46 applications, and that they are the focus of considerable r&d attention. Certain types of winding are commonly associated with specific types of e-machine, although the choice of winding configuration depends on factors such as motor design, the intended application, control strategy and performance requirements, so the associations are not set in stone. For example, concentrated windings, also known as single-layer windings, involve winding all the turns of a phase in a single layer, concentrated in one or a few slots. This shortens the mean length per turn of the conductor material, and with that the phase or winding resistance is kept as low as possible, Anders says. At the same time, the reduction in axial space occupied by the end-turns allows for additional active material, producing higher torque in a given volume, he adds. Concentrated windings are commonly associated with permanent magnet synchronous motors (PMSMs) and sometimes with brushless DC (BLDC) motors. They are relatively simple to construct and control, making them well suited to applications requiring precise torque control, high efficiency and compact design. As the name suggests, in distributed windings the turns of a phase are spread across multiple slots on the stator. Commonly found in IMs, synchronous reluctance motors (SynRMs) and some types of switched reluctance motors (SRMs), distributed windings help to reduce harmonics, improve torque production and minimise torque ripple, making them suitable for applications where smooth and efficient operation is prioritised. In lap windings, the end of one coil is connected to the start of the next in a continuous manner. Often used in DC motors, universal motors (which can operate on either AC or DC power) and in some types of AC motor, lap windings allow for more poles. In DC motors they enable better speed control and torque characteristics. Wave winding is a configuration in which each coil side progresses through a number of slots and then returns to the starting point. The end of one coil is connected to the start of another with either the same polarity or a different one, depending on the application. In the latter case, this allows for higher voltage ratings and improved cooling due to better air circulation between the coils. Wave windings are commonly used in high-speed AC motors and generators. With fractional slot winding there is a non-integer number of slots per pole per phase, with numbers such as 3/2, 5/4, 7/6, 9/8 and 11/10 being fairly common. Often found in interior permanent magnet (IPM) motors and some PMSMs, fractional slot windings offer lower torque ripple, higher power density and enhanced flux weakening. The latter reduces the back electromotive force (back EMF) as rpm rises to extend the speed range of a PMSM. Random windings feature coils placed randomly in the stator slots. Also known as distributed random windings or stochastic windings. In EVs, they are typically used in special applications where specific performance characteristics are required. Their irregular and distributed coil arrangement helps to reduce electromagnetic vibrations and noise from the motor, enhancing passenger comfort. They can also be easier to cool than other types, thanks to better air circulation and heat dissipation within the motor. Further, irregular coil distribution can help to minimise losses associated with coil proximity and slot effects, resulting in more uniform magnetic-flux distribution and reduced winding losses, in turn leading to increased efficiency. Finally, random windings tend to produce lower harmonic distortion than other winding types. Reducing harmonic content in the magnetic field results in smoother motor operation and reduced electromagnetic interference, further improving efficiency and reliability. Hairpin windings use pre-formed conductor bars that are shaped like hairpins and inserted into the slots of the stator core. Advantages include reduced end-winding losses, higher slot fill, improved thermal performance and increased power density. Hairpin windings are increasingly used in PMSMs for EV propulsion, more of which later. “You will find the more traditional distributed windings along with the concentrated windings are Insight | Coil windings May/June 2024 | E-Mobility Engineering Examples of wave windings (left), with each coil side passing through several slots before returning to the starting point, and concentrated windings (Image courtesy of Oswos)
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