Synchronous Motors Questions and Answers

Answer: A

In the inverted V-curves the power factor is plotted on the y-axis, and in the V-curves the armature current is plotted on the y-axis, with the field current on the x-axis in both cases.

The first option gives them in this order, power factor then armature current. The field current is the x-axis variable, not the y-axis, which rules out the second and third options, and the fourth option reverses the order. So the first option is correct.

Answer: C

The armature current decreases from a large value to a minimum value.

This describes a synchronous motor on constant load (its V-curve). With low excitation the motor is under-excited and draws a large lagging current to supply its own magnetisation. As the field current is raised towards normal, the reactive component falls, and at normal excitation the motor runs at unity power factor, where the armature current is at its minimum.

Increasing the excitation further would make the current rise again, now leading, which is what option D describes. The current never falls to zero, because the active component needed to drive the load remains.

Answer: B

Damper winding is used for eliminating hunting of the rotor.

The damper (amortisseur) winding is a set of copper bars in the pole faces, short-circuited at the ends. When the rotor runs exactly at synchronous speed no emf is induced in it. If the rotor oscillates about synchronous speed (hunting), currents are induced in the bars and produce a torque that opposes the oscillation and damps it. The same winding also lets the motor start as an induction motor.

It is not provided to raise efficiency, to limit temperature rise or to improve the power factor, which is controlled by the field excitation.

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.

Answer: A

A synchronous condenser operates at a leading power factor.

A synchronous condenser is a synchronous motor run without mechanical load and over-excited. Over-excitation makes it draw a current that leads the voltage by nearly 90 degrees, so it behaves like a capacitor bank and supplies reactive power to the system, improving the power factor of inductive loads and supporting the voltage.

At unity power factor it would supply no reactive power at all, and running at a lagging power factor (under-excited) would make it absorb reactive power like an inductor, the opposite of what a condenser is for.