Methods of Starting 3-Phase Induction Motors

The following four methods are extensively used for starting the three-phase induction motors −

• Direct-On-Line Starter

• Auto-Transformer Starter

• Star-Delta Starter

• Rotor Resistance Starter

In this chapter, let's discuss each of these starting methods in detail.

Direct On Line Starter

As the name implies, the direct-on line (D.O.L.) starter is one in which the three-phase induction motor is started by connecting it directly to a 3-phase balanced ac supply as shown in Figure-1.

In this method, the induction motor draws a very high starting current about 4 to 10 times of the full-load current. It is because, the impedance of the motor at standstill is low. Therefore, the direct-on line (D.O.L.) method of starting is suitable for the motor of low power rating, usually up to 7.5 kW.

Autotransformer Starter

In this method of induction motor starting, a three-phase autotransformer is used to supply three-phase electricity to the motor. The autotransformer is mainly used to reduce the 35. Methods of Starting 3-Phase Induction Motors supply voltage at starting and then connecting the motor to the full supply voltage as the motor attains a sufficient speed.

The circuit arrangement of autotransformer starter is shown in Figure-2. The tapings on the autotransformer used for starting the induction motors are provided in such a way that when it is connected in the circuit, 60% to 80% of the supply voltage is applied to the motor.

At the instant of starting, the autotransformer is put into the circuit and hence reduced voltage is applied to the motor. Consequently, the starting current is limited to a safe value. When the motor reaches about 80% of the rated speed, the autotransformer is removed from the circuit through a changeover switch, and the motor is then connected to the full supply voltage. The autotransformer starter has several advantages like low power loss, small starting current, etc. Therefore, this method is suitable for relatively larger induction motors of power ratings over 25 hp.

Star-Delta Starter

In this method, the three-phase induction motor is started as a star-connected motor and run as a delta-connected motor.

The induction motors which are started by star-delta starter has the stator windings which are designed for delta operation and are connected in star during the starting period. When the motor attains a sufficient speed, the winding connections are changed from star to delta.

Figure-3 shows the circuit arrangement of the star-delta starter. Here, the six terminals of the stator windings are connected to a changeover switch. At the instant of starting, the changeover switch connects the stator windings in star-configuration. As a result, each stator phase gets a voltage equal to V/3, where V is the full line voltage. In this way, the stator windings get a reduced voltage during starting period.

When the motor attains a specific speed, the changeover switch turns the connection of stator windings to delta. Each phase now gets the full line voltage V, and the motor runs at the normal speed. However, this method of starting of three-phase induction motor causes a large reduction in the starting torque of the motor. This method is best suited for medium sized induction motors, up to about 25 hp.

Rotor Resistance Starter

This method of starting is applicable to slip-ring induction motors only. In this method, a variable star-connected rheostat is inserted into the rotor circuit through slip rings, and full supply voltage is applied to the stator winding. The circuit arrangement of rotor resistance starter is shown in Figure-4.

At the instant of starting, the handle of the star-connected rheostat is set in the off position. Consequently, a maximum resistance is inserted in each phase of the rotor circuit, and reduces the starting current. At the same time, this resistance increases the starting torque.

When the motor picks up speed, the external resistance is gradually removed from the rotor circuit by moving the rheostat handle. Once the motor attained about 80% of the normal speed, the handle is switched to the on position, and thus the whole external resistance is removed from the rotor circuit.