64 March/April 2024 | E-Mobility Engineering Flavours of control Remarkably, the simple process of switching power transistors on and off is amenable to several methods of controlling AC motors with inverters. Control is key to the stability and reliability of the drive system, and is applied to motor parameters such as torque, speed, voltage and safety-related functions. All control schemes for EV motors use pulse width modulation (PWM). PWM controls the voltage applied to the motor by adjusting the width of the pulses in the waveform. Essentially, it varies the length of time that the switches are on; the longer they are on, the closer the output is to the maximum DC voltage. PWM’s main advantage is simplicity. Drawbacks include limited precision compared with more advanced methods and the potential for higher harmonic distortion in motor currents. A refined PWM scheme, known as field oriented control (FOC), aligns the statorcurrent vector with the rotor’s magneticflux vector to achieve independent control of torque and flux. FOC yields high efficiency and a quick, dynamic response. However, it requires accurate information on rotor position, which usually comes from encoders or sensors. Also, FOC algorithms are complex, primarily because of the need for coordinate transformations to align the vectors. Direct torque control (DTC) regulates torque and flux without needing coordinate transformations, yielding fast responses, so it is well suited to high-performance EVs. However, it can lead to torque and flux ripple, causing vibration, and it is sensitive to variations in motor parameters such as resistance and inductance in rotor and stator. Advanced control methods such as DTC and FOC implement a control loop connecting the traction inverter to the vehicle. “The normal approach is to give the motor-inverter set a torque-control command, says the expert at our traction inverter and BMS developer. “The vehicle sends a torque set-point request, and the inverter converts the desired torque command into phasecurrent output to command the motor. The feedback of the measured current is then used as feedback for the vehicle control unit (VCU) to calculate the real torque output. A good inverter calibration should be able to provide a torque accuracy better than +/-5 Nm at all operating points.” To try to simplify inverter control, our traction inverter developer is working on technology that turns a voltage-source inverter into a current-source inverter. “There’s still PWM and a voltage fed into the machine, but the processing of duty cycles is handled differently,” its expert says. “Basically, this is an AC peakcurrent controller. It can follow the same switching pattern as a space-vector modulation (SVM) controller, minimising the ripple current on the DC link.” Software boost In modern EVs, all these control methods are implemented in the software that runs in the inverter’s microcontroller, under the ultimate command of the VCU. The software affects system performance through the switching strategy, says the electric machines, power electronics and control expert. “Novel switching patterns can minimise losses for both the inverter and the motor, and can be introduced remotely via over-the-air updates. Derating models can also be improved over time to allow for more efficient usage of the traction system,” he says. Some suppliers of inverters Belgium Cissoid +32 10 48 92 10 www.cissoid.com Canada Exro Technologies +1 587 619 1517 www.exro.com Dana TM4 +1 450 645 1444 www.danatm4.com Denmark Danfoss +45 74 88 22 22 www.danfoss.com Germany Infineon Technologies +49 89 234 0 www.infineon.com Hofer Powertrain +49 7022 217 884 0 www.hofer.de Italy Marelli +39 051 615 7011 www.marelli.com Zapi +39 0522 960050 www.zapigroup.com Spain EPowerlabs +34 943 466 286 www.epowerlabs.com Switzerland ABB +41 43 317 7111 new.abb.com/power-converters-inverters STMicroelectronics +41 22 929 2929 www.st.com The Netherlands NXP +31 40 272 9999 www.nxp.com UK GKN Automotive +44 121 788 4000 www.gknautomotive.com McLaren Applied +44 1483 261 400 www.mclarenapplied.com USA Allegro Microsystems +1 603 626 2300 www.allegromicro.com/en BorgWarner +1 248 754 9200 www.borgwarner.com/technologies/ power-electronics Curtiss-Wright +1 704 869 4600 www.cw-industrial.com Eaton +1 248 226 6772 www.eaton.com Texas Instruments +1 972 995 2011 www.ti.com
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