Measurement of the pull-out torque of synchronous micromotors with PM rotor

The subject of this study is a 2-pole, 2-phase synchronous motor with rare-earth permanent magnetic rotor, having an overall diameter and length of 1 mm and 2 mm. Using hydrodynamic theory the viscous drag, exerted as load torque on the shaft of the micromotor, was calculated by extrapolation from the measurement results on the larger cylindrical body. By decreasing the stator currents to the point where the micromotor stalled, the pull-out torque for a given current was determined. By application of the same method to the micromotor without external load the friction torque, particularly in the dry bearings, could be estimated as well