Synchronous Machines MCQ [Free PDF] – 11 Objective Questions with Answers

Steady torque exists only in synchronism, and for the basic cylindrical-rotor model T ∝ sin δ. A conventional synchronous motor has no inherent starting torque, and maximum torque occurs near δ = 90°, not 180°.

A synchronous motor runs at synchronous speed, has zero inherent starting torque and holds speed as load changes. Torque depends on load angle, not slip, and converter-fed synchronous motors are used in variable-speed drives.

A conventional synchronous motor is not inherently self-starting. By adjusting field excitation it can operate at lagging, unity or leading power factor; over-excited operation is used for reactive-power support. Key relation: N_s=120f/P.

Exam focus: Excitation controls power factor; frequency and pole count set speed. Common trap: Load changes torque angle, not steady synchronous speed.

A — Absence of slip losses. A synchronous motor runs at zero slip, so there are no rotor slip (copper) losses of the induction-motor kind.

GPSC's final key accepts 'All of the mentioned'. A larger air gap reduces synchronous reactance and armature-reaction sensitivity, which improves stability/parallel operation; appropriate air-gap design also helps achieve a smoother magnetic-field distribution.

C — Hunting. Damper windings reduce the rotor oscillations that follow a sudden load change by producing a torque that opposes those oscillations.

For an alternator connected to an infinite bus, bus voltage and frequency are fixed. Increasing prime-mover mechanical input increases torque angle δ and therefore increases the active power delivered to the bus.

C — 100 A approximately. Electrical input is 50/0.90 = 55.56 kW, giving line current 55,555.56/(√3 × 400 × 0.8) = 100.23 A.

A — It increases up to the stability (pull-out) limit. With V and Xs constant, P = (VE/Xs) sinδ, so raising excitation E raises Pmax = VE/Xs.

The final GPSC answer is B. A field winding placed on the stator produces an air-gap field stationary with respect to the stator. A rotor moving at synchronous speed sees that stationary field moving backward at Ns, not at 2Ns.

A generator with more poles produces the same frequency at a lower mechanical speed; for fixed pole count, frequency is directly proportional to speed. f=PN/120. With P=8 and N=750 rpm, f=50 Hz; at 1500 rpm, f=100 Hz. Key relation: f=PN/120; N_s=120f/P.

Exam focus: Use rpm in the 120-factor formula. Common trap: Frequency is not independent of rotor speed in a synchronous generator.