Side-by-side comparison
| Parameter | Synchronous Generator | Motor |
|---|---|---|
| Energy Conversion | Mechanical → Electrical (prime mover drives rotor) | Electrical → Mechanical (stator field drives rotor) |
| Power Angle δ | δ > 0 (E_f leads V_t); machine supplies active power to grid | δ < 0 (E_f lags V_t); machine absorbs active power from grid |
| Excitation for Unity pf | Normal excitation — field current set for rated terminal voltage | Same — field current adjusted for unity power factor operation |
| Over-Excitation Effect | Supplies lagging reactive power to grid (capacitive to grid) | Draws leading current from grid (acts as capacitor bank) |
| Under-Excitation Effect | Absorbs lagging reactive power — stability risk | Draws lagging current from grid (acts as inductor) |
| Prime Mover | Required — steam turbine (500 MW), hydro turbine, diesel engine | Not required as prime mover; absorbs mechanical load |
| Application | All grid-connected power stations: thermal, hydro, nuclear, gas | Synchronous condensers for VAR compensation; large industrial drives |
Key differences
Power angle δ is the angle between excitation EMF E_f and terminal voltage V_t. In a generator, E_f leads V_t (δ positive) and active power P = (E_f × V_t / X_s) sinδ flows out. In a motor, E_f lags V_t (δ negative) and the machine absorbs power. A 500 MW, 21 kV turbogenerator at NTPC Ramagundam operates with δ ≈ 30–40° under full load. Over-excitation in a generator increases reactive power output (supplies MVAR to grid); over-excitation in a motor makes it draw leading current, acting like a capacitor bank — this is the synchronous condenser principle used in HVDC converter substations.
When to use Synchronous Generator
Use a synchronous generator when mechanical shaft power must be converted to grid-frequency AC — for example, a 210 MW, 15.75 kV, 50 Hz turbogenerator in a coal-fired power station connected to the 220 kV grid through a generator transformer.
When to use Motor
Use a synchronous motor (or synchronous condenser) when you need reactive power compensation or a large constant-speed drive — for example, an over-excited synchronous condenser at a 220 kV substation supplying 50 MVAR to maintain voltage during peak load hours.
Recommendation
In exam problems, identify energy flow: mechanical in → generator; electrical in → motor. Then use P = (E_f × V_t / X_s) sinδ for both — the sign of δ tells you direction. Over-excitation always makes the machine look like a capacitor to the external circuit, regardless of whether it is a generator or motor.
Exam tip: Examiners ask for the effect of changing field current in a generator on power factor — over-excitation increases E_f, raising reactive power output and making the generator supply lagging MVAR to the inductive grid; under-excitation makes the generator absorb MVAR.
Interview tip: Interviewers at NTPC or power plant EPC companies ask how active and reactive power are independently controlled — answer: active power is controlled by governor (steam admission/gate opening) which changes δ; reactive power is controlled by AVR (automatic voltage regulator) which changes field current and thus E_f.