Losses and thermal analysis of permanent magnet brushless motor under different drive modes

The effect of two different driving modes, square-wave and sine-wave drive modes, on the losses and temperature rises of permanent magnet brushless motor is researched, and the differences of losses and temperature rises under three square-wave drive conditions with the same equivalent voltage are compared. The currents of motor under two drive modes were simulated and harmonic analyses were performed respectively. An accurate loss model was used to calculate the losses of motor. Thermal field was analyzed using analytical lumped-circuit method based on the obtained losses and the result was validated by experiment. The analysis reveals that the phase current is smooth sinusoidal waves under sine-wave drive mode while significant magnetomotive force(MMF) time harmonics and pulse width modulation(PWM) carrier harmonics exist in the phase current under square-wave drive mode, causing larger rotor eddy current losses under square-wave drive mode. The temperature rise difference between rotor and stator is not significant under sine-wave drive mode, while large difference exists in temperature rises of rotor and stator under square-wave drive mode, and the difference becomes more significant with the decrease of PWM duty ratio and increased DC supply voltage.