Alternator / Synchronous Generator
AC Generator
The machine that makes almost all of the world's electricity: a DC-excited rotor field swept past stator windings induces three-phase AC at exactly the speed-locked frequency f = PN/120.
Schematic symbol
Generator / alternator
How alternator / synchronous generator appears in a circuit diagram.
Types & variants
Key specs
kVA rating
kVA / MVAApparent power output; the engine behind it sets the kW, the excitation sets the kVAR.
Voltage
V415 V for small gensets; 11 kV or higher for utility machines, stepped up by a generator transformer.
Frequency
HzLocked to speed: 50 Hz needs 3000 RPM (2-pole), 1500 RPM (4-pole), etc. Frequency droop means the engine is slowing.
Power factor rating
—Usually rated at 0.8 lagging — defines how much reactive power it can supply at full kVA.
Voltage regulation
%No-load to full-load voltage change; an AVR trims excitation to hold the terminal voltage.
Markings
Nameplate: kVA, V, A, PF, RPM, frequency, excitation V/A, insulation class. Genset panels add the AVR model and connection diagram (star with neutral brought out for 3-phase 4-wire).
Standard values
Portable gensets: 1–10 kVA single-phase 230 V. Commercial DG sets: 15–2000 kVA, 415 V, 1500 RPM, 0.8 PF, brushless with AVR. Car alternators: 12 V, 35–150 A internally rectified.
How to choose
1) Size kVA for the peak load including motor starting surges (a motor can demand 3× its rating at start). 2) Brushless + AVR for anything unattended. 3) Check step-load acceptance for sudden loads. 4) For paralleling with grid or other sets, you need synchronising gear: match voltage, frequency, phase sequence and phase angle before closing the breaker.
Pinout & package
Stator output terminals (U V W N), exciter/AVR wiring, and the shaft coupled to engine or turbine. Brushless sets have the exciter armature and rotating diode plate on the shaft — no slip rings to service.
Example circuits
- Diesel genset: engine + brushless alternator + AVR feeding a changeover panel
- Synchronising an incoming alternator to live busbars with lamp or synchroscope methods
- Car charging system: alternator + rectifier + regulator maintaining 13.8–14.4 V
- Grid alternator delivering kW set by turbine torque and kVAR set by excitation
Common failures
Failed rotating diodes on brushless sets (low or no output), AVR failure (over/under voltage), loss of residual magnetism after long storage (no voltage build-up — field flashing fixes it), bearing wear, and winding insulation breakdown from heat or moisture.
How to test
Measure output V and f at rated speed; check the exciter field with the AVR temporarily replaced by a battery (voltage should build); test rotating diodes with a multimeter diode range; megger stator to frame; on car alternators, look for ~14 V at idle and AC ripple under 0.5 V (bad diode if higher).
Substitutes
Genset alternator swaps need matching kVA, voltage, speed and mounting (single/two-bearing, SAE housing). Solar + battery inverter systems increasingly replace small gensets; for exam purposes the induction generator is the alternative for wind, but it can't self-excite the grid.