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A Switched Reluctance (SR) motor is a rotating electric machine
where both stator and rotor have salient poles. The stator winding
is comprised of a set of coils, each of which is wound on one pole.
SR motors differ in the number of phases wound on the stator. Each
of them has a certain number of suitable combinations of stator
and rotor poles.
The motor is excited by a sequence of current pulses applied at
each phase. The individual phases are consequently excited, forcing
the motor to rotate. The current pulses need to be applied to the
respective phase at the exact rotor position relative to the excited
phase.The inductance profile of SR motors is triangular shaped,
with maximum inductance when it is in an aligned position and minimum
inductance when unaligned. When the voltage is applied to the stator
phase, the motor creates torque in the direction of increasing inductance.
When the phase is energized in its minimum inductance position,
the rotor moves to the forthcoming position of maximal inductance.
The profile of the phase current together with the magnetization
characteristics define the generated torque and thus the speed of
the motor.
The SR motor requires control electronic for it's operation. Several
power stage topologies are being implemented, according to the number
of motor phases and the desired control algorithm. A power stage
with two independent power switches per motor phase is the most
used topology. This particular topology of SR power stage is fault
tolerant -- in contrast to power stages of AC induction motors --
because it eliminates the possibility of a rail-to-rail short circuit.
The SR motor requires position feedback for motor phase commutation.
In many cases, this requirement is addressed by using position sensors,
like encoders,Hall sensors, etc. The result is that the implementation
of mechanical sensors increases costs and decreases system reliability.
Traditionally, developers of motion control products have attempted
to lower system costs by reducing the number of sensors. A variety
of algorithms for sensorless control have been developed, most of
which involve evaluation of the variation of magnetic circuit parameters
that are dependent on the rotor position.
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