14-12-2012, 02:57 PM
Switched Reluctance Motor
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Introduction
The reluctance motor is an electric motor in which torque is produced by the tendency
of its moveable part to move to a position where the inductance of the excited
winding is maximized. The origin of the reluctance motor can be traced back to 1842,
but the “reinvention” has been possibly due to the advent of inexpensive, high-power
switching devices.
The reluctance motor is a type of synchronous machine. It has wound field coils of a
DC motor for its stator windings and has no coils or magnets on its rotor. Fig.1 shows
its typical structure. It can be seen that both the stator and rotor have salient poles;
hence, the machine is a doubly salient machine.
SRM Configurations
Switched reluctance motors can be classified as shown in Fig.3. The initial
classification is made on the basis of the nature of the motion (i.e., rotating or linear).
Rotary SRM
The rotary machine-based SRMs are further differentiated by the nature of the
magnetic field path as to its direction with respect to the axial length of the machine.
If the magnetic field path is perpendicular to the shaft, which may also be seen as
along the radius of the cylindrical stator and rotor, the SRM is classified as radial field.
When the flux path is along the axial direction, the machine is called an axial field
SRM.
Radial field SRMs are most commonly used. They can be devided into shorter and
longer flux paths based on how a phase coil is placed. The conventional one is the
long flux path SRMs, in which the phase coil is placed in the diametrically opposite
slots, as shown in Fig.1. In the shorter flux path SRMs, the phase coil is placed in the
slots adjacent to each other, as shown in Fig.4. Short flux path SRMs have the
advantage of lower core losses due to the fact that the flux reversals do not occur in
stator back iron in addition to having short flux paths. However, they have
disadvantage of having a slightly higher mutual inductance and a possible higher
uneven magnetic pull on the rotor.
Single-Phase SRM
Single-phase SRMs are of interest as they bear a strong resemblance to single phase
induction and universal machines and share their low-cost manufacture as well.
High-speed applications are particularly appealing for single-phase SRMs. When the
stator and rotor poles are aligned, the current is turned off and the rotor keeps moving
due to the stored kinetic energy. As the poles become unaligned, the stator winding
again is energized, producing an electromagnetic torque. A problem with single-phase
SRM operation arises only when the stator and rotor poles are in alignment at
standstill or the rotor is at a position where the torque produced may be lower than the
load torque at starting. This can be overcome by having a permanent magnet on the
stator to pull the rotor away from the alignment, or to an appropriate position, to
enable the generation of maximum electromagnetic torque, as shown in Fig.6.
Stator Inductance
The torque characteristics of switched reluctance motor are dependent on the
relationship between the stator flux linkages and the rotor position as a function of the
stator current. A typical phase inductance vs. rotor position is shown in Fig.8 for a
fixed phase current. The inductance corresponds to that of a stator-phase coil of the
motor neglecting the fringe effect and saturation. The significant inductance profile
changes are determined in terms of the stator and rotor arcs and number of rotor poles.
Power Converters for SRMs
Since the torque in SRM drives is independent of the excitation current polarity, the
SRM drives require only one switch per phase winding. Moreover, unlike the ac
motor drives, the SRM drives always have a phase winding in series with a switch.
Thus, in case of a shoot-through fault, the inductance of the winding limits the rate of
rise in current and provides time to initiate the protection. Furthermore, the phases of
SRM are independent and, in case of one winding failure, uninterrupted operation is
possible. Following are some configurations of converters used in SRM drives.
Asymmetric Bridge Converter
Fig.10a shows the asymmetric bridge converter. Turning on the two power switches in
each phase will circulate a current in that phase of SRM. If the current rises above the
commanded value, the switches are turned off. The energy stored in the motor phase
winding will keep the current in the same direct until it is depleted. The waveforms
are shown in Fig.10b and c with different switching strategies.
Bifilar Type Drive Topology
Fig.12a shows a converter configuration with one power switch and one diode per
phase but regenerating the stored magnetic energy to the source. This is achieved by
having a bifilar winding with the polarity as shown in the figure. The various timing
waveforms of the circuit are shown in Fig.12b. It is shown that the voltage across the
power switch can be very much higher than the source voltage. A disadvantage of this
drive is that the SRM needs a bifilar winding, which increases the complexity of the
motor.
Position Sensors
In the SRM drives, rotor position is essential for the stator phase commutation and
advanced angle control. The rotor position is usually acquired by the position sensors.
The commonly used position sensors are phototransistors and photodiodes, Hall
elements, magnetic sensors, pulse encoders and variable differential transformers.
Hall Position Sensors
The function of a Hall sensor is based on the physical principle of the Hall effect
named after its discoverer E. H. Hall: It means that a voltage is generated transversely
to the current flow direction in an electric conductor (the Hall voltage), if a magnetic
field is applied perpendicularly to the conductor.