Switched Reluctance Motor: Design and Control Strategies

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Introduction;
Switched reluctance motor is a brushless AC motor, it is always referred to as SR. This
motor has a very easy mechanical construction and it doesn't need a permanent magnet for it to
work perfectly. It rotor and starter always have main poles. There is no exact number of the
poles on the starter since this will always depend on the number of phases the motor is made to
work on. Usually, one phase of this motor is realized through having two stator poles at opposite
ends. And in this arrangement, a three-phase can be realized by having 6 starter poles. The
number of poles of the rotor is selected not to be the same to the number of poles of the stator
(Gieras, 2014).
SR motor will contain the phase winding only on its stator. Intense windings are always
employed for such motor (Miller, 2014). The windings are put onto the poles of the stator and
then connected in series in order to make one phase of the motor. For a three Phase motor, there
must be 3 pairs of intense windings where each pair of the winding should be connected in series
to make each phase respectively. The mechanical assembly of a 6/4 3-Phase motor is illustrated
in Figure 1 (Hughes, 2011). Other arrangements of such motor are like 4/2 2-Phase SR motor
(having uneven rotor) and 8/6 4-Phase motor is illustrated in Figure 2 and Figure 3 as shown
below.

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Fig 1. Fig 2 Fig 3
Fig 1: Showing the arrangements of 4/2 2-Phase SR motor (having uneven rotor) and 8/6 4-
Phase motor. (Hughes, 2011)
A real structure of the switched reluctance motor for a 4- phase machine having 4 rotor- poles
and 3 rotor pair is shown in the diagram below;
Fig 2: Showing a real structure of the switched reluctance motor for a 4- phase machine having 4
rotor- poles and 3 rotor pair. (Janardnan, 2014)

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The main aim of this project is to come up with a controller for the stepper motor and relate its
design to the controller of the switched reluctance motor (Janardnan, 2014). The micro controller
is then interfaced with the stepper motor as seen in the diagram below;
Fig 3: Showing controller module interfaced with the stepper motor. (Boldea, 2012)
Reflected back from 1980s, the motor drive of a switched reluctance has been considered as one
of the best, reliable and promising speed adjusting driving systems. In both the ac and dc drives
of motors, the SRD has become a good and sufficient competitor as a result of it being simple,
robust, and reliably efficient and possession of high power density. The products that relates to
the switched reluctance motors have been in the late past been applied in various fields and
sectors which includes: electric driving vehicle, appliances used in the household, system of the

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servo, aviation industry and other many more. This has been widely achieved through the
product having much market potential.
It is however difficult creating accurate mathematical models for SRM due to the fact that
magnetic saturation causes many problems relating to nonlinear properties. In addition the SRM
mathematical model inaccuracies are also caused by the effect originating from the eddy together
with hysteresis effects. Nevertheless, the constant change in the strategy of control of the SRM
are always in consideration to the structure and parameters of the SRM thus leading to the
difficulty in achieving a reliable performance of control through the use of a pure traditional
strategy of the PID. This literature therefore discusses the fuzzy PI strategy through a
combination of PID algorithm together with fuzzy algorithm and eventually the 4 phase designs,
the 8/6 pole SRD that is basically grounded on the 32 bit digital signal processor with a general
motive of solving the previously mentioned challenges and problems. The results which are
obtained from the experiment clearly show that certain extents of the challenges and problems
originating from the effects of the nonlinear properties of the SRM can be solved by the scheme.
Strategy of control and structure of the system
The power converter, SRM, detector and the controller are usually the main or major
components of the SRD system. The SRM structure is shown in the below figure 1 of the
literature. Through the control of the motors current of winding switch, the 4-phase, SRM 8/6
pole, power converter energy is transferred. The SRD systems normally have the controller as
the core part or component which is capable of adjusting the current at the phase and the SRM
speed of motor. In addition, its properties greatly and directly have impacts on the SRD system
performance. Rotor position detection and the detection of the winding current forms the general

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components of the detector module. The detector module mainly performs the role of providing
important information of rotor position and the motor winding of the switch state.
Fig. 4 the 4 phase 8/6 pole SRM structure (Athani, 2014)
Maintenance of the SRM speed within the desired parameters is normally the key function or the
obligation of the system of the SRD and the significant point of the system of driving is usually
the torque control. Turn on angle, turn off angle, voltage of the phase and the current of the
phase are the common parameters of control possessed by the system of the SRD. Some of the
strategies for control entails control foe the angular position, chopped current control and finally
the control of voltage pulse width modulation. Angular position control mainly deals with the
speed control and SRM torque through regulation of the turn on angle and turn off angle of the
devices of the main switch while maintaining the input voltage. This strategy is very applicable
and suitable for the high speed, efficient and torque maximum control but extremely not for the
low speed control due to the high peak of current.
Chopped current control is often achieved via control of the switch devices IGBT with an aim
limiting the peak of winding current with an aim of achieving the torque control while
maintaining the turn on angle and turn off angle. Voltage PWM control have a number of
advantages including a possibility of it regulating indirectly the winding current through

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adjustments of the average value of phase windings. In addition, voltage PWM control is
propitious to both high speed and low speed drive systems. Nevertheless, it also possess a faster
dynamic response on disturbances opposing the load which is eventually significant as it aids in
achievement of an effective and reliable SRD operation performance.in consideration to all the
above characteristics or properties, this literature mainly considers voltage PWM as the key
strategy for control.
Design for the system hardware
Diagram below represents the general structure of the hardware with the control chip DSP
TMS320F2812 as a basis. In addition, figure 3 illustrate a picture of TMS320F2812 system of
PCB. TMS320F2812 as a component of the advanced 32 bit DSP chip of a fixed point, has the
capability of reaching 150MIPS as a processing speed. It too have a capability relating to signal
processing and can efficiently achieve very complicated algorithm of control.
In figure 2, the dotted line partially represents the TMS320F2812 inclusive of the all parts of
control of the system which entails the control of current, control of speed and eventually the
control for motor commutation which are fulfilled by DSP software. The power circuit is driven
by the output logic level of the PWM signal originating from the DSP hence controlling the
current and speed of the SRM. Hall current sensors detects the feedback signals which are
delivered to DSP unit of ADC with an intention of implementing the current closed loop control
as the photoelectric sensors detects the feedback signals of the rotor position and is delivered to
the DSP CAP unit with an intention of achieving the speed closed loop control. Nevertheless,
significant information relating to the speed of the motor, current of the winding phase and the
torque output are sent to the display of the LCD through the DSP interface.

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Fig 5. The switched reluctance drive system block diagram (Acarnley, 2013)
Fig 6. TMS320F2812 32 bit digital signal processor photograph (Bartelt, 2013)
The power converter design
In order to improve or enhance the operational reliability, a circuit of half bridge asymmetry is
implanted as the SRD system circuit of power. This enables each phase to be independently
controlled avoiding the phase connections and the idea of the direct connection phenomenon
existing between the arms power circuit bridge. The asymmetry half bridge possesses an
improved utilization of power in comparison to other circuits of power making it more efficient
and reliable. Figure 4 of the diagram clearly represent the asymmetry half bridge topology