Slip recovery and prevention in Pseudo Direct Drive permanent magnet machines

A Pseudo Direct Drive (PDD) is a permanent magnet machine with an integrated magnetic gear. It has two magnetically coupled rotors. When subjected to a load torque higher than the pull-out torque slip between the two rotors may occur. Although, this may not cause any damage to the PDD or the load, it could, if left uncontrolled, result in undesirable consequences such as loss of power transfer, excessive speed on the high-speed rotor, incorrect commutation signal and/or loss of load speed/position control. This paper proposes a technique to control slip caused by external overload torques and enable a swift recovery when these are removed for a PDD equipped with a position/speed sensor on each rotor. However, when the PDD is equipped with only one sensor on the high-speed rotor, it is impossible to determine the position/speed of the low-speed rotor using an observer after slip occurs, which precludes a swift and controlled recovery. Therefore, a novel technique which prevents the slip between the two rotors under overload conditions is also proposed. Both techniques have been successfully implemented and demonstrated on a prototype PDD drive.