44 May/June 2024 | E-Mobility Engineering Peter Donaldson takes electric machines apart to examine the inner workings of coil windings In the loop At first glance, electric machines are simple. They have one moving part, the rotor, which either rotates in response to an input of electrical energy or puts out an electric current in response to an input of mechanical energy that rotates it. Inside the machine, things get more complicated as its basic functions – besides crucial parameters, such as torque, power, speed, energy efficiency and controllability – depend on the interaction of magnetic fields that push and pull each other. Some electrical machines create all their magnetic fields by passing electric currents though coils of wire wound onto cores, while others use a combination of permanent magnets and coil windings. These windings may be in the machine’s rotor or stator, or both. In all cases, they are essential components whose design has a profound effect on the behaviour of the machine and its suitability for the intended application. Windings affect performance through numerous interacting factors, including switching sequences and frequencies, resistance, reluctance, voltage and current constants, heat dissipation and mechanical strength. They can also be made of different conductive materials and in various multiple configurations, with pros and cons for each application, bringing complexity to the choices that engineers must make. Coil anatomy When an electric current flows through the windings (which form a closed loop) it generates a magnetic field made up of lines of force in accordance with Ampere’s law. Consisting of wire looped around an iron core, the induction coil functions as an electromagnet and, like a permanent magnet, it has north and south poles. The core provides a path for the magnetic field to move through, and the conductor typically fits into slots in An example of a stator with a braided winding configuration and copper hairpin conductors. Its compact winding structure reduces losses at the expense of complex construction (Image courtesy of ZF)
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