EEIR11 – Basics of Electrical and Electronics

Engineering

Dr. P. Maheswaran
Assistant Professor, Dept. of ECE
NIT Trichy

mahes@nitt.edu
 Overview of the unit
Unit 2:

DC & AC Machines: DC Motor, Induction motor, Synchronous motor, Synchronous generator

and Transformers- construction, principle of operation, types and applications.
Three phase induction motor: Concept of slip rings and brush assembly

The slip ring and brush assembly is used whenever there is a need of connecting the
rotating member of the machine to stationary external circuit.
Consider the star connected winding in the figure.
External star connected stationary resistors connected to rotor windings.
Contact between the two even during rotation.
Slip rings and brushes enable this.
Slip rings are mounted on same shaft as windings.
Each terminal of winding connected to a slip ring.
The three ends of R-Y-B of winding available
at 3 slip rings.
This assembly play an important role in induction motor.
Three phase induction motor: Construction
Consists of two parts:
The part which is stationary called the stator (three phase windings).
The part which rotates and is connected to the mechanical load through shaft called rotor.

 The conversion of electrical power to mechanical power takes place in a rotor.

The rotor develops a driving torque and rotates.
Three phase induction motor: Construction - Stator

It has lamination type stampings which are 0.4 to 0.5 mm thick.
Stampings slotted on the periphery to carry the 3P stator
windings.

Windings connected either in star or delta network.
Stampings are insulated from each other.

Iron losses are minimized.
A number of stampings are stamped together to build the stator
core.
The built up core is fitted in a casted steel frame.
Stampings are made from: silicon steel, minimizes hysteresis
loss.
Three phase induction motor: Construction - Stator

Stator winding are wound for definite number of poles.

The windings when excited by 3P supply creates RMF.
Choice of number of poles depend on speed of the RMF.
Radial ducts are provided for the cooling purpose.
All six terminals of three phase stator winding are brought out
sometimes.

Gives flexibility to connect them either in star or delta.
Three phase induction motor: Construction - Rotor

Rotor is placed inside stator.

The rotor core is also laminated in construction.

Cylindrical with slots on periphery.
Rotor conductors or windings is placed in the rotor slots.
Two types of rotor constructions used for induction motors are:

Squirrel case rotor.

Slip ring or wound rotor.
Three phase induction motor: Construction – Squirrel
cage rotor

Rotor consists of uninsulated copper or aluminum bars

called rotor conductors.
The bars are placed in the rotor slots.
These bars are permanently shorted at each end with
conducting copper ring called end ring.
The bars are brazed to the end rings for mechanical
support.
The entire structure looks like a cage, forming closed
electrical circuit.
So the rotor is called squirrel cage rotor.
Three phase induction motor: Construction – Squirrel cage rotor

The bars are permanently shorted to each other through end ring, the entire rotor
resistance is small.

This rotor is also called short circuited rotor.
 No external resistance will have any effect on rotor resistance.
No external resistance can be introduced in the rotor circuit.
Slip ring and brush assembly is not required for rotor.
Rotor construction is simple.
Three phase induction motor: Construction – Slip ring or wound rotor

The rotor winding is exactly similar to the stator.

The rotor carries a three phase star or delta connected, distributed winding, wound for the
same number of poles as that of stator.
The slots contain the rotor winding.
The three phase windings are connected in star or delta.
The remaining ends are connected permanently to slip rings.
Slip rings are mounted on same shaft.
External resistance can be added in series to rotor
winding (using slip rings and external circuit).
Rotor resistance per phase can be controlled.
Speed, starting torque can be controlled.
Three phase induction motor: Construction – Slip ring or wound rotor

In running condition, slip rings are shorted.


By connecting a metal collar which gets pushed and connects all the slip ring together.
External resistance addition is the main feature of this rotor.

https://learnengineering.org/slip-ring-induction-motor-how-it-works.html
Three phase induction motor: Comparison of squirrel cage and wound rotor

Wound or slip ring rotor Squirrel cage rotor

Rotor consists of a three phase winding Rotor consists of bars which are shorted at
similar to stator winding the ends with the help of end rings
Resistance can be added externally External resistance cannot be added

Only 5% of induction motors in industry use Very common and almost 95% induction
slip ring rotor motors use this type of rotor
Rotor must be wound for the same number of The rotor automatically adjusts itself for the
poles as that of stator same number of poles as that of stator
Speed control by rotor resistance possible Speed control by rotor resistance not
possible
Used for lifts, hoists, cranes, elevators, Used for lathes, drilling machines, fans,
compressors, etc. blowers, water pumps, grinders, printing
machines, etc.
Three phase induction motor: Working principle

Works on the principle of electromagnetic induction.

RMF of constant magnitude is produced with three phase supply.
The speed of this RMF is Ns rpm.
RMF creates the effect of rotating poles around rotor.
Assume RMF rotates clockwise.
Three phase induction motor: Working principle

In (a), rotor is stationary and stator flux is rotating.

There is a relative motion between the RMF and rotor conductors.
RMF gets cut by rotor conductors as RMF sweeps over.
Cutting flux generates EMF in the conductors (rotor induced EMF).
Induced EMF circulate rotor current in the closed rotor circuit as in (b).
Rotor current produces rotor flux.
Direction of flux found by right thumb rule.
Three phase induction motor: Working principle

Two fluxes:
Rotor flux
RMF flux
 Both the fluxes interact with each other as shown in
figure.
In the rotor conductor:
Two fluxes add up in the left.
Two fluxes cancel each other in the right.
Flux lines act as stretched rubber band.
High flux area exerts a push on rotor conductor towards
the low flux area.
Three phase induction motor: Working principle

High flux area exerts a push on rotor conductor towards

the low flux area.
All the rotor conductors experience a force, develops
torque and rotates the rotor.
Interaction of the two fluxes is very essential for a motoring
action.
The direction of force is same as that of RMF.
Rotor rotates in same direction as RMF.
Three phase induction motor: Working principle

Lenz’s law: Induced current in the rotor conductor opposes

the cause producing it.
The cause of rotor current is induced EMF.

Induced due to relative motion of rotor and RMF.
To oppose the relative motion, the rotor experiences a
torque in the same direction as that of EMF.
Rotation of rotor tries to catch up with the speed of RMF.
Ns – speed of RMF in rpm.
N – speed of rotor in rpm.
Ns – N – relative speed between the two.
Three phase induction motor: Slip of induction motor

Rotor rotates in the same direction as RMF, but in steady state it attains speed less than
synchronous speed.
The difference between synchronous speed of RMF (Ns) and rotor speed (N) is called slip
speed.
Slip speed is generally expressed as the percentage of synchronous speed.

The actual speed of motor N is

As start N = 0, so s = 1.
0 < s <= 1.
Single phase induction motor:

Single phase ac supply is commonly used in households, shops and offices.

Motors that run on single phase ac supply are called single phase induction motor.
Applications of single phase ac motor:

Fans

Water pumps
Single phase induction motor: Construction

Two main parts:


Stationary stator and rotating rotor.
Stator:

Laminated construction, made up of stampings, slotted on the periphery to carry the
winding called stator winding or main winding.

This is excited by a single phase ac supply.

Laminated construction keeps iron losses to minimum.

Stator windings wound for definite number of poles.

Number of poles decide the synchronous speed of the motor.
Single phase induction motor: Construction

Two main parts:


Stationary stator and rotating rotor.
Rotor:

Squirrel cage type construction.

Consists of uninsulated copper or aluminum bars placed in the slots.

The bars are permanently shorted at both the ends with the help of conducting rings
called end rings.
Single phase induction motor: Construction

Rotor:

Resistance of rotor is very small as bars are shorted.

The air gap between stator and rotor is kept uniform and small.

Rotor adjusts itself automatically for the same number of poles as that of stator winding.
Single phase induction motor: Working principle

Two fluxes interact with each other for motor action and produce torque.
Stator carries ac supply and produces alternating flux. This is the main flux.
This flux induces EMF in the rotor.
Induced EMF drives current through the rotor as it has closed circuit.
The rotor current produces rotor flux required for motor action.
The single phase induction motors are not self starting.
Double revolving field theory helps in understanding why single phase induction motors are
not self starting.
Single phase induction motor: Double revolving field theory

This theory says:


Any alternating quantity can be resolved into two rotating components which rotate in
opposite directions.

Each component has magnitude as half of the maximum magnitude of the alternating
quantity.
There is only one phase winding in the stator.

The field can act only in one direction.

The magnitude of the field varies with ac current.
We can represent the varying magnitude as the combined effect of two fields rotating in
opposite directions.
Single phase induction motor: Double revolving field theory

(a) The two fields are aligned and their sum gives
the peak magnetic field created by the
one-phase system.

(b) Vertical components add up, but horizontal

components cancel.

(c) No vertical component, horizontal components

opposite to each other. Zero field.

(e) Same as (a) but opposite direction.

Single phase induction motor: Double revolving field theory


If rotor is moving, it moves in the direction of one of these fields.

Achieved slip is between 0 and 1.

Rotor also rotates in the direction opposite to the other field.

Gives rise to breaking action.

Slip is between 1 and 2.

Total rotor action is the sum of two actions.

Motor action is stronger than breaking action.
Single phase induction motor: Double revolving field theory
Torque-speed characteristic:

Two oppositely directed torques and the resultant torque can be shown effectively with
torque-speed characteristic.

At start, N = 0 and the resultant torque is zero.

If the rotor is given an initial rotation in any direction, the resultant average torque increases
in the direction in which rotor is rotated.

Motor starts rotating in that direction.

But giving initial torque externally is not
always possible.

Modification needed in motor for self starting

capability.
Single phase induction motor: Cross field theory


Induction motor in standstill in top figure.

Stator winding excited by a single phase ac supply.

The supply produces alternating flux Φs along the axis of stator.

EMF gets induced in the rotor conductors due to this flux
(transformer action).

As the rotor forms closed circuit, EMF circulates current through
rotor conductors.

Direction of rotor current is shown in top figure.

The rotor current opposes the status flux Φs (Lenz’s law).
Single phase induction motor: Cross field theory


Fleming’s left hand rule gives the direction of the force
experienced by the rotor conductors.

Φs is upwards increasing positively.

The conductor on the left experiences force from the left to the
right.

The conductor on the right experiences force from the right to
the left.

Overall force experienced by the rotor is zero.

Not torque on rotor and it does not rotate.
Single phase induction motor: Cross field theory


RMF can only be created with two fluxes separated by some
angle.

According to cross field theory, stator flux can be split into two
mutually perpendicular components:

One component along the stator winding

Other component perpendicular to it

Assume initial push is in anti-clockwise direction.
Single phase induction motor: Cross field theory


Rotor physically cuts the stator flux due to rotation.

EMF is induced in the rotor.

This is the speed EMF or rotational EMF.

Direction of the EMF found using Fleming’s right hand rule.

This EMF is in phase with stator flux Φs.

The direction of EMF is shown in the figure.
 Let this EMF be E2.
 This EMF circulates current I2 through the rotor.
 I2 produces flux Φr.

Φr is 90 deg to Φs. (Cross field).
Single phase induction motor: Cross field theory


Φr and Φs produce cross fields. In turn gives RMF.

The direction of RMF is the same as the direction of initial push
given.

Rotor experiences torque in the same direction as that of the
RMF (direction of initial push).

Rotor accelerates in the anticlockwise direction.
Single phase induction motor: Cross field theory
Single phase induction motor: Types


In practice, arrangements are provided in single phase induction motors to change the
magnetic flux from alternating type to rotating type.

RMF rotates in one particular direction.

Under RMF, induction motor becomes self starting.

Motor rotates in the same direction as that of RMF.

Based on the method of producing the RMF, single phase induction motors are classified
as:

Split phase induction motor.

Capacitor start induction motor.

Capacitor start capacitor run induction motor.

Shaded pole induction motor.
Single phase induction motor: Types


To produce RMF, minimum two alternating fluxes with a phase difference between the two
are needed.

The interaction of two fluxes produce a resultant flux which is RMF.

Attempt is made in single phase motors to produce an additional flux with a phase
difference from the stator flux.

Two fluxes with a phase difference of α is shown in figure.

More the phase difference α, more is the starting torque.
 Once the motor starts, Φ2 may be removed.

After start, RMF created with

Stator flux and rotor flux.
Single phase induction motor: Split phase induction motor


It has single phase stator winding called main winding.

Stator also carries one more winding called auxiliary or starting
winding.

Auxiliary winding carries series resistance such that it is highly
resistive.

The main winding is inductive in nature.

Im – current through main winding

Ist – current through auxiliary winding
Single phase induction motor: Split phase induction motor


Main winding is inductive:

Im lags behind V by Φm.

Ist is almost in phase with V (since aux winding is highly resistive).

A phase difference of α between Im and Ist.

Two fluxes produced by Im and Ist also have the same phase
difference.

The resultant of these two fluxes is an RMF.

The starting torque is produced in only one direction, and rotates
the rotor.
Single phase induction motor: Split phase induction motor


Aux winding has centrifugal switch in series with it.

When motor gathers a speed upto 75% to 80% of Ns, centrifugal
switch opens mechanically.

Aux winding remains out of the circuit in running condition.

Motor runs only on stator winding.

Aux winding is designed for short time use.

Main winding is designed for long time use.

Im and Ist are split from each other by angle α.

Hence the name split phase motor.
Single phase induction motor: Split phase induction motor


The torque speed characteristic is shown in the bottom figure.

Starting torque Tst is proportional to the split angle α.

Poor starting torque (125 to 150% of full load torque).

The direction of rotation of motor can be reversed by reversing the
terminals of either main winding or aux winding.

This changes the direction of RMF and the rotor.
Applications:

Motor has low to moderate starting torque.

Used for:

Rans, blowers, grinders, centrifugal pumps.

Washing machines, office equipments.
Single phase induction motor: Capacitor start induction motor


In aux winding, instead of resistor, capacitor is connected.

Capacitive circuit draws leading current.

Used to increase the split angle α between Im and Ist.

In capacitor start motor, capacitor is disconnected from the circuit
using centrifugal switch.

The current Im lags the voltage by Φm.

The current Ist leads the voltage by angle Φst.
Single phase induction motor: Capacitor start induction motor


The phase difference between the two currents is almost 90 deg.

90 deg is the ideal case.

Large α gives large starting torque.

The motor is called capacitor start as capacitor is used only during
starting and disconnected after 75 to 80 % speed is attained.
Single phase induction motor: Capacitor start capacitor run
induction motor


No centrifugal switch in this motor as opposed to capacitor start
motor.

Capacitor remains in the circuit permanently.

This improves the power factor of the motor.

Improved power factor draws less current from supply.

The direction of rotation in this motor also can be changed by
interchanging the connections of main winding or aux winding.

These motors are costlier than split phase motors.
Single phase induction motor: Capacitor start capacitor run
induction motor


The capacitor value can be selected as per the required starting
torque.

Starting torque can be as high as 350 to 400 % of full load torque.
Applications:

High starting torque applications.

Used for:

Compressors, conveyors

Grinders, fans, blowers

Refrigerators, air cons.

Ceiling fans use capacitor start capacitor run motor.
Synchronous Generators or Alternators


Machines generating AC EMF are called alternators or synchronous generators.

Machines accepting input from AC supply to produce mechanical output are synchronous
motors.

Both these machines work at a specific constant speed called synchronous speed.
Synchronous Generators or Alternators
Difference between DC and AC generators

Induced EMF in DC generator is alternating type.

Commutator and brush assembly converts the alternating EMF to DC.

If commutator is dropped and induced EMF is tapped from armature directly, AC EMF is
available. Such machine without commutator providing AC are called alternators.
Synchronous Generators or Alternators
Concept of slip rings and brush assembly

This arrangement is used to collect the AC EMF from rotating armature and make it
available to stationary circuit.

In case of three phase alternators, the armature consist of three phase winding.

AC EMF gets induced in these windings.

Armature is rotating. Stationary load cannot be connected directly.

Slip rings (conductors) mounted on shafts.

Each winding terminal connected to slip ring.

Brushes rest of slip rings to make contact.
Synchronous Generators or Alternators


The induced EMF is the effect of the relative motion between an armature and the field.

Relative motion is achieved by rotating armature with the help of prime mover in case of DC
generator.

As armature is connected to commutator in a DC generator:

Armature must be a rotating member while the field is stationary.

But in alternator, it is possible to have:

The rotating armature and stationary field.

The rotating field and stationary armature.

Practically, most of the alternators prefer rotating field type construction with stationary
armature due to certain advantages.
Synchronous Generators or Alternators
Advantages of rotating field over rotating armature:

The generation level of AC voltage is higher as 11 kV to 33 kV which is induced in the
armature.

Larger space can be provided in stationary armature to accommodate larger number of
conductors and insulation.

It is better to protect high voltage winding from the centrifugal forces caused by rotation.

High voltage armature is generally kept stationary. This avoid the interaction of
mechanical and electrical stresses.

It is easier to collect larger current at high voltages from a stationary member that from the
slip ring and brush assembly.

The voltage for the field is very low (110 V to 220 V DC), which can be easily supplied
with slip ring and brush assembly.
Synchronous Generators or Alternators
Advantages of rotating field over rotating armature:

The problem of sparking at slip ring can be avoided by keeping field rotating (low voltage
circuit) and high voltage armature as stationary.

Field side is low voltage circuit:

Less insulation is required.

Field system has very low inertia. Effort required is less for rotating low inertia system.

For rotating field, two slip rings are enough.

For rotating armature, three slip rings are needed. Insulation is also a problem (due to
high voltage).

Ventilation can be improved if high voltage side is kept stationary.
Synchronous Generators or Alternators: Construction


In practice, most of the alternators have:

Stator as armature, rotor as field.
Stator:

This consists of a core and the slots to hold the armature winding.

Stator build on laminated steel stampings insulated from each other.

This keeps down eddy currents losses.

The core is fabricated in a frame made
of steel plates.

Slots in the core house armature conductors.
Synchronous Generators or Alternators: Construction
Rotor:

Two types of rotors used in alternators:

Salient pole type

Smooth cylindrical type
Salient pole type rotor:

AKA projected pole type as all the poles are projected out from the surface of the rotor.

Built up of thick steel lamination

Poles are bolted to the rotor.

The field winding is provided on the pole shoe.

The rotors have large diameter.

The limiting factor for the size of the rotor is centrifugal force.

Less mechanical strength. Low speed applications (125 to 500 rpm)

Prime mover: water turbines.
Synchronous Generators or Alternators: Construction
Smooth cylindrical type rotor:

AKA non salient type or non-projected pole type of rotor.

Consists of smooth solid steel cylinder, with number of slots to accommodate field coil.

The slots are covered at the top with steel or manganese wedges.

The unslotted portions of the cylinder act as the poles.

The poles are not projected out, rotor surface is smooth.

Maintains uniform air gap between rotor and stator.

Rotors have small diameter.

The advantage of this type rotor is high mechanical strength.

Preferred for high speed alternators.

1500 to 3000 rpm (turboalternators).

Prime movers: steam turbines, electric motors.
Synchronous Generators or Alternators: Working principle


When there is relative motion between the conductors and the flux,
EMF is induced in the conductors.

For understanding, consider relative motion between rotating
conductor and flux from stationary field winding.

Two stationary poles, and single conductor for example.

P1: dφ/dt = 0 (flux parallel to motion). Induced EMF = 0.

P1 to P2: Part of velocity component becomes perpendicular to flux
lines. EMF induced proportional to that. Magnitude of EMF increases
from position 1 to position 2.

P2: Entire velocity component is perpendicular to flux lines. Maximum
cutting of flux lines. Induced EMF is maximum.
Synchronous Generators or Alternators: Working principle


P2 to P3: Similar to P1 to P2. But induced EMF decreases.

P3: Same as P1. Induced EMF = 0.

P3 to P4: Velocity component perpendicular to flux lines increase.
Direction of velocity opposite to that in P1 to P2. Induced EMF is also
opposite.

P4: Induced EMF achieves maxima in opposite direction.

P4 to P1: Induced EMF decreases.

Cycle continues as conductor rotates at a constant speed.
Synchronous Generators or Alternators:
Mechanical and electrical angle

For 2 pole alternator, one mechanical revolution corresponds to one
electrical cycle of induced EMF.

Consider 4 pole alternator. Magnetic axis exists diagonally (dotted
lines).

P1, P3, P5, P7: Velocity component is parallel to flux.
Induced EMF = 0.

P2, P4, P6, P8: Velocity component and flux are perpendicular.
Induced EMF is maximum.

During one complete revolution, induced EMF experience:

Four times maxima, four times zero.
Synchronous Generators or Alternators:
Mechanical and electrical angle

The number of cycles of electrical EMF depends on the number of
poles of alternator.

360 deg mechanical = 720 deg electrical.

The general relation between mechanical and electrical angle is
Synchronous Generators or Alternators:
Frequency of the induced EMF

Let


We get


There are P/2 cycles per revolution.

Speed is N rpm, in one second rotor will complete N/60 revolutions.

Machines with fixed relation

between P, N, and f are called
synchronous machines.
Hence, alternators are
synchronous generators.
Synchronous Generators or Alternators:
Synchronous speed (Ns)

For a fixed number of poles, alternator has to be rotated at a particular speed to keep the
frequency of the generated EMF constant at the required value.

Such a speed is called synchronous speed of the alternator denoted by Ns.
Synchronous Motor:

A synchronous motor is electrically identical with an alternator.

Theoretically, a synchronous machine can be used as either an alternator or as a motor.
Characteristics of synchronous motors:

It runs either at synchronous speed or not at all.

That is, while running, it maintains a constant speed.

Only way to change its speed is to vary the supply frequency.

It is not self starting.

It has to be run up to the synchronous speed by some means before it can be
synchronized to the supply.
Synchronous Motor: Principle of operation


A three phase winding fed by a three supply creates a rotating magnetic field.

Consider the two pole stator of the figure shown.

Two stator poles Ns and Ss.

Rotating at synchronous speed in clockwise direction (say).

With the shown rotor position, suppose that stator poles are situated at A and B.

N and Ns will repel each other. Rotor torque in anti-clockwise direction.

Half a period later, stator poles rotate around and interchange positions.

Ns attracts S and Ss attracts N. Clockwise torque develops.

Due to the rapid rotation of stator poles, the rotor torque also rapidly reverses.

Rotor cannot respond to this due to its large inertia of rotor.

Rotor remains stationary.
Synchronous Motor: Principle of operation


Consider the condition in figures (a) and (b).

The stator and rotor poles attract each other.

Suppose rotor is rotating clockwise:

It turns one pole-pitch by the time the stator poles interchange positions.

In (b), again the stator and rotor poles attract each other.

If the rotor poles shift their position along with stator poles, they will
continuously experience unidirectional torque.
Synchronous Motor: Methods of starting


The unexcited rotor is speeded up to synchronous/near synchronous speed by some
arrangement.

After it reaches near synchronous speed, the field coil is excited with DC source.

The moment the synchronously rotating rotor is excited, it is magnetically locked to the
stator poles.

Stator and rotor poles are engaged and both run synchronously in the same direction.

Because of this interlocking, the motor runs either synchronously or does not run at all.
Synchronous Motor: Methods of starting


The arrangement between the stator and rotor poles is not absolutely rigid.

As the load increases, the rotor progressively tends to fall back in phase by some angle.

But the speed does not reduce as in induction motor or DC motor.

The value of the load angle or coupling angle depends on the amount of load to be met by
the motor.

The torque developed by the motor depends on this angle.
Synchronous Motor: Methods of starting


The working of synchronous motor is analogous to transmission of mechanical power by a
shaft.

Pulley P transmit power to pulley Q.

P and Q are keyed (similar to rotor and stator).

When Q is loaded, it slightly falls behind due to twist in the shaft (correspond to α in motor).

The angle of twist is a measure of the torque transmitted.

Pulleys run at the same speed.

If shaft does not break due to load.
Synchronous Motor: Methods of starting


Almost all synchronous motors are equipped with dampers or squirrel cage windings
consisting of:

Copper bars embedded in the pole-shoes and short-circuited at both ends.

Such a motor starts readily acting as induction motor during the start.
Synchronous Motor: Methods of starting

The procedure:

Line voltage is applied to the stator. Field circuit is left unexcited.

Motor starts as an induction motor.

DC field is excited when the motor reaches 95% of its synchronous speed.

The stator and rotor poles are engaged with each other and the motor is pulled into
synchronism.
Synchronous Motor: Methods of starting

Points to note:

The rotor is stationary when the voltage is applied to stator.

RMF of stator induces very large EMF in the rotor during the starting period.

This EMF reduces as the rotor speed increases.

Normally field windings are meant for 110 V to 220 V.

But during the starting period, many thousands of volt is induced in them.

Proper insulation is needed.