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Transcript
Beyond Bits
Motor Control Edition
Switched Reluctance Motors
Control techniques using Freescale solutions
Switched Reluctance
Motor Features
A switched reluctance (SR) motor is a
rotating electric machine where both
stator and rotor have salient poles.
The stator winding comprises a set
of coils, each of which is wound on
one pole. The rotor is created from
lamination in order to minimize the
eddy current losses.
SR motors differ in the number of
phases wound on the stator. Each
has a certain number of suitable
combinations of stator and rotor poles.
Figure 1 illustrates a typical two-phase
SR motor with a 4/2 (stator/rotor) pole
configuration and a stepped gap. The
stepped gap is used to eliminate dead
zones, where motor torque is zero at a
symmetrical SR motor and it ensures
motor startup in the proper direction.
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. Figure 2 illustrates
the idealized triangular-like inductance
profile of both phases of an SR
motor with phase A highlighted. The
individual phases A and B are shifted
electrically by 180 degrees relative
to each other. When the respective
phase is powered, the interval is
called the dwell angle: Qdwell. It is
Figure 1: Two-Phase 4/2 Switched Reluctance (SR) Motor
Stator
(4 Poles)
Phase B
Phase A
Rotor
(2 Poles)
Aligned Rotor Position
defined by the turn-on Qon and the
turn-off Qoff angles.
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 movement is defined
by the magnetization characteristics
of the motor. A typical current profile
for a constant phase voltage is shown
in figure 2.
Control of SR Motor
The SR motor is driven by voltage
strokes coupled with the given
rotor position. The profile of the
phase current together with the
magnetization characteristics defines
the generated torque and thus the
speed of the motor. Due to this fact,
the motor requires electronic control
for operation. Several power stage
topologies are being implemented,
according to the number of motor
phases and the desired control
algorithm. The particular structure of
Unaligned Rotor Position
the SR power stage defines
the freedom of control for an
individual phase.
There are a number of control
techniques for SR motors. They
differ in the structure of the control
algorithm and in position evaluation.
Three basic techniques for controlling
SR motors can be distinguished,
according to the motor variables
that are being controlled:
• Angle control
• Voltage control
• Current control
In angle control techniques, the
constant full voltage is applied in the
SR motor. The speed of the motor is
controlled by changing on/off angles.
The speed controller processes the
speed error (the difference between
the desired speed and the actual
speed) and calculates the desired
on/off angles. This technique is not
suitable for full speed range operation
since during low-speed operation the
maximal voltage amplitude generates
high current peaks in the motor
phases. This technique is used to run
Beyond Bits
Figure 2: Ideal Phase Inductance and Current Profile
Stator Phase A
Aligned
Just in Touch
Aligned
Rotor
iphA
LB
LA
Motor Control Edition
switching of individual phases.
Also, the motor structure causes
noise and torque ripple. The greater
the number of poles, the smoother
the torque ripple, but motor
construction and control electronics
become more expensive. Torque
ripple can also be reduced by
advanced control techniques such
as phase current profiling.
Freescale Enablement
ON
the SR motor over nominal speed. At
the nominal speed, the full voltage is
applied on the motor phases and by
properly adjusting on/off angles the
motor can achieve operation over the
nominal speed.
Dwell
OFF
Position /Time
current reaches desired amplitude,
the current controller keeps the phase
current at the desired level.
As is apparent from the description,
the SR motor requires position
feedback for motor phase
commutation. In many cases, this
requirement is addressed by using
position sensors, such as encoders
and Hall sensors. 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.
In voltage control techniques, the
speed of the motor is defined by
the voltage applied to the motor
phases. The voltage applied to the
phase is directly controlled by a
speed controller. The speed controller
processes the speed error (the
difference between the desired speed
and the actual speed) and generates
the desired phase voltage. The
desired voltage is generated by the
SR inverter using PWM modulation.
During PWM modulation, the on/off
times are constant. Once the applied
voltage has achieved its maximal
value, the motor speed can be
increased over the nominal speed by
changing on/off times.
SR Motor Applications
In the case of current control, there is
one more control loop: inner current
control loop employed in the control of
the SR motor. In this type of control,
the output of the speed controller
defines the required current amplitude
in the motor phase. Based on the
required current amplitude, the new
on/off times are calculated. Once the
The SR motor itself is a cost-effective
machine of simple construction. Since
high-speed operation is possible,
the motor is suitable for high-speed
applications, such as vacuum
cleaners, fans and white goods. As
discussed above, the disadvantage
of the SR motor is the need for shaftposition information for the proper
The selection of suitable MCUs for
control of SR motors depends on
selected algorithms and required
speed range. In the case of sensor
application and low-speed range,
the 8-bit MCU with a PWM module
is a sufficient option. The Freescale
MC9S08MP16 MCU includes
a 6-channel PWM module with
commutation support, which is
very important for applications with
high-speed range where precise
commutation is required. The
sensorless algorithms are too complex
for 8-bit devices without additional
external components. The Freescale
DSC family provides an optimal
solution. This family offers advanced
PWM modules, very fast ADC with
PWM to ADC synchronization and
a powerful DSP core, such as an
MC56F8006 or MC56F8013. Both
are the smallest representatives of
the DSC family and offer the best
performance/price ratio. Several other
members of the Freescale DSC family
are suitable for these applications.
Reference designs, application
notes and software solutions for
SR motor control are available at
freescale.com/motorcontrol.
Beyond Bits
Motor Control Edition
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