The windings of motors are available in many different forms. This article will discuss 3-phase distributed wound AC motors. These are the most common AC motors used in industry. This discussion is applicable equally to induction motors and permanent magnet synchronous engines.

This distributed winding creates a smooth sinusoidal magneto-motive force (MMF) that flows through the motor's air gap. The MMF forms when balanced three-phase AC currents flow through the windings. The motor's magnetic design creates a traveling wave in the air, producing the torque needed for the motor to work.

The windings consist of several coils made from copper wire or, in certain cases, aluminium. Multiple strands can be connected parallel to create a single conductor. This is then wound up into a coil with multiple turns. Number of turns depends on the design.

As shown in the image below, a distributed winding is made up of multiple coils that are inserted into slots on the stator of the motor. Number of coils depends on number of slots in the stator, number of phases (3 for us) and number motor poles.

Each coil spans several slots. The average coil span of a full-pitch will be equal to 360deg/p or the number slots in the winding. A short-pitch will only have fewer slots. This figure shows the full-pitch for a 4 pole motor.

A 4-pole motor stator with a 3-phase distributed winding

The rest will go in the end windings which do not contribute to the motor torque. Rest of the winding will be placed in end winds which does not affect motor torque. To avoid unnecessary copper waste, careful design is required. 

Achieving good thermal performance also drives the requirement for high slot-fill and thermal end-winding management. The manufacturing processes often limit these factors. An ideal distributed winding would have infinite coils in infinite slots, so that the MMF distribution space is perfect. In practice, this is not feasible so the best compromise to achieve performance requirements must be made.

To avoid failures or short-circuits, coils of different phases must be isolated from one another and the core of the stator. The insulation will act as a thermal barrier, limiting the transfer of heat from inside the machine to outside. 

There will be air voids between winding cables and the insulation. The voids between the winding wires and insulation, as well as between the stator core and winding are filled using a resin by an impregnation procedure. This improves heat transmission while also improving winding insulation.

There are many different applications for electric motors. Motor design is affected by different applications. The winding will impact on several of these requirements.

· Maximisation of efficiency by minimising harmonic losses

· Reduce torque pulsations

· Reducing acoustic and vibration noise

There are several winding arrangements that can achieve the same performance. These layouts are determined by production constraints, which in turn is influenced heavily by the automation level used for winding.

This table shows some common configurations of winding as well as the key selection criteria.

There are clear compromises to be made when it comes to the technical requirements, automation, and costs. Motor designers must work with the manufacturing engineers to determine the best solution.