Question

(a) Explain the construction and working principle of Star-Delta Starter with the help of circuit diagram. What are the advantages and limitations of using a Star-Delta Starter for starting an induction motor. (10)

(b) Describe the maintenance procedures for a motor starter. What are the common faults, and how would you troubleshoot them. (6)

Answer 1

Part (a)

Construction and working principle of a Star-Delta starter:

Construction: the starter has three contactors - a main (line) contactor, a star contactor, and a delta contactor - together with a timer (time delay relay), an overload relay, fuses/MCCB, start and stop push buttons, and an indicating lamp. The motor has six terminals (U1, V1, W1 and U2, V2, W2) brought out to the starter.
Working principle: on pressing start, the main and star contactors close, connecting the motor windings in star. Each winding then receives phase voltage (line voltage / root 3), so the starting current is reduced to about one third of the DOL value, and the starting torque to about one third. The motor accelerates under reduced voltage. After a preset time (set by the timer, allowing the motor to reach near rated speed), the star contactor opens and the delta contactor closes, reconnecting the windings in delta so each winding receives full line voltage and the motor runs at rated condition.
Advantages:
Starting current is reduced to about one third of the DOL starting current, reducing the voltage dip on the supply.
Simple, robust, and relatively cheap.
No extra losses during running (the motor runs in delta at full voltage).
Reduces mechanical shock during starting.
Limitations:
Starting torque is reduced to about one third of the DOL torque, so it cannot start a load requiring high starting torque.
The changeover from star to delta causes a current and torque surge.
Only suitable for motors that can be started on reduced voltage and that run in delta (normally delta-connected motors).
The motor must be designed for star-delta starting (six terminals available).

Part (b)

Maintenance procedures for a motor starter, common faults and troubleshooting:

Maintenance: regularly inspect and clean the contactor contacts (check for pitting, burning, and correct contact pressure); check and tighten all electrical connections; check the timer setting; test the overload relay and reset it; check the fuses/MCCB; lubricate moving parts; check the coil for correct operation and voltage; check for loose wiring, moisture, and dust; verify the earth connection; periodically test the starter operation and the insulation resistance.
Common faults and troubleshooting:
Motor does not start: check the supply, fuses/MCCB, control circuit, start button, contactor coil, and overload relay (may be tripped). Check for a blown fuse or an open circuit in the control wiring.
Contactor chatters or fails to hold: check the coil voltage, the holding contact, and the supply; a low voltage or a faulty holding contact causes chattering.
Motor starts in star but fails to change to delta: check the timer, the delta contactor, and its coil and contacts.
Overload relay trips frequently: check for an actual overload, incorrect relay setting, single-phasing, or a faulty relay.
Burnt or pitted contacts: caused by frequent operation, arcing, or a faulty coil; clean or replace the contacts.
Single-phasing: check the fuses and connections on all three phases; a blown fuse causes the motor to run on two phases and overheat.
Always isolate and lock off the supply before working on the starter.

✓ Verified Model Answer (Text Solution)

Construction and working principle of a Star-Delta starter:

Maintenance procedures for a motor starter, common faults and troubleshooting: