For a long time, induction motors could only offer fixed-speed operation. With the advent of reliable and cost-effective power electronic switches, motor designers developed a method to achieve variable-speed operation from AC induction motors. A DC power supply converts AC wall plug power to DC. Leveraging sophisticated power electronics enabled by insulated-gate bipolar transistors (IGBTs), an AC inverter drive uses pulse-width modulation (PWM) to chop the DC output into something that simulates a variable-frequency sine wave.
AC inverters can provide useful solutions for applications like fans and pumps where energy saving can be achieved by using variable speed control to replace inefficient bypass or throttling control. More sophisticated versions can be used for simple point-to-point motion from one or two axes in machines that don’t require high-speed or highly synchronized motion.
SR systems comprise a motor and a drive and offer an alternative to inverter-fed induction or PM machines, particularly in applications that are required to operate with high efficiency across a wide speed and load range. SR motors don’t include brushes, rotor windings, commutators, or magnets. Instead, their rotors consist purely of a stack of electrical steel laminations mounted about a shaft. Torque is produced by the magnetic attraction of the steel rotor poles to stator electromagnets. The rotor position relative to the stator is detected using a simple hardware sensor or by electronic "sensorless" means. The controller then energizes each stator winding only when it can produce useful torque. By suitable timing of the stator excitation, the machine can operate as a motor or generator, with good efficiency over a wide range of speed and torque.
The SR motor concept originated more than a hundred years ago, but only with the advent of cheap, powerful microprocessors did the approach become practical. Thirty years later, the technology has graduated from a laboratory curiosity to a viable solution for applications ranging from crushers and weaving machines to automotive and white goods, particularly in light of the high cost of rare earth magnets.
SR rotors feature a so-called "salient" design, which refers to gaps between the rotor poles. This design significantly reduces the rotor weight. “The designs remove mass from the periphery of the rotor, and not only does that make the rotor lighter, but it makes it substantially lower in inertia, typically less than half that of the equivalent induction machine,” says Cummins.
That makes them effective for applications that require fast dynamic performance such as high-speed traction drives. Lower rotor inertia also benefits gear-driven applications, which experience large transient loads, such as rotors, in which the lower rotor inertia of the SR motor reduces shock loading on the primary of the gearbox.
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