Direct torque control of permanent magnet drives

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Abstract
Many permanent magnet motor drives use an open loop form of torque control, based on the assumption that output torque is proportional to applied current. In a practical motor this assumption may not always be correct, due to sub-optimal alignment of magnets, nonuniformity of magnetic material, current sensor nonlinearities and current controller limitations. These factors, together with nonoptimised current references, can lead to high values of torque ripple and copper loss. This paper describes a method of estimating the electromagnetic torque from the rate of change of co-energy with respect to position, thus taking account of mutual torque, reluctance torque and saturation effects. The paper shows how the estimator can be used in a direct torque control scheme. The direct torque controller maximises the torque/copper loss ratio. Implementation of the direct torque controller in a DSP based drive system is described, with steady-state and transient experimental results illustrating the effectiveness of the direct torque control scheme.