Zero Synchronous Speed Stable Operation Strategy for Speed Sensorless Induction Motor Drive with Virtual Voltage Injection

In speed sensorless induction motor drives (SSIMD), the rotor speed is unobservable when synchronous speed $\omega_{e}$ equals to zero. Consequently, SSIMD system has unstable problem when input frequency of motor is closed to zero or equal to zero. This unstable problem results in that the SSIMD technology cannot be applied to motor drive system with gravitational load. In this paper, a virtual voltage injection method, based on adaptive full order observer (AFO), is proposed to solve the unstable problem of SSIMD when the synchronous speed $\omega_{e}$ equals to zero. The virtual voltage injection means the voltage signal is only injected into AFO, there is no signal being injected into the motor. When the synchronous speed $\omega_{e}$ equals to zero, the traditional rotor speed estimation method become invalid, but rotor speed still can be estimated by the injected voltage in the AFO. Finally the feasibility of proposed method is verified by experiments.