Sinusoidal Multilevel Inverters for Electric Vehicle Motors

Abstract

The traction motor of an Electric Vehicle (EV) manages a load with high-dynamic torque and speed, specially on the city, where the motor stop and go repeatedly. The ac motors are very effective and efficient in EV applications, but are powered by conventional converters with very undesirable voltages, producing losses, high THD, lower performance, torque jerk and isolation damage on the motor. Multilevel converters have been presented as a solution to improve the voltages, specially the cascaded multilevel converters, which produce many voltage levels with an optimal number of semiconductors, but they have three main disadvantages; use more than one isolated power source, decrease the voltage quality with the speed and regenerate even when the machine is motoring. The authors developed a novel control-system for cascaded multilevel converters (symmetrical, asymmetrical and/or hybrid) to keep all their advantages and eliminate their three main disadvantages. The converter uses only one power source, maintains the voltage quality at all speed range and automatically avoids the regeneration when the machine is motoring. The control-system controls the amplitude and frequency of the motor voltage using a variable dc source and the cascaded multilevel converter respectively. So the voltage waveform (quality) is maintained and the regeneration in motor mode is avoided at all speed range. The multilevel converter uses capacitors as isolated power supplies to use only one power source. These capacitors keep a constant voltage relationship with the single dc source using two possible methods, by hardware or software: i) Capacitors supplied from a High Frequency Link (HFL); and/or ii) Floating and auto-balanced capacitors working in the points of operation where their average power is zero. This paper presents the control-system concept, its applications in different cascaded multilevel converters and the experimental results in a specific converter, an Asymmetric Cascaded H-bridge (ACHB) inverter. The control-system showed to be a satisfactory solution to EV motors because it can be supplied by a single source (battery pack, fuel cells, etc.), deliveries high-quality voltages at all speed range, avoids regeneration in motor mode and is quite simple.