Question

Draw power and control circuit for definite time-limit starter for slip-ring induction motor. Explain its working.

Answer 1

Definite Time-limit Starter for Slip-ring Induction Motor: This starter is used for automatic control of acceleration of slip-ring induction motor at the time of starting. In this starter, the accelerating contactors close after pre-set time delays determined by the timers. The time periods are so adjusted that when a resistance step is cut off, the resulting current peaks remain within limits. The time delay between energisation of successive contactors can be obtained by using any of the following types of time-delay elements;

i) Individual timers ii) Motor driven cam timer iii) Timer heads mounted on contactors iv) Flux decay relays

The power circuit is as shown below. The slip-ring induction motor is started by rotor resistance starter having resistance steps R1, R2, R3 and R4. At the time of starting, full resistance R4 is inserted in each rotor phase. With preset time delays the contactors are operated in sequence A-B-C-D and resistance is cut in steps as R1-R2-R3-R4 during starting and finally total resistance is cut-out from the rotor circuit.

The control circuit using individual timers is as shown in figure above. Pressing of ON-pushbutton energizes control relay CR. Contactor M and 1T also get energized as contact CR2 closes. Contactor M starts the motor with full resistance R4 in the rotor circuit. When timer 1T times out, its delayed contact 1T closes to energize contactor A and timer 2T. Energization of contactor A causes cutting off of resistance R1 from the three rotor phases and motor accelerates. When timer 2T times out, its contact 2T closes to energize contactor B and timer 3T. The closing of contactor B shorts second step of resistance i.e R2 and the motor accelerates further. In this way, the contactors C and D are closed with time delay determined by timers 3T and 4T respectively and the resistance is cut-off in steps for further acceleration of motor. Finally, when contactor D is closed, the full resistance R4 is cut-off from each phase, shorting the rotor winding terminals and final acceleration of the motor takes place.