LEC - 5 (IM Fundamentals)

7/28/2019 LEC - 5 (IM Fundamentals)

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AC Machines

B.Sc. Electrical Engineering

Lecture # 5

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Engr. Riaz Ahmad Rana

Assist. Prof. FOE, UCP Lahore

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Learning Objectives Working Principle of Induction Motor

Advantages

Disadvantages Applications

Construction

Rotor Types

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Why is an induction motor named as an

induction motor?

Convert electrical AC energy to mechanical

energy.

Rotor does not receive any electric power by

conduction but onl b induction similar to

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secondary winding of the transformer which

receives power from primary winding.

That is why such motors are known as

induction motors.


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Working Principle

When three phase supply is given to 3-phase statorwindings, a rotating magnetic field is produced whichrotates around the stator at synchronous speed(Ns=120 f/P).

The rotating field passes through the air gap and cutsthe rotor conductors which are yet stationary. Due to

the relative speed between the rotating flux and the

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stationary rotor, emf is induced in the rotorconductors. As the rotor is short circuited, so thecurrents start flowing in the rotor conductors.

As the current carrying rotor conductors are placed inthe magnetic field produced by the stator, somechanical force acts on the rotor conductors andproduce a torque which rotates the rotor in the same

direction as the rotating field.


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Advantages

1. Widely used in industry.

2. Simple in construction.

3. Mechanically strong.

4. Cheaper in cost.

5. More reliable.

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6. Self-starting.

7. High efficiency.

8. Minimum maintenance.9. No excitation at rotor.

10.No brushes needed for squirrel cage IM.

11.No. brush contact loss.


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Disadvantages

Starting torque is lower than DC shunt motor.

Speed decreases as load increases.

Efficiency changes with change in speed.

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Speed control arrangements increase losses.

Brushes needed for slip ring (Wound) IM.

5-10 times more starting current than rated

current.


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Winding Connections

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Stator

Stationary part.

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Small air-gap (0.4 mm to 4 mm).

Stator made up of steel stampings or laminations.

Laminations reduce the eddy current losses. 3 phase windings placed in stator core.

Stator consists of three parts: frame, core andwindings.


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Rotor

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Rotor

Rotating part of an IM.

When 3-phase ac supply is given to the stator

windings, then a rotating magnetic field of

constant ma nitude is roduced. This rotatin

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field induces current in the rotor by

electromagnetic induction.

Rotor core is made of silicon steel laminations

having the slots in its outer cylindrical surface.


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Squirrel-Cage Rotor

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One copper or aluminum bar is placed in eachslot and all these bars are joined at each end by

metal rings called end rings. This forms apermanently short-circuited winding. The entireconstruction (bars and rings) resembles a squirrelcage and hence the name.


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Advantages of Squirrel Cage Rotor

Cheaper and robust.

High power factor.

No slip rings and brushes etc.

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Minimum maintenance required.

Comparatively higher efficiency.

Relatively better cooling conditions.

Comparatively better overload capacity.


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Disadvantages of Sq. Cage Rotor

Low starting torque.

No speed control.

Needs a starter.

Higher starting current (5-6 times of ratedcurrent).

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,so it is not possible to add any external resistanceto the rotor circuit to have a large starting torque.

Applications Flour mills.

Printing machinery, etc.


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Wound / slip Ring Rotor

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Slip Ring Rotor

Laminated cylindrical core carries 3-phase

winding, similar to one on the stator. Rotor winding is uniformly distributed in the slots

and is usually star connected.

The open ends of the rotor winding are broughtout and are joined to three insulated slip rings

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mounted on the rotor shaft.

The three brushes are connected to a three phasestar connected rheostat.

The external resistances are added in the rotorcircuit to give a large starting torque. Theseresistances are gradually reduced to zero as themotor runs up to speed.


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Advantages of wound / Slip-Ring Rotor

Much higher starting torque (by inserting

resistances in rotor).

Comparatively less starting current (2-3 times

of rated current).

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starting torque.

Speed control (by varying resistances in rotor).

Can be started directly on lines, (resistances in

the rotor circuit acts like a starter and reduces

the starting current).


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Disadvantages of SR Rotor

High cost Comparatively low efficiency

Low power factor at high loads

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their brushes and slip rings.

Extra losses in external resistances specially whenoperated at reduced speed.

Extra slip rings, brush gears, etc


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Applications of SR Rotor

Pumps

Lifts

Generators

Winding machines

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Line shafts

Mills etc.


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Advantages of skewed slots in the rotor of a

squirrel cage motor

Skewed slots mean slots are not parallel to the

shaft axis.

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No chance of jamming the rotor.

It increases the starting torque.

It reduces the starting current

LEC - 5 (IM Fundamentals)

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