Question

A synchronous motor operates at rated voltage and frequency and has a load torque angle of 30°. If both the terminal voltage and frequency are reduced by 10%, then
1. the load torque angle remains the same
2. the load torque angle increases
3. the load torque angle decreases
4. nothing can be said about the torque angle 

Answer 1

Correct Answer - Option 1 : the load torque angle remains the same

Concept:

The power in a synchronous motor is given by,

\(P = \frac{{EV}}{{{X_s}}}\sin \delta\) 

\(P \propto \left( {\frac{{{V^2}}}{f}} \right)\sin \delta\) 

\(T = \frac{P}{{{\omega _0}}};\;\;T \propto \left( {\frac{{{V^2}}}{{{f^2}}}} \right)\sin \delta \)

Application:

The terminal voltage and frequency are reduced by 10% i.e. V/f ratio is constant and the torque angle δ is constant.

Therefore, the load torque remains the same.