Generator Running but Low Voltage or No Output: Causes and Checks

In short. If the engine runs but the voltmeter shows little or nothing, the fault is almost always in the excitation system — the automatic voltage regulator (AVR), the exciter, the rotating diodes, or a set that has lost its residual magnetism after standing. If the voltage is low and the frequency is also low, the engine is running slow and the alternator is simply following it. Check the frequency first; it tells you which half of the set to look at.

Who this page is for: For operators whose engine runs normally but the alternator gives low, unstable or no voltage: separate engine-speed problems from alternator and AVR problems, and know what must be left to a qualified engineer.

How the voltage is made

A brushless alternator has a small exciter on the same shaft as the main rotor. The AVR senses output voltage and feeds a controlled DC current to the exciter’s stator. The exciter’s rotor produces AC, which rotating diodes turn into DC for the main rotor’s field. More field current means more output voltage. A small amount of magnetism left in the iron gets the process started when the set begins to turn. A break anywhere in that chain — sensing, AVR, exciter, diodes, rotor — shows up as low or no voltage.

Diagnostic decision table

Reading	Most likely cause	Next step
Voltage low, frequency low	Engine running slow: governor, fuel restriction, overload	Check speed and load first — see the hunting and black-smoke pages
No voltage at all, frequency normal	Lost residual magnetism, AVR failed, open circuit in exciter field, tripped breaker	Check breaker and AVR fuse; engineer to test and flash field
Voltage low at no load, frequency normal	AVR set low, failed rotating diode, AVR sensing fault	Engineer: adjust AVR, test diodes
Voltage normal at no load, collapses on load	Failed rotating diode, weak AVR, overload, large motor start	Engineer: diode test; measure load
Voltage very high	AVR sensing lost, AVR failed full-on, leading power factor from capacitors	Stop; check APFC and AVR
Voltage unequal between phases	Unbalanced load, loose connection, winding fault	Measure each phase; tighten connections
Voltage swinging	AVR stability setting, engine hunting, loose sensing wire	See the hunting page; engineer for AVR

Safe checks you can make

1. Note voltage and frequency on the panel at no load, and again on load.
2. Check the output breaker is on and has not tripped.
3. Look for an obvious loose or burnt cable at accessible terminals — with the set stopped and isolated.
4. Note any recent change: AVR replaced, set moved, long idle period, new capacitor bank.

Do not open the alternator terminal box or AVR cover with the set running. Field flashing, diode testing and AVR adjustment must be done by a qualified person.

Worked example (illustrative)

An illustrative case, not a project record. A 125 kVA set reads 415 V and 50 Hz at no load. When the building load of about 60 kW is applied, voltage drops to 340 V and lights dim, though frequency stays at about 49.5 Hz. Because frequency holds, the engine is not the problem. A test of the rotating diodes finds one open. With one diode out, the exciter cannot supply full field current under load. Replacing the diode set restores normal voltage on load.

When the usual assumptions fail

• “New AVR, so it’s not the AVR.” A replacement AVR must suit the alternator and be set up correctly; wrong sensing connections give wrong voltage.
• “Voltage is fine at no load.” Diode and exciter faults often appear only on load.
• Capacitor banks. High voltage on DG supply is often caused by APFC capacitors, not by the alternator.
• Insulation. A set that has stood in damp conditions should have its insulation resistance checked before it is loaded.

Prevention

• Insulation resistance checked periodically, especially after the monsoon
• Terminal and sensing connections checked and tightened at service
• Capacitor banks switched off on DG supply
• Regular runs on load to keep the excitation system exercised

The excitation chain under load: what changes and why faults hide

At no load the alternator needs only a modest field to make rated voltage. As load is added, and especially as the power factor falls from unity towards 0.8 lagging, armature reaction opposes the main field and the AVR must push far more current into the exciter to hold voltage. On a typical brushless machine the exciter field current at full load, 0.8 power factor, is commonly two to three times its no-load value (illustrative; the alternator data sheet gives the real figures). This is why a weakened link in the chain — one open rotating diode, a tired AVR output stage, an exciter winding with a partial fault — can hold voltage perfectly at no load and then fail as soon as the plant is switched on.

Open diode, shorted diode: two different signatures

An open diode turns the three-phase bridge into a partial bridge. The main field still receives DC, but less of it, so the AVR drives harder to compensate and the voltage sags once the field requirement rises with load. A shorted diode is more aggressive: it short-circuits one exciter phase each cycle, the exciter rotor heats, and the AVR runs at or near full output simply to make no-load voltage. Many AVRs then trip on over-excitation, which an operator sees as voltage that builds normally and then collapses after a few seconds or minutes. A surge suppressor across the main rotor that has failed short gives a similar picture. Both need a stopped, isolated machine and a qualified person to test.

Under-frequency roll-off is protection, not a fault

Most AVRs reduce voltage in proportion to speed once frequency falls below a set “knee”, commonly somewhere around 47–48 Hz on a 50 Hz set (illustrative). Holding full voltage at low speed would over-flux the alternator and overheat the exciter. So a set loaded beyond what the engine can carry will show falling frequency and falling voltage together. Turning the voltage potentiometer up in that situation simply hides an overload and stresses the windings.

What the AVR is actually sensing

Many smaller sets use an AVR that senses only one line-to-line voltage, often between two of the three phases. The AVR holds that pair at the set value and knows nothing about the third phase. With well-balanced load this does not matter. With heavy single-phase loading — common in commercial buildings, hostels and mixed-use sites in Telangana and Andhra Pradesh — the unsensed phase can sit well below the others while the AVR reports that all is well. Three-phase sensing, where fitted, averages all three phases and behaves better, but it cannot correct unbalance either; it only shares the error.

Moisture, insulation and the monsoon

An alternator that stands idle through the monsoon, particularly in a basement, an unheated canopy or near the coast, absorbs moisture into its winding insulation. Low insulation resistance does not always stop the set making voltage, but it can lead to tracking, earth-fault trips, or a winding failure when the set is first loaded after a long idle spell. Typical good practice, carried out by a qualified person with the AVR and electronic devices disconnected:

• Measure insulation resistance of the main stator, and where accessible the exciter and rotor, at a test voltage suited to the machine.
• Record the winding temperature with each reading — insulation resistance falls sharply as temperature rises, so readings are only comparable at similar temperatures.
• Where readings are low, take a timed series (for example at 1 minute and 10 minutes). A reading that rises well over time suggests a clean, dry winding; a flat reading suggests moisture or contamination.
• Dry a damp winding gently with anti-condensation heaters or controlled low-voltage heating before any load is applied. Running a wet machine on load to “dry it out” is how windings fail.

Anti-condensation heaters, where fitted, should be supplied whenever the set is at rest. A heater circuit with a tripped breaker is a common finding at post-monsoon inspections. For winding repairs and insulation work, see our alternator service page.

Electronic AVRs and controller alarms on newer sets

Newer CPCB IV+ sets usually pair a digital AVR with a controller that records alternator alarms alongside engine fault codes. Typical entries include loss of sensing, over-excitation, under-voltage, over-voltage and under-frequency. The order of events in the log is often the quickest clue: an engine speed or fuel-system code just before an under-voltage alarm points to the engine; an over-excitation alarm on its own points to the rotating rectifier or the exciter. Note the full alarm list, with times, before anyone resets the controller — a reset can clear the very record that shows the cause.

Voltage survey worksheet

Fill this in during a planned run. Readings are taken from the panel meters by site staff; readings at the load end are for a qualified electrician with suitable instruments.

Reading	No load	About 50 % load	Full normal load
Frequency (Hz)
Voltage R–Y / Y–B / B–R (V)
Voltage R–N / Y–N / B–N (V)
Current R / Y / B (A)
kW and power factor shown on controller
Voltage at the main load panel (qualified person)
Controller alarms or AVR indications
Capacitor bank status (on / off)

Escalation guide

Finding from the worksheet	Can the set keep supplying load?	Who acts next
Phase currents differ by more than about 20 % of their average, voltages within a few volts	Yes, if no phase exceeds rated current	Site electrician to rebalance single-phase loads
Any phase current above the alternator’s rated current	No — reduce load on that phase now	Site electrician; engineer if heating or alarms seen
One phase voltage low under load while the sensed pair is correct	Limited — sensitive equipment at risk	Rebalance load; engineer to review AVR sensing
Voltage builds then collapses, over-excitation alarm	No	Engineer: rotating rectifier, surge suppressor, exciter
Voltage and frequency both fall together on load	Reduce load	Engine-side diagnosis — speed, fuel, overload
Earth-fault trip or low insulation after a long idle spell	No	Engineer: insulation test and drying

Worked example 2: the phase the AVR cannot see

Illustrative case. A 160 kVA set supplies a commercial building. The rated current of the alternator is 160,000 ÷ (1.732 × 415) ≈ 223 A per phase. Tenants on one floor complain of low voltage on DG supply, and the B-phase cable at the DG panel runs noticeably warm,, though the panel reads 415 V line to line.

The worksheet shows line-to-neutral voltages of R 240 V, Y 238 V and B 222 V, with currents of R 120 A, Y 130 A and B 250 A.

• Average phase voltage = (240 + 238 + 222) ÷ 3 ≈ 233.3 V. The largest deviation is 233.3 − 222 = 11.3 V, a voltage unbalance of 11.3 ÷ 233.3 ≈ 4.9 %.
• Total load = (240 × 120 + 238 × 130 + 222 × 250) ÷ 1,000 = 28.8 + 30.9 + 55.5 ≈ 115.2 kVA, which is about 72 % of the set’s rating. The panel’s overall load figure looks comfortable.
• But B-phase current is 250 ÷ 223 ≈ 1.12, or about 112 % of rated current. That phase winding is overloaded while the set as a whole is not.

The AVR senses R–Y, so it keeps that pair at 415 V and cannot raise B. The fix is not in the alternator: the building’s single-phase circuits are redistributed so that each phase carries roughly 160–175 A (500 A in total ÷ 3 ≈ 167 A). A diode fault, an AVR change or a larger set would all have missed the cause.

Common mistakes

• Adjusting the AVR potentiometer before checking engine speed — low speed gives low voltage.
• Flashing the field without first finding why residual magnetism was lost.
• Changing the AVR when the fault is a failed rotating diode or a loose sensing lead.
• Measuring voltage at the far end of a long cable and blaming the alternator for cable drop.

What to send us

Send the set’s kVA, the voltage and frequency shown on no load and on load, whether voltage is zero, low or unstable, any recent work on the alternator or panel, and the AVR type if you know it. Use the service and AMC enquiry, or WhatsApp the details and photographs.

Related pages

• Why DG Sets Trip When the APFC Panel Is On
• Generator Hunting
• Generator Not Starting? A Step-by-Step Diagnosis for DG Sets
• Generator Troubleshooting
• Breakdown Diagnostics

Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 3 November 2026.