Torque saturation and inherent temperature compensation of hybrid stepper motor used in space application

Abstract

Hybrid stepper motors used in space applications will face varying supply voltage and a wider range of ambient temperatures. A higher reversible temperature coefficient of B_r for SmCo₅, Sm₂ Co₁₇, or NdFeB magnets in these motors will cause undesirable changes in their performance. In this article, a method to achieve constant torque as an inherent feature in a hybrid stepper motor used as the drive motor of a solar array of a satellite is described. Holding torque specification of the motor in prime/redundant mode is 0.55 Nm±10%0.55 Nm±10% and in synchronized mode–a mode in which both prime and redundant windings are simultaneously energized—it is 0.75 Nm±10%:0.75 Nm±10%: the supply voltage ranging from 28 to 42 V dc and ambient temperature varying from −55 °C to +75 °C. Based on finite element analysis (FEA) investigation on the operation of this motor, a “torque saturation region” has been incorporated in the design of the motor to permit constant torque despite voltage variations. Measured holding torques at the ambient temperature of +25 °C and at 28, 35 and 42 V are 0.49 Nm, 0.57 Nm, and 0.63 Nm in the prime/redundant mode and 0.76 Nm, 0.80 Nm, and 0.81 Nm in the synchronized mode against the FEA calculated values of 0.49, 0.55, 0.61, 0.73, 0.75, and 0.76 Nm, respectively: these values meet the requirements. The maximum voltage 42 V will be available in the raw bus during the time when the solar array is able to see the sun during which the ambient temperature will be maximum: +75 °C. Measured holding torque in the prime/redundant mode in this condition is the same as that at the nominal condition. That means the adverse effect caused by the increase in operating temperature is completely neutralized by the increase in voltage. Similar phenomenon occurs for lower temperatures also. Altogether, the torque variation is from 0.55 Nm (at −55 °C and 28 V) to 0.57 Nm (at +75 °C and 42 V) in prime/redundant mode. In synchronized mode, the corresponding values are 0.79 and 0.78 Nm. Thus, a combination of torque saturation phenomenon and the occurrence of simultaneous variation in voltage and temperature can achieve nearly constant torque from the hybrid stepper motor.