Starting of Induction Motor | Electrical Machines - Electrical Engineering (EE) PDF Download

There are two main types of three-phase induction motors: squirrel cage and slip ring motors. They both start differently. Let's look at each one.

Starting Methods for Squirrel Cage Induction Motors

1. Direct On-Line (D.O.L.) Starting

This method starts the motor by directly connecting it to the three-phase power supply. The motor uses a lot of current when starting (about 4 to 7 times its normal current) and has a low power factor. It's best for smaller motors (up to 10 kW).

2. Stator Resistance Starting

Here, external resistance is added to each phase of the stator winding during starting. This reduces the voltage available to the motor, which lowers the starting current. As the motor speeds up, these resistances are gradually removed. However, this method reduces the starting torque and wastes power in the resistances.

3. Autotransformer Starting

An autotransformer is used to lower the starting voltage of the motor. Initially, 60 to 80% of the line voltage is applied to the motor, and as it speeds up, the full line voltage is used. This method limits the starting current and is good for larger motors over 25 hp.

4. Star-Delta Starting

Here, the motor starts in star (low voltage) and then switches to delta (full voltage) once it's up to speed. During starting, the stator windings are connected in a star configuration to reduce the voltage. When the motor reaches about 80% of its speed, the connection switches to delta. This method reduces the starting torque due to the lower voltage at the start and is used for medium-sized motors up to 25 hp.

Starting Methods for Slip Ring Induction Motors

There are four ways to start a slip ring induction motor:

1. Direct On-Line (D.O.L.) Starting

Just like with squirrel cage motors, you connect the motor directly to the power supply to start it.

2. Stator-Resistance Starting

This method is the same as for squirrel cage motors, where external resistance is added to the stator windings during starting.

3. Autotransformer Starting

Again, similar to squirrel cage motors, an autotransformer is used to lower the starting voltage.

4. Rotor-Resistance Starting

This is the main method used in industries for slip ring motors. Here, a variable resistance connected in the rotor circuit through slip-rings is used.

In the rotor-resistance starting method:

• When starting, the maximum resistance is added in series with each phase of the rotor circuit by setting the variable resistance to 'OFF'. This reduces the starting current and increases the starting torque.
• As the motor speeds up, the external resistance in the rotor circuit is gradually taken away.
• When the motor reaches its rated speed, the variable resistance is switched to 'ON', removing all the external resistance from the rotor circuit.