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Switched Reluctance (SR) Motor

Switched Reluctance or variable reluctance motor does not contain any permanent magnets. Here are a few key points about switched reluctance motors:

  • The stator is similar to a brushless DC motor. However, the rotor consists only of iron laminates
  • The iron rotor is attracted to the energized stator pole
  • The polarity of the stator pole does not matter
  • Torque is produced as a result of the attraction between the electromagnet and the iron rotor
  • Switched reluctance motor control is simple to implement but this type of motor is not commonly available
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Key Characteristics of the Switched Reluctance Motor
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  • Synchronous
  • Simple construction, but difficult to control for smooth torque operation
  • More suitable for high-speed/power density applications
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How it Works
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The Switched Reluctance (SR) or variable reluctance motor contains no permanent magnets, has a rotor consisting of iron laminates, and a stator similar to the DC Brushless motor. The iron rotor is attracted to the energized stator pole. Torque is produced as a result of this attraction between the electromagnet and the iron rotor. The simple, inexpensive construction of a Switched Reluctance motor requires a microcontroller for speed control and to reduce torque ripple and audible noise.

The rotor forms a magnetic circuit with the energized stator pole. The reluctance of a magnetic circuit is the magnetic equivalent to the resistance of a electric circuit. The reluctance of the magnetic circuit decreases as the rotor aligns with the stator pole. When the rotor is inline with the stator the gap between the rotor and stator is very small. At this point the reluctance is at a minimum. This is where the name "Switched Reluctance" comes from, the inductance of the energized winding also varies as the rotor rotates. When the rotor is out of alignment, the inductance is very low. And the current will increase rapidly. When the rotor is aligned with the stator, the inductance will be very large and the slope decreases. This is one of the difficulties in driving a switched reluctance motor.

Switched Reluctance Motor Control
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Input

  • Speed
  • Direction
  • Torque

Control

  • Comparators for over-current sensing or feedback
  • ADC to sense phase currents

Feedback

  • Tachometer wheels
  • Optical encoders
  • Sensorless for instantaneous speed and position sensing

Driver

  • Multiple switches (MOSFETS)
Applications
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  • Fuel pump
  • Throttle control
  • Oil pump
  • ABS
  • Vacuum cleaner
  • Lawnmower
  • Washing machine
  • Automatic doors in buildings and vehicles
  • Fans