1. Induction Machines1.1 Introduction

• Induction machines are rotating electromechanical

energy converting devices.

• Their operating principle is similar to all rotating

electrical machines.

• Operating principle of all rotating electrical machines is based on the two electromagnetic laws.

1. Generator action

e = BLV, voltsWhere, e - induced emf , v B - magnetic field density, wb

L - length of the conductor ,m V - velocity of the conductor, m/s B, L,V, are orthogonal to each other.

2. Motor action

F = BLI, N where, I - current , A

For rotating object, torque is expressed as:-

T = Fxr = BLIr, N-m

where, r - Radius of the rotating object.

• An induction machine is an ac machine which can work as an induction generator and as induction motor.

• Application of induction machine as a generator is limited to some special purposes, whereas induction motors are widely used for various industrial and domestic applications. They are the Workhorses of industries.

• Generally induction motors are of two types

1. Poly phase IM. (usually 3-phase)

2. Single phase IM.

Some Advantages and Disadvantages In using IM

Advantages:• Its losses are reduced. • It has a very simple and extremely rugged, almost unbreakable

construction (especially squirrel cage type), thus requires minimum of maintenance.

• It has low cost compared with other motors of the same power out put.• It has sufficiently high efficiency. Squirrel cage types use no brushes,

hence frictional losses are minimum and reasonably good power factor.

• It starts up from rest and needs no extra starting motor and has not to be synchronized. Its starting arrangement is simple especially – for squirrel- cage type motor.

Disadvantage • Its speed cannot be varied without sacrificing some of its efficiency.• Just like a d.c. shunt motor, its speed decreases with increase in load• Its starting torque is somewhat inferior to that of a d.c shunt motor

2. Construction of poly phase Induction Motor

A typical motor consists of two parts:

1- An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field,

2- An inside rotor attached to the output shaft that is given a torque by the rotating field.

- The rotors of induction motors are of two types:-

- squirrel cage rotor. - Wound rotor

3-phase squirrel cage IM

3-phse Wound rotor IM

2. Bearings shield 3. Enclosure; 4.Stator core with windings 5.Rotor core 6.Bearings shield 7.Ball bearings; 8.Shaft ; 9.Terminal box ; 10. Basement; 11. slip-rings

Induction motor components.

•Stator construction–• Stator of an IM consists of:-

- stator frame, stator core, 3-phase/single phase

distributed winding, two end covers, bearings,

etc.• It is a stack of steel laminations (0.35-o.5 mm

thick) with slots similar to a stator of a synchronous machine. –

• Coils are placed in the slots to form a three or single phase winding.

• Stator iron core without windings

• Single-phase IM stator with windings.

• INDUCTION MOTORS MAGNETIC

CIRCUIT

Stator iron core

Rotor iron core

Stator

Slots

Rotor Slot

Squirrel-Cage Rotor–Rotor is made from punched laminations ( 0.35-0.5mm thick) of steel

core with slots to provide rotor windings.

–Metal (Aluminum) bars are molded in the slots instead of a winding.

–Two rings short circuits the bars.

–Most of single phase induction motors have Squirrel-Cage rotor.

–One or 2 fans are attached to the shaft in the sides of rotor to cool

the circuit.

Squirrel cage Rotor of a large induction motor. (Courtesy Siemens).

Advantages of squirrel cage rotor• No slip rings, brushes, brush holders, & rotor

terminals; thus less operating troubles.• Star-Delta starter is sufficient for its starting• Its construction is robust and cheep• It has higher efficiency• Rotor to slots space factor is better, shorter

overhang, thus smaller copper loss• With bare end rings, it has better ventilation

opportunity.• With smaller overhang leakage, it has better

power factor, greater pull-out torque and overload capacity.

Disadvantages• It is not possible to insert external resistance

- to increase starting torque and

- to decrease starting current• They have small starting torque but large starting

current as compared to the wound rotor.• From the rotor side, speed control is impossible.

Wound Rotor

It is usually for large 3 phase induction motors. • Rotor has a winding the same as stator and the end

of each phase is connected to a slip ring.

• Three brushes contact the three slip-rings to three connected resistances (3-phase Y) for reduction of starting current and speed control.

• Wound rotor induction motor was the standard form for variable speed control before the advent of semiconductor devices.

schematic and real diagram of wound-rotor

• 3-phase wound –rotor induction motor with external starting rheostat.

3-phasesupply

stator

rotor

Slip-rings

StartingRheostat

Advantages of wound rotor• The locked-rotor

current can be reduced by inserting external resistances in series with the rotor windings

• The speed can be varied by varying the external resistances.

• The wound rotor motor is ideally suited to accelerate high inertia loads

Disadvantages• Has slip rings, brushes,

brush holders, & rotor terminals; thus greater operating troubles and require frequent maintenance.

• It has large overhang leakage, more copper loss, thus, less power factor and inferior efficiency compared to squirrel cage motor.

• Compared to squirrel cage rotors, wound rotor motors are expensive, so it is not so common in industry applications

Basic operating principles • An AC current is applied in the stator armature

which generates a flux in the stator magnetic

circuit.

• This flux induces an emf in the conducting bars of rotor as they are “cut” by the flux while the magnet is being moved (E = BVL (Faraday’s Law))

• A current flows in the rotor circuit due to the

induced emf, which in tern produces a force,

(F = B I L ) can be changed to the torque as the

output.

Rotating Magnetic Field

• Operation of an IM is based on the development and existence of rotating magnetic field.

• The 3 windings in the stator of a 3-phase IM are positioned from each other by 120o electrical.

• When a balanced three-phase voltage is applied to the stator windings, currents ia, ib and ic, each of equal magnitude, but differing in phase by 120° flow in the stator winding.

• Each phase current produces a magnetic flux and there is physical 120 °shift between each flux.

• The total flux in the machine is the sum of the three fluxes.

• The summation of the three ac fluxes results in a rotating resultant flux, which turns with constant speed and has constant amplitude.

• Such a magnetic flux produced by balanced three phase currents flowing in thee-phase windings is called a rotating magnetic flux (RMF).RMF rotates with a constant speed (Synchronous Speed).

• Existence of a RFM is an essential condition for the operation of an induction motor.

A

B

C

120

120

120

Flux wave formPositive direction of flux

Graphical analysis of resultant rotating magnetic field

• Let the maximum value of flux of each phase be m • The resultant flux r, at any instant, is given by the vector sum of the individual fluxes A, B and c

• Let us consider values of r at four instants 1/6 time- period apart, corresponding to points marked 0,1,2 and 3.

i) when = 0o, i.e. corresponding to point 0,

,2

3,

2

3,0 mCmBA

BC

0θi)

mr 1.5ΦΦ

mmmr 2

3

2

3x3

2

60Cos

2

3x2

0

ii) When = 600 , i.e. corresponding to point 1,

mA 2

3

mB 2

3

C = 0

mmmr 2

3

2

3x330Cos

2

3x2

A

B

60θii)

mr 1.5ΦΦ

iii) When = 1200, i.e. corresponding to point 2

mcBmA 2

3,0,

2

3

A

C

120θiii)

mr 1.5ΦΦ

mmmr 2

3

2

3x330Cos

2

3x2

iv) When = 1800, i.e. corresponding to point 3,

mCmBA 2

3,

2

3,0

BC

180θiv)

mr 1.5ΦΦ

mmmr 2

3

2

3x330Cos

2

3x2

SUMMARY

• In all four cases, it is found that the resultant flux is1.5 m ; but has rotated clockwise through an angle of 600 in each of the case.

BC

0θi)

mr 1.5ΦΦ

A

B

60θii)

mr 1.5ΦΦ

A

C

120θiii)

mr 1.5ΦΦ

BC

180θiv)

mr 1.5ΦΦ

• The resultant flux is of constant value ; i.e. 1.5 times the maximum value of the flux of each phase.

• The resultant flux rotates around the stator at synchronous speed given by:-

Where, f - is supply frequency

P – is number of poles

P

fN s

.120

Rotating magnetic field and Operating Principles of Induction Motors

1. If stator is energized by an ac current, RMF is generated due to the applied current to the stator winding.

2. This flux produces magnetic field and the field revolves in the air gap between stator and rotor.

3. So, the magnetic field induces a voltage in the short-circuited bars of the rotor. This voltage drives current through the bars.

4. The interaction of the rotating flux of the stator and the flux in the rotor developed by the rotor current generates a force that drives the motor and a torque is developed consequently.

5. The torque is proportional with the flux density and the rotor bar current (F = BLI, N ).

6. The direction of the rotation of the rotor is the same as the direction of the rotation of the revolving magnetic field in the air gap

7. The rotor speed is less than the synchronous speed of the rotating magnetic field. WHY?

• An induction motor running at no load will have a speed very close to synchronous speed and therefore e.m.f. in the rotor winding will be very small .

• This small e.m.f. gives a small current producing a torque just sufficient to overcome the losses such as due to friction and windage and maintain the rotor in rotation.

• As the mechanical load is applied on the motor shaft, it must slow down because the torque developed at no load will not be sufficient to keep the rotor revolving at the no load speed against the additional opposing torque of load.

• As the motor slows down, the relative motion between the magnetic field and the rotor is increased

• This results in greater rotor e.m.f., rotor current and greater developed torque. Thus, as the load is increased, the motor slows down until the relative motion between the rotor and the rotating magnetic field is just sufficient to result in the development of the torque necessary for that particular load.

Slip• In practice the rotor never succeeds in catching up with the

stator field. If it really did so, then there would be no relative speed between the two hence no rotor e.m.f. no rotor current and so no torque to maintain rotation. It is due to this relative motion that torque is developed.

• The difference between the synchronous speed Ns and the

actual speed N of the rotor is known as slip speed. ns = Ns - N

and, the relative speed expressed in % is called slip.

Where, Ns – synchronous speed of RMF

N – speed of the rotor

S% - slip

100% XN

NNS

s

s

• N0tice that, if the rotor turns at synchronous speed, S = 0; while if the rotor is stationary, S = 1.

• All normal motor speeds fall some what between these two limits.

• Thus, S = (1-nrot/nsync)100%

nrot = (1-S) nsync

and, ωrot = (1-S) ωsync