Synchronous Motors Questions and Answers

Answer: C

The rotor has to be accelerated the last fraction of a per cent by the synchronising torque alone, and a heavy inertia may swing past and fail to lock. Applying the field at a favourable rotor position makes the transition gentler.

Answer: D

Correct answer: short circuited through a discharge resistance

Left open, the field winding would have a very large voltage induced across it during the induction start, since it has many turns and the field is sweeping past at slip frequency. Closing it through a resistance limits that voltage while still allowing the winding to contribute a little starting torque.

Answer: A

Correct answer: its damper winding acting as a cage rotor

The damper bars in the pole faces behave exactly as a squirrel cage, so the machine starts as an induction motor and accelerates to near synchronous speed. The same winding then serves its other purpose in running, damping the oscillations that would otherwise develop into hunting.

Answer: B

Correct answer: fail to accelerate, the average torque over a revolution being zero

With the field excited from standstill, the rotor poles are alternately attracted and repelled as the stator field sweeps past them, so the torque reverses twice each revolution and averages to nothing. The rotor merely vibrates. The field must be applied only once the machine is turning near synchronous speed.

Answer: C

Correct answer: a sudden change of load or of supply

The rotor sits at a load angle determined by the load, and a sudden change moves the equilibrium point. The rotor swings towards the new angle, overshoots because of its inertia, and oscillates about it. The damper winding dissipates that oscillation; without it, or with a periodic disturbance near the natural frequency, hunting can build up until synchronism is lost.

Answer: D

It behaves as a torsional oscillator whose stiffness is the synchronising coefficient and whose mass is the inertia, so the frequency is the root of their ratio. A load fluctuating near that frequency excites the oscillation strongly.

Answer: A

Any relative motion between rotor and field induces currents in the bars whose torque opposes the swing, which damps the oscillation out. A flywheel of suitable inertia is sometimes added to shift the natural frequency away from the disturbing one.

Answer: B

With no mechanical load it draws only enough real power for its own losses, and its excitation determines how much reactive power it exchanges. Static compensators have largely replaced it, but the principle is identical.

Answer: C

Below the excitation the terminal voltage requires, the machine draws reactive current from the network, which is useful on a lightly loaded cable system with a rising voltage. That two-way capability is what distinguishes it from a fixed capacitor bank.

Answer: D

Correct answer: a controllable power factor and a higher efficiency

These are the two reasons for choosing a synchronous machine. Its power factor is set by the field current and can be made leading, so the machine supports the network instead of loading it, and its efficiency is higher because the rotor is separately excited rather than drawing magnetising current through the air gap.