Cogging Torque Measurement Using the Electromotive Force in Surface-Mounted Permanent-Magnet Motors

This paper presents a new, fast, and inexpensive method for the measurement of the cogging torque in mass-produced surface-mounted permanent-magnet motors from the open-circuit voltage electromotive force (EMF), thereby obviating the need for mechanical torque meters. Conventional procedures involve rotating the motor at very low speed while the cogging torque measurement is performed. These procedures are very time-consuming and require expensive equipment, and hence cannot be used to verify all the units in mass production. The proposed method relates the cogging torque to EMF through the spatial induction wave in the air gap and can be fully automated in the manufacturing process without any additional time consumption. Once the foundations of the method are presented, proposals to overcome the limitations imposed by the presence of the slots and by the type of winding are addressed. The technique has been satisfactorily tested both numerically and experimentally on 40 motors, showing good agreement with the theoretical expectation.