In short. The alternator is the half of a generating set that actually makes electricity, and its nameplate kVA is only the start of the assessment. Steady-state rating, motor-starting capability, behaviour with non-linear loads, sustained short-circuit current, temperature rise in the real room and the winding connection all have to be checked against the load. The Reddy Generators engineering library covers each of those topics in its own guide, and the sections below explain how they fit together.
How a brushless generator alternator is built
Almost every diesel generating set from a few kVA to several MVA uses a brushless synchronous alternator. The main stator carries the output winding. The main rotor carries the field winding, which is fed with direct current to create the rotating magnetic field. Instead of slip rings and brushes, the field current comes from an exciter mounted on the same shaft: the exciter’s stationary field is fed by the automatic voltage regulator, its rotating armature generates three-phase AC, and a rotating rectifier on the shaft converts that to DC for the main field. On many machines a small permanent magnet generator at the non-drive end supplies the regulator with clean, independent power. The regulator and the excitation system have their own pages because they decide how the machine behaves under disturbance.
Speed, poles and frequency
A synchronous machine’s frequency is locked to its speed and pole count.
Ns = 120 × f ÷ P
Here Ns is synchronous speed in rpm, f is frequency in hertz and P is the number of poles. For the Indian 50 Hz supply a four-pole alternator runs at 1,500 rpm, which is the standard for engines up to a few MW; a six-pole machine runs at 1,000 rpm and appears on larger, slower engines. As a worked example (illustrative), a four-pole alternator driven at 1,530 rpm produces 51 Hz, and a set whose governor sags to 1,440 rpm under load is delivering 48 Hz, which is outside what most loads and protection relays will accept for long. The formula tells you that frequency problems are engine speed problems, and that voltage problems are excitation problems; they are diagnosed by different people with different instruments.
What the rating actually covers
An alternator nameplate states kVA at 0.8 power factor lagging, a voltage, a frequency, an insulation class, a temperature rise, an ambient temperature basis of 40 degrees and an altitude basis of 1,000 m. Each of those qualifies the kVA figure. Standby ratings usually accept a class H temperature rise, while prime and continuous ratings use a lower class F rise on the same insulation and therefore quote a lower kVA for the same frame. Load above 0.8 power factor lagging, or any leading power factor, changes the excitation demand and must be checked. Temperature, ambient and enclosure effects are explained under temperature rise, insulation and IP.
The set as a whole is rated on the lower of engine kW and alternator kVA at the site conditions. It is common for the alternator to be one or two frame sizes larger than the engine strictly needs, because an oversized alternator is the cheapest way to improve motor-starting and non-linear-load performance without buying a larger engine.
The five checks that decide suitability
| Check | Question it answers | Data needed | Where it is explained |
|---|---|---|---|
| Steady-state capacity | Can it carry the running load at the real ambient and power factor? | Load list, power factor, room temperature, altitude | Rating and derating pages |
| Motor starting | How far does voltage dip when the largest motor starts, and does it recover? | Motor kW, starting method, transient reactance, excitation type | Motor-starting performance |
| Non-linear loads | Will UPS, drives and LED lighting distort the waveform or overheat the machine? | Share of non-linear load, subtransient reactance, winding pitch | Alternators and non-linear loads |
| Fault current | Can it deliver enough current for long enough to trip downstream breakers? | Subtransient and transient reactance, excitation support | Reactance and short-circuit performance |
| Connection and voltage | Is the winding connected for 415/240 V star, and is the neutral available and sized? | Lead count, nameplate connection diagram, earthing arrangement | Windings and voltage |
A machine can pass the first check and fail any of the others. The usual example in Telangana industry is a pump house or a crusher where the running load fits comfortably but the first direct-on-line start pulls voltage down far enough to drop the contactors.
Mechanical integration
The alternator is bolted to the engine flywheel housing through a standard SAE adaptor, and on most sets below about 1 MVA it is a single-bearing design whose rotor is carried at the drive end by the engine crankshaft through flexible coupling discs. That makes alignment simple at the factory and makes the combination of engine and alternator a single torsional system that must be checked when a non-standard pairing is built. Bearing types, greasing, vibration limits and the torsional question are covered under bearings and torsional considerations.
Condition and life
Alternators fail slowly. Insulation ages with heat and moisture, bearings wear, rotating diodes fail one at a time, and the first symptom is often a voltage that is slightly unstable or slightly low under load. An insulation resistance test at each major service, a polarisation index on older machines, a thermal image of the terminal box under load, and a check of the rotating rectifier take less than an hour and give years of warning. In Hyderabad’s monsoon, an alternator in an open shed can absorb enough moisture during a long idle period to fail an insulation test, and anti-condensation heaters wired to run when the set is stopped are a cheap protection. Where a winding has failed, rewinding to the original specification is practical for most frames.
Related photographs and videos
Common mistakes
- Selecting an alternator on running kVA alone; the first large motor start or the UPS load reveals a machine that was never suitable.
- Reading the nameplate kVA without the temperature rise class and ambient basis; a standby-rated machine is run continuously in a 48 degree room and its insulation life is spent in a few years.
- Diagnosing a frequency complaint as an alternator fault; frequency is engine speed, and the governor or fuel system is where the problem lies.
- Skipping insulation testing because the set “runs fine”; moisture absorbed during an idle monsoon shows up as a winding failure at the next outage.
What to send us
Send a photograph of the alternator nameplate and the genset nameplate, your load list with the largest motor and its starting method marked, the share of UPS or drive load, the room ambient and the fault or symptom if there is one. For a new set, the single-line diagram and the site location help with ambient and altitude. Send your application details and we will tell you which of the five checks matter for your load.
Questions engineers ask
Why do generators state kVA and engines state kW?
The engine delivers real power, measured in kW. The alternator must carry the full current, which depends on apparent power in kVA. The two are linked by power factor, and the industry convention is to rate sets at 0.8 power factor lagging, so a 500 kVA set delivers 400 kW.
Can an alternator be run above its nameplate kVA for a short time?
Most designs tolerate a stated overload, commonly 10 percent for one hour in twelve, provided the engine can supply it. Beyond that the winding temperature rises quickly and insulation life is consumed. Repeated overload is a sizing problem, not an operating strategy.
What is the difference between a two-thirds pitch and a full pitch winding?
A two-thirds pitch winding is wound so that third-harmonic voltages cancel, which keeps the line-to-neutral waveform clean and prevents circulating currents when sets are paralleled or when the neutral is solidly earthed. It is the usual choice for generator alternators in India for that reason.
How long does a generator alternator last?
With clean, dry operation within its temperature class, several decades. Bearings and rotating diodes are the wear items. Life is shortened by heat, moisture, overload, harmonic-rich loads and vibration, so the installation environment matters more than the calendar.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.