This guide covers how alternator faults show up, how they are tested, and how to decide between repair, rewind and replacement.
How alternator problems show up
Common symptoms include voltage that will not build up, unstable or drifting voltage, tripping on earth fault or overcurrent, overheating, burning smells and bearing noise. Many of these are not winding faults at all. A failed automatic voltage regulator, a diode in the rotating rectifier, a loose connection or a faulty sensing cable can all look like a dead alternator.
Tests before any decision
Before anything is pulled apart, a service technician should carry out insulation resistance tests on the stator, rotor and exciter windings, a polarisation index on larger machines, winding resistance balance checks, and a check of the voltage regulator, rectifier diodes and surge suppressor. Bearing condition can be assessed by vibration and temperature. These results usually point clearly to one of three outcomes: clean and dry, repair a component, or rewind.
Repair options
| Finding | Typical fix |
|---|---|
| Low insulation from moisture or dirt | Clean, dry out with controlled heat, retest |
| Failed regulator or diodes | Replace the component on site |
| Bearing wear | Replace bearings, check shaft and housing fits |
| Winding fault in exciter | Rewind or replace the exciter |
| Main stator or rotor winding failure | Workshop rewind or replace the alternator |
Moisture is a frequent cause of low insulation on standby sets in humid coastal and tropical sites. Anti-condensation heaters, kept working, prevent much of it.
Rewind or replace: the African reality
A quality rewind in a capable workshop can restore an alternator to good condition, and southern Africa has workshops able to rewind large machines. The decision depends on the size and age of the alternator, the condition of the core and shaft, how long the site can be without the set, and the lead time for a new unit shipped to site. For a remote site, a rewind close to home can be faster than waiting for a new alternator to clear customs; for a large or old machine with core damage, replacement may be the only sound answer. Our page on service vs new unit decisions applies the same thinking to whole sets, and customs clearance of spares covers the import side.
What to ask from a rewind workshop
Ask for the insulation class used, the varnish process, core loss testing before rewinding, test reports after rewinding (insulation resistance, winding resistance and high voltage test) and a balance report for the rotor. A rewind without test records is a gamble.
Prevention on African sites
Dust, heat and humidity are the enemies. Keep air filters on enclosures clean, keep anti-condensation heaters working on idle sets, avoid overloading in hot weather, and include insulation testing in the annual service. On remote sites, carrying a spare regulator and a diode set costs little compared with the downtime of waiting for one. See our guide to spare parts across Africa for stocking advice.
Field service across Africa
USP&E has worked in African power for 24+ years and has delivered 150+ power projects in 45+ countries, with 25,000 MW contracted since 2002. Our teams in Johannesburg and across the region support mixed fleets of diesel, HFO and gas units. See our operations and maintenance service for how we support sites in the field.
Offices: Colorado Springs (1755 Telstar Dr, Suite 300), Knoxville (800 S Gay St, Suite 700), London (162-168 Regent Street), Johannesburg (22 Kildoon Rd, Bryanston), Cape Town (31 Brickfield Road, Woodstock) and China (No. 1 Ningzhou Road, Wuxi). Regions: USA | AFRICA | UK | UAE | CHINA. ISO 9001 and ISO 45001 certified.
Alternator repair questions
Can a generator alternator be rewound?
Yes. Most alternators can be rewound in a capable workshop if the core and shaft are in good condition, with full test records afterwards.
How do I know if my alternator or my regulator has failed?
Insulation and winding resistance tests, plus checks on the voltage regulator and rectifier diodes, show whether the fault is in the windings or in a component.
Is it better to rewind or replace an alternator in Africa?
It depends on size, age, core condition and lead time. A local rewind can be quicker than importing a new unit; core damage often means replacement.
What causes low insulation resistance on generators?
Moisture and dirt are common causes, especially on idle standby sets in humid sites. Drying out and anti-condensation heaters often restore it.
Alternator problem on site?
Tell us the set, the alternator data and what you are seeing, and we will help decide between repair, rewind and replacement.
Send drawings, load data or questions via the Africa service desk; the switchboard is +27 10 822 2324 and the inbox is info@uspeglobal.com. Replacement sets and alternator-matched packages are in the diesel generator listings.