The Switch to Switched Reluctance

Misconceptions
Emerson engineers say there is a misconception that SR motors tend to step when operated at low speeds. In reality, they say, torque production is relatively continuous, so there is no stepping behavior. They also say the energy efficiency of SR motors is at least as good as the best ac machines operating at their sweet spot. Engineers point out that the energy efficiency of ac motors drops dramatically when they operate at less than 50% load or when used in the field-weakening range at higher speeds. In contrast, complete SR systems (including all motor and inverter losses) can have an efficiency of well over 90% under a wide span of load conditions.

There is no fundamental high-speed limit for SR motors. Emerson says it has run some units at 70 krpm and is evaluating operation at 100 krpm for certain small machines. High speeds are constrained only by the bearing system and the yield strength of the rotor steel. Moreover, SR motors generate no back-EMF so there is no need to put energy into field weakening at high speeds, as is the case with permanent-magnet drives.

Drive electronics for SR motors resemble those for conventional variable-speed drives to a degree. Ordinary six-switch inverters for VFDs and SR motor drivers both contain an identical number of power switches (usually IGBTs) and diodes.

However, the SR drive has efficiency advantages compared to VFD drives that use relatively high PWM frequencies to synthesize sinusoidal ac waveforms and thus keep down harmonic content. Switching losses can be appreciable in these VFDs, thus the inverters tend to run hotter.

It is interesting to compare the operation of 12 or 18-switch VFDs with the drive for a typical eight-pole SR motor. Here switching takes place at eight times the physical rotation speed of the motor. Thus for a 3,600-rpm motor, switching frequency per phase is 480 Hz, about 10 times slower than for the equivalent inverter. Switching losses are 10 times lower as well. Emerson says all in all, power loss in an SR inverter can be as much as half compared with an inverter for an ac motor.

Another common misconception is that SR motors cannot serve as generators because they have no rotor magnets. Actually these motors can become generators by altering the timing of phase excitation — that is, switch on the electromagnets when the stator and rotor poles are separating, rather than when they are approaching. Switching on a stator pole when it aligns with the rotor puts a magnetic field through the rotor. The mechanical load does work on this magnetic field as it pulls the magnetized poles apart. That action increases the stored energy in the magnetic field. This energy then returns to the power supply when the IGBTs controlling the phase winding turn off.

Efficiency concerns
Torque production in an SR motor is proportional to the amount of current put into the windings. Torque production is also unaffected by motor speed, unlike the case in ac motors where, in the field-weakening region, rotor current increasingly lags behind the rotating field as motor speed rises.

SR-torque density can easily exceed that of ac motors. But SR motors do have a somewhat lower torque density than permanent-magnet ac motors. PM motors operating at their design sweet spot can have a hightorque output per unit of stator current because the magnetic field has already been “paid for” by inclusion of permanent magnets. (In other words, no stator current is necessary to magnetize the motor.)

There are trade-offs, however, because efficiency falls off faster at light loads for a PM motor than for SR motors. And there is a loss of efficiency with PM motors at speeds high enough to necessitate use of field weakening to prevent the back- EMF from exceeding the power supply voltage. Applying a current to temporarily weaken the magnets causes losses in the windings and commensurate losses in efficiency. Loss of control under field weakening conditions in a PM machine is dangerous, too, because the back- EMF then exceeds the power supply voltage resulting in uncontrolled generating and, consequently, high currents.