A 170-year-old electrical technology called variable switched reluctance (VSR) motors was resurrected in the 1980s with the advent of electronic controllers. Texas-based Le Tiurneau.inc has developed a wheeled front loader for mining work that uses high-power VSR motors for its drive wheels. Basically, a VSR motor includes a rotor and a stator with a stator winding. The rotor, consisting of a permeable material laminated with teeth, is a passive device with no winding or permanent magnets.
The stator typically consists of slots containing a series of coil windings, the activation of which is electronically switched to generate a moving field. When a stator coil is ignited, a magnetic flux path is generated around the coil and rotor. The rotor experiences torque and moves in line with the energized coils, minimizing the flux path. by coarse switching and energizing of the stator coils, the rotor can be stimulated to rotate at any desired torque and speed.
VSR Motors offers the following advantages: Since there is no brush noise, there is no need for commutator maintenance. the motor is more robust as there are no coils or moving parts. A VSR motor can maintain higher torque and efficiency over wider speed ranges than is possible with other advanced variable speed systems. Also, since the commutation can be precisely controlled with respect to the rotor angle, the motor will operate at its intended high efficiency. With VSR technology it is possible to design a low cost motor with over 90% system efficiency and variable speed.
VSR motors can be programmed to precisely match the loads they serve, and their simple, rugged construction has no expensive magnets or squirrel cages like the induction motor. VSR motors are smaller than DC motors. The VSR motor is inherently resistant to overload and immune to single point failure. They have a high level of fault tolerance and are immune to switching failures. According to their Le Tourneau spokesperson, while the initial cost of the SR motor and control is slightly more expensive than the standard DC system, in the event of a failure, there may be little or no difference in manufacturing costs due to decreased of the prices of electronic components. However, VSR engines are not without their drawbacks. The most significant disadvantage is acoustic noise and large vibration caused by the high pulsating magnetic flux of the motor. Another limitation is torque ripple. But while these drawbacks have an effect on low-output VSR engines, they are of no importance in high-output traction motors.