Slip Power Recovery Scheme (Static Scherbius System)

Slip Power Recovery Scheme (Static Scherbius System) AU : May-08, 11, 13, 14, 16,17, Dec.-07, 11, 12, 13, 14, 15

The efficiency of rotor resistance speed control method is low as the slip power in the rotor circuit is wasted as I2R losses. It is possible to recover the slip power from the rotor and fed back to the supply using static devices. Such a method is called slip power recovery scheme of controlling the speed of the induction motor. It is also called static Scherbius system.

The Fig. 7.12.1 shows the slip power recovery scheme of controlling the speed of slip ring induction motor.

In this method, the rotor a.c. slip power is converted to d.c. using bridge rectifier using diodes. The rectified current is smoothed by using a smoothing inductor L. This d.c. output of rectifier is given as the input ta a line commulated inverter using triacs.

The inverter converts d.c. power to the a.c. power. As the voltage across the slip rings is limited, a three phase transformer is used at the output of an inverter. The output of the transformer is fed back to the a.c. supply.

As the slip power flows only in one direction, this method offers speed control below synchronous speed only. This method provides a constant torque control and its main limitation is lower power factor at reduced speeds.

This method is used in large power fan and pump drives which require speed control in narrow range.

The main advantages of this method are high efficiency and low cost. If maximum slip of induction motor is sm then the power ratings of bridge rectifier, inverter and transformer are sm times the rating of the motor hence the cost is very low.

Review Questions

1. Explain the static Scherbins system of slip power recovery scheme.

2. Write a note on slip power recovery scheme.

3. Describe in detail the slip power recovery scheme of three phase induction motor. AU : May-13, 14, 16,17, Dec.-13, 14, 15, Marks 8