In short. Rewinding a generator alternator is justified only after the winding has been proven to have failed, the cause has been identified and the machine is confirmed as still suitable for its duty. A good rewind reproduces the original winding design, insulation class and connection, is verified by insulation resistance, polarisation index, winding resistance and surge tests, and is followed by a loaded run before the set returns to standby service.
When rewinding is the right decision
An alternator stator or rotor is rewound when the insulation system has failed and the core, frame and shaft remain serviceable. Typical triggers are a phase-to-phase or phase-to-earth fault, a burnt coil group after sustained overload, or insulation resistance that stays low after drying out. Rewinding is a repair of last resort because it is labour-intensive and because the result depends entirely on the workshop reproducing the original design.
Before authorising a rewind, three questions need answers. Has the winding actually failed, or is low insulation resistance simply moisture that a dry-out would cure? What caused the failure, and has that cause been removed? Are the bearings, exciter, rotating rectifier and core in a condition that justifies the spend against a replacement quotation for the same frame? See alternator service for the inspection work that comes before this decision.
Why windings fail in Indian operating conditions
Most winding failures we see around Hyderabad trace back to one of five causes, and each leaves its own evidence on strip-down.
| Cause | What you find on strip-down | What must change before the rewind |
|---|---|---|
| Sustained overload or unbalanced load | Uniform browning of all coils, brittle insulation, one phase darker than the others | Load study, correction of phase imbalance, protection settings reviewed |
| Moisture and contamination | Tracking marks at coil ends, deposits, low insulation resistance that recovers on heating | Canopy sealing, space heaters, monsoon exercise routine |
| Blocked cooling | Hot spots at the air-outlet end, dust packed in ducts | Clean air path, correct louvre arrangement, ambient derating check |
| Overvoltage or excitation fault | Punctured slot insulation, often with a failed AVR or shorted rotating diode | Excitation chain repaired, AVR overexcitation limit confirmed |
| Vibration or bearing failure | Chafed slot liners, loose wedges, rotor rub marks on the stator bore | Bearing, alignment and coupling corrected, core checked |
If the cause is not corrected, the new winding fails the same way, usually within the first year.
Confirming failure before committing
The test sequence is short and should be recorded with the machine identity, ambient temperature and humidity. First, measure insulation resistance of each winding to earth with a 500 V or 1000 V insulation tester, chosen according to the winding’s rated voltage. A reading below 1 megohm on a 415 V stator is unacceptable; a reading between 1 and 5 megohms suggests moisture and calls for a controlled dry-out and a repeat test rather than an immediate rewind. Second, take a ten-minute polarisation index; a ratio of 2 or more generally indicates dry, healthy insulation, and below 1.5 suggests contamination or ageing. Third, measure DC resistance of each phase with a micro-ohmmeter; the three readings should agree within about 2 to 3 percent, and a larger spread points to shorted turns or a bad joint. Fourth, where the equipment is available, a surge comparison test between phases reveals turn-to-turn shorts that resistance readings can miss.
The manufacturer’s limits take precedence where published. A winding that fails the resistance balance or surge test has a genuine defect; one that fails only insulation resistance may simply be wet. The guide on temperature rise, insulation and IP ratings explains the insulation classes these readings relate to.
What a correct rewind reproduces
A rewind is a manufacturing operation and its quality depends on copying the original design exactly. Before stripping, the workshop records the number of slots, coils per group, turns per coil, conductor size and strands in hand, coil pitch, connection (star, series or parallel star, and the twelve-lead arrangement on dual-voltage machines), insulation class and the position of any winding temperature detectors. Where the data plate and winding data are missing, the stripped winding is the only record and must be documented coil by coil.
Three changes are commonly made without authority and all are redesigns, not repairs. Changing the conductor size alters current density and slot fill. Changing the coil pitch alters the voltage waveform and the harmonics delivered to UPS and VFD loads; two-thirds pitch windings are built that way deliberately to suppress the third harmonic. Changing turns per coil to correct a voltage complaint that was really an AVR fault leaves a distorted waveform permanently. Any departure from the original design needs a written technical basis and the owner’s acceptance before work starts. The guide on alternator windings and voltage covers the connection options.
Process and curing
The old winding is removed after a controlled burn-out in a temperature-limited oven; a torch burn-out distorts the core and raises iron losses. The core is tested for interlaminar shorts before new liners, coils and wedges go in, and the connections are brazed. The winding is then impregnated and cured for the full time at the varnish manufacturer’s temperature. A rushed cure is the most common hidden defect in a cheap rewind: the varnish stays soft, the coils move under load and the insulation chafes through within months.
Tests that prove the repair
A completed rewind should come back with a test record that identifies the machine by serial number and states the ambient conditions. The minimum set is insulation resistance to earth and between phases, polarisation index, DC resistance balance, a surge comparison test, and a high-voltage withstand test at the manufacturer’s value for a repaired winding, which is normally lower than the factory value for a new one. After reassembly the set runs at no load to confirm voltage build-up and phase sequence, then under load to confirm temperature rise, regulation and absence of vibration. Where site load is not available, load-bank testing provides the controlled run. The record should also say what was not tested: a rewound stator proves nothing about the exciter, diodes, AVR or bearings, which should be inspected while the machine is open. The alternator parts page lists what is normally checked at the same time.
Common mistakes
- Rewinding a stator that was only wet after the monsoon, because the dry-out option was never tried; the owner pays for a rewind the machine did not need.
- Rewinding without finding the cause, so an overloaded or badly cooled machine burns its new winding within a year.
- Accepting a test record that shows insulation resistance only, with no resistance balance or surge test, so turn-to-turn faults escape detection until the set is loaded.
- Allowing the workshop to change pitch or turns to fix a voltage complaint that was an AVR fault, leaving a distorted waveform permanently.
- Returning the set to automatic standby without a loaded run, so the first real test of the rewind is a mains failure.
What to send us
Send a photograph of the alternator nameplate and the generator set nameplate, the insulation resistance and phase resistance readings you already have with the tester voltage used, a description of what happened when the fault occurred, the hours run and the type of load the set carries. Use the service and AMC enquiry form to send the details.
Questions engineers ask
How do I know whether the winding is wet or actually failed?
Measure insulation resistance, dry the machine with space heaters or a low-current heating method, let it cool and measure again. A wet winding recovers to several megohms with a healthy polarisation index; a failed winding does not recover, or shows unbalanced phase resistances or a failed surge test. A dry-out costs a day and a rewind costs weeks.
Is rewinding cheaper than a new alternator?
For small frames up to roughly 125 kVA a new alternator is often comparable in price and comes with new bearings, exciter and diodes. Above that size rewinding usually wins on cost if the core and frame are sound. Compare the rewind quotation, including transport and downtime, with a replacement quotation for the same frame and coupling.
How long does a rewind take?
Typically two to four weeks from removal to reinstallation, depending on frame size and workshop loading. For a standby set that means arranging rental cover for the period, which is another reason to try the dry-out first.
Can a rewind change the output voltage?
Technically yes, by changing turns or the connection, but it is a redesign that affects current rating, reactances and AVR sensing. Many alternators already have twelve-lead windings that can be reconnected for a different voltage without rewinding, so check the connection diagram first.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 19 October 2026.