Investigation of a Two-Stage Boost Converter Using the Neutral Point of a Motor

Control strategy and loss evaluation is discussed for a proposed dc/ac two-stage boost converter using the neutral point of a motor. The proposed converter consists of a small boost chopper and a three-phase inverter, which has a boost-up function from the use of the leakage inductance of a motor, instead of a boost-up reactor, for dc to ac conversion. A six-step operation strategy is proposed for the circuit. When the inverter part outputs a square waveform of 180°, the input current has a distortion because the neutral point voltage of the motor fluctuates at a frequency three times the output frequency. The input current is also shown distorted due to the neutral point voltage fluctuation. Feedforward compensation is proposed to reduce the input current fluctuation. In addition, the dc current in the proposed circuit is imposed into the phase current of the motor. Loss evaluation is therefore implemented to investigate the influence of the imposed dc current on the experimental results. Also, finite-element method was used to analyze the motor loss for a 1.5-kW interior-permanent-magnet motor.