Synchronous Generators Questions and Answers

Answer: C

It is the field current for rated voltage on open circuit divided by that for rated current on short circuit, which comes to the reciprocal of the reactance. A larger ratio means a stiffer machine with better regulation and a larger fault current.

Answer: D

Correct answer: good voltage regulation and a large fault current

The short circuit ratio is the reciprocal of the per unit synchronous reactance, so a large ratio means a small reactance. A small reactance means a small voltage drop on load, hence good regulation - and, by the same token, little to limit the current into a fault. The two consequences are inseparable, and machine design trades one against the other.

Answer: A

Iron along the pole axis and air between the poles give quite different reluctances, so the armature current is resolved into direct and quadrature axis components with a reactance for each. A cylindrical rotor machine needs no such treatment.

Answer: B

Correct answer: larger than the quadrature axis reactance

The direct axis lies along the pole, where the magnetic path is mostly iron and the reluctance low, while the quadrature axis passes through the wide interpolar gap. Lower reluctance means higher permeance and higher reactance, so \( X_d > X_q \), typically by a factor of about two. The difference is what gives a salient pole machine its reluctance torque.

Answer: C

Driving the machine at a little away from synchronous speed with a small unexcited voltage applied makes the armature current swing between two limits as the poles slip past the field. The ratio of voltage to current at each limit gives the two reactances.

Answer: D

The difference between the two axis reactances gives a torque that tries to align the low-reluctance axis with the field. It is the basis of the synchronous reluctance machine and it shifts the pull-out angle below ninety degrees.

Answer: A

Flux is trapped in the damper winding and the rotor body for the first cycles, giving the smallest reactance and the largest current. As those currents decay the reactance rises through the transient to the synchronous value.

Answer: B

Correct answer: subtransient, transient, synchronous

Immediately after a fault the damper windings and the rotor body screen the flux, giving the very small subtransient reactance; as those currents decay the field winding continues to screen, giving the transient value; and finally the steady state synchronous reactance applies. So \( X_d'' < X_d' < X_d \), and the fault current is largest at the first instant.

Answer: C

Correct answer: voltage, frequency, phase sequence and phase angle

All four must match before the switch is closed. A mismatch in any of them puts a difference voltage across the machine’s own small impedance and draws a large current, and a wrong phase sequence in particular can wreck the machine and its coupling. Synchroscopes and check-synchronising relays exist to enforce exactly these four conditions.

Answer: D

Its pointer rotates in one direction if the machine is running fast and the other if slow, and it comes to rest at the twelve o'clock position when the two are in phase. The operator closes the breaker as the pointer creeps slowly towards that.