Permanent magnet AC (PMAC) motor drives are finding expanded use in high-performance applications where torque smoothness is essential. This paper reviews a wide range of motor- and controller-based design techniques that have been described in the literature for minimizing the generation of cogging and ripple torques in both sinusoidal and trapezoidal PMAC motor drives. Sinusoidal PMAC drives generally show the greatest potential for pulsating torque minimization using well-known motor design techniques such as skewing and fractional slot pitch windings. In contrast, trapezoidal PMAC drives pose more difficult trade-offs in both the motor and controller design which may require compromises in drive simplicity: and cost to improve torque smoothness. Controller-based techniques for minimizing pulsating torque typically involve the use of active cancellation algorithms which depend on either accurate tuning or adaptive control schemes for effectiveness. In the end, successful suppression of pulsating torque ultimately relies on an orchestrated systems approach to all aspects of the PMAC machine and controller design which often requires a carefully selected combination of minimization techniques.

Pulsating torque minimization techniques for permanent magnet AC motor drives-a review

Permanent magnet synchronous machines generate parasitic torque pulsations owing to distortion of the stator flux linkage distribution, variable magnetic reluctance at the stator slots, and secondary phenomena. The consequences are speed oscillations which, although small in magnitude, deteriorate the performance of the drive in demanding applications. The parasitic effects are analyzed and modeled using the complex state-variable approach. A fast current control system is employed to produce high-frequency electromagnetic torque components for compensation. A self-commissioning scheme is described which identifies the machine parameters, particularly the torque ripple functions which depend on the angular position of the rotor. Variations of permanent magnet flux density with temperature are compensated by on-line adaptation. The algorithms for adaptation and control are implemented in a standard microcontroller system without additional hardware. The effectiveness of the adaptive torque ripple compensation is demonstrated by experiments.

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Identification and compensation of torque ripple in high-precision permanent magnet motor drives

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 suboptimal alignment of magnets, nonuniformity of magnetic material, current sensor nonlinearities, and current controller limitations. These factors, together with nonoptimized current references, can lead to undesirable levels of torque ripple and copper loss. This paper describes a method of estimating the electromagnetic torque from the rate of change of coenergy 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 maximizes the torque:copper loss ratio. Implementation of the direct torque controller in a digital signal processor (DSP)-based drive system is described, with steady-state and transient experimental results illustrating the effectiveness of the direct torque control scheme.

Direct torque control of permanent magnet drives

This paper describes an improved implementation of direct torque control (DTC) to a permanent-magnet brushless dc (BLDC) drive. The commutation torque ripple, which occurs every 60deg elec. in a conventional three-phase BLDC machine with two-phase 120deg elec. conduction, is reduced by employing a hybrid two- and three-phase switching mode during the commutation periods. It is based on the criterion of minimizing the error between the commanded torque and the estimated torque and does not require knowledge of the conduction duration of the three-phase switching mode. It adaptively adjusts the phase-current waveform to maintain constant electromagnetic torque so that commutation torque ripple, which would have resulted with conventional DTC, particularly at high rotational speeds, is effectively eliminated, as confirmed by both simulations and measurements.

Commutation Torque Ripple Minimization in Direct Torque Controlled PM Brushless DC Drives

Motion control techniques have been developed to exploit the high efficiency and extremely fast dynamic response capabilities of permanent-magnet AC (PMAC) machines. Control techniques are reviewed separately for the two major classes of PMAC machines referred to as trapezoidal (i.e., brushless DC) and sinusoidal machines. While trapezoidal PMAC machine drives are distinguished by their controls simplicity and minimal sensor requirements, sinusoidal PMAC machine drives offer opportunities for extremely smooth torque production and extended high-speed operating ranges. Advanced PMAC machine control topics including sensor elimination techniques and robust servocontrol algorithms are reviewed, concluding with a discussion of PMAC machine drive application trends. >

Motion control with permanent-magnet AC machines

An original analytical study is developed concerning the torque undulation due to phase commutation on brushless DC motors. The results indicate that the relative ripple is independent of current, varies with speed, and may reach 50% of the average torque. The amplitude of the torque undulation and the duration of the commutation are analyzed, showing how this affects the torque-speed curve of the drive. Simulation and experimental results are presented that validate the theoretical analysis. >

R. Carlson

Papers

Analysis of torque ripple due to phase commutation in brushless DC machines