Key takeaways
- A distributed winding spreads each phase over several slots and produces a smoother magnetomotive force.
- A concentrated winding puts one coil on each tooth, which shortens the end turns and saves copper.
- The shorter end turns of a concentrated winding usually come with more harmonic content and more torque ripple.
- Slot fill, insulation and impregnation decide whether a good electromagnetic design becomes a reliable stator.
What a motor winding has to do
A motor winding turns current into a rotating magnetic field. The stator carries coils in slots, the rotor responds, and the interaction produces torque. The winding has to produce the right flux, with the right number of poles, at an acceptable loss and temperature.
The winding also has to fit. The slot area is fixed by the lamination, and the copper, the slot liner, the phase insulation and the wedges all share it. A design that ignores the fill will be rewound or rejected on the line.
Three routes are common: distributed windings, concentrated windings and formed or hairpin windings. They differ in how the coils are placed, how much copper sits in the end turns and how the magnetomotive force is shaped.
Distributed windings
In a distributed winding, each phase is spread over several slots per pole. A coil may span several teeth, and the coils of a phase overlap in a pattern that approximates a sinusoidal distribution. This is the classic induction motor winding.
The distributed layout produces a smoother magnetomotive force and lower harmonic content. That means lower torque ripple, lower iron loss from harmonics and quieter operation. It is the reason most mains-fed induction motors use it.
The cost is in the end turns. The coils have to travel from one slot to another, so the copper outside the core is longer. Longer end turns mean more resistance, more copper loss and a longer axial length. They also add inductance, which affects the starting behaviour.
Distributed windings are usually inserted as preformed coils or wound in place. The process needs more labour and more insulation between phases, and the end winding needs to be braced against the magnetic forces.
Concentrated windings
A concentrated winding places a coil around a single tooth. Each coil is separate, the end turns are short and the winding is easy to automate. The pattern is common in brushless DC and permanent magnet synchronous motors.
The short end turns reduce copper and resistance, which helps efficiency and axial length. The slot fill can be high because each tooth is wound on its own, and the coils can be wound on a separate machine before assembly.
The magnetomotive force is less sinusoidal. A concentrated winding produces more harmonic content, which can increase torque ripple, rotor loss and noise. The designer can reduce it with a fractional slot combination, skew or a different pole and slot count, but the trade is rarely free.
The winding sits around the tooth, so heat has a short path to the core. That helps the thermal design, provided the slot liner and impregnation do not block it.
Pole and slot count is a winding decision
The combination of poles and slots sets the winding factor, the harmonic content and the cogging torque. It should be chosen with the winding layout, not after the lamination is fixed.
Hairpin and formed windings
A hairpin winding uses rectangular copper bars formed into a U shape, inserted into the slots and welded at the other end. The bars fill the slot more completely than round wire, which lowers the resistance for a given slot area.
The process is common in traction motors and other high power density machines. The fill factor is high, the end turns are short and the welding can be automated. The tooling and the welding equipment are the barrier.
The rectangular conductor also changes the loss behaviour. At high frequency, the current distribution in a bar is different from a round strand, and the ac loss can rise. The design has to check the loss at the switching frequency, not just at the fundamental.
Hairpin windings are less flexible for low volume. The forming tools, the welding program and the inspection are all specific to the stator, so the route pays at volume and is hard to justify for a small batch.
Winding factor and harmonics
The winding factor is the ratio of the flux actually linked by the winding to the flux that would be linked by an ideal concentrated coil. A higher factor means more torque for the same current, so the designer tries to keep it high.
Distributed windings can achieve a high fundamental winding factor with low harmonic content. Concentrated windings can also achieve a good fundamental factor with certain slot and pole combinations, but the harmonic spectrum is different and often richer.
Harmonics matter beyond torque ripple. They add iron loss in the stator and the rotor, they can excite a mechanical resonance and they can produce a cogging torque that affects low speed control. A winding that looks good on fundamental torque can still fail a noise or vibration target.
Use the actual slot and pole combination and drive waveform: a sinusoidal supply and a pulse width modulated inverter produce different losses in the same winding.
Slot fill and wire choice
Slot fill is the ratio of copper area to slot area. A higher fill lowers resistance and improves the thermal path, but leaves less room for insulation and makes insertion harder, and too high a fill damages the enamel.
Round wire fill is limited by the way the wires pack. A layer wind in a slot is more orderly than a random fill, but the slot shape and the insertion method decide what is possible. Rectangular wire achieves a higher fill because the bars tile the slot.
The wire grade has to match the insulation class and the impregnation. A class H motor with class B enamel is a weak point. The enamel also has to survive the insertion, the forming and the varnish cure without cracking.
The wire diameter affects the loss and the fill. Finer wire is easier to wind but has more resistance for the same copper area. Stranded wire helps at high frequency, but the strand insulation takes space and reduces the fill.
Insulation, impregnation and bracing
A motor winding has to be insulated from the core, between phases and between turns. The slot liner, the phase insulation, the wedge and the end winding sleeving all contribute. The creepage and clearance at the terminals are part of the same design.
Impregnation bonds the winding and fills the gaps. In a motor, it also improves the thermal path from the copper to the core, reduces the movement that wears the enamel and shortens the path that sets the hot spot. Vacuum pressure impregnation is common on larger machines.
The end winding carries the highest mechanical load during a start or a fault. The magnetic forces push the coils, and an unbraced end winding can move, crack the insulation and fail. The bracing and the ties are part of the winding design, not an afterthought.
The impregnation resin has to be compatible with the enamel, the liner and the magnet wire. A resin that attacks the enamel or a liner that does not wet will leave voids and a weak thermal path.
Tests and records
A motor winding is checked for resistance per phase, phase balance, insulation resistance and high potential withstand. A surge test compares the winding with a known good unit and can find a shorted turn. The turns ratio and the polarity matter for a three phase winding.
The first article should include the winding layout, the wire grade, the insulation materials, the slot fill calculation and the impregnation record. A stator that measures correctly can still have the wrong layout, and the error may only show as a loss or a noise problem.
A no-load run gives current, loss and vibration signature. A load test gives efficiency and temperature rise. Agree the acceptance criteria before the test, with the ambient and cooling conditions stated.
| Feature | Distributed | Concentrated | Hairpin |
|---|---|---|---|
| Coil placement | Several slots per pole | One tooth per coil | Rectangular bars per slot |
| End turns | Long | Short | Short |
| Magnetomotive force | Smoother, lower harmonics | More harmonic content | Depends on layout |
| Slot fill | Medium | Medium to high | High |
| Automation | Moderate | High | High at volume |
| Typical use | Induction motors | BLDC and PM motors | Traction and high density |
The right route depends on the duty, the volume and the loss target. A single machine may use more than one route across its range.
Frequently asked questions
What is the difference between distributed and concentrated windings?
A distributed winding spreads each phase over several slots, which gives a smoother magnetic field and lower harmonics but longer end turns. A concentrated winding puts a coil on each tooth, which shortens the end turns and saves copper but raises harmonic content.
Why do concentrated windings produce more torque ripple?
The magnetomotive force has a richer harmonic spectrum, and the interaction with the rotor magnets produces ripple and cogging. The slot and pole combination, skew and the drive waveform all affect how much appears.
What is a hairpin winding?
A hairpin winding uses rectangular copper bars bent into a U shape, inserted into the slots and welded at the other end. It gives a high slot fill and short end turns, and it is common in high power density traction motors.
Does a higher slot fill always improve a motor?
It lowers resistance and improves the thermal path, but it leaves less room for insulation and makes insertion harder. A fill that is too high damages the enamel and raises the reject rate, so the gain has to be proved on the line.
Sources and further reading
- IEC 60034 series, Rotating electrical machines
- IEC 60085, Thermal evaluation and classification of electrical insulation
- Qumek motor winding and stator DFM notes
Qumek Engineering Desk
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