Synchronous Generators Questions and Answers

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.

Answer: A

Correct answer: the setting of its prime mover governor

Real power comes from the prime mover, so more power means more steam, water or fuel - a change to the governor setting. Excitation controls the reactive power and the terminal voltage, not the real power. This separation of the two controls is the foundation of power system operation.

Answer: B

Raising the excitation above the value the terminal voltage requires makes the machine export reactive power, and lowering it makes the machine absorb. That independence from the active power is what makes a synchronous machine so useful to a network.