Question

(a) Explain the effect of making incorrect phase and starter connections. (6)

(b) A 440V shunt motor takes an armature current of 30A at 700 rev/min. The armature resistance is 0. 7ohm. If the flux is suddenly reduced 20 percent, to what value will the armature current rise momentarily? Assuming unchanged resisting torque to motion, what will be the new steady values of speed and armature current? Sketch graphs showing armature current and speed as functions of time during the transition from initial to final, steady-state conditions. (10)

Answer 1

Part (a)

Incorrect phase and starter connections in electric motors can cause:

Incorrect Phase Connections:

Reversing any two phases in a three-phase motor will reverse its rotation direction.
Incorrect phase connections can result in an unbalanced power supply, leading to uneven current distribution across the motor windings.
Motors may run noisily, vibrate excessively, or operate at reduced performance levels.

Incorrect Starter Connections:

Star-delta starters are commonly used to reduce starting current. Incorrect wiring can prevent the motor from transitioning from star to delta connection, or prevent motor from starting.
Incorrect starter connections can cause the motor to draw excessive current, leading to overheating.
Continuous operation under incorrect starter conditions can stress the motor components, leading to premature failure.

✓ Verified Model Answer (Text Solution)

Incorrect phase and starter connections in electric motors can cause: