A simple sensorless control for medium voltage synchronous motor drives

When synchronous motors are driven without speed and position sensors, the uncertainty of magnetic flux position makes it hard to start motors, and it also causes a hunting phenomena of motor torque due to the deviation of phase between the inverter and the motor while accelerating the motors. In this paper, a simple control method for synchronous motors without speed and position sensor is proposed. A simple phase control to make rotor be on the position that can make starting torque and decrease the reversal angle has been developed. The hunting frequency caused by a deviation of phase angle between the inverter and the motor while accelerating can be estimated from the value of the current and inertia. The gain of the proposed hunting suppression control block can be designed simply from the estimated hunting frequency. The proposed control method enables smooth starting and acceleration of existing high and medium voltage synchronous motors. The steady operation of this method was confirmed by simulation and experiments by the multi-level (H-bridge cascaded) medium voltage inverter for 6 kV/1250 kW wound-rotor synchronous motor.