The cogging torque measurement through a new validated methodology

The recent activity of the authors, concerning different types of small PM machines, brought to the necessity to measure the respective cogging torques. Normally such a measure (including the hysteresis and bearing torque components), is realized with a prime motor to drag the machine under exam at very low speed and adopting a very good gearbox (classical method). During the activity a new method has been introduced adopting a step motor dragging directly the machine to be tested. The measure has been carried out with both systems; the results obtained with the classical method constitute the reference for the new one adopting the step motor, with a very good agreement. The reason to adopt the new method is related to the great elasticity which the step motor and its control may allow; that appears useful, for example, to try to separate the actual cogging torque from the hysteresis and bearing torque components, which results impossible with the classical method.