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Switched Reluctance (SR) Motors and Drives
LTI Power Systems’ Switched Reluctance (SR) drive systems, consisting of motors and controls, are proven to deliver reliable, high torque performance in even the most demanding heavy equipment applications. Remarkably simple in its design, the SR drive system has established new standards for performance, long life, reliability, and ease of maintenance.
The system consists of a brushless switched reluctance motor combined with high-power semiconductor switches and digital controls. The SR motor includes a rotor with no magnets or windings and a stator whose poles contain a winding, similar to a field of a DC motor. The SR system employs the principle of magnetic attraction to move the motor’s rotor from pole to pole, creating rotation.
How It Works
The unique SR motor achieves rotation by the sequential energizing of stator poles. When the stator pole winding is energized, the nearest rotor pole is attracted into alignment with that stator pole. (see figure 1) The rotor will follow this sequence, attempting to align rotor poles with energized stator poles. However, as the rotor and stator poles align, the stator poles switch off and the next group of stator poles switch on, continuing the rotation of the rotor.
The switched reluctance motor generates continuous movement by consecutively switching the currents on and off, thus ensuring the poles on the rotor are continually chasing the stator current. The movement achieved
figure 2
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is a function of the current through the winding and the characteristics of the iron in the rotor. For added reliability all LTI SR systems feature sensorless technology that allows the position of the rotor to be determined without external sensors.
(see figure 2)
IGBT Switching
The success of Switched Reluctance technology has been made possible by utilizing the principle of magnetic attraction in conjunction with today's high-power semiconductor switches and digital control techniques. IGBT (insulated gate bi-polar transistor) switching provides the required precise control of high horsepower SR motors. In addition, SR drives feature a motor coil in line with each IGBT, providing system impedance and a higher level of control over any type of system fault, resulting in a durable and reliable system.
Benefits of SR Drive
SR drive offers a number of key benefits. It offers high-torque capability down to and including zero (0) speed, with full control at zero speed with, or without speed sensors. In addition, there are no windings on the rotor, adding to the durability of the motor. There is also no commutator maintenance and no brushes or brush rigging to maintain, so the system requires very little attention. And since the system features low rotor inertia, it benefits gearing life, especially if the application involves frequent stopping and starting.
Other features of the SR system include:
- High level of fault tolerance
- High-speed capability limited only by bearing speed rating
- Consistent high efficiency over wide speed and load ranges
- Inherently 4-quadrant systems; runs forward or backward as either a motor or generator
- Shape adaptable – can be designed to match installation requirements, vertical or horizontal
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