When does a motor need rewinding?

Rewinding is usually needed for:

1. 01an interturn short;
2. 02an interphase short;
3. 03a breakdown of the winding to ground;
4. 04significant thermal damage;
5. 05systemic insulation aging;
6. 06numerous local defects;
7. 07damage to a large portion of the coils;
8. 08an incident after which local repair cannot deliver the needed reliability;
9. 09the need for a full replacement of the insulation system;
10. 10the impossibility of restoring the winding’s electrical parameters any other way.

But the final decision is made after a set of checks.

Rewinding is a full or partial replacement of an electric machine’s winding. For a stator the process can include:

• recording the old scheme;
• removing the winding;
• cleaning the slots;
• inspecting the core;
• manufacturing new coils;
• inserting them;
• phasing;
• lashing/bracing;
• impregnation;
• drying;
• electrical testing.

For a wound rotor or an armature the technology differs, but the logic is the same: the old winding is replaced with a new one.

Rewinding is a significant intervention into the machine’s construction. During the repair it is necessary to:

• safely remove the old winding;
• avoid damaging the core;
• exactly reproduce the winding data;
• select the right insulation;
• ensure mechanical retention;
• perform impregnation;
• run tests.

If the old factory winding is sound, there is no point replacing it just because of the motor’s age.

An interturn short occurs when the insulation between adjacent turns is damaged. A circulating current arises in the shorted part of the coil, causing local overheating, destruction of the insulation, and growth of the defective zone.

Does an interturn short always mean a full rewind? Not always. In some large machines, replacing a single coil or section may be possible, but only when the defect is clearly localized, the rest of the winding is in good condition, the insulation shows no systemic aging, and the new section can be technologically installed correctly.

Interphase short

A short between conductors of different phases. It usually has more serious consequences — the coils, interphase insulation, end windings, slot area and core can all be damaged. In many cases a full rewind is needed after such a defect.

Breakdown of the winding to ground

An electrical contact forms between the current-carrying part and the core or housing. Causes: aging, moisture, mechanical damage, insulation abrasion, partial discharges, overheating.

Can a breakdown be repaired without rewinding? If the defect is local, accessible, and has not damaged a large part of the winding, local repair is sometimes possible. But if the breakdown occurred deep in the slot, involved melting, damaged the core, or covered a large area, rewinding is usually the better option.

Overheating can substantially change the insulation’s properties: loss of elasticity, cracking, charring, reduced dielectric strength, and destruction of the impregnation. If the insulation has degraded throughout the whole winding, local repair no longer makes sense.

Signs of severe thermal damage: heavy discoloration, blackening, charring, brittle insulation, cracks, delamination, a characteristic smell. But the final conclusion should not be based on appearance alone.

Insulation ages under the influence of temperature, vibration, the electric field, moisture, contamination, and thermal cycling. If defects already appear in many places, that is a sign of systemic aging — in that case, repairing one spot may not deliver sufficient remaining life. Signs of systemic aging: numerous cracks, brittleness, delamination, frequent local breakdowns, unstable insulation resistance, signs of partial discharge, weakened mechanical strength.

If the winding has already been locally repaired several times and new defects keep appearing, that can indicate the whole insulation system has run out of life. In that case, a full rewind can be technically more reliable than yet another local repair.

Mechanical damage to the winding can arise from loose slot wedges, coil movement, vibration, a short circuit, or rotor-to-stator contact. If a significant number of coils are damaged, rewinding may be necessary.

The rotor can touch the stator due to bearing damage, a bent shaft, eccentricity, or incorrect assembly. This can damage the teeth, slot insulation, coils, and core — if the winding sustains serious mechanical damage, it needs to be replaced.

Water ingress alone does not automatically mean rewinding is required. Cleaning, drying, an insulation resistance check, and other electrical tests may be performed first. But if the water caused corrosion, tracking, insulation destruction, or persistently low resistance, rewinding may be needed.

An aggressive chemical environment — acids, alkalis, saline solutions, process vapors — can damage insulation. If the materials have lost their properties, simple cleaning may not be enough.

For DC motors, a circular flashover can severely damage the commutator, the armature, the winding, and the lashing. If the winding damage is significant, a full rewind of the armature may be needed.

Rewinding a wound rotor is necessary for an interturn short, a break in a significant part of the winding, a breakdown to the core, widespread insulation damage, or destruction of the end windings. In that case the stator winding can remain completely sound.

For DC machines, rewinding the armature may be needed for an interturn defect, a broken section, a breakdown, severe overheating, or winding damage after a commutator failure.

Low insulation resistance can be caused by moisture, contamination, aging, or a breakdown. Low resistance does not automatically mean rewinding is necessary — the cause needs to be identified first.

If resistance recovers after drying, that can mean moisture was the main problem — if the other tests are normal, rewinding may not be needed. If resistance does not recover, serious aging, tracking, damage to the ground-wall insulation, or contamination inside the system are possible — then a deeper inspection is needed.

Checks that help make the decision:

• a visual inspection;
• insulation resistance;
• the polarization index;
• phase resistance;
• a phase comparison;
• a surge (interturn) test;
• a high-voltage test;
• partial discharge monitoring;
• thermal imaging;
• a core check.

If one phase shows a significant deviation in resistance, that can indicate a break, a connection defect, an incorrect number of turns, or damage to a parallel branch. Not every such issue requires a full rewind, but it does need to be localized.

A surge (interturn) test is a very important tool for assessing interturn insulation condition. It can reveal a weakness that does not yet show up as a breakdown to ground or low insulation resistance. When a systemic interturn defect is confirmed, rewinding is often the most reliable solution.

A high-voltage test helps assess the strength of the main insulation. If a new or repaired winding cannot withstand the specified level, the defect needs to be located, the feasibility of local repair assessed, or the winding rewound.

For high-voltage machines, elevated partial-discharge activity can indicate voids in the insulation, delamination, corona-protection problems, or aging. That does not automatically mean a full rewind, but it can be an important factor when assessing remaining life.

Does a motor need rewinding just because of its age? No. Age by itself is not a technical reason. An old motor can have a good core, a stable factory winding, and normal insulation. Conversely, a relatively new motor can suffer a serious failure. What matters is the technical condition.

Is rewinding needed when replacing bearings? No — if the winding is sound, bearing replacement has nothing to do with rewinding.

Is rewinding needed for high vibration? Not necessarily. Causes of vibration include imbalance, misalignment, a bearing, the shaft, the foundation, the coupling, and the rotor — the winding can be completely sound.

Is rewinding needed if the motor does not start? Not always. Possible causes: a lost phase, a bearing, jamming, the supply, the starter, the rotor — diagnostics is needed first.

Is rewinding needed if the breaker trips? Not necessarily. The cause could be incorrect protection, a short circuit, mechanical jamming, a cable problem, or the winding itself — rewinding is only done once a winding defect has been confirmed.

Yes, in some machines. It is possible to replace one coil, several sections, or a damaged lead. But the condition of the whole winding needs to be assessed.

A local replacement is not advisable if the insulation is brittle throughout the whole stator, there are many defective zones, the winding operated under prolonged overheating, or the condition of the other coils is unstable. In that case, a local repair may have little remaining life.

Is rewinding needed after a local breakdown? Not always. If the defect is accessible, the damage is small, and the rest of the winding is in good condition, repair is possible. But afterward, repeat electrical tests are needed.

Sometimes a winding is changed not because of a failure, but to change the machine’s parameters — a different voltage, a different frequency, adaptation to a drive, an insulation upgrade. But that is no longer an ordinary repair — it is a technical upgrade.

Can rewinding increase power? Simply changing the wire or the number of turns does not automatically make a motor more powerful. Power is limited by the core, magnetic saturation, cooling, current loading, and the mechanical construction. Any upgrade requires calculation.

1. 01Diagnostics.
2. 02Recording the winding data.
3. 03Removing the old winding.
4. 04Cleaning the slots.
5. 05Inspecting the core.
6. 06Repairing the core — if necessary.
7. 07Manufacturing the coils.
8. 08Inserting the winding.
9. 09Installing the wedges.
10. 10Connecting the phases.
11. 11Lashing/bracing.
12. 12Intermediate electrical tests.
13. 13Impregnation.
14. 14Drying.
15. 15Final tests.
16. 16Assembling the motor.
17. 17Bench control.

Why is the core checked after a burned winding? A severe short circuit or fire can damage the core’s inter-laminar insulation, creating eddy currents and local heating. If a new winding is installed in a damaged core, it can overheat again.

Why is it important to find the cause of the failure? Rewinding removes the consequence but not always the cause. The winding may have burned due to overload, a lost phase, mechanism jamming, poor cooling, frequent starts, or an incorrectly configured drive. If the cause is not fixed, the new winding can burn again.

The main factors:

• the number of damaged coils;
• the age of the insulation;
• the degree of overheating;
• the technical condition of the rest of the winding;
• the accessibility of the defect;
• the machine type;
• the cost of a local repair;
• the expected remaining service life.

When is a full rewind more economical? For example, if many sections need repair, the winding shows systemic aging, local work would be very labor-intensive, or a partial repair would leave the other coils with little remaining life. In such a situation a full rewind can be more cost-effective in the long run.

The need for rewinding can only be determined from photos in the case of obvious severe damage — a charred winding, melting, or significant mechanical destruction. In most cases, electrical measurements are required.

For a preliminary assessment it helps to send:

• a photo of the nameplate;
• a photo of the motor;
• a photo of the winding, if the motor is disassembled;
• a description of the fault;
• the reason for the shutdown;
• information about any overheating;
• measurement results, if available.

1. 01Rewinding any old motor.
2. 02Basing the conclusion only on the winding’s color.
3. 03Treating a good megohmmeter reading as proof of full soundness.
4. 04Ignoring the interturn test.
5. 05Locally repairing a systemically aged insulation system.
6. 06Rewinding a stator because of a mechanical defect.
7. 07Not checking the core after a failure.
8. 08Not looking for the cause of the burnout.

Things to avoid:

• rewinding a sound winding without a technical reason;
• leaving a confirmed interturn short unaddressed;
• repairing one coil when the whole system is already critically aged;
• installing a new winding into an unverified core after a severe failure;
• treating rewinding as the only method of motor repair;
• changing the winding data without a calculation.

The decision to rewind should be based on the technical condition, not a single indicator. It makes sense to assess the winding, the insulation, the core, the mechanical retention, and the history of the failure. Rewinding is appropriate when local repair cannot deliver the needed reliability, does not eliminate a systemic defect, is not economically justified, or does not allow confirming the machine’s remaining service life.

The motor burned due to a lost phase

The winding has significant thermal damage. Inspection confirms destroyed insulation and interturn defects. A full rewind is needed. But the external supply system also needs to be checked so the failure does not repeat.

One section had a local breakdown

The rest of the high-voltage winding is in good condition. The design allows replacing the section. In this case, a full rewind may not be needed.

The old winding has numerous cracks

Even if it still has acceptable insulation resistance, its mechanical and thermal condition may be unsatisfactory. Locally repairing several zones does not solve a systemic problem. A full rewind is more appropriate.

The motor overheated but the insulation is sound

The cause is a contaminated cooling system. After cleaning, checking the winding, and bench testing, rewinding is not needed.

The megohmmeter reading is normal, but the motor runs hot

A surge test reveals an interturn defect. This is an example of why insulation resistance alone is not enough.

The motor does not develop torque

The stator winding is sound. A squirrel-cage rotor defect is found. The rotor is repaired, and the stator is not rewound.

• Can a motor be repaired without rewinding?
• What does a capital repair of a motor include?
• How is a motor winding tested?
• What are stator and rotor windings?
• Why does a motor overheat?
• What tests does a motor go through after repair?
• What is a motor high-voltage test?
• Synchronous motors: design and operating principle

Elektropromremont determines whether rewinding is necessary after diagnostics and inspection of the electric machine. Before the decision, it can assess:

• the winding condition;
• insulation resistance;
• phase resistance;
• interturn condition;
• mechanical retention;
• the core condition;
• the nature of the failure.

If the winding can be reliably restored locally, a full rewind is not mandatory. If the insulation system shows systemic aging or significant damage, a full rewind may be recommended. After rewinding, electrical checks, impregnation, drying, high-voltage tests, assembly, bench control, and a load test can be performed, depending on the repair program.

A motor needs rewinding not simply because it is old or has stopped running, but because the winding has lost the technical reliability it needs. The main grounds are: interturn defects, an interphase short, a breakdown, significant thermal damage, systemic insulation aging, and widespread mechanical damage.

But even with a serious fault, the first question needs to be: is the problem in the winding at all?

The main principle: rewinding should be a technically justified decision, not the standard answer to any motor fault. If the winding is sound, it is worth keeping. If its condition no longer allows the electrical, thermal, and mechanical reliability the machine needs, a full rewind becomes the right way to restore it.