Instantaneous torque analysis of hybrid stepping motor

The hybrid stepping motor is promising as a high-torque and low-speed servomotor, since the motor can be considered as a multipole synchronous motor. For this application, it is necessary to have a clear equivalent circuit under the sinusoidal current drive just like the conventional brushless DC motor. For the given stator/rotor teeth and winding arrangements of the hybrid stepping motor, the permeance distribution with respect to the rotor position is obtained by a well-known permeance-based method. Based on the calculated permeance distribution, an equivalent magnetic circuit of the given motor can be obtained, from which the basic voltage equation can be deduced. Following this procedure, electric parameters in the voltage equation are related to the machine construction constants such as stator-rotor pole dimensions, air-gap length, number of turns of the stator windings, and magnetomotive force (MMF) of the permanent magnet. The analysis shows that a fundamental component of the permeance distribution produces the average torque and that harmonic components produce the ripple torque under the sinusoidal current drives. The cogging torque is also produced by the 4th harmonic component of the permeance distribution. For the laboratory testing, the two-phase hybrid stepping motor connected to the current-controlled voltage source inverter was used. The experimental results showed quite reasonable agreement with the predicted instantaneous torque.