Question

Which of the following statement(s) is/are correct about synchronous motor?

• A. When motor acting as lagging load, terminal voltage $V_t$ is less than excitation voltage.
• B. Maximum power input in synchronous motor is when
  $\delta ={\theta }_s,where\delta$ is load angle and ${\theta }_s$ is impedance angle.
• C. Under the maximum active power input, reactive power is flowing from infinite bus to synchronous motor.
• D. When excitation voltage is greater than terminal voltage, synchronous motor is always delivering reactive power to infinite bus.

Answer 1

The correct option is C Under the maximum active power input, reactive power is flowing from infinite bus to synchronous motor.

For maximum power input conditions,
$\delta =180-{\theta }_s$
When motor acting as lagging load, $V_t$ can be greater or less than excitation voltage.
When $E_f>V_t$
It can be working on lagging or leading pf so it is not always delivering reactive power input to bus.
Under maximum power input conditions motor works on lagging p.f. so it will absorb reactive power from infinite bus.