34 November/December 2023 | E-Mobility Engineering Nick Flaherty explains the various ways to create the perfect sine wave to maximise the efficiency of electric motor control Looking for the perfect wave Improving the efficiency of e-mobility motor control is a key focus for developers. Design technologies to squeeze the last microamp out of the battery can boost the range of EVs significantly, while improving the efficiency of the motor control can reduce losses, cutting the amount of thermal management needed and so reducing the complexity and weight of a motor. All of this comes down to the quality of the sine wave that controls the motor. Generating it is a complex task and varies with the individual motor design. Any deviation from the perfect sine wave, from jagged peaks to variable timing, lowers the efficiency of the output from the gate drivers that feed the switching transistors. These can be IGBT silicon transistors, silicon carbide (SiC) MOSFETs or gallium nitride (GaN) transistors. Control algorithms are optimised for these different types of transistors, in a complex development process for hardswitching topologies. Hard-switching is the most common technique for DC-AC power converters but it has numerous drawbacks, the largest of which is the introduction of switching losses when a transistor fully transitions between On and Off states. These losses are responsible for a large percentage of power converter losses. Pulse width modulation Pulse width modulation (PWM) is the most common hard-switching technique for generating a sine wave. The size and timing of each pulse builds up the wave, and on an engineer’s bench it is possible to generate a suitable sine wave. However, with constant changes in load, current, voltage and temperature it is difficult to reduce the total harmonic distortion. EVs are mostly driven in the low to mid-power range, so reducing the switching losses in this range will greatly improve the efficiency in terms of battery usage time and mileage. SPWM (sinusoidal PWM) is the most predominant PWM technique in inverter technology. The pulses are generated by comparing a reference sinusoidal signal as a modulating signal with the carrier signal. The carrier wave determines the switching frequency and the modulating wave that determines the frequency of the output wave. The frequency and amplitude of the carrier wave frequency are higher than the modulating wave. The amount of harmonic distortion is minimised by varying the modulation index, which is determined by the amplitude of the generated pulse.
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