Theory and steady-state analysis of series-connected wound-rotor induction motor in sub-synchronous mode

The wound-rotor induction machine is capable of electromechanical energy conversion, if the stator and rotor windings are connected in series with a proper phase sequence. As a motor, the operation can be in one of two modes. In the first mode, the motor runs at exactly twice the synchronous speed for stable loading conditions. In the second mode, the motor runs at a speed below the synchronous speed (sub-synchronous) with self-starting capability in a manner similar to that of the normal operation mode as an induction motor. In this paper, the operation of the motor in the sub-synchronous mode is analyzed using the physical relationships between the stator and rotor mmfs. The analysis includes the calculation of losses and efficiency. The obtained laboratory experimental results proved the validity of the proposed analysis.