The windings on the stator of an SR motor are concentrated rather than distributed as they are in induction motors and some types of PM motors. This minimizes the length of the non-torque producing "end-turn" section of the winding and enables more torque to be produced from a given motor size, or alternatively yields a smaller motor for a given continuous torque requirement.
The PWM switching used by AC inverters creates losses every time the IGBTs switch, reducing efficiency. SR drive systems switch as well, but for a given motor speed around an order of magnitude less frequently than an inverter. This results in lower switching losses and, therefore, more efficient operation. SR systems offer comparable system efficiency to inverter-fed induction machines at rated speed and power. However, as operation moves away from rated speed and power, SR systems maintain higher efficiency over a wider range of speed and load than either induction or PM systems.
SR systems offer the ability to deliver rated power over a wide range of speed. This is a vital attribute in many traction-type applications such as mining vehicles, off-road vehicles, and, more recently, in commercial vehicle and automotive applications. “One of our licensees uses our technology in their wheel loader vehicles to replace brushed DC machines that were expensive to produce and maintain,” says Cummins.
There aren't any perfect solutions, and SR systems are no exception. They can't operate direct online as an induction motor can, and even though they're smaller than a comparable induction motor, in applications that are required only to run over a narrow speed range they remain somewhat larger than their rare earth PM counterparts. As in all types of engineering, it comes down to a series of tradeoffs. Designers need to consider all aspects of their application, from torque, size, and duty cycle to cost and time-to-market.
The REE market has become steadily more rational, and new sources coming online will continue to reduce costs. Still, it is unlikely that prices will drop to their former lows. Fortunately, engineers have a number of options when it comes to balancing cost and performance. By exploring the types of design alternatives we’ve discussed in this five-part series, OEMs can develop a system that will best meet all of their objectives.