Synchronous Motors Questions and Answers

Answer: C

The first two are definitions. The third is false: the field current for unity power factor rises with the load, because there is more armature reaction to offset, so normal excitation traces a locus rather than a point.

Answer: D

Correct answer: 1-Q, 2-R, 3-P

Three curves describing three relationships. The V-curve plots armature current against field current and dips to a minimum at normal excitation. The inverted V-curve plots power factor against field current and peaks at the same point. The power-angle curve is different in kind, plotting power against load angle at fixed excitation.

Answer: A

Correct answer: 1-R, 2-P, 3-Q

Three torques belonging to three stages. Starting torque is what the damper winding develops at standstill. Pull-in torque is the load against which the machine can be locked into step as the field is applied. And pull-out torque is the load beyond which synchronism is lost, corresponding to a load angle of ninety degrees.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Normal excitation is the field current giving unity power factor, and as the load rises the armature current rises with it, so its demagnetising effect grows and more field current is needed to offset it. Normal excitation is therefore a curve rather than a single value.

Answer: C

A revolving stator field sweeping past a stationary rotor field gives an alternating torque whose average over a revolution is nothing. The reason is exactly why the field has to wait until the rotor is nearly up to speed.

Answer: D

Correct answer: False, since the speed is locked rigidly to the supply frequency

The rotor is locked to the revolving field, so it turns at exactly \( 120f/P \) at every load up to the pull-out point. What changes with load is the angle at which it sits behind the field, not its speed - and beyond pull-out it does not slow down but falls out of step altogether.

Answer: A

Over-excited operation exports reactive power whatever mechanical duty the machine is performing, so the two benefits come together. That double return is a large part of why such machines were installed in works with many induction motors.

Answer: B

Correct answer: a change in power factor with no change in speed

Excitation controls the reactive power and therefore the power factor; the prime mover, or in a motor the mechanical load, controls the real power. Since the speed is fixed by the supply frequency, changing the field current moves the machine along its V-curve and does nothing else.

Answer: C

Correct answer: the rotor field axis and the stator field axis

The load angle is the angular displacement between the two fields, expressed in electrical degrees, and it is what determines the power transferred through \( P = (EV/X)\sin\delta \). It is not a mechanical measurement, and in a multi-pole machine the physical displacement is smaller by the number of pole pairs.

Answer: D

One describes the phase between voltage and current at the terminals and the other the spatial displacement of the two fields. Both appear on the phasor diagram and both change with load, but they are not the same thing.