the
eddy
currents
which
induce
while
operating
on
AC.
The rotary armature is of wound type having straight or skewed slots and commutator with brushes resting on it.
The commutation on AC is poorer than that for DC. because of the current induced in the armature coils. For
that reason brushes used are having high resistance.
Electrical engineering & Control Systems(15EE232)
Working of Universal Motor
A universal motor works on either DC or single phase AC supply. When the universal motor is fed with a DC
supply, it works as a DC series motor. (see working of a DC series motor here). When current flows in the field
winding, it produces an electromagnetic field. The same current also flows from the armature conductors. When
a current carrying conductor is placed in an electromagnetic field, it experiences a mechanical force. Due to this
mechanical force, or torque, the rotor starts to rotate. The direction of this force is given by Fleming's left hand
rule.
When fed with AC supply, it still produces unidirectional torque. Because, armature winding and field winding
are connected in series, they are in same phase. Hence, as polarity of AC changes periodically, the direction of
current in armature and field winding reverses at the same time.
Thus, direction of magnetic field and the direction of armature current reverses in such a way that the direction
of force experienced by armature conductors remains same. Thus, regardless of AC or DC supply, universal
motor works on the same principle that DC series motor works.
Speed/Load Characteristics
Speed/load characteristics of a universal motor is similar to that of DC series motor. The speed of a universal
motor is low at full load and very high at no load. Usually, gears trains are used to get the required speed on
required load. The speed/load characteristics are (for both AC as well as DC supply) are shown in the figure.
Applications Of Universal Motor


Universal motors find their use in various home appliances like vacuum cleaners, drink and food mixers,
domestic sewing machine etc.
The higher rating universal motors are used in portable drills, blenders etc.