In short. The alternator is the electrical machine on the end of the engine that turns shaft power into 415 V, 50 Hz output. Its stator, rotor, exciter, rotating rectifier, AVR and optional permanent magnet generator decide voltage regulation, how deep the voltage dips when a motor starts, how well it tolerates UPS and drive loads, and how hot it runs. Two sets with the same engine and kVA can perform quite differently because their alternators differ.
How a brushless alternator works
Almost every generator set sold in India above a few kVA uses a brushless synchronous alternator. The main rotor carries a DC field winding and turns inside the main stator, whose three windings produce the output. Field current comes not from brushes but from a small exciter on the same shaft: its stationary field is fed by the AVR, its rotating armature produces AC, and diodes on the shaft rectify that AC into the DC the main rotor needs. The AVR senses output voltage and adjusts the exciter field to hold the set point. On larger machines a permanent magnet generator on the shaft end feeds the AVR, so excitation does not depend on output voltage and the machine can sustain short-circuit current and recover from a deep dip. With no wearing contacts beyond the bearings, alternators routinely run for decades between rewinds.
| Component | Function | Typical failure and symptom |
|---|---|---|
| Main stator | Three output windings, star connected, neutral brought out | Insulation breakdown after moisture or overheating; earth fault or one phase low |
| Main rotor | DC field winding producing the rotating flux | Winding failure; low voltage that the AVR cannot correct |
| Exciter | Supplies field power without brushes | Field open circuit; no voltage build-up |
| Rotating rectifier | Diodes converting exciter AC to rotor DC | Shorted or open diode; unstable or low voltage, overheated exciter |
| AVR | Controls exciter field to regulate output voltage | Component failure or wrong setting; voltage hunting, over or under voltage |
| PMG | Independent power source for the AVR | Magnet or winding damage; loss of voltage under fault or heavy load |
| Bearings | Support the rotor; single bearing on most sets with flexible disc coupling to the engine | Grease failure or misalignment; vibration, noise, eventual seizure |
The AVR and PMG excitation pages explain the control loop in more depth.
Reactances: the figure behind every voltage dip
When a motor starts or a transformer is energised, the alternator sees a sudden demand for reactive current and its terminal voltage falls before the AVR can respond. How far it falls in the first cycles is set by the sub-transient reactance, X”d, expressed as a percentage on the machine’s own rating.
Voltage dip (approximate, percent) ≈ (Starting kVA × X”d) ÷ (Starting kVA × X”d + Alternator kVA) × 100
Here starting kVA is the motor’s locked-rotor demand, X”d is the sub-transient reactance as a per-unit value and alternator kVA is the machine’s rating. As an illustrative example, a 45 kW motor on direct-on-line start demanding about 330 kVA, fed by a 500 kVA alternator with X”d of 0.12 per unit, produces an initial dip of roughly 330 × 0.12 ÷ (330 × 0.12 + 500), which is about 7 percent; the same motor on a 250 kVA alternator with X”d of 0.16 dips about 17 percent (illustrative). Most contactors drop out somewhere below 70 to 80 percent voltage, so the second case is marginal if anything else is running. Lower-reactance alternators cost more because they contain more copper and iron, but specifying one is often cheaper than stepping the whole set up to the next engine size. The reactance and short-circuit page and the voltage dip page cover the calculation and the recovery that follows.
Nonlinear loads and temperature rise
UPS rectifiers, variable-frequency drives, LED drivers and switched-mode power supplies draw current in pulses rather than a sine wave. Those harmonic currents flow through the alternator’s reactance and distort the terminal voltage, which can confuse the AVR, heat the rotor through induced currents and overheat the stator beyond what the RMS current suggests. The usual safeguards are a 2/3 pitch stator winding that suppresses the third harmonic, a larger alternator frame than the kW alone would call for, and an AVR that senses true RMS or average on all three phases rather than the peak of one. Where more than about a quarter of the load is rectifier-fed, the alternator should be selected with its harmonic capability stated in writing; the nonlinear loads page sets out the derating approach.
Temperature rise is the other limit. Alternators are built with Class H insulation but normally rated for a Class F rise of 105 K above a 40 °C ambient, with the margin to Class H held as a reserve. Running at 45 °C ambient, with harmonic heating, or with a blocked air inlet erodes that reserve and halves insulation life for every 10 K of sustained overheating. Standby-rated alternators use more of the thermal margin than prime-rated ones, which is part of why the same frame carries a different kVA at each rating class.
Diagnosing a voltage problem
A set with no voltage, low voltage or unstable voltage does not automatically need a new AVR, although that is the part most often changed first. The measured approach starts at the output terminals under known load, checks residual voltage with the AVR disconnected to prove the main machine, measures the exciter field and the rotating diodes, confirms the sensing wires reach the AVR with the right voltage, and checks engine speed, because a governor fault that drops speed a few percent looks exactly like a weak alternator. Only when the chain is proven is the AVR condemned. The alternator service page describes the tests, and the alternator parts page the diodes, AVRs and bearings most often replaced.
Related photographs and videos
Common mistakes
- Replacing the AVR for every voltage complaint; a shorted rotating diode or a loose sensing wire is the real cause and the new AVR fails the same way.
- Selecting the alternator on kVA alone for a plant with large direct-on-line motors; the voltage dip at starting drops contactors across the plant.
- Feeding a UPS of more than a quarter of the set rating without checking harmonic capability; the alternator overheats and the AVR hunts.
- Starting a set after the monsoon without an insulation resistance test; damp windings fail on the first load.
- Loading one phase heavily with single-phase circuits; the rotor overheats although total load is modest.
What to send us
Send a photograph of the alternator nameplate showing make, model, frame, kVA, voltage, connection and insulation class, and of the AVR with its type number, together with the symptom in your own words, the voltage readings on all three phases under load and at no load if you have them, and the engine speed or frequency at the time. For a new set, send the largest motor rating and starting method and the UPS or drive kVA. Use the generator sales enquiry for a new machine or the spare parts enquiry for AVRs, diodes and bearings, and we will respond with a diagnosis path or a selection.
Questions engineers ask
What is the difference between a brushless alternator and one with slip rings?
A brushless machine feeds its rotor field through a shaft-mounted exciter and rotating diodes, so there are no brushes to wear or spark. Slip-ring machines, now rare in gensets, pass field current through carbon brushes that need regular inspection and replacement. Virtually all industrial generator alternators today are brushless.
Why does a PMG matter on a generator?
A permanent magnet generator gives the AVR a power source that does not collapse when the output voltage collapses, so the alternator can deliver about three times rated current into a short circuit for several seconds to trip breakers, and can recover from the deep dip of a large motor start. Self-excited machines without a PMG lose excitation under those conditions.
What voltage regulation should a generator alternator hold?
A modern AVR holds steady-state voltage within about ±1 percent from no load to full load at constant speed and power factor. Transient dips during load changes are a separate matter governed by the alternator reactances and the excitation system, not by the AVR’s steady-state accuracy.
Can an alternator be rewound, and is it worthwhile?
Yes, stator and rotor rewinding is routine for machines above about 100 kVA, and it is usually economic where the frame, shaft and bearings are sound. For small alternators a replacement is often cheaper than a rewind. The rewound machine should be tested for insulation resistance, winding resistance balance and voltage build-up before it goes back into service.
How often should alternator bearings be greased or replaced?
Many smaller alternators have sealed bearings rated for 20,000 to 40,000 hours and are replaced rather than greased; larger machines have regreasing points with intervals stated in hours on the nameplate or manual. Vibration and temperature at the bearing housing are the early warnings.
Generators by rating
- Under 15 kVA generators
- 15–50 kVA generators
- 50–125 kVA generators
- 125–250 kVA generators
- 250–500 kVA generators
- 500–1000 kVA generators
- 1–2 MVA generators
- 2 MVA and above generators
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.