22-10-2012, 03:26 PM
Switched Reluctance Motor Control – Basic Operation and Example Using the TMS320F240
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ABSTRACT
This report describes the basic operation of switched reluctance motors (SRMs) and
demonstrates how a TMS320F240 DSP-based SRM drive from Texas Instruments (TI]) can
be used to achieve a wide variety of control objectives.
The first part of the report offers a detailed review of the operation and characteristics of
SRMs. The advantages and disadvantages of this type of motor are cited.
The second part of the report provides an example application of a four-quadrant, variable
speed SRM drive system using a shaft position sensor. The example has complete hardware
and software details for developing an SRM drive system using the TMS320F240. The SRM
operation is described, along with the theoretical basis for designing the various control
algorithms. The example can be used as a baseline design which can be easily modified to
accommodate a specific application.
This report contains material previously released in the Texas Instruments application report
Developing an SRM Drive System Using the TMS320F240 (literature number SPRA420),
and has been updated for inclusion in the Application Design Kit (ADK) for switched
reluctance motors.
Introduction
Electric machines can be broadly classified into two categories on the basis of how they produce
torque - electromagnetically or by variable reluctance.
In the first category, motion is produced by the interaction of two magnetic fields, one generated
by the stator and the other by the rotor. Two magnetic fields, mutually coupled, produce an
electromagnetic torque tending to bring the fields into alignment. The same phenomenon causes
opposite poles of bar magnets to attract and like poles to repel. The vast majority of motors in
commercial use today operate on this principle. These motors, which include DC and induction
motors, are differentiated based on their geometries and how the magnetic fields are generated.
Some of the familiar ways of generating these fields are through energized windings, with
permanent magnets, and through induced electrical currents.
Motor Characteristics
The basic operating principle of the SRM is quite simple; as current is passed through one of the
stator windings, torque is generated by the tendency of the rotor to align with the excited stator
pole. The direction of torque generated is a function of the rotor position with respect to the
energized phase, and is independent of the direction of current flow through the phase winding.
Continuous torque can be produced by intelligently synchronizing each phase’s excitation with
the rotor position.
By varying the number of phases, the number of stator poles, and the number of rotor poles,
many different SRM geometries can be realized. A few examples are shown in Figure 1.
Power Electronics Hardware
The amount of current flowing through the SRM windings is regulated by switching on or off
power devices, such as MOSFETs or IGBTs, which connect each SRM phase to a DC bus. The
power inverter topology is an important issue in SRM control because it largely dictates how the
motor can be controlled.
There are numerous options available, and invariably the decision will come down to trading off
the cost of the driver components against having enough control capability (independent control
of phases, current feedback, etc.) built into the driver. A popular configuration, and the one used
in this application report, uses 2 switches and 2 diodes per phase. This topology is depicted in
Figure 11.