Discussion
A 3-phase synchronous motor driving a constant load torque draws power from the infinite bus at a leading power factor. If the excitation is increased :
Answer: A
Both the power angle and the power factor decrease.
With a constant load torque the motor draws constant active power, \(P = \frac{EV}{X_{s}}\sin\delta\). Raising the excitation increases E, so \(\sin\delta\) must fall to keep P constant: the power angle decreases.
The motor is already over-excited, since it runs at a leading power factor. Further excitation increases the leading reactive current while the active component of current stays the same, so the current grows and its phase angle widens. The power factor therefore falls further (more leading).
The option where power factor increases describes a motor that starts at a lagging power factor, where extra excitation first brings it towards unity. The options with an increasing power angle contradict the power equation.
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