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The variable reluctance motor is based on the principle that
an unrestrained piece of iron will move to complete a magnetic flux path
with minimum reluctance, the magnetic analog of electrical resistance. (Figure below)
If the rotating field of a large synchronous motor with salient poles
is de-energized, it will still develop 10 or 15% of synchronous torque.
This is due to variable reluctance throughout a rotor revolution. There
is no practical application for a large synchronous reluctance motor.
However, it is practical in small sizes.
If slots are cut into the conductorless rotor of an induction motor,
corresponding to the stator slots, a synchronous reluctance motor
results. It starts like an induction motor but runs with a small amount
of synchronous torque. The synchronous torque is due to changes in
reluctance of the magnetic path from the stator through the rotor as the
slots align. This motor is an inexpensive means of developing a moderate
synchronous torque. Low power factor, low pull-out torque, and low
efficiency are characteristics of the direct power line driven variable
reluctance motor. Such was the status of the variable reluctance motor
for a century before the development of semiconductor power control.
If an iron rotor with poles, but without any conductors, is fitted to
a multi-phase stator, a switched reluctance motor, capable of
synchronizing with the stator field results. When a stator coil pole
pair is energized, the rotor will move to the lowest magnetic reluctance
path. (Figure below) A switched reluctance motor is also known as a
variable reluctance motor. The reluctance of the rotor to stator flux
path varies with the position of the rotor.
Reluctance is a function of rotor position in a
variable reluctance motor.
Sequential switching (Figure below) of the stator phases moves the
rotor from one position to the next. The mangetic flux seeks the path of
least reluctance, the magnetic analog of electric resistance. This is an
over simplified rotor and waveforms to illustrate operation.
Variable reluctance motor, over-simplified
operation.
If one end of each 3-phase winding of the switched reluctance motor
is brought out via a common lead wire, we can explain operation as if it
were a stepper motor. (Figure above) The other coil connections are
successively pulled to ground, one at a time, in a wave drive
pattern. This attracts the rotor to the clockwise rotating magnetic
field in 60o increments.
Various waveforms may drive variable reluctance motors. (Figure
below) Wave drive (a) is simple, requiring only a single ended unipolar
switch. That is, one which only switches in one direction. More torque
is provided by the bipolar drive (b), but requires a bipolar switch. The
power driver must pull alternately high and low. Waveforms (a & b) are
applicable to the stepper motor version of the variable reluctance
motor. For smooth vibration free operation the 6-step approximation of a
sine wave (c) is desirable and easy to generate. Sine wave drive (d) may
be generated by a pulse width modulator (PWM), or drawn from the power
line.
Variable reluctance motor drive waveforms: (a)
unipolar wave drive, (b) bipolar full step (c) sinewave (d) bipolar
6-step.
Doubling the number of stator poles decreases the rotating speed and
increases torque. This might eliminate a gear reduction drive. A
variable reluctance motor intended to move in discrete steps, stop, and
start is a variable reluctance stepper motor, covered in another
section. If smooth rotation is the goal, there is an electronic driven
version of the switched reluctance motor. Variable reluctance motors or
steppers actually use rotors like those in Figure below.
Variable reluctance motors are poor performers when direct power line
driven. However, microprocessors and solid state power drive makes this
motor an economical high performance solution in some high volume
applications.
Though difficult to control, this motor is easy to spin. Sequential
switching of the field coils creates a rotating magnetic field which
drags the irregularly shaped rotor around with it as it seeks out the
lowest magnetic reluctance path. The relationship between torque and
stator current is highly nonlinear-- difficult to control.
Electronic driven variable reluctance motor.
An electronic driven variable reluctance motor (Figure below)
resembles a brushless DC motor without a permanent magnet rotor. This
makes the motor simple and inexpensive. However, this is offset by the
cost of the electronic control, which is not nearly as simple as that
for a brushless DC motor.
While the variable reluctance motor is simple, even more so than an
induction motor, it is difficult to control. Electronic control solves
this problem and makes it practical to drive the motor well above and
below the power line frequency. A variable reluctance motor driven by a
servo, an electronic feedback system, controls torque and speed,
minimizing ripple torque. Figure below
Electronic driven variable reluctance motor.
This is the opposite of the high ripple torque desired in stepper
motors. Rather than a stepper, a variable reluctance motor is optimized
for continuous high speed rotation with minimum ripple torque. It is
necessary to measure the rotor position with a rotary position sensor
like an optical or magnetic encoder, or derive this from monitoring the
stator back EMF. A microprocessor performs complex calculations for
switching the windings at the proper time with solid state devices. This
must be done precisely to minimize audible noise and ripple torque. For
lowest ripple torque, winding current must be monitored and controlled.
The strict drive requirements make this motor only practical for high
volume applications like energy efficient vacuum cleaner motors, fan
motors, or pump motors. One such vacuum cleaner uses a compact high
efficiency electronic driven 100,000 rpm fan motor. The simplicity of
the motor compensates for the drive electronics cost. No brushes, no
commutator, no rotor windings, no permanent magnets, simplifies motor
manufacture. The efficiency of this electronic driven motor can be high.
But, it requires considerable optimization, using specialized design
techniques, which is only justified for large manufacturing volumes.
Advantages
- Simple construction- no brushes, commutator, or permanent magnets,
no Cu or Al in the rotor.
- High efficiency and reliability compared to conventional AC or DC
motors.
- High starting torque.
- Cost effective compared to bushless DC motor in high volumes.
- Adaptable to very high ambient temperature.
- Low cost accurate speed control possible if volume is high enough.
Disadvantages
- Current versus torque is highly nonlinear
- Phase switching must be precise to minimize ripple torque
- Phase current must be controlled to minimize ripple torque
- Acoustic and electrical noise
- Not applicable to low volumes due to complex control issues
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