Vector Control of AC Motor in Six-Step Operation Based on Variable DC-link Voltage

This paper proposes a cooperative control method of power factor correction (PFC) circuit and inverter for reduction of the heatsink of the electric drivetrain in air conditioning systems. The air conditioning system with inverter-based variable speed drives operates at the rated operating point at the initial start-up and then at a light-load condition by lowering the motor speed. The six-step operation reduces the heatsink volume and weight by reducing the maximum total semiconductor losses of the system. When operating with a fixed DC-link voltage, additional loss occurs due to the d-axis current. The proposed method conducts a six-step operation taking advantage of a variable DC-link voltage attributed to the PFC in the front end. It is possible to independently control the d/q-axis current with two degrees of freedom, namely the variable DC-link voltage and the phase angle. The proposed method reduces losses of the inverter significantly at the rated operating point, which enables the reduction of the heatsink. The validity of the proposed method is verified by a 7 kW PMSM drive system in commercial air conditioners.