TABLE OF CONTENTS The operating speed range of both hybrid and variable-reluctance stepping motors is proportional to the phase resistance (eqns (5.15) and (5.31)). As the phase resistance can be controlled by changing the forcing resistance, it is possible to operate stepping motors at very high speeds using the simple drive circuits (described in Chapter 2) with a large forcing resistance. However, the supply voltage must also be increased to maintain the phase current at its rated value when the motor is stationary, and consequently a large dc power supply is needed. For small motors this may be a perfectly satisfactory method of obtaining a wide speed range, because the size of power supply is unimportant. With larger motors, however, the power supply may have to have a capacity of several kilowatts if the system is to operate over a satisfactory speed range. In these circumstances it is worth reconsidering the design of the drive circuit.
A large supply voltage and phase resistance are only required when the motor is operating at high speeds. If the motor is stationary the phase currents dissipate a substantial part of the supply output in the series forcing resistance, and the heat produced can cause problems if the forcing resistances cannot be cooled. The simple series forcing resistance is therefore an inefficient method of improving the speed range; power is wasted in the resistances at low speeds so that the mechanical output power (torque speed) can be improved at...
