45 Coil windings | Insight E-Mobility Engineering | May/June 2024 “This can be done either by using a better conductive material than copper, by keeping the temperature of the wires as low as possible, or by simply getting more of the conductive material into the slot, thereby increasing the so-called slot fill factor,” he says. Slot fill refers to the percentage of the slot area occupied by the stator windings. As well as reducing ohmic losses, increasing slot fill can enhance the electromagnetic performance of the motor by increasing the ampere-turns and improving the torque density. “Resistance increases as the temperature goes up,” Anders says. “Generally, this will mean we need to have sufficient direct cooling of the wires/coil sides/windings. Another measure that supports the loss reduction is to get the mean length per turn within a coil as short as possible, again helping to reduce the coil resistance or overall phase resistance.” Juho Montonen of Danfoss Editron agrees higher slot fill is a key factor in improving energy efficiency, along with the exploitation of a higher space factor. As a concept, the space factor is related to slot fill, but it is not identical as it refers to the ratio of the crosssectional area of copper conductors to the total slot area, including both the copper/conductor and the insulation. It provides a measure of the efficiency with which the available slot space is used for carrying current. Increasing the space factor includes optimising the design to minimise the amount of insulation and maximise the amount of copper/conductor. Increasing torque density usually necessitates more effective cooling to the windings, which is another reason to adopt direct cooling, Montonen says. The catch is that higher power and torque density do not always mean higher efficiency. “The smaller the motor gets, the bigger the share of losses where effective cooling takes place.” Winding taxonomy Given their importance, it is not surprising there are many types of windings in use in motors and generators developed for e-mobility the core, which is usually laminated with insulating material between thin layers of iron to minimise heat-generating eddy-current losses. Magnetic forces act in a direction that tends to shorten the field lines between opposing poles, attracting north and south to each other. Torque at the rotor of an electric machine is related to the force acting on a current-carrying conductor in a magnetic field (Lorentz force) and it changes in reluctance, which is to magnetic fields what resistance is to electric currents. Both allow the conversion of electrical energy into mechanical energy and vice versa. In alternating current (AC) machines, for example, such as induction motors (IM) and synchronous motors (SM), the windings are arranged in specific patterns, and switched on and off in sequence to create a rotating magnetic field that causes the rotor to spin. The windings are typically arranged in phases, which are electric circuits isolated from one another, so they can be switched on and off separately. In a three-phase AC induction motor, for example, the phases are arranged 120o apart to ensure smooth power delivery. By changing the amplitude, frequency and phase of the current flowing through the windings, the motor’s speed, torque and direction of rotation can be controlled. Also, by optimising factors such as the number of turns, wire gauge and winding pattern, engineers can improve the motor’s efficiency, reducing energy losses and boosting overall performance. In pursuit of efficiency Therefore, to meet demand for greater energy efficiency, and higher torque and power density from electrical machines, engineers pay close attention to the windings. They often focus on reducing ohmic losses (electrical resistance), says Markus Anders from Siemens Digital Industries Software, a manufacturer of simulation-based, e-machine design tools. APP 310 electric motor for the VW ID.3 electric car with an innovative hairpin stator and conductors made from coated copper (Image courtesy of Zeiss)
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