Synchronous Machine Interview Questions

Q1. Why is a synchronous motor not self-starting?

At standstill, when 3-phase AC is applied to the stator, the rotating magnetic field spins at synchronous speed while the rotor is stationary — the stator field passes the rotor poles so rapidly (50 times per second) that the rotor experiences alternating pull and push in successive half cycles and produces zero average torque. To start a synchronous motor, a squirrel cage (damper) winding is embedded in the pole faces, allowing it to start as an induction motor and then synchronise when near synchronous speed. Modern installations use variable-frequency drives to ramp frequency from zero, bringing the motor up smoothly.

Follow-up: What is the role of the damper winding during steady-state operation of a synchronous motor?

Q2. What is the significance of excitation in a synchronous generator?

Excitation current in the rotor field winding creates the magnetic flux that, when the rotor spins, induces the stator EMF — increasing excitation increases terminal voltage (on an isolated machine) or reactive power output (when connected to an infinite bus). A 210 MW turbo-generator at a thermal power station is controlled by an automatic voltage regulator (AVR) that adjusts field current in milliseconds to maintain terminal voltage. Over-excitation makes the generator supply reactive power (capacitive load) and under-excitation makes it absorb reactive power — this is the fundamental control mechanism for power factor correction in the grid.

Follow-up: What happens to a synchronous generator if it becomes under-excited beyond the stability limit?

Q3. Explain V-curves and inverted V-curves of a synchronous motor.

V-curves plot stator current versus field current at constant real power — stator current is minimum at unity power factor (normal excitation), increases with under-excitation (lagging pf), and increases with over-excitation (leading pf), forming a V-shape. Inverted V-curves plot power factor versus field current, forming an inverted V with peak at unity. A 1 MVA synchronous compensator at a 33 kV substation uses over-excitation to supply reactive power to the grid, operating on the right branch of the V-curve — this is the same principle as a STATCOM but using a rotating machine.

Follow-up: What is a synchronous condenser and how is it different from a synchronous motor on load?

Q4. What is hunting in synchronous machines and how is it damped?

Hunting is the oscillation of rotor angle about the steady-state load angle position when a sudden load change disturbs the machine — the rotor swings back and forth around the equilibrium like a pendulum. Without damping, a 100 MVA synchronous generator can sustain oscillations for 10–15 seconds, threatening transient stability. Damper (amortisseur) windings in the pole faces carry induced currents whenever rotor speed deviates from synchronous speed, producing an opposing torque proportional to the velocity deviation — exactly like viscous friction — and damp the oscillation within 2–3 cycles.

Follow-up: What is the difference between transient stability and steady-state stability in a synchronous machine?

Q5. What conditions must be met before synchronising an alternator to the grid?

Four conditions must be satisfied before closing the synchronising breaker: the incoming machine voltage must equal the bus voltage, frequency must match, phase sequence must be identical, and the incoming voltage must be in phase with the bus voltage. Synchroscopes and sync-check relays verify these conditions — in a modern 60 MW power plant, automatic synchronisation equipment checks all four conditions and closes the breaker only when phase difference is within ±10°. Violating the phase condition can cause a current surge of 10–20 times rated current that can damage windings and couplings.

Follow-up: What are the consequences of closing the synchronising breaker with a 180° phase difference?

Q6. What is the load angle (torque angle) of a synchronous generator and what limits it?

Load angle δ is the angular displacement between the rotor magnetic axis and the resultant air gap flux axis — it equals the electrical phase difference between the induced EMF (Ef) and the terminal voltage (Vt). For a cylindrical rotor generator, power P = (Ef × Vt / Xs) × sin δ, reaching maximum at δ = 90° — beyond this the machine loses synchronism and pulls out of step. A 100 MW steam turbine generator typically operates at δ = 20–30° at full load, giving a stability margin of 60–70° before the pull-out condition.

Follow-up: How does increasing the excitation of a generator affect the load angle at the same real power output?

Q7. What is the difference between a salient pole and a cylindrical rotor synchronous machine?

A cylindrical rotor (round rotor) has uniformly distributed field windings in slots along the rotor length, giving uniform air gap — used in high-speed 2-pole or 4-pole turbo-generators (3000 RPM at 50 Hz) driven by steam turbines. A salient pole machine has projecting poles with concentrated windings, non-uniform air gap, and is used in low-speed hydro-generators (12–200 RPM) with 24–120 poles. The salient pole machine has two reactances (Xd and Xq) due to the asymmetric air gap, requiring two-reaction theory for analysis, while the cylindrical rotor has a single synchronous reactance Xs.

Follow-up: What is reluctance torque in a salient pole synchronous machine and at what load angle is it maximum?

Q8. How does a synchronous generator respond to a sudden loss of load?

When load is suddenly disconnected, the turbine input exceeds the electrical output, accelerating the rotor and increasing rotor angle above equilibrium — the governor detects speed rise and reduces steam/water input. Simultaneously, terminal voltage rises because the demagnetising effect of lagging load current disappears, and the AVR reduces excitation. In a 210 MW unit on the Indian grid, sudden load rejection can cause a frequency overshoot of 1–2 Hz within seconds if governor response is slow — this is why turbine runback protection and load shedding relays are coordinated.

Follow-up: What is the role of the governor droop characteristic in frequency regulation after a load loss event?

Q9. What is armature reaction in a synchronous generator and how does it affect terminal voltage?

Armature reaction in a synchronous generator is the effect of armature (stator) current on the air gap flux — at unity power factor, the effect is purely cross-magnetising (distorts but doesn''t change net flux), at lagging power factor it is demagnetising (reduces flux and terminal voltage), and at leading power factor it is magnetising (increases flux and can cause over-voltage). A generator supplying a lightly loaded long cable (capacitive load) can experience leading power factor armature reaction severe enough to require under-excitation — the Ferranti effect on long cables makes this a real operational concern.

Follow-up: How is the effect of armature reaction modelled in the equivalent circuit of a synchronous generator?

Q10. What is per-unit reactance and why is it used in synchronous machine analysis?

Per-unit (pu) reactance expresses the machine''s synchronous reactance as a fraction of the base impedance (V_rated²/S_rated) — a 100 MVA generator with Xs = 1.2 pu means its synchronous reactance is 1.2 times the base impedance. Using per-unit values eliminates transformer voltage ratios and allows machines of different ratings to be directly compared and combined in power system studies. In a fault analysis involving three generators of 100, 200, and 50 MVA, all reactances are converted to a common MVA base for Thevenin equivalent calculation.

Follow-up: How do you convert a machine's per-unit reactance from its own rated MVA base to a system base?

Q11. What is the short circuit ratio (SCR) of a synchronous generator and what does it indicate?

SCR is the ratio of field current required for rated open-circuit voltage to the field current required for rated short-circuit current — it is approximately the reciprocal of the per-unit synchronous reactance. A generator with SCR = 0.6 (Xs = 1.67 pu) has lower stability margin than one with SCR = 1.0 (Xs = 1.0 pu). Hydro-generators typically have SCR of 0.9–1.2 (more stable) while turbo-generators have SCR of 0.5–0.7 because their smaller air gap gives higher reactance — lower SCR machines need more careful AVR tuning for stable parallel operation.

Follow-up: How does SCR relate to steady-state stability limit of a synchronous generator?

Q12. How does the two-reaction theory apply to salient pole synchronous machines?

Two-reaction theory resolves the armature MMF into two components: the direct axis (d-axis) component aligned with the field pole axis, and the quadrature axis (q-axis) component at 90° to it — each sees a different reluctance and thus different reactance (Xd > Xq). For a salient pole hydro-generator with Xd = 1.0 pu and Xq = 0.65 pu, the voltage regulation and power-angle equations must use both reactances, giving a power expression with an extra reluctance torque term: P = (Ef×Vt/Xd)sinδ + (Vt²/2)(1/Xq - 1/Xd)sin2δ. This reluctance torque exists even with zero excitation, which is exploited in synchronous reluctance motors.

Follow-up: At what load angle does the reluctance torque component reach its maximum value?

Q13. What is the effect of sudden 3-phase short circuit on a synchronous generator?

At the instant of a 3-phase short circuit, the sub-transient reactance Xd'' (typically 0.1–0.2 pu) limits the initial current — this is the highest current, perhaps 10–15 times rated, lasting only 2–3 cycles. The current then reduces to the level limited by transient reactance Xd'' (0.2–0.4 pu) during the transient period (0.1–1 s), and finally settles at the steady-state short circuit current limited by synchronous reactance Xd (0.8–2.0 pu). Circuit breakers must interrupt within the sub-transient or transient period, and switchgear is rated based on sub-transient fault current for the worst case.

Follow-up: What is the X/R ratio of synchronous machine and why does it affect the DC offset in fault current?

Q14. What is the role of the automatic voltage regulator (AVR) in a synchronous generator?

The AVR continuously compares the terminal voltage with a reference setpoint and adjusts the field current (excitation) through the exciter to maintain constant voltage under changing load conditions. Modern digital AVRs on large generators like the 500 MW units at Vindhyachal STPS respond within 50–100 ms to a step load change. The AVR also provides power system stabiliser (PSS) functionality — injecting a stabilising signal derived from rotor speed or power to damp inter-area oscillations that would otherwise persist for many seconds.

Follow-up: What is the difference between a brushless exciter and a static excitation system?