A 170 year old electrical technology, called variable switched reluctance (VSR) motors was resurrected in the 1980s with the advent of electronic controllers. The Texas based Le Tiurneau.inc has developed a wheeled front end loader for mining work which use large horsepower VSR motors for its wheel drives. Basically, a VSR motor comprises a rotor and a stator with a coil winding in the stator. The rotor, which consists of a laminated permeable material with teeth, is a passive device with no coil winding or permanent magnets.
The stator typically consists of slots containing a series of coil winding, the energization of which is electronically switched to generate a moving field. When one stator coil is set on, a magnetic flux path is generated around the coil and the rotor. The rotor experiences a torque and is moved in the line with the energized coils, minimizing the flux path. with the approximate switching and energization of the stator coils, the rotor can be encouraged to rotate at any desired torque and speed.
VSR Motors offers the following advantages: Since there are no brushes ringing, there is no requirement of commutator maintenance. the motor is more robust since there are no coils or moving parts. A VSR motor can maintain higher torque and efficiency over broader speed ranges than is possible with other advanced variable speed systems. In addition, as the commutation can be accurately controlled with respect to the rotor angle, the motor will operate at its predicted 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 the serve, and their simple rugged construction has no expensive magnets or squirrel cages like the induction motor. VSR motors are smaller than DC motors. 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 faults. According t a spokesman of Le Tourneau, While the initial cost of SR motor and control is a little more expensive than standard DC system, in the failure, there may be little or no difference in the manufacturing costs due to decreasing prices of electronic components. VSR motors are not without their drawbacks, however. The most significant downside is the acoustic noise and the large vibration caused by the motor's high pulsating magnetic flux. Another limitation is torque ripple. But while these drawbacks have an effect in small horsepower VSR motors, they are of no significance in large horsepower traction motors.
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