Comparing Open-Loop and Closed-Loop Operation of a Synchronous Motor

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This Demonstration compares open-loop and closed-loop operation of a non-salient pole synchronous motor. Non-salient implies that the reactance is equal along every axis due to the uniform airgap between stator and rotor. The voltage vectors and the current vector in the - coordinate system are shown for the case of open-loop and closed-loop operation. The motor torque is proportional to the component of the current vector . In the case of closed-loop operation the current vector is always parallel to the axis.

Contributed by: Frank Brechtefeld (April 2012)
Open content licensed under CC BY-NC-SA



The current of a non-salient pole synchronous motor fulfills the equation


In this equation is the supply voltage, is the back-EMF, is the resistance, is the reactance, and is the imaginary unit.

The voltages , , , and are represented by phasors in the - coordinate system. The axis, which is the imaginary axis, is parallel to the phasor of the back-EMF . The axis, which is the real axis, is perpendicular to the phasor of the back-EMF .


[1] J.R. Hendershot and T.J.E. Miller, Design of Brushless Permanent-Magnet Motors, Hillsboro, OH: Magna Physics Publications, 1994.

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