In short. The excitation system powers the voltage regulator and, through the exciter, the main rotor field. A shunt or self-excited alternator draws that power from its own output terminals, so it weakens exactly when a motor start or a short circuit pulls the voltage down. An auxiliary-winding or permanent magnet generator machine keeps the regulator fully powered regardless of terminal voltage, so it holds voltage better on heavy starts and sustains fault current long enough to trip breakers.
How the field gets its current
A brushless alternator has no electrical connection to its rotor. The main field is fed by a rotating rectifier, which is fed by the exciter armature on the same shaft. The exciter’s stationary field is driven by the automatic voltage regulator, so the regulator controls terminal voltage at one remove. Where the regulator itself gets the power to drive that exciter field decides how the machine behaves when the terminal voltage is disturbed. The regulator’s control functions are described under alternator AVR.
Three excitation architectures
Shunt or self-excited
The regulator is powered from the main stator output, sometimes through a small transformer. It is the simplest and cheapest arrangement and is adequate for lighting, heating and modest motor loads. Its weakness is circular: when a large motor starts and the terminal voltage falls to 75 percent, the regulator’s supply falls with it, its ceiling drops, and recovery is slower than it need be. On a bolted short circuit the regulator loses its supply and the fault current decays to a fraction of rated within a few cycles, which may be too little to trip the downstream breaker. A shunt machine also depends on residual magnetism to begin building voltage, which is why field flashing is occasionally needed after long storage.
Auxiliary winding
Here one or two extra windings are embedded in the main stator slots. One produces a voltage proportional to terminal voltage; the other picks up flux from load current and its harmonics. Their combined output powers the regulator, so when load current rises during a start or a fault, the regulator supply rises with it. The arrangement supports about three times rated current for several seconds and gives good motor-starting recovery at a cost close to a shunt machine. Manufacturers use their own trade names for it, and it is the default on many mid-range sets sold in India.
Permanent magnet generator
A small pilot alternator with a permanent magnet rotor is mounted at the non-drive end of the shaft. It produces a clean three-phase supply, commonly 150 to 220 V at a frequency several times the output frequency, that depends only on shaft speed. The regulator is therefore fully powered from the moment the engine reaches speed, through the deepest dip and through a sustained short circuit, and is immune to the waveform distortion that non-linear loads impose on the main stator. PMG machines sustain around 300 percent rated current for up to ten seconds, build voltage without residual magnetism, and give the best transient recovery of the three.
| Characteristic | Shunt | Auxiliary winding | PMG |
|---|---|---|---|
| Regulator supply | From output terminals | From dedicated stator windings | From separate pilot alternator |
| Behaviour during a deep voltage dip | Supply falls with the dip | Supply rises with load current | Supply unaffected |
| Sustained short-circuit current | Decays within cycles | About 3 times rated for several seconds | About 3 times rated for up to 10 s |
| Sensitivity to non-linear load distortion | High | Moderate | Low |
| Dependence on residual magnetism | Yes | Reduced | No |
| Relative cost and complexity | Lowest | Low | Higher, extra rotating component |
The sustained current figures are typical values and must be confirmed from the data sheet of the specific machine.
Why it matters for motor starting
When a motor starts direct-on-line, the alternator sees several times the motor’s running kVA at a very low power factor. Voltage falls according to the machine’s transient reactance, and the regulator forces the exciter field towards its ceiling. How high that ceiling is, and how fast the regulator reaches it, decides whether the dip is 15 or 30 percent and whether recovery takes half a second or three. With a shunt supply the ceiling shrinks during the event; with a PMG it does not, so for the same frame a PMG or auxiliary-winding machine starts a larger motor within a given dip limit. The calculation is worked through under alternator motor-starting performance.
Why it matters for protection
Downstream breakers are selected to clear faults fed by the utility, where fault current is many times the breaker rating. On generator supply the fault current is limited by the machine’s reactance to perhaps six to ten times rated in the first cycles, and on a shunt machine it then collapses towards or below rated current. A breaker two or three levels downstream may never see enough current for long enough to trip, and the generator’s own protection clears the fault by shutting down the whole installation. A PMG or auxiliary-winding machine holding three times rated current for several seconds gives the breakers time to operate selectively. The study method is covered under reactance and short-circuit performance.
Choosing between them
- Lighting, HVAC and small motor loads with no paralleling and simple downstream protection: a shunt machine is adequate.
- Large direct-on-line motors, pumps, compressors and crushers, or a site that parallels sets: auxiliary winding or PMG, chosen by the dip calculation.
- Data halls, UPS-heavy loads, drives and LED lighting that together exceed about a quarter of the load: PMG, because the regulator supply is then independent of the distorted stator waveform. The effect of such loads is explained under alternators and non-linear loads.
- Installations where selective tripping on generator supply has been studied and depends on sustained fault current: PMG or auxiliary winding, with the sustained current confirmed in writing.
Diagnosing excitation faults
Excitation faults present as no voltage, low voltage, unstable voltage or voltage that collapses under load, and they overlap with regulator faults. On a PMG machine the first test is the PMG output itself: three balanced phases of the stated voltage at rated speed, measured at the regulator’s power terminals. On any machine the exciter field resistance, the rotating diodes and the surge suppressor are tested next, then the main field resistance and insulation. A diode that fails short lets the regulator drive hard with little result and overheats the exciter; one that fails open gives low, unstable voltage. Replacement AVR, PMG and exciter parts are matched by alternator serial number, and our AVR and excitation service carries the test gear to do this on site.
Common mistakes
- Specifying a shunt machine for a pump house because the running kVA fits; the first direct-on-line start drops the voltage enough to release the contactors.
- Fitting a PMG-type regulator to a shunt machine, or the reverse, because the connector matched; the regulator either never builds voltage or is destroyed.
- Assuming “brushless” implies sustained short-circuit capability; all three architectures are brushless, and only two sustain fault current.
- Flashing the field on a machine that never needed it; the real fault was a sensing fuse or a diode, and the flashing damaged the regulator.
What to send us
Send a photograph of the alternator nameplate, a photograph of the non-drive end showing whether a PMG is fitted, the regulator label, and your load list with the largest motor, its kW and starting method marked. If the enquiry concerns a fault, describe whether voltage is absent, low or unstable and whether it changes with load. Send a spares or service enquiry and we will identify the excitation type and the parts or tests that apply.
Questions engineers ask
Can a PMG be added to an existing shunt alternator?
On some frames the manufacturer offers a PMG kit with a matching regulator, fitted at the non-drive end. It requires the correct shaft extension, the kit for that exact frame and a regulator change. Where no kit exists, the practical upgrade is a larger or differently excited alternator.
Does a PMG improve steady-state voltage regulation?
Not noticeably; steady-state accuracy is set by the regulator. What the PMG changes is behaviour during disturbances: a higher excitation ceiling during motor starts, sustained current on faults, and immunity to the distorted waveform of non-linear loads.
What does three times rated current for ten seconds actually buy me?
Time for downstream protection to operate selectively. A 500 kVA machine at 415 V holds about 2,000 A for ten seconds, which will trip a correctly rated 400 A or 250 A downstream breaker on its overload or short-time element, leaving the rest of the installation live. A shunt machine may not.
Why has my generator lost its residual magnetism?
Long idle periods in humid conditions, reverse-polarity connection during testing or a large demagnetising event can reduce residual flux below what a shunt regulator needs to start. Flashing the field through a blocking diode restores it. PMG machines do not have this problem.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.