29-05-2012, 04:18 PM
Switched Reluctance Motor (SRM)
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INTRODUCTION
The Switched Reluctance Motor (SRM) has been used not only for low performance
applications such as fans, pumps and hand-tools but also for high performance applications such
as centrifuges, electric vehicles and spindles. The SRM is known to be highly cost-effective and
reliable due to its simple structure and the unidirectional operation of its converter. Its rotor is
made of steel laminations without magnets and windings, and its stator is also made of steel
laminations with short-pitched and concentrated windings as
manufacturing cost of the SRM is relatively low and it can be operated at very high speed
without mechanical problems. In addition, the unidirectional nature of the converter for the SRM
provides room for unique and diverse designs such as [1] - [5] and eliminates the problems
common in bidirectional converters such as requiring dead-time to prevent the shoot-through
of upper and lower switches. By connecting switches always in series with a phase winding it is
not necessary to add lock-out circuitry and in case of a failure, enough time can be provided to
shut off the converter to prevent further damage. There is also a greater degree of independence
between phases than is possible in other motor drives due to the winding and converter
configurations. A fault in one phase in the motor or in the converter generally affects only the
flawed phase and other phases can continue to operate independently. Therefore, uninterrupted
operation of the motor drive is possible although with reduced power output.
Conventional Operation of the SRM
an example of an ideal inductance profile of phase a for an SRM
with 8 stator poles and 6 rotor poles. Conventionally, it is called an 8/6 SRM. It is shown for
only 60 mechanical degrees because of its periodicity. Fig. 1.2 portrays an ideal situation while
in reality the inductance and torque are nonlinear functions of the current and rotor position. This
ideal inductance profile of the SRM is assumed to produce torque in the conventional drives.
State of the Art
In earlier work given in [7] [8], the aligned inductance max L and the unaligned
inductance min L were analytically estimated and the self inductance was modeled as a piecewise
linear function similar to that of Fig. 1.2. All the above methods have considerable error in
predicting the unaligned inductance and they dont provide enough information about inductance
at positions in between. Hence, to obtain an actual inductance profile either Finite Element
Analysis (FEA) [9] - [15] and or measurements have been adopted.
Scope
The review in the previous sections provides insight into the development of the different
control schemes for the SRM and their advantages and limitations for different applications.
Based on this review, the following are identified as the key objectives to be achieved in this
dissertation for high performance SRM drives.