Start-up transient improvement for sensorless control approach of PM motor

PM motor´s speed sensorless control does not work well in the low speed. To avoid this issue, open-loop control is usually used to start the motor, and then it is switched to sensorless close-loop control. The transition between the two control modes can cause a transient during the starting process. This transient can be undesirable especially for motor drives with front-end rectifier and small energy storage components, such as small dc capacitors in the case of voltage source inverter (VSI) drive. This paper studies the principle of dc bus transient in sensorless control start-up process and proposes a method to reduce the oscillation: before closing the sensorless loop, the reference current is adjusted to continuously track the real motor rotor position to the estimated rotor position, and the speed regulator is pre-calculated to generate the q-axis reference current before closing the speed loop. An experiment is conducted using a Vienna-type rectifier and a VSI based motor drive with back-EMF observer based sensorless control on a PM motor. Experimental results show that with the proposed method, the DC bus voltage transient is obviously reduced.