What is a motor stator: design, winding, faults and repair

The stator is the stationary part of a motor or generator, usually consisting of:

1. 01a frame;
2. 02a core made of electrical steel;
3. 03slots;
4. 04the stator winding;
5. 05slot and phase insulation;
6. 06slot wedges;
7. 07the winding’s end-turns;
8. 08a winding-bracing system;
9. 09terminals;
10. 10temperature sensors — where fitted.

The stator’s main job is to produce the magnetic field needed for electromagnetic torque. In a generator, the energy-conversion principle runs the other way: the rotor’s electromagnetic field interacts with the stator winding, inducing an electrical voltage in it.

The stator surrounds the rotor and stays stationary while the machine runs. In simplified form, the construction can be represented as:

frame → stator core → slots → winding → air gap → rotor

The air gap sits between the stator’s inner surface and the rotor’s outer surface. Its size is an important parameter of the electrical machine. An excessive or non-uniform air gap can affect magnetic flux, current, efficiency, reactive power, vibration, noise, heating, and electromagnetic forces.

Construction depends on the machine type, but the main elements are:

• the frame — provides mechanical rigidity and mounts the active part;
• the core — forms the stator’s magnetic circuit;
• slots — hold the winding’s conductors or coils;
• the stator winding — produces the electromagnetic field, or, in generator mode, is where electrical energy is formed;
• insulation — electrically separates turns, coils, and phases, separates the winding from the core, and separates live parts from the frame;
• slot wedges — hold the winding in the slots;
• end-turns — connect the slot sections of the coils to each other;
• terminals — connect the machine to the grid or a converter.

The frame can be made of cast iron, steel, a welded steel structure, an aluminum alloy, or special constructions for high-power machines.

The frame performs several functions: it holds the core, provides geometric rigidity, carries mechanical loads, serves as a base for the bearing brackets, participates in heat removal, and protects the internal parts.

In large electrical machines, the frame can be a complex welded structure with stiffening ribs, ventilation ducts, cooling chambers, support elements, inspection hatches, and sensor mounts.

The core is part of the motor’s magnetic system. It is not made from a solid piece of steel, but is usually built up from a large number of thin electrical-steel laminations. Each lamination has an insulating coating that limits eddy currents between laminations. This reduces eddy currents, iron losses, local heating, and the machine’s overall losses.

As the motor runs, the magnetic field changes over time. If the core were made from a solid mass of electrically conductive steel, significant eddy currents would arise in it, causing extra energy losses, strong heating, and reduced efficiency. That is why the core is laminated from thin sheets, electrically insulated from one another.

The core includes the back-iron, teeth, slots, ventilation ducts, and pressure elements. The stator teeth sit between the slots and carry magnetic flux to the air gap. The core back-iron closes the magnetic flux between pole zones.

The winding sits in the core’s slots. Slots can be open, semi-closed, semi-open, closed, rectangular, or specially shaped. Slot design depends on the winding type, voltage, power, conductor size, coil-insertion method, and the required magnetic regime. Large high-voltage machines often use open or semi-open slots, which allow pre-formed, pre-insulated coil sections to be fitted.

The stator winding is a system of electrically connected conductors or coils placed in the core’s slots. In a three-phase motor, it usually consists of three phases. Once connected to a three-phase supply, the currents in the windings create a rotating magnetic field, which interacts with the rotor and produces torque.

These terms are often used interchangeably, but technically they don’t always mean the same thing. The stator winding is the complete system of conductors for all stator phases. A stator coil is an individual structural element of the winding with one or more turns. A stator section (or bar) is a term often used for an individual pre-formed winding element, especially in large electrical machines.

Depending on the machine design, the following are used:

• random-wound windings;
• coil-type;
• section (bar) type;
• single-layer;
• double-layer;
• distributed;
• concentrated;
• lap;
• wave;
• bar-type and other special constructions.

The choice depends on power, voltage, pole count, slot count, connection scheme, cooling method, and manufacturing technology.

Random-wound winding

A random-wound winding is typical for many low-voltage motors of small and medium power. Conductors are inserted into the slots as the winding is formed, usually using insulated round or profiled magnet wire. Advantages: relatively simple technology, lower cost, and suitability for machine winding. For large high-voltage motors, this design is often unsuitable because of geometry and dielectric-strength requirements.

Form-wound coils and stator sections

Pre-manufactured sections are used for high-power and high-voltage machines. A typical process includes selecting the conductor, winding, forming the geometry, applying turn insulation, forming the ground-wall insulation, pressing or heat-treating (depending on the system), dimensional checks, electrical testing, inserting into the slots, connecting the sections, bracing the end-turns, and impregnation or another specified process.

Windings are mostly made with copper magnet wire — round, rectangular, profiled, enamel-coated, taped, glass-fiber insulated, or with a combined insulation system. In large machines, one section can consist of several parallel conductors; certain designs use conductor transposition to reduce stray losses.

When manufacturing or rewinding, it is essential to know the exact slot, pole, and phase counts, the winding pitch, turn count, number of parallel paths, conductor cross-section, winding direction, phase sequence, connection scheme, and terminal layout. Changing even one parameter can change the motor’s characteristics.

A three-phase winding can be connected in a star (Y) or delta (Δ) scheme. In a star connection, the three phase ends join at a common point; in a delta connection, the phases form a closed loop in series. The scheme is set by the motor’s design and its rated data — it must never be changed arbitrarily without analyzing the winding and the supply parameters.

The parts of the coils that extend beyond the core are called end-turns (end-windings). They are not held in slots, so they need separate mechanical bracing. They are subject to electrodynamic forces, vibration, thermal expansion, starting forces, and short-circuit forces. In large machines, end-turn bracing is one of the critical design elements.

Bracing tape, glass tape, cord ties, support rings, spacer blocks, insulating blocks, glass-laminate parts, composite materials, and special brackets can all be used. The goal is to prevent the winding from shifting under electromagnetic and mechanical forces.

The insulation system is one of the stator’s most important parts. It must provide dielectric strength between turns, between coils, between phases, between the winding and the core, and between live parts and the frame. At the same time, the insulation must withstand temperature, vibration, electrodynamic forces, moisture, contamination, thermal cycling, and surge overvoltages.

A design can use:

• turn insulation — separates adjacent turns;
• ground-wall insulation — separates a coil or section from the grounded core;
• slot insulation — sits between the winding and the slot walls;
• phase insulation — separates parts of different phases;
• end-turn insulation — protects and separates coils outside the core;
• terminal insulation — provides dielectric strength at the connection point.

Electrical machines use insulation systems of various thermal classes — commonly B and F. An insulation system’s class describes its allowable thermal level under the applicable requirements.

Modern high-power machines widely use thermosetting insulation systems. After curing, they form a rigid structure with high mechanical strength, good thermal stability, vibration resistance, geometric stability, and high dielectric strength. High-voltage sections can use systems based on mica, glass fabric, epoxy binders, or composite materials.

Thermoplastic systems can soften and change their mechanical properties on heating. Such systems are typical of a number of older machine designs. During repair, it is important to correctly identify the original insulation type, since the technology for removal, repair, and restoration can differ substantially.

Once the winding is fitted in a slot, it must be securely held in place. Slot wedges do this job and can be made of glass-laminate, laminate (textolite), specially treated wood in older designs, composite materials, or magnetic wedges. A wedge must provide mechanical retention, winding stability, vibration resistance, and the required dielectric properties.

Some machines use magnetic or semi-magnetic wedges. Their use can affect the slot zone’s magnetic reluctance, field harmonics, noise, no-load current, and electromagnetic losses — so a magnetic wedge should never be replaced with an ordinary glass-laminate one without an engineering calculation.

After the winding is inserted, it can be impregnated with an insulating varnish or resin compound. The main goals of impregnation: filling voids, fixing the conductors in place, increasing mechanical strength, protecting against moisture, improving heat transfer, reducing vibration, and extending service life.

Vacuum removes air and some of the moisture from the winding’s voids before the impregnating material is introduced, allowing the varnish or resin to penetrate more deeply. This technology can be used when manufacturing a new winding, rewinding, performing a major overhaul, or restoring certain insulation systems.

VPI (Vacuum Pressure Impregnation) is a vacuum-and-pressure impregnation process. A typical cycle can include pre-drying, evacuation, feeding the impregnating material, a soak period, applying excess pressure, draining the material, and curing. Specific parameters are set by the materials and the process documentation.

A stator generates significant heat from copper losses, iron losses, additional electromagnetic losses, and high-frequency components under converter supply. Heat is removed using external air cooling, internal ventilation, independent ventilation, heat exchangers, water cooling, or combined systems. In large machines, the core can have radial ventilation ducts.

High-power motors and generators can have temperature sensors embedded in the stator — Pt100, thermocouples, PTC sensors, and other types. They monitor winding temperature, core temperature, cooling-air temperature, and sometimes specific zones. This data is used for alarms, emergency protection, operating-mode analysis, and condition forecasting.

In a three-phase induction motor, the stator produces a rotating magnetic field whose speed is set by supply frequency and pole count. The rotor turns slightly slower than the stator field — this difference is what induces current in a squirrel-cage or wound rotor. An induction machine’s stator can have a low-voltage random-wound winding, a form-wound coil winding, or high-voltage sections.

In a synchronous motor, the stator also produces a rotating magnetic field. The rotor has its own magnetic field, produced by a field winding or permanent magnets. In steady synchronous operation, the rotor turns in synchronism with the stator field. For high-power synchronous machines, insulation dielectric strength, mechanical bracing of the sections, core condition, air-gap uniformity, and the cooling system are all especially important.

In many generators, the main power winding sits in the stator. As the rotor’s magnetic field rotates, an electrical voltage is induced in the stator winding. For large generators, the stator is one of the machine’s most complex and expensive components. Its repair can include replacing individual bars or sections, insulation and terminal repair, restoring bracing, core repair, sensor replacement, and partial or complete rewinding.

Electrical causes

• an interturn fault;
• a phase-to-phase fault;
• a ground fault;
• overvoltage;
• phase asymmetry;
• surge overvoltages;
• terminal damage.

Thermal causes

• overload;
• insufficient cooling;
• clogged ventilation;
• frequent starts;
• high ambient temperature;
• local core overheating.

Mechanical causes

• vibration;
• loosened wedges;
• coil movement;
• end-turn damage;
• frame deformation;
• rotor-to-stator contact.

Operational causes

• moisture;
• dust;
• oil;
• an aggressive environment;
• incorrect supply;
• operation outside the design regime.

An interturn fault occurs when insulation between adjacent turns of the same coil breaks down. A closed loop of very low resistance forms, which can carry a significant circulating current and cause local heating. The process can develop very quickly: insulation damage → a shorted turn → local current → heating → destruction of adjacent insulation → a phase-to-phase fault or a ground fault. Early detection of an interturn defect is therefore very important.

A phase-to-phase fault occurs between the live parts of different phases, caused by damaged phase insulation, end-turn displacement, overheating, contamination, moisture, mechanical damage, or a developing interturn defect. It is often catastrophic and can cause significant damage to the winding and core.

A winding ground fault is a breakdown of insulation between a live part and the grounded core or frame. Typical causes: insulation aging, overheating, mechanical abrasion, damaged slot insulation, moisture, contamination, partial discharges, and surge overvoltages. An electrical arc at the fault point can damage not only the winding but also the core laminations.

A loosened wedge lets the winding shift within the slot, which can lead to vibration, insulation abrasion, breakdown of the impregnation, conductive dust, coil damage, and a ground fault. For large machines, checking the slot bracing is an important part of the inspection.

End-turn damage arises from electrodynamic forces, frequent starts, short circuits, vibration, loosened bracing, material aging, or an improper repair. Signs: cracked varnish, abrasion dust, movement marks, loosened cord ties, cracked insulation, deformation, and local heating.

The core can develop interlaminar shorts, melting, dents, tooth damage, loosened pressing, corrosion, local overheating, and rotor-rub marks. Damage to the interlaminar insulation is especially dangerous — eddy currents rise at that spot and cause local heating, which can damage a new winding again after a rewind.

If the air gap is disturbed, the rotor can touch the stator’s inner surface. Causes: bearing damage, a bent shaft, frame deformation, incorrect assembly, eccentricity, core displacement, or a loosened bearing bracket. Consequences: tooth damage, shorted laminations, winding destruction, heating, vibration, and rotor damage.

Diagnostics should be comprehensive. Depending on design and condition, these methods can be used:

1. 01Visual inspection.
2. 02Insulation-resistance measurement.
3. 03Polarization-index determination.
4. 04Phase DC-resistance measurement.
5. 05A symmetry check.
6. 06A surge test.
7. 07A dielectric-strength (hipot) test.
8. 08Partial-discharge monitoring — for suitable machines.
9. 09A core inspection.
10. 10Slot-wedge inspection.
11. 11End-turn inspection.
12. 12Thermal imaging.
13. 13Vibration diagnostics.
14. 14Checking the temperature sensors.

A single test does not give a complete picture of a stator’s condition.

Visual diagnostics

During inspection, look at insulation color, overheating marks, cracks, delamination, dust, oil, moisture, signs of electrical discharge, the condition of the wedges and bracing, end-turn position, terminal condition, local burn marks, and rotor-rub marks. An experienced inspection often points to the direction of further diagnostics before any electrical testing.

Electrical tests

Insulation resistance is measured between phases, between each phase and the frame, and for the whole winding to ground. The result depends on temperature and humidity, so comparing measurements without accounting for conditions is not valid. Low resistance can result from moisture, contamination, aging, damage, or conductive dust. Sometimes cleaning and drying substantially restore the readings, but that does not automatically rule out other defects.

Phase resistances are compared with each other, accounting for winding temperature. A deviation can indicate a poor connection, an open parallel path, a wiring error, a terminal defect, or damage to part of the winding. Low-resistance windings need an appropriate measuring instrument, since contact and lead resistance can be comparable to the measured value.

A surge test is used to find interturn-insulation defects — the response of different phases, similar coils, and reference characteristics is compared. Interturn-insulation damage changes the winding’s electrical parameters, which shows up in the pulse waveform.

Core inspection

After removing the old winding, it is important to check the core before installing the new one — otherwise a new winding can end up in a core with a local defect that will cause overheating again. Tooth condition, interlaminar insulation, pressing tightness, rub marks, local melting, gouges, corrosion, and mechanical damage are all checked.

One method for a thermal core test involves creating a magnetic flux in the core and monitoring the temperature field — areas with damaged interlaminar insulation can run hotter. Temperature sensors, thermal imaging, or specialized core-diagnostic systems are used for monitoring. The method and flux level must match the machine’s design and the process documentation.

A complete rewind can be necessary for:

• a significant winding breakdown;
• systemic insulation aging;
• numerous interturn defects;
• a phase-to-phase short circuit;
• significant thermal damage;
• destroyed end-turns;
• a repair that cannot be done locally;
• a need to change the insulation system.

But rewinding should not be an automatic response to every defect. Sometimes cleaning, drying, terminal repair, restoring bracing, replacing a single section, re-wedging, local insulation repair, or re-impregnation is enough — if the technology allows it.

A typical rewind process includes:

1. 01Incoming diagnostics.
2. 02Recording the old winding scheme.
3. 03Measuring the geometry.
4. 04Marking the terminals.
5. 05Removing the old winding.
6. 06Cleaning the core.
7. 07Inspecting the core for defects.
8. 08Repairing teeth and slots.
9. 09Preparing the insulation components.
10. 10Manufacturing new coils or sections.
11. 11Inserting the winding.
12. 12Fitting the slot wedges.
13. 13Connecting the sections.
14. 14Forming the phases.
15. 15Soldering or welding the connections.
16. 16Forming the end-turns.
17. 17Bracing.
18. 18Electrical testing.
19. 19Impregnation.
20. 20Drying or curing.
21. 21Final electrical testing.
22. 22Assembling the motor.
23. 23Bench testing.

The exact process depends on the machine design. Before disassembly, the scheme, turn count, pitch, conductor cross-section, number of parallel paths, phase layout, connection type, coil geometry, end-turn length, and terminal position must all be recorded. This is especially important for old motors, where the original design documentation may be unavailable — but copying the old winding without verification is also risky, since the machine may have already been repaired incorrectly in the past.

The term "manufacturing a stator" can cover different scopes of work: manufacturing a wound stator (using the existing frame and core but making a new winding), manufacturing a core (making a new electrical-steel stack), or fully manufacturing a stator (frame, core, pressure elements, winding, insulation, wedges, terminals, sensors, and bracing system). Replicating a stator requires restoring not just its geometry but also the original design’s electromagnetic parameters.

Manufacturing a stator core can include developing or restoring the lamination drawing, selecting the electrical-steel grade, producing the laminations or segments, applying or checking the insulating coating, stacking the pack, forming ventilation ducts, pressing, clamping the pack, machining, checking the geometry, and verifying the magnetic condition. For large machines, the core can be assembled from segments directly inside the frame.

In many cases a stator can technically be manufactured from a sample, but a single physical sample may not be enough. The geometry, steel grade or its properties, stack length, slot count, slot shape, pole count, winding scheme, turn count, conductor cross-section, insulation class, cooling system, supply parameters, and rated characteristics of the machine all need to be determined. Manufacturing a replica should therefore begin with technical analysis, not simply copying external dimensions.

A stator as a spare part

A stator can be supplied as a separate assembly at various stages of completion: a frame without a core, a frame with a core, an unwound core, a wound stator, a fully complete stator with terminals, a stator with sensors, or a stator after a major overhaul. It’s therefore important, when ordering, to clarify exactly what is meant by "motor stator."

The decision between a new or a rebuilt stator depends on the condition of the frame, core, fits, winding, insulation, geometry, and economic viability. If the frame and core are in good condition, manufacturing a completely new stator is often unnecessary — manufacturing a new winding and a full restoration is enough. If the core has critical damage, it may need repair or a new stack.

1. 01Removing the old winding without recording its scheme.
2. 02Copying the old winding without verification.
3. 03Overheating the core while burning out the old winding.
4. 04Damaging the teeth during removal.
5. 05Ignoring interlaminar shorts.
6. 06Using an unsuitable conductor.
7. 07Changing the turn count.
8. 08Using the wrong winding pitch.
9. 09Getting the connection scheme wrong.
10. 10Insufficient slot insulation.
11. 11Damaging the insulation during insertion.
12. 12Unreliable end-turn bracing.
13. 13Weak slot wedges.
14. 14Using an unsuitable impregnation material.
15. 15Using the wrong curing regime.
16. 16Skipping interturn testing.
17. 17Testing with a megohmmeter alone.
18. 18Skipping the core inspection.
19. 19Wiring the temperature sensors incorrectly.
20. 20Skipping bench testing after repair.

What not to do:

• rewind a stator without inspecting the core;
• copy an old winding without verifying its parameters;
• change the turn count without a calculation;
• change the conductor cross-section just because it’s available;
• use arbitrary insulation materials;
• fit a different type of slot wedge without analysis;
• overheat the core while removing the old winding;
• leave sharp edges in the slots;
• do the final impregnation before intermediate electrical checks;
• assess a winding with a megohmmeter alone;
• ignore local core overheating;
• start the motor after a repair without acceptance testing.

When repairing an electrical machine, it’s important not to limit the inspection to the single question of "is the winding OK, or does it need rewinding?" A stator must be treated as a system: frame + core + winding + insulation + bracing + cooling + terminals + sensors. Replacing only the winding without fixing the root cause of its failure can let the defect recur.

The winding burned out, but the cause wasn’t the winding

After a failure, you might see a badly damaged winding and draw the obvious conclusion that a rewind is needed. But the root cause could have been bearing damage, rotor-to-stator contact, blocked ventilation, voltage asymmetry, a converter fault, frequent overloads, a loosened wedge, or a core defect. Simply rewinding the stator without finding the root cause can let the new winding fail again.

Local overheating after a rewind

After a new winding is fitted, one zone of the stator runs hotter. Possible causes: damaged interlaminar core insulation, an incorrect connection, a coil defect, disrupted ventilation, mechanical damage during insertion, or a local impregnation defect. Checking the core before fitting a new winding is therefore fundamentally important.

Good insulation resistance, but the motor overheats

High insulation resistance does not prove the winding is fully sound. Possible causes: an interturn fault, an incorrect scheme, phase asymmetry, a core defect, disrupted cooling, overload, or incorrect supply parameters. A megohmmeter is therefore an important tool, but not the only one.

The stator vibrates heavily

The cause may not be in the stator itself. Rotor balance, bearings, the shaft, the air gap, the foundation, motor mounting, electromagnetic symmetry, core condition, and frame rigidity all need checking. Diagnostics should treat the motor as a single electromechanical system.

• Synchronous motors: design and operating principle
• What is a motor rotor?
• What are stator and rotor windings?
• Why does a motor stator overheat?
• How is a motor stator rewound?
• How are stator coils manufactured?
• What is an interturn fault?
• How do you measure insulation resistance?
• What is vacuum impregnation?
• What is glass-laminate (fiberglass)?

Electropromremont LLC performs diagnostics, repair, rewinding, and restoration of electrical-machine stators, as well as manufacturing windings, coils, and stator sections for industrial motors and generators.

Depending on the equipment’s design and technical condition, the following can be performed:

• stator diagnostics, winding inspection, core checks;
• insulation-resistance and DC-resistance measurement, interturn diagnostics;
• terminal and phase-connection repair;
• replacement of individual coils or sections, complete stator rewinding;
• manufacture of stator sections, coils, and insulation components;
• slot-insulation replacement, manufacture and replacement of slot wedges;
• end-turn repair, bracing;
• vacuum or vacuum-pressure impregnation — depending on the specific machine’s technology;
• drying and curing, core repair;
• restoration of individual frame components;
• electrical and bench testing of the electrical machine after assembly.

Whether a specific stator can be manufactured or repaired is determined after analyzing the machine type, power, voltage, winding design, dimensions, technical condition, available documentation, and testing requirements.

The stator is one of the key components of a motor or generator. Its technical condition is not defined by the winding alone. A proper assessment must cover: frame → core → slot system → winding → insulation → bracing → terminals → cooling.

Main stator defects:

• interturn faults;
• phase-to-phase faults;
• ground faults;
• insulation aging;
• loosened slot wedges;
• end-turn movement;
• core damage;
• local overheating;
• contamination and moisture ingress.

A quality stator restoration includes diagnostics, a core inspection, restoring or manufacturing the winding, choosing the right insulation materials, reliable mechanical bracing, impregnation, and final electrical testing.