Synchronous Motor Drives Questions and Answers

Answer: C

Correct answer: suppress hunting

A damper winding is a set of bars in the pole faces, short-circuited at the ends like a squirrel cage. When the rotor oscillates about its steady torque angle there is relative motion between the bars and the stator field, currents are induced and a torque is produced opposing that motion, which damps the oscillation. The same winding also serves as a starting cage on a fixed supply. It does nothing for power factor or synchronous reactance.

Answer: D

Correct answer: oscillation of the rotor about its steady torque angle

Hunting is a swinging of the rotor about its equilibrium torque angle following a disturbance - a load step, a supply dip, or a pulsating load such as a reciprocating compressor. The rotor and the electromagnetic stiffness of the coupling form a lightly damped oscillator, and if the disturbance is periodic and near the natural frequency the swing can grow until synchronism is lost. Damper windings exist to suppress it.

Answer: A

With more excitation than the load requires, the machine supplies reactive power to the network and its current leads the voltage. That capability is what lets a synchronous machine correct the power factor of a whole installation.

Answer: B

Correct answer: a lagging power factor

An under-excited synchronous machine produces less excitation emf than the terminal voltage requires, so it must draw magnetising reactive power from the supply, exactly as an induction motor does, and the current lags. Over-excitation reverses this: the machine supplies reactive power and the current leads. The transition between them occurs at the excitation giving unity power factor, which is the minimum of the V-curve.

Answer: C

Over-excited and unloaded, it behaves as a variable capacitor whose output is adjusted by the field current. Static compensators have largely replaced it, but the principle is the same as an over-excited motor supplying reactive power.

Answer: D

Correct answer: armature current against field current

The V-curve plots armature current against field current at constant load, and takes its name from its shape: the current falls as excitation is raised, reaches a minimum at unity power factor, and rises again as over-excitation makes the machine supply reactive power. There is one curve for each load, the family nesting one above another, and the locus of the minima is the unity power factor line.

Answer: A

Correct answer: unity power factor at that load

At the bottom of the V the armature current is the smallest that can carry the load, which means it is carrying real power only - no reactive component in either direction. Under-excited operation lies to the left, with lagging current, and over-excited to the right, with leading current, and both draw more current for the same power. Running at the minimum therefore gives the least armature copper loss.

Answer: B

Correct answer: increases the torque angle and the armature current

The machine cannot slow down, so the only way it can develop more torque is by increasing the angle \( \delta \) at which the rotor lags the stator field, since \( P = (EV/X_s)\sin\delta \). The armature current rises with the power delivered. The field current is set externally and does not change of itself, and the speed remains locked to the supply frequency throughout.

Answer: C

Correct answer: a brushless exciter on the same shaft

A modern large machine uses a shaft-mounted exciter whose armature is on the rotor, feeding a rotating rectifier that supplies the main field winding directly - so no slip rings or brushes are needed for the field current. This removes the brush wear, carbon dust and inspection that a slip ring arrangement demands, which matters greatly on a machine expected to run continuously for years.

Answer: D

Correct answer: loss of synchronism and heavy pulsating current

With the field lost, the excitation emf collapses and so does the pull-out power \( EV/X_s \). Unless the load is very light the machine cannot hold synchronism; it slips poles, drawing a heavy pulsating current from the supply and producing pulsating torque in the shaft. A wound field machine may run on briefly as a reluctance motor at reduced capability, but the condition is a fault and field failure protection is fitted to trip it.