1 Dr. Firas Obeidat Faculty of Engineering Philadelphia University

Starting of Induction Motors

1- Star Delta Starter

The method achieved low starting current

by first connecting the stator winding in star

configuration, and then after the motor

reaches a certain speed, throw switch

changes the winding arrangements from star

to delta configuration.

By connecting the stator windings, first in

star and then in delta, the line current

drawn by the motor at starting is reduced to

one-third as compared to starting current

with the windings connected in delta. At the

time of starting when the stator windings are

start connected, each stator phase gets

voltage VL/√3, where VL is the line voltage.

Since the torque developed by an induction

motor is proportional to the square of the

applied voltage, star- delta starting reduced

the starting torque to one-third that

obtainable by direct delta starting.

2 Dr. Firas Obeidat Faculty of Engineering Philadelphia University

Starting of Induction Motors

2- Auto transformer starter

One way to reduce the motor's terminal voltage during starting by using

autotransformers to step its terminal voltage down.

During starting, contacts 1 and 3 are shut, supplying a lower voltage to the

motor. Once the motor is nearly up to speed, those contacts are opened and

contacts 2 are shut. These contacts put full line voltage across the motor.

3 Dr. Firas Obeidat Faculty of Engineering Philadelphia University

Starting of Induction Motors

3- Rotor resistance starter

This method of starting is only used for slip ring induction motors because in case of squirrel

cage induction motors, rotor has copper bars, which are short circuited at both ends. Hence it is

not possible to add any external resistance in the rotor circuit.

In case of slip ring induction motors, rotor has 3-phase star connected winding whose terminals

are connected to three respective slip rings.

In rotor resistance starter the three terminals of the rotor winding are connected to a variable

external resistances through slip rings. Full supply voltage is applied across the stator.

Resistances are fully in the circuit at starting, so that the starting current is reduced.

The External variable resistance connected in each phase of the rotor circuit not only reduces

the current at starting but increases the starting torque also due to improvement in power

factor.

The rotor circuit resistance is gradually cut out, as the motor speeds up and during normal

running condition, the rotor circuit resistance is completely cut out and the slip rings are short

circuited.

4 Dr. Firas Obeidat Faculty of Engineering Philadelphia University

Starting of Induction Motors

4- Full-voltage or across-the-line magnetic induction motor starter.

When the start button is pressed, the relay (or contactor) coil M is

energized, causing the normally open contacts M I, Mb and M3 to

shut. When these contacts shut, power is applied to the induction

motor, and the motor starts. Contact M4 also shuts, which shorts out

the starting switch, allowing the operator to release it without

removing power from the M relay. When the stop button is pressed,

the M relay is de-energized, and the M contacts open, stopping the

motor.

A magnetic motor starter

circuit of this sort has several

built-in protective features:

1. Short-circuit protection

2. Overload protection

3. Under-voltage protection

5 Dr. Firas Obeidat Faculty of Engineering Philadelphia University

Speed Control of Induction Motors

1- Induction Motor Speed Control by Pole Changing.

2- Induction Motor Speed Control by Changing the line frequency.

3- Induction Motor Speed Control by Changing the line voltage.

4- Induction Motor Speed Control by Changing the Rotor Resistance.

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