Abstract—The paper
proposes the SRM drive system using the information of DClink current and
rotor position, from which the phase currents of SRM can be easily estimated
and also they can be used in driving SRM instead of the threephase currents. Comparing to the general drive system based on the threephase
currents, it is verified through the simulation that the SRM drive system based
on the proposed estimation has also the good performance in dynamic and
steadystate responses of the speed control. Using the DClink current and the
rotor position, the multiphase currents of SRM can be simply estimated and the
number of current sensors can be at minimum decreased to a single current
sensor.
I.
INTRODUCTION
Switched reluctance motor (SRM) drives are gradually penetrating
the market with applications already developed or being developed for the
consumer products, aerospace, home appliances, and automobile
industries [1][6]. The SRM drives
have the attractive characteristics
of fault tolerance and absence of magnets. However, the control of an SRM
depends on the phase current and the
commutation of the stator phases in synchronism with the rotor position. The
currentsensing and positionsensing
requirements increase the overall
cost and calls for extra space and complexity [1][2]. This drawback excludes the SRM from many cost sensitive
industrial applications. Sensorless operation is a key requirement for the
success of SRM drives in various industries. Nowadays sensorless field in study has been progressed with much interest
[1][2].
In detecting the stator
current of the SRM, the number of current sensor is increased in proportional
to the number of stator phase. The algorithm using the minimum sensor or the
pure sensorless algorithm is required for keeping the overall cost lower.
Solving the above mentioned, this paper proposes the SRM drive system using the information of
DClink current and rotor position, from which the phase currents of SRM can be
easily estimated and also they can be used in driving SRM instead of the
threephase currents.
RMxprt is used for
designing the SRM model with nonlinear characteristics. Simplorer available to multidomain of power, electronics, and system is used for analyzing the overall system
consisting of power circuit, control, and mechanical system. [7][8].
It
will be verified through the simulation using RMxprt and Simplorer that the
proposed algorithm based the information of DClink current and rotor position
is very effective.
II.
Principle of Switched
reluctance motor
A. Voltage and torque equation of the SRM
The voltage equation for one phase is followed as;
_{} (1)
where _{} is phase voltage, _{} is stator winding
resistance, _{} is selfinductance, and _{} is phase current.
Introducing the concept of coenergy such as equation (2), the developed torque of the SRM in equation (3) can be obtained by
partial derivatives.
_{} (2)
_{}
(3)
where the
developed torque of the SRM is proportional to the current square. And the sign
of torque is dependant on the rotor position, because the sign of torque is varied according
to the up or downslope of the inductance [4][5].
B. Excitation current control according to rotor position
Fig. 1. Relation between phase current and
inductance according to rotor position.
The torque is independent
of the direction of the current. Its direction depends only on the sign of _{}. When the rotor poles
are approaching the aligned position, this is positive, and the positive torque
is produced, regardless of the direction of the stator current. When the rotor
poles are leaving the aligned position and approaching the unaligned position,
the torque is negative, regardless of the direction of the stator current.
Therefore the ideal motoring current waveform is a rectangular pulse (in
practical, the trapezoidal waveform in Fig.1 is used for reducing the torque
ripple) that coincides with the rising inductance. Similarly, the ideal braking
current waveform is a rectangular pulse that coincides with the falling
inductance. The implication is that the current should be switched on and off
in synchronism with the rotor position; in other words, the SRM is a
shaftpositionswitched machine like the squarewaveform BLDC motor.
III.
Estimation
Algorithm of phase currents Using DCLink current
Fig. 2.
Relations of phase current
and DClink current by switch state.
When the switch is turnon, the DClink current flows as shwon in
Fig.2(a). aphase current and DClink current have the same flow and the
relation are followed as;
_{} (1)
Otherwise when the switch is turnoff,
DClink current flows as shown Fig.2(b). aphase current and DClink current have
the reverse flow and the relation are followed as;
_{} (2)
Therefore, using the relations of phase
current and DClink current shown in Fig.1, phase current can be estimated as
the following.
_{} (3)
where _{} is the estimated aphase current.
These relations are equally available to the estimation of other
phase currents. Therefore, using the information of the rotor position and
DClink, each phase current can be estimated as followings;
_{}
(4)
_{}
(5)
_{} (6)
where _{}is the estimation
function of each phase current, _{} and _{} are b and cphase estimated
currents.
IV.
Simulation Analysis
Using Simplorer and the RMxprt available
to the multidomain simulations of the powerelectronicssystem, the virtual
system is implemented to verify the effectiveness of the proposed algorithm.
A. Model of Switched Reluctance Motor
Fig.3(a) and (b)
shows the appearance and the crosssection of the SRM used in simulation.
The specification of SRM is shown in TABLE I. Its information and shape are used for modeling the SRM by RMxprt, which can consider a nonlinearcharacteristic of the SRM and produce the practical
model. Fig.4 and 5
show the currentflux curve of SRM and the inductance profile of SRM, which be
produced by RMxprt.
TABLE I
SPECIFICATION OF
SRM USED IN SIMULATION
Items

unit

value

rated
power

W

80

rated
voltage

V

220

rated
speed

rpm

815

resistance
/ phase

ohm

113

inductance
/ phase

mH

9.678

number
of stator / rotor pole


12/8

air
gap

mm

0.45

number
of turns / phase

turn

650

outer
diameter of stator

mm

132.0

inner
diameter of stator

mm

74.5

Fig. 3.
Appearance and crosssection of
SRM used in simulation (12/8).
Fig. 4.
Currentflux curve of SRM used in
simulation.
Fig. 5. Inductance profile of SRM used in simulation.
B. System Configuration
After producing the SRM model
with a nonlinear characteristic by RMxprt, the overall speed control drive
system of the SRM in Fig.6 and 7 is implemented by Simplorer. DCLink voltage
is 220V and proportional gain and integral time of the speed controller are set
as 100Hz and 1ms, individually. In addition, the current
controller is configured by hysteresis controller.
Fig. 6. SRM drive system by Simplorer.
Fig. 7. Blockdiagram of SRM control system.
C. Simulation Result
Fig.8 and 9 shows the simulation results, individually based on
actual phase currents and estimated phase currents. Through the simulation
results, the good responses about the reference speed and disturbance can be
confirmed. And also the phase currents can be well estimated using DClink
current and rotor position.
V.
conclusion
The paper proposes the algorithm that three phase currents of SRM
can be estimated using only the DClink current sensor. Simplorer and RMxprt, which can support the multidomain simulation
of power, electronics, and system, are used to verify the proposed algorithm
applied to SRM drive. Comparing to the general drive system based on the phase
currents, it is verified through the simulation that the SRM drive system based
on the proposed estimation has the good performance in dynamic and steadystate
responses of the speed control. Using the DClink current, the multiphase
currents of SRM can be easily estimated and the number of current sensors can
be decreased to a single current sensor. RMxprt is used to model the SRM under
consideration of the nonlinear characteristics, and the model has the nearly
equal characteristics of the practical motor.
References
[1] RR. Krishna,
“Sensorless Operation of SRM Drives: R & D Status,” Proceedings of IEEE
IECON’01, pp.14981503, 2001.
[2] M. Ehsani, B. Fahimi, 'Elimination of Position Sensors in Switched
Reluctance Motor Drives: State of the Art and Future Trends,' IEEE Trans. on Industrial
Electronics, vol.49, no.1, pp.4047, Feb. 2002
[3] K. Ohyama, 'Recent Advances of Reluctance Torque Asisted Motors (in Japanese),' IEEJ Trans. on Industry
Application, vol.123, no.2, pp.6366, 2003
[4] PJ Lawenson et al.,
'VariableSpeed Switched Reluctance Motor,' IEE, vol. 127, pp.253165, July 1980
[5] V.N. Walivadekar, V.V.,Sr. Athani, G.N. Acharya, 'Equivalent
Circuit for Switched Reluctance Motor,' Proceedings of IEEE TENCON '93, pp.568571, Oct. 1993
[6] Swithced Reluctance Motor
Drives, Interec
Communications Inc., T.J.E. Miller, 1988
[7]
http://www.ansoft.kr, (Ansoft Korea HP)
[8] http://www.ansoft.com, (Ansoft
Headquarter HP)
Fig. 8. Simulation result based on phase
current information.
Fig. 9. Simulation result based on estimated
phase currents.