Control and operation of PM synchronous motor drives in the field-weakening region

Due to the upper limit on the available DC link voltage and current rating of a given inverter, the motor input voltage and current ratings are limited. The voltage and current limits impact the maximum speed upto which constant rated torque is available and the maximum torque producing capability of the motor drive system, respectively. It is required and desirable to produce the rated power with the highest attainable speed for many applications such as electric vehicles, golf carts, people carriers in airport lobbies, forklifts, machine tool spindle drives etc. This paper reviews the operation of the permanent magnet synchronous motor drives (PMSM drives) when they are constrained to be within the permissible envelope of maximum inverter voltage and current to produce the rated power and to provide this with the highest attainable rotor speed. The rated power is for steady state operation and multiple times that is preferred for fast accelerations and decelerations during transient operation. Effective current control during the field weakening operation with transient capability is preferable compared to the saturation of the current loop resulting in higher peak currents and hence higher copper losses. From these considerations, this paper addresses the steady state operation, transient operation, and control techniques for the PMSM drives in the field-weakening region