Alternator rewinding rebuilds a burnt stator or rotor so a generator produces full output again. A failed winding stops the set instantly, and the choice is a workshop rewind or a new alternator. Knowing the cost and the test readings turns that decision into simple arithmetic.
Quick Answer
Rewinding replaces the copper windings inside a failed stator or rotor.
It makes sense when the core, shaft and bearings are still sound.
A rewind usually costs 30 to 50 percent of a brand new alternator.
Insulation class F allows winding temperature up to 155 celsius.
A good workshop proves the repair with insulation and surge tests before dispatch.
What is alternator rewinding?
This process refers to stripping the old copper from the stator and rotor and fitting new insulated windings on the same core.
It means restoring the magnetic circuit instead of buying a complete replacement machine.
alternator rewinding in practice
When a rewind is the right call, and when only replacement will do:
| Condition | Rewind | Replace |
|---|---|---|
| Burnt stator winding | Yes | Not needed |
| Scored or melted core | Usually no | Yes |
| Failed bearings only | Bearings only | Not needed |
| Cracked rotor shaft | No | Yes |
| Open circuit winding | Yes | Option |
| Water damage and rust | Rarely | Often |
What does a generator alternator rewind cost in Nigeria?
A rewind for a 100 kVA alternator costs about 450,000 to 900,000 naira in Lagos.
A 250 kVA unit runs about 900,000 to 1,800,000 naira, depending on core condition.
A new alternator of the same size usually costs 2 to 3 times as much.
Insulation upgrade and vacuum dipping add about 10 to 20 percent.
Bearing and shaft work, when needed, adds 80,000 to 250,000 naira.
Most workshops finish a rewind in 5 to 14 working days.
How is alternator rewinding carried out?
- Isolate the set and lock off the supply before work starts.
- Test the windings and record the insulation and resistance readings.
- Remove the alternator and mark the coupling, alignment and terminals.
- Strip the failed copper and inspect the core, slots and shaft.
- Fit new insulated windings, then dip and bake the assembly.
- Reassemble, re-align and load test the set before handover.
Sources
https://www.nerc.gov.ng, https://nemsa.gov.ng, https://www.cummins.com, https://www.stamford-avk.com
Why windings fail
Heat is the main enemy of any winding.
Overload and high ambient raise the temperature.
Moisture and dust weaken the insulation.
A single short circuit can burn a whole phase.
Most failures give warning weeks before they stop the set.
Insulation classes
Insulation class B allows about 130 celsius.
Class F allows about 155 celsius.
Class H allows about 180 celsius.
A higher class costs more but lasts longer in Nigerian heat.
Always match or exceed the original class.
Proving the repair
An insulation test checks resistance to earth.
A surge test compares each phase to the others.
A balanced result shows a sound winding.
The workshop should record every reading.
Nothing ships without a signed test sheet.
Cost versus replacement
A rewind keeps the original frame and mounting.
That saves time on alignment and cabling.
But a damaged core can defeat the savings.
Compare the rewind to a new unit honestly.
Factor in transport and downtime before deciding.
Choosing a workshop
Ask for the insulation class they will fit.
Ask whether they vacuum dip and bake.
Ask for a sample test record from another job.
Confirm the warranty length in writing.
Check they can re-align the set on site.
Keeping a new winding alive
Keep the set within its rated load.
Keep the room cool and well ventilated.
Guard against moisture during the rains.
Service the cooling fan and filters.
Watch for any smell of hot varnish.
Related guides from this series
- Generator alternator insulation and megger testing in Nigeria
- Generator alternator guide in Nigeria
- Test a generator AVR automatic voltage regulator
About the author
Written by the Illuminate Engineering technical team. We repair and rewind generator alternators across Nigeria.
Frequently Asked Questions
How much does alternator rewinding cost in Nigeria?
A small unit starts near 450,000 naira; a large one can reach 1,800,000 naira.
How long does a rewind take?
Most workshops return a unit in 5 to 14 working days.
Can every alternator be rewound?
No. A damaged core or cracked shaft makes replacement the better choice.
What warranty should I expect?
A good workshop gives 6 to 12 months on the winding and its workmanship.
Conclusion
Rewind when the core is sound, and replace when it is not. A tested, properly dipped winding restores full output for a fraction of a new machine.