E-Mobility Engineering | November/December 2023 35 Motor control | Focus Each winding is energised by a separate inverter leg triggered by individual PWM signals at intervals of 400o out of phase with each other. Naturally the nine-phase inverter consists of nine legs, each leg consisting of two semiconductor transistor switches connected in series. That means they are in the same leg and will be in the On or Off state depending on control signals generated by the PWM signal. SiC MOSFETs are used as semiconductor switches on each leg for the higher frequency, and for the simulation the load is a star-connected RL load. Another important factor in designing a nine-phase inverter is the non-linear characteristic of the EV power profile. Non-linear loads cause harmonics in electrical power converters, resulting in a poor power factor and high switching losses. To overcome these issues, various PWM techniques are used that vary the modulation index to minimise harmonic content. These include SPWM and space vector PWM (SVPWM). Simulations show that SVPWM produces fewer ripples than SPWM, while the power output of SPWM can be improved by using a suitable filter circuit. Compared with SPWM, SVPWM has better performance but is more complex to implement. New techniques Another new variable switching frequency PWM (VSFPWM) strategy aims to improve EV inverter efficiency. An SiC MOSFET inverter has excellent switching characteristics, enabling pulse width modulation control with a high switching frequency. The high switching frequency can reduce the voltage ripples in DC-link capacitors, which enables their use at a reduced capacitance. The switching frequency is typically set to a level that is within the limits of the voltage ripple in the maximum output region. Given that the same switching frequency is applied to the entire operating region, there is sufficient margin with respect to the limits of the voltage ripple in the low-to-medium output range. VSFPWM is designed to consider the real-time minimum switching frequency by considering the voltage ripple of the capacitor during operation under loaded conditions, thereby minimising the switching loss. In addition, the method is suitable for high-switching frequency control because the calculation time is short. Because it does not require additional hardware, it is also easy to apply to an existing inverter. In an inverter design, the DC-link capacitors, power semiconductors, cooling systems, connectors, and control units account for 23%, 17%, 20%, 15% and 25% of the power losses respectively. The switching performance of a SiC MOSFET allows a reduction in the size of the cooling system as well as the size of the DC-link capacitor. The design of a DC-link capacitor should consider its lifetime and capacitance, which is determined by the root-meansquare (RMS) value of the current ripple of the capacitor. The variation in the RMS current ripple with respect to the switching frequency is insignificant. The capacity of the DC-link capacitor is determined by the voltage ripple. Given that this ripple exhibits a large variation with respect to the switching frequency and has an inverse relationship with it, it is possible to reduce the capacitor’s capacity by increasing the switching frequency. The switching frequency satisfies the voltage ripple condition of the capacitor in the maximum output range of the load. When the inverter is driven according to the constant switching frequency PWM strategy, the voltage ripple has a sufficient margin compared with the limit value in the operating range lower than the maximum output. With an IGBT inverter, even if a separate switching frequency is selected in real time according to the capacitor voltage ripple margin, the The higher the carrier frequency, the smoother the output waveform. IGBT devices can only switch at 10-20 kHz, SIC up to 200 kHz and GaN up to 2 MHz. Multiple phases Multi-phase machines have many advantages such as smoother torque and low stator current in the motor at each phase without increasing the stator voltage per phase. By implementing a nine-phase drive system the stator current is reduced at each phase, in turn reducing the size of bulky components used in a conventional three-phase system and reducing the need for series connection of large numbers of batteries, improving the safety of the system. The nine-phase machine consists of nine stator windings operating at an angle of 400 phase shift (3600 ÷ 9). Examining the performance of motor control algorithms boosts the range of EVs (Courtesy of Renesas Electronics)
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