Generator problems
Generator Running but No Power Output
The short answer
When the engine runs but nothing has power, first find out whether the generator itself is making voltage. If the controller or panel meter reads close to 0 V with the engine at normal speed, the alternator is not building up, usually because of lost residual magnetism, the AVR, the rotating diodes or the exciter. If the generator shows normal voltage but the building is dark, the power is being lost on the way: a tripped breaker, a changeover that has not moved across, or a cable fault.
- Check the generator’s own output breaker is on
- Read the voltage and frequency on the controller or meter
- Portable units: unplug, then reset a tripped breaker or RCD once
- Never open the alternator terminal box or the changeover panel
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Where is the power missing?
| Symptom | Likely causes | Repair route |
|---|---|---|
| Close to 0 V, engine at normal speed | Likely causesResidual magnetism lost on a self-excited alternator, an open main stator winding or output lead, or a fault in the controller’s voltage reading | Repair routeResidual voltage and winding tests by a technician |
| A few volts that never build up | Likely causesFailed AVR or AVR fuse, a broken lead to the exciter field, failed rotating diodes | Repair routeAVR and excitation tests |
| Voltage builds for a few seconds, then drops away | Likely causesThe AVR’s over-excitation protection cutting out because a rotating diode or a winding has failed | Repair routeDiode and winding tests |
| Normal voltage at the generator, building dark | Likely causesOutput breaker off or tripped, the ATS not transferring, generator voltage or frequency outside the changeover’s limits, a burnt terminal or cable fault | Repair routeChangeover and ATS checks |
| Electrical trip or earth-fault alarm on the controller | Likely causesThe output breaker or the controller’s protection tripped on an overload, a short circuit or earth leakage | Repair routeFind the fault before resetting |
| Engine sounds slow or stays at idle | Likely causesSpeed too low for the AVR to build voltage, or for the controller to close the breaker: a governor, fuel or idle-control fault | Repair routeSpeed and frequency faults |
| Portable unit runs, sockets dead | Likely causesSocket breaker or RCD tripped, worn brushes or slip rings, a failed capacitor, an inverter module fault | Repair routePortable generator repair |
How it works
How an alternator builds voltage, and where it stops
Most standby and mid-size sets have a brushless alternator. When the engine starts, the small amount of magnetism left in the alternator’s iron, chiefly the poles of the exciter field, produces a residual voltage of a few volts at the output. On a self-excited alternator the AVR uses that voltage to feed DC to the exciter field; the exciter’s output is rectified by the rotating diodes on the shaft, and that current magnetises the main rotor. Within a second or two the output climbs to its set voltage.
A break anywhere in that chain leaves the engine running and the sockets dead. Alternators with a permanent magnet generator (PMG) feeding the AVR do not depend on residual magnetism to start, so on those the PMG itself, the AVR, its fuse, the diodes and the windings are the places to look.
Smaller portable generators often work differently. Many feed the rotor through brushes and slip rings, some regulate with a capacitor instead of an AVR, and inverter generators convert the output electronically. Worn brushes, a failed capacitor or a faulty inverter module all give a running engine with dead sockets.
- Residual voltage, AVR, exciter, diodes, main rotor
- A PMG powers the AVR from its own magnets
- Small units: brushes, a capacitor or an inverter module
Causes explained
Why the chain breaks
Residual magnetism fades
It weakens when a set stands unused for months, after a short circuit, or after the alternator has been stripped. A set that stood unused through a long break in load shedding can start perfectly when the outages return and give nothing. Restoring it means a controlled field flash: DC applied briefly to the exciter field with the correct polarity.
The AVR or its fuse fails: AVR repairs
Lightning surges in the Highveld storm season, heat in a cramped canopy, a fault further down the excitation circuit or plain age can all kill an AVR. It is tested before it is replaced, because a regulator that has lost its supply, or whose lead to the exciter field has broken, behaves exactly like a dead one.
Rotating diodes fail
An overload or a short circuit can blow a diode on the rotor. With a diode shorted, the exciter is heavily loaded, the regulator runs flat out to compensate, and its protection may shut the output down after a few seconds.
The engine is too slow: Surging and hunting
Many AVRs lower the voltage on purpose when the speed falls, and many controllers will not transfer the load until voltage and frequency are within limits. An engine stuck at idle can therefore look like an alternator fault.
The power is lost after the generator: ATS and changeover problems
The output breaker may be off or tripped by the controller’s protection, the ATS may refuse to transfer, or a terminal may have burnt open. If the controller reads normal voltage, the alternator is probably healthy.
The mains reached the alternator
If a faulty changeover or a plug-to-plug lead ever lets the mains supply connect to the generator’s output, the AVR, the diodes and even the windings can be destroyed in an instant. The set then runs normally and produces nothing.
Safe checks from outside the generator
Work from the outside of the generator and panels only. Each of these takes a minute and tells us a lot.
- The generator’s output breaker, on the set or in its panel, is switched on
- The controller or meter: note the voltage and frequency it shows, and any alarm text
- Whether the engine sounds like it is at its normal running speed or stuck at a low idle
- On a portable unit, the socket breaker and any earth-leakage (RCD) button: unplug everything, reset it once, and if it trips again, leave it off
- On a portable unit, a different socket and a known-good appliance
- On a manual changeover, that the switch is in the generator position
- Whether the whole building is dark or only some circuits
- What happened just before: a storm, an overload, months unused, or recent work on the set
A photo of the controller screen and the set’s nameplate, sent when you call, helps us bring the right parts.
Diagnosis
How we trace the missing power
- 01
Measure at the alternator terminals
Voltage on each phase and the frequency, with the load disconnected, separates an alternator fault from everything after it.
- 02
Check speed and residual voltage
Speed is corrected first. The residual voltage then shows whether the rotor still holds its magnetism.
- 03
Follow the excitation
The AVR’s supply, its sensing and its output to the exciter field show whether the regulator is working and being obeyed.
- 04
Test the rotor and windings
Rotating diodes, the surge suppressor and the exciter and main windings are tested for shorts, open circuits and insulation breakdown.
- 05
Prove it under load
After the repair the set is loaded and the transfer checked, so you know power reaches the building, not just the terminals.
Repairs, from a reset to a rewind
| Cause found | What the repair involves |
|---|---|
| Breaker or RCD tripped | What the repair involvesFind why it tripped (overload, earth leakage or a faulty appliance), then reset or replace it |
| Residual magnetism lost | What the repair involvesA controlled field flash, then a check that the alternator builds up on its own after a restart |
| AVR or AVR fuse failed | What the repair involvesFind what caused it, then fit a matching AVR and set it up at rated speed and under load |
| Rotating diodes failed | What the repair involvesReplace the diode set and surge suppressor, then check the AVR and windings they may have stressed |
| Brushes or slip rings worn | What the repair involvesReplace the brushes and clean the slip rings (brushed alternators) |
| Capacitor failed | What the repair involvesReplace it with one of the correct rating (capacitor-regulated units) |
| Inverter module failed | What the repair involvesRepair or replace the module |
| Changeover not transferring | What the repair involvesRepair the contactor, motorised switch, sensing or settings |
| Winding burnt or earthed | What the repair involvesRewinding or a replacement alternator, quoted before work starts |
Long idle spells and storms: when it tends to happen
Sets that stand for weeks or months between outages are the ones most likely to have lost their residual magnetism, so a generator that ran well through one round of load shedding can give nothing when outages resume. Running the set regularly, with some load on it, and including an output check in every generator service catches this before the power goes off.
Summer thunderstorms are the other common trigger. A nearby lightning strike can damage the AVR or the controller’s sensing, and the first sign is a set that starts on cue but supplies nothing. If that happens, note when it started and what the weather was doing, and call us so the excitation circuit can be tested before anything else is replaced.
FAQ
Frequently asked questions
Why is my generator running but not producing power?
Either the alternator is not building voltage (the AVR, residual magnetism, the rotating diodes or the exciter), or the power it makes is not getting through (a breaker, the changeover or a cable). The controller’s voltage reading tells you which: close to 0 V points to the alternator, normal voltage to what comes after it.
What is residual magnetism, and how is it lost?
It is the small magnetism left in the alternator’s iron, chiefly the exciter field poles, that lets a self-excited AVR start building voltage. It can fade during long storage, or be lost after a short circuit or when the alternator is dismantled.
Can I flash the field myself?
No. It means connecting a battery or another DC source to the excitation circuit, close to terminals that are live while the alternator turns, and a mistake can destroy the AVR or give a serious shock. A technician also checks why the magnetism was lost, which a flash alone does not answer.
Is it always the AVR?
No. Tripped breakers, lost residual magnetism, failed diodes, a slow engine and changeover faults all give the same symptom. The AVR should be tested before it is replaced, otherwise a new one can fail the same way.
My inverter generator runs but the sockets are dead. What can I check?
Switch off and unplug everything, let the unit cool if it has been working hard, then restart it and try one small appliance. Many inverter units cut their output after an overload and only reset when restarted. If the sockets stay dead or the overload light stays on, the inverter module or its wiring needs testing.
Can I power my house by plugging the generator into a wall socket?
No. It is dangerous and does not comply with the wiring regulations: it makes the plug pins live and can feed power back into the supply network. A generator must be connected through a properly installed changeover; see generator installation.
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