higher stepping rates.

3.9 SWITCHED RELUCTANCE MOTOR (SRM)

3.9.1 Introduction
The switched reluctance motor is a double salient, singly excited motor, i.e., it has
salient polesin both the rotor and stator, but only the stator carries windings. The rotor
has no windings, magnets. It works on variable reluctance principle. Operates on the
principle of Faraday's law of clectromagnetic induction.

Faraday's Law
Whenever the flux linkages associated with a electric circuit, an emf is induced in it.
Flux linkage à = NÙ
SRM are used in adjustable-speed drives because of the following features:
Motor is simple in construction with nowinding on rotor and simple concentrated
coils on stator.
" It can be run at high speeds (about 2 x 10 rpm) because of no windings on
rotor and rugged rotor construction.
" Stator windings can be cooled easily.
 Machínes
178 Special Electrical
unidirectional drive circuits.
operated from
" ItSRM
can be
operates successfully though at reduced output, even if one or more phases

due to some fault.

are out of circuit large number of stator and
manufactured with
" These motors
can be
volume.
rotor teeth,
give large torque per unit
they
Construction of SRM.

Block Diagram
Power

DC
Semiconductor SRM
Switching
Supply Circuitary

Current Signal

Controller RPS

Fig. 3.27

Constructional Diagram
A1
0= 0° Yoke

Stator Pole

B
C
B1
Rotor
a B2
B

Fig.3.28
Terminal Board

Aj4; B B; c{C;
Stator
Stator is made up of silicon steel stampings with inward projectedpoles.
number.
" The number of poles of the stator can odd
be either in even number or
Most of the motors have even number
of stator poles.
 Stepper Motors and
Switched Reluctance
poles carry field coils (o) Motors 17g
connected in series such stator windings. The
Allthese
poles
are
that mmfs are field coils of
phase
windings. Phase windings are additive and they areopposite
connected to the terminals called
of the motor
Rotor

made up of silicon steel

also

The
is
stampings
consists of rotor position sensor. Number with outward projected poles
rotor

The
shafi

or
number of stator poles.
of poles of the
from
the rotor are different
Figure 3.27 shows the block
diagram of SRM. DC supply is
semiconductor
switching circuitary, which given to the power
is connected to the
windings of SRM, various phase
anS which is mounted on a shaft
of the SRM, provides signal to the controller
about the position of the rotor with respect to the
Controller collects these information and also
reference axis.
reference speed
eon and off the concern
power semiconductor devices ofsignal and suitably
sIuch that the
desired phase winding is connected to the DC switching circuit.
The current signal is also a feedback to supply.
the controller circuit to unit the motor
current within permissible limits.

39.2 Principle of Operation

30°

Rotor b

Fig. 3.29

=)

aue n mum
Stator AjA and rotor poles a,a' ue in alignnent, heyposiuon ductase
Poes concerned ie, at thus
reluctance
B'
position so far as phase'A' is
phase winding 1s neither maximun nor munmum

In this
condition = ).
180 Special Electrical Machines

position, if the phase winding B is

Now there axists At this
then the motor develops atorque because of variable, reluctance e principle becauseenergized,

of
The motor develops atorque which is called as "electromagnetic ctorque which
L2B The direction of this torque is such that B,B and b,8 try
isequal to
2
to get alignment.
" If the torque is more than the opposing load and frictional torque, the rotor

occupies the position such that BB' and bb'

begins to rotate, when the shaft
because 1S zero.
is developed
in alignment, then no torque
ons.
off and phase winding 'c is turned
Now phase winding 'B' is switched
supply.
exists, the electromagnetic torque
The load experiences a torques as

developed OC The motor continues to rotate.

to 'C winding is zero, Then
" When the rotor moves further 30° the torque due
a continuous and
it is switched off and phase winding ´A' is energized. This is
starting motor.
cyclic process. Thus rotor starts. It is a self
the average
" The speed increases, the load torque requirement also changes, when
developed torque is more than the load torque.
" The rotor attains dynamicequilibrium condition when developed torque is equal
to load torque.
Aj
0= 60°

--.
B
C
B1
Rotor
B

Fig. 3.30
 Stepper Motors and Switched Reluctance Motors 181
At this steady state condition. the power
drawn from the mains is equal to time
rate of change of energy stored in
magnetic
When the load is increased. speed of the field.
motor tends to fall. So that power
balance 1s maintaincd, then toraue devcloped is to be
the current. Thus more power is drawn from the incrcased by increasing
mains vice versa takes place
when load is reduced. Thus clectrical to mechanical
place. power conversion takes

29.3 Voltage and Torque Equation of a SRM

Basic voltage equation of SRM
da
V= iR +
R’ resistance of phase winding
2’ flux linkage
da d(Li) dL
dt + io
dt

V= iR +L+i0

where ’ slope of magnetic curve

•L ’ incremental inductance
iR ’ resistive drop
L ’ emf due to incremental inductance
dL
io ’e’"Self emf" depends on currents speed and rate of change of
inductance with rotor angle.
Thus the equivalent circuit of SRM consists of each phase, a resistance, an
di
incremental inductance and self emf. L emf due to incremental inductance is zero.
inductance is
During the flat-top period, emf 'e' is constant. Atsome instant the
di will be constant.
Constant, 'e' will be zero and L
mainly on inductance and
Thus the equivalent circuit of SRM changes from being
emi. "X plying Eqn. (1) by(i) on both sides
JL ...2)
V,= 'R+i+to

watts.
V; ’ electrical energy supplied in
iR ’resistive loss.
incremental inductance.
Li- Power associated with
dr
 ei ’ Power due to self emf (i.e.)
(i) Power associated with change in stored energy.
mechanical.
(i) Power converted into

Stored energy in the magnetic field

=
2
Li?

" Power associated with change in stored energy = dWst

dt

2 2
di •L
= Li +
dt 2
Power converted into mechanical, ..3)
P, = Vi- PR - Power associated with change in stored
energy.
Substituting Eqn. (2) and (3) in (4), we get ..4)
di 1 dL
= Vi-iR- Li
di
=?R-iR+Li-Li +i'o
1
P m

P..
m
= oT
Troque developed by an SRM is

T= 2 L N-m
2
.:. Applications of SRM
Washing machine
Vacuum cleaners
" Fans
Future automobile applications
" Rototics control
applications.
3.9.4 Operating Modes
These are two types of operating
modes of SRM:
1. Single-pulse mode
2. Pulse-width
modulation (PWM) mode.
1. Single Pulse Mode
In single pulse mode, also called high speed mode, the current Iises in within limits
duringsmall interval of each phase excitation. This built up Current limit is due to
 Stepper Motors and Switched Reluctance Motors 183

winding indductance and motional counter emf generated in stator

waveform and variation of L with is winding. An ideal
CuTent
shown in Fig. 3.31(a).
When LO) begins to 0, switch is closed and current at once
reaches a const. value.
ahen L() stops rising, switch is opened and current decays to 0 immediately. During
time current pulse is const. and +ve, L(O) is rising and therefore
+ve torque is
produced. Actually ideal waveforms are non-existant.
In Fig. 3.31(6) V, should be applied to winding some what in advance of instant
when L(0) begins to rise. Low L allows i(0) to build up faster to large value before
torque producing region begins,

iL
i(0)
2(0)

L(e)

i(0)

Switch Switch + Vs
Close open

- Vs

(a) (b)
Fig. 3.31
motional emf switch is
After ,, current i0) begins to fall of rising L(0) and
phase winding. This
opened at ,. At this rotor angle ,, -ve voltage is applied to
extinction angle ,. Angle
cause rapid fall of current and i(0) finally reduces to 0 at
diode conduction angle of an
(&, -0,) is transistor conduction angle and (0, - 0,) is
inverter.
0,) and energy is feedback by motor
Source delivers energy to motor during (0, -
SOurce during (0, 0,). The angle (0, - ), called , (conduction angles), is an
to e, is switch off angle.
mportant control parameter. , is called switch on angle and
(8, -,) is called angle of advance. be applied much before
voltage to phase winding at switch off 0, should decays to 0' and
Lne -ve reduced. This is done to ensure that i(0)
c max. value of LO) is
Ve torque region is avoided.
2. PWM Mode: (For Chopping) period
mode,each phase winding gets excited for a
B o d e , also called
low-speed of motor and
current rise within acceptable ratings
sufficiently
long. In order to keep
Inverter components, a current limiting device is
incorporated before
SRM.
 Machines

Electrical
Special
184
3-¢ Diode
0000 SRM
bridge
3-¢ Diode C Rectifier
bridge
30 Rectificr

Controller
Rotor Position
Set Speed Signal Sensor
(a)

L(0)
i(0)

Imax

min

I-Vs
on Toff Ton Tof
(b)
Fig. 3.32
sensor in each phase so as to om
This is achieved by installing a current instants of powe
controls alternately the on and off
Current. The current sensor then a
current between permissible uppel
Converter components in order to hold the position is shown in
lower levels. The chopped phase current as a function of rotor
Fig. 3.32(b). phase
levels of choppedI
In Fig. 3.32, ma, and /mi, are upper and lower prescribed 3.32(b).
current and I, is its mean value. The variation is shown in Fig. mean
down, then
up or
If upper and lower limits of each phase current are shiftedControl ' s p e e

and hence
value of motor current would alter. This will results in torque
adjustment as desired.
Merits
1, Construction is simple and
robust.
2. Rotor carries no windings, no brushes. less
3. There is no
maintenance.
permanent magnets.
 Stepper Motors and Switched Reluctance Motors 185

4. No shoot through fault.

5. Switching circuitry is simple.
6. It is self-starting machine.
7. Possibility to get very high speed.
8. Operation of machine can be casily change from motoring mode to generating
mode.

Demerits

1. Stator phase winding should be capable of carrying magnetising current also.

2. It requires a rotor position sensor.
and motor and
3. No. of power Wires between the power semiconductor circuitry
no. of controlcables from controller to Dower semiconductor switching circuitry
are more.

4. For high speed current waveform has undesirable harmonics.

produces
5. For high speed operation developed torque has undesirable ripples which
noise.

CIRCUITS FOR SRM

3.10 POWER SEMICONDUCTOR SWITCHING
(POWER CONTROLLER SWITCHING CIRCUITS)
the application. One of the main aspects of
The selection of aconverter depends upon
converter design. The main objectives of the
the research inSRM drives has been the the drive.
performance of the drive and cost of
design of the converter are
Basic Requirements
other phases.
phase of SRM should be able to conduct independent of the
(i) Each before it steps into the
should be able to demagnetize the phase to
asamotor and should be able
(ii) The converter
machine is operating
generating region if the generating region if operated
as a
it steps into the
excite the phase before
generator. chopping period to
be able to freewheel during the
(iii) The converter should
reduce the switching frequency. energy from the
be able to utilize the demagnetization using
(iv) The converter
should
either feeding it back to the source or
useful way by
off going phase in a
conducting phase.
it in the next
3.10.1 Clamic Converter
converter with two
converter topologies
the clamin bridge
versatile SRM Fig. 3.33.
Ihe most perphase shown in mode is
switches and two diodes
T, and I, are in onstate. This phase
pOwer phase A, that the
conduction mode of stator pole overlap. So increase this
During the the rotor and to
initiated before the start of phase inductance begins
Usually value, before the
OVert reaches
the reference switching one of
ripple. value by
helps to reduce the torque
reference
maintained at the
commutation
current is continuously on
till the
the
m this mode
while leaving
the other one
pnase switches
time is reached.
186 Special Electrical Machines

A
B. C
DC
Supply Q0 D,

A B

Fig. 3.33

Both of the phase switches are turned off to initiate commutation. The phase
starts, todemagnetize through the two diode and the energy transfers from the motor
phase to the dc source.
During the commutation the off going phase winding for a voltage of -V,
3.10.1.1 Operation
Phase winding Ain connected to dc supply through power semiconductor switches T,
and T,, depending upon the rotor position. When phase winding Ais to be energized
the device T, and T, are turned on. When T, and T, are turned off, the phase winding
A is to be disconnected from the supply. But stored energy in the
tends to maintain the current in the same direction. This current phase winding A
passes through diode
D, and D, to the supply. Thus stored energy is feedback to the mains,
windings B and C are switched on and off from the supply in a then phase
cycle manner.
Merits

(i) Controlof each phase is completely

independent of the other phases.
(ii) The converter is able to Freewheelduring the
which helps to reduce the switching frequencychopping
and then
period of low speeds
the switching laws o
the converter.
(ii) The energy from the off going phase is feedback
useful utilization of the energy. to the source which results
Demerits
High number of switches required in
and also used for low voltage each phase which makes the converter expensive

application.
 Stepper Motors and Switched Reluctance Motors 187

40.2 (n + 1) Power Semiconductor Switches and (n + 1) Diodes per

Phase, where 'n' is Number of Phase Windings
Cioure 3.34 shows an (n + 1) power semiconductor switches and (n + 1) diodes per
phase where 'n' is number of phase windings.

D,A D,A

A1
B

A C D

T,

Fig. 3.34
The commutation is initiated by turning both the common switch and the phase
switch off.
and the
The phase demagnetizes through the common diode and the phase diode
energy is transferred back to the source. During the commutation, the phase winding
sees a voltage of -V:
completey
The next phase cannot be turned on until the off going phase has not been
demagnetized.
common switch has to be
In order to start the magnetization of the next phase
turned on.
off-going phase forcing
This gives a path to the demagnetization currents of the
demagnetization prOcess.
zero voltage across it and therefore allowing the

3.10.2.1 Operation of the Circuit

diodes when IGBTs Tand T, are turned on.
lT 3O motor requires 4 transistors our
winding 'A' connected to the de supply.
Fhase winding 'A' is energized then phase winding 'A' is disconnected from the
When IGBTS T and T, are turned off phase
fedback to the mains through diodes D and
supply.But energy stored in the winding in
D: turned on, the phase winding B is conneted to
the
When IGBTs T and T, over
fedback tode
T and T, are turned off. Stored energy in phase winding B is
dans.
mains through diodes D and D,
Similarly phase winding C" is energized
the cycle is repeated.
188 Special Electrical Machines

Merits
of switching
devices which reduces the cost
lower number
(i) The converter uses
chopping then
of the converter.
to freewheel during the reducing the
(ii) The converter is able losses.
and
switching frequency
switching
devices and the diodes are Vdc which is
all the
(ii) Voltage rating of
relatively low. transferred back in
to the source which
off going phase is
improvesthe
(iv) The energy for the
results in useful
utilization of the energy and also efficiency.
Demerits
going phase is still demagnetizing
magnetize a phase while the off commutation.
() Disabilityto torque ripple during
which results in higher enough becaiuce
higher speed theoff going phase cannot be -energized fast
(ii) A keeps turning on
intermediately disabling forced
the common switch T
demagnetization. switchine
for allthe phase and thus,has higher
(iii) The common switch conducts
stress.

Wires
3.10.3 Phase Winding using Bipolar
+4
B"

0 00 0 00 rO0 0 0
T; D, T, D;

Fig. 3.35

3.10.4 Split Power Supply Converter

Operation: Figure 3.36 shows the circuit of split power supply converts the main
power supply 2V is split into two helver using split capacitors. During conduchon
energy in supplied to the phases by one half the power supply. During commutation
period the phases demagnetize into other half of the power supply.
When IGBTT,is turned on the phase winding Iis energized by capacitor Cwhen
IGBT T, isturned off. Stored energy is the phase winding I in fedback to the capacitor
C, through diode D,.
When IGBT T, in turned on by capacitor C, and phase winding 2 is energz
when IGBT T, is turned off. Stored energy in the widing 2 is fedback to the capacito
C through diode D,. The similar operation taken place in the remaining winding also.
 Stepper Motors and Switched Reluctance Motors 189

3.10.4.7 Operation

Figure
3.36 showsthe above switching circuit of phase winding using bipolar wires.
pole carriers a coil using bipolar wire. Each phase group has two exactly
Each stator
áentical phase windings and they have a common magnetic case T, is turned on, phase
winding Ais energised winding is conneccted to the dc mains.

00 AD;
00
2 Vcc
00 00 00

Fig. 3.36 Split power supply Converter

disconnected from the supply then the stored
energy
T,,is turned off winding A is
transferred to the second coil through mutual
in the first coilof phase winding A is
winding A is fed back to the supply through
Induction. Then the stored energy in phase
place in the B and Cphase windings.
diode D,. The similar operation takes
Merits:
number of switching devices thus reducing the cash
() The converter used lower
on the converter.
commutation.
last demagnetization of phases during
(i) The converter allows

Demerits:
chopping as the phases. See
the -V this
not possible during
() Free wheding in current and torque during chopping.
causes a higher ripples in spikes
coupling between the two winding causes voltage
(0) The implication in the
during turn-off.
with the auxiliary winding are unacceptably high
anociated
( )The clipper lower
for many applications.
 Machines
Electrical
190 Special biased and the energy from
forward
period, D, is the
commulation
capacitor C
During the dump
transferred to the transferred
into the
machine phase is capacitor C is source
energy from
the dump Energy is transferred to the by
excess , is off. source
The when the switch
9,
turning on the switch
through D
Merits:
lower number ofswitching devices and has only one switch
topology uses
() This
voltage drop. regenerative
capability.
has full commutation
(i)) The converter demagnetization of phases during
(i) There is faster
allow fast
Demerits:

voltage is maintained 2 V, to demagnetization b

(i) Dump
capacitor
inductor in the dump circuit and also the voltage
capacitor and an
use of a voltage.
rating of the devices
is twice the by
dump capacitor voltage C, and controlar. The dump switch
(i) Monitoring of the does
converter very complicated and also the converter
0, makes the
allow free wheeling.

Merits:
switching devices.
() Itrequires lower number of
during commutation.
(ii) Faster demagnetization at phases
Demerits:
voltage
() During chopping, free wheeling is not possible as the phase see the
V2 this causes higher switching frequency and more lesses.
(ii) This is not feasible for low voltage application.
(iii) The converter is less fault tolerant and fault in any phase will unbalance the
other phase that in connected toit.

3.10.5 Dump 'C' Converter

L
O0 0 0

D,
D,

D
C

Fig. 3.37 Basic 'C dump

converter circuit
 Stepper Motors and Switched Reluctance Motors 191

Figure 3.37 above shows the dump C converter switching circuit for SRM. The
int a capacitor to achieve fast
energy from the off going phase in dumped into
demagnetization. This energy is then returned to the source from the capacitor.
The conduction of the phase initiated by turning on the phase switch
T, for l
by turned off the
connected in series with the phase. The phases are demagnetized
phase switch T.
 Switched Reuctance Mots
Flux based method
Passive
methods