A three-level traction inverter with IGBTs for EMU

In order to reduce acoustic noise, size and weight of a traction inverter for electric multiple units (EMUs) connected to a DC 1500 V catenary line, the authors developed a three-level inverter using 2000 V class insulated gate bipolar transistors (IGBTs) and a novel PWM control method. This inverter consists of water cooled power units with a newly developed delta snubber circuit, which results in much lower loss compared to a conventional snubber circuit. The novel PWM control method, or multiple mode PWM control method, consists of three fundamental PWM modes and two transition modes. It is adopted not only to cover continuously the whole operational output voltage range, but also to reduce acoustic noise by making good use of the high switching frequency of the IGBTs of over 1.5 kHz. This PWM control method has five modes: the dipolar, partial dipolar, unipolar, over modulation, and one pulse modes. Transitions of each PWM mode are carried out smoothly just by controlling the modulation index and the bias. The authors also compare experimental waveforms and of the developed and the conventional two-level PWM using the developed inverter rated at 1600 k VA. The motor currents are less distorted than the conventional PWM because of both the multiple mode PWM at switching frequencies over 1.5 kHz and the three-level configuration. In particular, the motor currents are almost sinusoidal in the range between the dipolar and unipolar modes. The authors also verify that the acoustic noise from the induction motor is successfully reduced by approximately 15 dBA.<
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