Q6Advanced Electric Drives
Question
2 marks
Q.6. List the starting methods of induction motors.
Answer
- Direct-On-Line (DOL) starting: connecting the motor directly to full-rated supply voltage, simplest but producing the highest starting current (typically 5-7 times full-load current).
- Star-Delta starting: temporarily connecting a normally delta-run motor's windings in star during starting, reducing the phase voltage (and hence starting current and torque) to about 1/3 of the direct-delta values.
- Auto-transformer starting: applying a reduced voltage to the motor via a tapped autotransformer during starting, then switching to full voltage once the motor accelerates, allowing the starting current/torque reduction ratio to be tuned via the transformer tap setting.
- Rotor resistance starting (for wound-rotor induction motors): inserting external resistance in the rotor circuit via slip rings during starting, increasing starting torque per unit starting current compared to a squirrel-cage motor's fixed low rotor resistance.
- Soft starting (power-electronic): using a controlled AC voltage controller (thyristor-based soft starter) to gradually ramp up the applied voltage from a low starting value to full voltage, providing smoother, more controllable starting than the fixed-ratio electromechanical methods above.