Question

Synchronous motor can be used for power factor improvement when it operates with:
1. Over excitation
2. No excitation
3. Normal excitation
4. Under excitation

Answer 1

Correct Answer - Option 1 : Over excitation

Applications of Synchronous motors:

• The important characteristics of the synchronous motor is its constant speed irrespective of the load conditions and variable power factor operation.
• Due to constant speed characteristics, it is used in machine tools, motor generator sets, synchronous clocks, stroboscopic devices, timing devices, belt driven reciprocating compressors, fans and blowers, centrifugal pumps, vacuum pumps, pulp grinders, textile mills, paper mills line shafts, rolling mills, cement mills etc.
• Over-excited synchronous motors draw leading current and hence acts as synchronous condenser. So, they are used to improve the power factor and to improve the voltage regulation.
• Synchronous motors are generally used for voltage regulation of long transmission lines but not the current regulation.

 

Important Points:

• A synchronous motor is capable of operating at all types of power factor i.e. either upf, leading or lagging power factor.
• Lagging power factor: If field excitation is such that Eb < V the motor is said to be under excited and it has lagging power factor.
• Leading power factor: If field excitation is such that Eb > V the motor is said to be over excited and it draw leading current. So that power factor improves.
• Unity power factor: If field excitation is such that Eb = V the motor is said to be normally excited.
• A synchronous motor running with no load will lead the current i.e. leading power factor like a capacitor. This synchronous motor running without load i.e. over excited is synchronous condenser.