How are stator coils manufactured?

Soft random-wound coils are made mostly from round enameled wire — typical of low-voltage motors of small and medium power with semi-closed slots. The advantage is simple technology and flexibility during insertion; the drawback is harder control over the inner turns and heavy dependence of the result on impregnation quality.

Hard formed coils are made from rectangular wire or insulated copper bar and are used in high-voltage motors, large generators, and synchronous machines. Such a coil has its final geometry and ground-wall insulation already set before it goes into the stator, so form and individual-test requirements are much stricter. Large generators and specialized machines may use bar-type elements with transposed conductors and internal cooling — a separate, highly specialized operation.

A coil cannot be made just from the approximate shape of an old one — the number of slots, poles and phases, the winding type, the pitch, the turn count and parallel conductors, wire size, connection scheme, and insulation type all need to be established. Sources of this data include the factory drawing, repair documentation, an undamaged old coil, or an electromagnetic calculation — the most reliable approach compares several independent sources.

The old winding may already have been repaired, and the turn count, pitch, wire cross-section, or connection scheme may have been changed in the process — so blindly copying an old coil risks repeating the earlier mistake.

A winding can be single- or double-layer, concentric, lap, or wave, with full or fractional pitch. Pitch — the distance between a coil’s two slot sides, expressed as a number of slot spans (e.g. 1–10) — affects the induced EMF, the field’s harmonic content, end-winding length, and noise. Fractional pitch is used to suppress specific harmonics and save copper, but pitch must never be changed just to make insertion easier.

Turn count affects the magnetomotive force, the working magnetic flux, no-load current, starting torque, and heating — even a small error can change the machine’s characteristics. It is established by direct count during teardown, checking wire length, an electromagnetic calculation, and comparison with a similar machine, ideally checking several old coils from different phases.

Round wire is used mostly in low-voltage motors and random-wound coils for its flexibility and easy winding. Rectangular wire is used in large and high-voltage machines for better slot fill and more stable geometry in formed coils — but it bends less easily and demands tighter control of bend radius and transposition.

A wire’s measured outer dimension includes the copper, the enamel, and any contamination, so cross-section should never be judged with calipers alone — a control sample needs to be stripped, or the cross-section calculated from resistance and length. Increasing cross-section without a calculation can complicate insertion, hurt varnish penetration, and rule out fitting the slot wedges — thicker wire is not always better.

For a soft low-voltage coil, the main turn insulation is the wire’s enamel, supplemented with glass tape or insulating sleeving; ground-wall insulation is largely provided by the slot liner and the impregnating material. A formed high-voltage coil has reinforced turn and ground-wall insulation, a semi-conductive slot coating to even out the electric field, and a graded corona-protection coating at the slot exit — it is almost a finished insulated product before it ever goes into the core.

Using one high-temperature-class material does not automatically raise the class of the whole winding — the weak link can remain the wire’s enamel, a tape, a varnish, or a phase separator. For motors run from a variable frequency drive, elevated interturn voltage from steep pulse fronts needs to be accounted for — an inverter-duty wire, reinforced turn insulation on the first turns of a phase, or an output filter may be needed.

Dimensions for the form are taken from an undamaged coil, the stator before teardown, or a drawing — an overheated failed coil is not a reliable reference, since it may have deformed or expanded. The form must give the right pitch, consistent length across every coil, and easy removal without damaging the enamel.

Before winding a production batch, one control coil is made and checked for pitch, turn count, dimensions, mass, and resistance, and where possible test-fitted into the stator before the whole batch is produced.

Before winding, the wire’s grade and condition are checked; coils for one phase should ideally come from a single confirmed batch of material. The turn counter must be verified and backed up by process control — a coil with the correct turn count but wrongly positioned leads can be unusable for mounting.

Wire tension must give a tight, stable shape without stretching the copper or damaging the enamel. Too little tension produces a loose coil and unstable geometry; too much damages the enamel, reduces the cross-section, and cracks rectangular wire at the bends. When a turn is made of several parallel conductors, equal length and correct transposition are essential — transposition should never be altered on one’s own initiative, since it equalizes induced voltages and reduces circulating currents.

After winding, the coil is given the spatial shape needed for slot insertion — spreading the slot sides, bending the end turns, forming the leads. The wrong shape can make insertion impossible, cause excessive overhang, or bring the coil into contact with a neighboring phase.

For a formed coil, ground-wall insulation on the slot section is applied as a controlled number of tape layers with a set overlap — too little overlap leaves weak spots prone to partial discharge, too much makes slot insertion harder. In high-voltage coils, the slot section gets a semi-conductive coating to even out the field, and the slot exit gets a graded anti-corona coating; an error in the length or resistance of these layers can cause surface discharges and premature breakdown.

Formed coils are pressed to compact the insulation, remove air, and stabilize dimensions — too little pressure leaves voids, too much can squeeze out resin and damage the wire. Impregnating before insertion allows each coil to be controlled individually and gives better geometric stability, but makes the coil stiffer and harder to install.

During curing, the coil’s actual temperature, duration, and cooling rate are all monitored — insufficient curing leaves the turns able to shift and the adhesion weak, while excessive temperature damages the enamel and speeds up aging.

Coil resistance is measured with the four-wire method to check turn count, cross-section, and the integrity of parallel conductors, always recording temperature. An interturn defect can go unnoticed in an insulation-resistance-to-frame test — so a surge test is used instead, revealing an interturn short, a wrong turn count, or weak insulation from the shape of the decaying waveform.

Formed high-voltage coils additionally undergo a high-voltage withstand test, partial-discharge monitoring, and a dissipation-factor (tan δ) measurement. After shaping and curing, the coil is checked with gauges for pitch, slot-section length, and end-turn overhang — it must fit the control gauge without damage or excessive force. Coils of identical design are also weighed: a mass difference can point to a wrong turn count or uneven impregnation.

Every coil needs identification — coil number, phase, group, start and finish, slot number, and the date made; the marking must survive curing without damaging the insulation. Finished coils are protected from moisture, impact, and deformation, with formed coils stored on dedicated racks in a set orientation with no load on the leads — improper stacking can distort the geometry or damage the insulation before the coil ever reaches the stator.

• starting to wind without confirmed winding data — the mistake will repeat on every coil;
• copying only the outer shape of an old coil, ignoring the slots, pitch, and actual scheme;
• treating the enamel as part of the copper cross-section when sizing the conductor;
• using damaged wire — even a local scratch can become an interturn breakdown site;
• winding without a verified turn counter, or allowing uncontrolled tension;
• forming a hard coil with impacts;
• impregnating a coil before checking its turns and geometry — correcting it after curing is difficult;
• judging a coil by appearance alone, without electrical testing;
• installing coils with different resistance without finding the cause — that creates phase asymmetry.

• How is a motor stator rewound?
• What is stator and rotor winding?
• Motor winding insulation
• Why does an interturn short circuit occur?
• What is an electric motor stator?
• How do I measure insulation resistance?

EPR (Elektropromremont) manufactures coils, sections and complete stator windings for industrial electric motors and generators.

The scope of work includes:

• recovering winding data and building forms;
• winding soft random-wound and hard formed coils;
• applying turn, ground-wall and semi-conductive insulation;
• pressing, impregnating and curing coils;
• resistance measurement, surge testing and high-voltage testing;
• geometric inspection and weighing of coils;
• inserting coils into the core and fully impregnating the stator.