DISTINCTION BETWEEN SWITCHED
RELUCTANCE MOTOR AND THE VARIABLE RELUCTANCE STEPPER MOTOR
The conduction angle for phase currents is controlled and synchronized with the rotor position, usually by means of a shaft position sensor.
Thus SR motor is exactly like a brushless dc motor. But the stepper motor is usually fed with a square-wave of phase current without rotor position feedback.
SR motor is designed for efficient power conversion at high speeds comparable with those of the PM brushless dc motor. The stepper motor is usually designed as a torque motor with a limited speed range.SR motor is more than a high-speed stepper motor. Its performance and low manufacturing cost make it a competitive motor to PM brushless dc system.
1. Merits of SRM
1. Construction is simple and robust, as there is no brush.
2. Rotor carries no windings, no slip rings and brush-less maintenance.
3. No permanent magnet, neither in the stator nor in the rotor.
4. Ventilating system is simpler as losses takes place mostly in stator.
5. Power semiconductor switching circuitry is simpler.
6. No shoot-through fault is likely to happen in power semiconductor circuits.
7. Torque developed does not depend upon the polarity of the current in the phase winding.
8. The operation of the machine can be easily changed from motoring mode to generating mode by varying the region of conduction.
9. It is impossible to have very high speeds.
10. Depending upon the requirement, the desired torque speed characteristics can be tailor made.
11. It is a self-starting machine.
12. Starting torque can be very high without excessive inrush currents.
2. Demerits of SRM
1. Stator phase winding should be capable of carrying the magnetizing current also, for setting up
the flux in the air gap.
2. For high speed operations, the developed torque has undesirable ripples. As a result it develops undesirable acoustic losses (noise).
3. For high speeds, current waveform also has undesirable harmonics. To suppress this effect alarge size capacitor is to be connected.
4. The air gap at the aligned axis should be very small while the air gap at the inter-polar axis should be very large. It is difficult to achieve. No standardized practice is available.
5. The size of the motor is comparable with the size of variable speed induction motor drive.
6. Number of power wires between power semiconductor circuitry and the motor and the number of control cables from one controller to the power semiconductor circuitry are more and all to be properly connected.
7. It requires a position sensor.
3. Application of SRM
1. Washing machines
2. Vacuum cleaners
4. Future automobile applications
5. Robotic control applications