View
225
Download
0
Category
Preview:
Citation preview
8/13/2019 Notes-Single Phase Motors
1/156
Chapter (9)
Single-Phase Motors
Introduction
As the name suggests, these motors are used on single-phase supply. Single-phase
motors are the most familiar of all electric motors because they are extensively
used in home appliances, shops, offices etc. It is true that single-phase motors are
less efficient substitute for 3-phase motors but 3-phase power is normally not
available except in large commercial and industrial establishments. Since electric
power was originally generated and distributed for lighting only, millions of homeswere given single-phase supply. This led to the development of single-phase
motors. ven where 3-phase mains are present, the single-phase supply may be
obtained by using one of the three lines and the neutral. In this chapter, we shall
focus our attention on the construction, wor!ing and characteristics of commonly
used single-phase motors.
9.1 Types of Single-Phase Motors
Single-phase motors are generally built in the fractional-horsepower range andmay be classified into the following four basic types"
#. Single-phase induction motors
$i% split-phase type $ii% capacitor type
$iii% shaded-pole type
&. A.'. series motor or universal motor
3. (epulsion motors
$i% (epulsion-start induction-run motor$ii% (epulsion-induction motor
). Synchronous motors
$i% (eluctance motor $ii% *ysteresis motor
9.2 Single-Phase Induction Motors
A single phase induction motor is very similar to a 3-phase s+uirrel cage induction
motor. It has $i% a s+uirrel-cage rotor identical to a 3-phase motor and$ii% a single-phase winding on the stator.
8/13/2019 Notes-Single Phase Motors
2/156
8/13/2019 Notes-Single Phase Motors
3/156
nli!e a 3-phase induction motor, a single-phase induction motor is not self-
starting but re+uires some starting means. The single-phase stator winding
produces a magnetic field that pulsates in strength in a sinusoidal manner. The
field polarity reverses after each half cycle but the field does not rotate.
'onse+uently, the alternating flux cannot produce rotation in a stationary s+uirrel-cage rotor. *owever, if the rotor of a single-phase motor is rotated in one direction
by some mechanical means, it will continue to run in the direction of rotation. As
a matter of fact, the rotor +uic!ly accelerates until it reaches a speed slightly
below the synchronous speed. nce the motor is running at this speed, it will
continue to rotate even though single-phase current is flowing through the stator
winding. This method of starting is generally not convenient for large motors. or
can it be employed fur a motor located at some inaccessible spot.
8/13/2019 Notes-Single Phase Motors
4/156
/ig. $0.#% shows single-phase induction motor having a s+uirrel cage rotor and asingle-phase distributed stator winding. Such a motor inherently docs not develop
any starting tor+ue and, therefore, will not start to rotate if the stator winding is
connected to single-phase a.c. supply. *owever, if the rotor is started by auxiliary
means, the motor will +uic!ly attain me final speed. This strange behaviour of
single-phase induction motor can be explained on the basis of double-field
revolving theory.
9.3 Double-ield !e"ol"ing Theory
The double-field revolving theory is proposed to explain this dilemma of no
tor+ue at start and yet tor+ue once rotated. This theory is based on the fact that an
alternating sinusoidal flux $1 mcos t% can be represented by two revolving
fluxes, each e+ual to one-half of the maximum value of alternating flux $i.e.,
m2&% and each rotating at synchronous speed $s1 #& f24, 1 &f% in opposite
directions.
The above statement will now be proved. The
instantaneous value of flux due to the stator current
of a single-phase induction motor is given by5
=m
coswt
'onsider two rotating magnetic fluxes # and & each of
magnitude m2& and rotating in opposite directions with
angular velocity 6See /ig. $0.&%7. 8et the two fluxes start
rotating from 9 axis at
ig.#9.1$
ig.#9.2$
8/13/2019 Notes-Single Phase Motors
5/156
8/13/2019 Notes-Single Phase Motors
6/156
t 1 . After time t seconds, the angle through which the flux vectors have rotated is at.
(esolving the flux vectors along-9-axis and :-axis, we have,
8/13/2019 Notes-Single Phase Motors
7/156
(
cost)+ = cos t
mmTotal 9-component = cos t + cos t = cos t
m&&
mmTotal :-component = sin t sin t = &&
&&(esultant flux, =mm
Thus the resultant flux vector is 1 m cos t along 9-axis. Therefore, an
alternating field can be replaced by two relating fields of half its amplitude
rotating in opposite directions at synchronous speed. ote that the resultant
vector of two revolving flux vectors is a stationary vector that oscillates in
length with time along 9-axis. ;hen the ig.#9.3$ rotating flux vectors are inphase 6See /ig.
$0.3 $i%%7, the resultant vector is 1 m5 when out of phase by #
8/13/2019 Notes-Single Phase Motors
8/156
ig.#9.%$
#ii$ !otor running
ow assume that the rotor is started by spinning the rotor or by using auxiliary circuit, in say
cloc!wise direction. The flux rotating in the cloc!wise direction is the forward rotating flux
$f% and that in the other direction is the bac!ward rotating flux $ b%. The slip w.r.t. the forward
flux will be
-ss == s
f s
where s 1 synchronous speed 1 speed of rotor in the direction of
forward flux
The rotor rotates opposite to the rotation of the bac!ward flux. Therefore, the slip w.r.t. the
bac!ward flux will be
-$-% + & - +s ssss = ==
b ss s
& $ %ss== & s
s s
s = & sb
Thus fur forward rotating flux, slip is s $less than unity% and for bac!ward rotating flux, the slip
is & s $greater than unity%. Since for usual rotor resistance2reactance ratios, the tor+ues at slips
of less than unity arc greater than those at slips of more than unity, the resultant tor+ue will be
in the direction of the rotation of the forward flux. Thus if the motor is once started, it will
develop net tor+ue in the direction in which it has been started and will function as a motor.
8/13/2019 Notes-Single Phase Motors
9/156
8/13/2019 Notes-Single Phase Motors
10/156
/ig. $0.@% shows the rotor circuits for the forward and bac!ward rotating fluxes.
8/13/2019 Notes-Single Phase Motors
11/156
8/13/2019 Notes-Single Phase Motors
12/156
&
8/13/2019 Notes-Single Phase Motors
13/156
1 (
8/13/2019 Notes-Single Phase Motors
14/156
&
8/13/2019 Notes-Single Phase Motors
15/156
2&, where (
8/13/2019 Notes-Single Phase Motors
16/156
&
8/13/2019 Notes-Single Phase Motors
17/156
is the standstill rotor resistance i.e., r
8/13/2019 Notes-Single Phase Motors
18/156
&
8/13/2019 Notes-Single Phase Motors
19/156
is e+ual to
8/13/2019 Notes-Single Phase Motors
20/156
half the standstill rotor resistance. Similarly, x
8/13/2019 Notes-Single Phase Motors
21/156
&
8/13/2019 Notes-Single Phase Motors
22/156
1 9
8/13/2019 Notes-Single Phase Motors
23/156
&
8/13/2019 Notes-Single Phase Motors
24/156
2& where 9
8/13/2019 Notes-Single Phase Motors
25/156
&
8/13/2019 Notes-Single Phase Motors
26/156
is the standstill rotor reactance. At standstill, s 1 # so that impedances of the two circuits are
8/13/2019 Notes-Single Phase Motors
27/156
e+ual. Therefore, rotor currents are e+ual i.e., I
8/13/2019 Notes-Single Phase Motors
28/156
&f
8/13/2019 Notes-Single Phase Motors
29/156
1 I
8/13/2019 Notes-Single Phase Motors
30/156
&b
8/13/2019 Notes-Single Phase Motors
31/156
. *owever, when the rotor rotates, the impedances of the two rotor circuits are une+ual and the
rotor current
8/13/2019 Notes-Single Phase Motors
32/156
I
8/13/2019 Notes-Single Phase Motors
33/156
&b
8/13/2019 Notes-Single Phase Motors
34/156
is higher $and also at a lower power factor% than the rotor current I
8/13/2019 Notes-Single Phase Motors
35/156
&f
8/13/2019 Notes-Single Phase Motors
36/156
. Their m.m.f.s, which oppose the stator m.m.f.s, will result in a reduction of the bac!ward
rotating flux. 'onse+uently, as speed increases, the forward flux increases, increasing the
driving tor+ue while the bac!ward flux decreases, reducing the opposing tor+ue. The motor-
+uic!ly accelerates to the final speed.
8/13/2019 Notes-Single Phase Motors
37/156
ig.#9.&$
9.% Ma'ing Single-Phase Induction Motor Self-Starting
The single-phase induction motor is not self-starting and it is
undesirable to resort to mechanical spinning of the shaft or pulling a
belt to start it. To ma!e a single-phase induction motor self-starting, we
should somehow produce a revolving stator magnetic field. This may
be achieved by converting a single-phase supply into two-phase supply
through the use of an additional winding. ;hen the motor attains
sufficient speed, the starting means $i.e., additional winding% may be
removed depending upon the type of the motor. As a matter of fact,single-phase induction motors are classified and named according to
the method employed to ma!e them self-starting.
$i% Split-phase (otors-started by two phase motor action through the use of an
auxiliary or starting winding.
8/13/2019 Notes-Single Phase Motors
38/156
8/13/2019 Notes-Single Phase Motors
39/156
$ii
8/13/2019 Notes-Single Phase Motors
40/156
%
8/13/2019 Notes-Single Phase Motors
41/156
8/13/2019 Notes-Single Phase Motors
42/156
)apacitor (otors
8/13/2019 Notes-Single Phase Motors
43/156
-started by two-phase motor action through the use of an auxiliary winding and a capacitor.
8/13/2019 Notes-Single Phase Motors
44/156
8/13/2019 Notes-Single Phase Motors
45/156
ig.#9.*$
8/13/2019 Notes-Single Phase Motors
46/156
8/13/2019 Notes-Single Phase Motors
47/156
$iii
8/13/2019 Notes-Single Phase Motors
48/156
%
8/13/2019 Notes-Single Phase Motors
49/156
8/13/2019 Notes-Single Phase Motors
50/156
Shaded-pole (otors
8/13/2019 Notes-Single Phase Motors
51/156
-started by the motion of the magnetic field produced by means of a shading coil around a
portion of the pole structure.
8/13/2019 Notes-Single Phase Motors
52/156
&&
9
+:
= $m% +$% =
m
and
=
&
22
ig.
#9.+$
9.& !otating Magnetic ield ro( 2-Phase Supply
As with a 3-phase supply, a &-phase balanced supply also produces a rotating magnetic field of
constant magnitude. ;ith the exception of the shaded-pole motor, all single-phase induction
motors are started as &-phase machine. nce so started, the motor will continue to run on
single-phase supply.
8et us see how &-phase supply produces a rotating magnetic field of constant magnitude. /ig.
$0.# $i%% shows &-pole, &-phase winding. The phases 9 and : are energi>ed from a two-phase
source and currents in these phases arc indicated as Ix and Iy 6See /ig. $0.# $ii%%7. (eferring to
/ig. $0.# $ii%%, the fluxes produced by these currents arc given by5
:
=m
sin t and 9
=m
sin$ wt + 0.% 1fm
coswt
*ere m is the maximum flux due to either phase. ;e shall now prove that this &-phase supply
produces a rotating magnetic field of constant magnitude e+ual to m.
$i% At instant # 6See $/ig. 0.# $ii%% and /ig. $0.# $iii%%7, the current is >ero in phase : and
maximum in phase 9. ;ith the current in the direction shown, a resultant flux is establishedtoward the right. The magnitude of the resultant flux is constant and is e+ual to m as proved
under"
At instant #, wt = . = and =ig.#9.,$
: 9m
& & (esultant flux, r=
$ii% At instant & 6See /ig. $0.# $ii%% and /ig. $0.# $iii%%7, the current is still in the same
direction in phase 9 and an e+ual current flowing in phase :. This establishes a resultant fluxof the same value $i.e., r 1 m% as proved under"
m mAt instant &, t = )@. =
&
(esultant flux, r= $
9% + $
:%
. & . & m
.
m
.
=..+..= m & &
8/13/2019 Notes-Single Phase Motors
53/156
8/13/2019 Notes-Single Phase Motors
54/156
ote that resultant flux has the same value $i.e.
8/13/2019 Notes-Single Phase Motors
55/156
8/13/2019 Notes-Single Phase Motors
56/156
m
8/13/2019 Notes-Single Phase Motors
57/156
% bu
8/13/2019 Notes-Single Phase Motors
58/156
t
8/13/2019 Notes-Single Phase Motors
59/156
turned )@
8/13/2019 Notes-Single Phase Motors
60/156
=
8/13/2019 Notes-Single Phase Motors
61/156
cloc!wise from position #
8/13/2019 Notes-Single Phase Motors
62/156
.
8/13/2019 Notes-Single Phase Motors
63/156
$iii% At instant 3 6See /ig. $0.# $ii%% and /ig. $0.# $iii%%7, the current in
phase. 9 has decreased to >ero and current in phase : has increased to
maximum. This establishes a ig.#9.9$resultant flux downward as proved under"
ig.#9.1$
8/13/2019 Notes-Single Phase Motors
64/156
8/13/2019 Notes-Single Phase Motors
65/156
At instant 3, wt
8/13/2019 Notes-Single Phase Motors
66/156
=
8/13/2019 Notes-Single Phase Motors
67/156
0
8/13/2019 Notes-Single Phase Motors
68/156
.
8/13/2019 Notes-Single Phase Motors
69/156
8/13/2019 Notes-Single Phase Motors
70/156
8/13/2019 Notes-Single Phase Motors
71/156
8/13/2019 Notes-Single Phase Motors
72/156
=
8/13/2019 Notes-Single Phase Motors
73/156
8/13/2019 Notes-Single Phase Motors
74/156
and f
8/13/2019 Notes-Single Phase Motors
75/156
=
8/13/2019 Notes-Single Phase Motors
76/156
8/13/2019 Notes-Single Phase Motors
77/156
&&
9
+ $:
% = $% + $m
% =m
r 9: . ... m
ig.#9.11$
&&
9
+:
= $m
%
+ $%
=m
: m 9
& & r=
ote that resultant flux has now turned 0= cloc!wise from position #. The reader may
note that in the three instants considered above, the resultant flux is constant and is
e+ual to m. *owever, this constant resultant flux is shining its position $cloc!wise in
this case%. In other words, the rotating flux is produced. ;e shall continue to consider
other instants to prove this fact.
$iv% At instant ) 6See /ig. $0.# $ii%% and /ig. $0.# $iii%%7, the current in phase 9 has reversed
and has the same value as that of phase :. This establishes a resultant flux e+ual to m turned
)@= cloc!wise from position 3.
m mAt instant ), t =#3@.:
= and 9
= &&
& & .m
.&
.m
.&
=+= + = & &
$v% At instant @ 6See /ig. $0.# $ii%% and /ig. $0.# $iii%%7, the current in phase 9 is
maximum and in phase : is >ero. This establishes a resultant flux e+ual to m toward left $or
0= cloc!wise from position 3%.
At instant @, wt 1#
8/13/2019 Notes-Single Phase Motors
78/156
8/13/2019 Notes-Single Phase Motors
79/156
9.*
8/13/2019 Notes-Single Phase Motors
80/156
Split-Phase Induction Motor
8/13/2019 Notes-Single Phase Motors
81/156
The stator of a split-phase induction motor is provided with an auxiliary or starting winding S in
addition to the main or running winding D. The starting winding is located 0= electrical from
the main winding 6See /ig. $0.#3 $i%%7 and operates only during the brief period when the motorstarts up. The two windings are so resigned that the starting winding S has a high resistance and
relatively small reactance while the main winding D has relatively low resistance and large
reactance as shown in the schematic connections in /ig.
$0.#3 $ii%%. 'onse+uently, the currents flowing in the two windings have reasonable phase
difference c $&@= to 3=% as shown in the phasor diagram in /ig. $0.#3 $iii%%.
ig.#9.13$
peration
$i% ;hen the two stator windings are energi>ed from a single-phase supply, the main
winding carries current Im while the starting winding carries current Is.
$ii% Since main winding is made highly inductive while the starting winding highly
resistive, the currents Im and Is have a reasonable phase angle a $&@= to 3=% between them asshown in /ig. $0.#3 $iii%%. 'onse+uently, a wea! revolving field approximating to that of a &-
phase machine is produced which starts the motor. The starting tor+ue is given by5
T = !I Isin as ms
where ! is a constant whose magnitude depends upon the design of the motor.
$iii% ;hen the motor reaches about C@? of synchronous speed, the centrifugal switch opens the
circuit of the starting winding. The motor then operates as a single-phase induction motor andcontinues to accelerate till it reaches the
8/13/2019 Notes-Single Phase Motors
82/156
8/13/2019 Notes-Single Phase Motors
83/156
normal speed. The normal speed of the motor is below the synchronous speed and
depends upon the load on the motor.
8/13/2019 Notes-Single Phase Motors
84/156
)haracteristics
$i% The sinning tor+ue is #@ to & times the full-loud tor+ue mid $lie starting current is B to ing reactance(H& 1 resistance of the rotor referred to the stator9H& 1 lea!age reactance of the rotor referred to the stator
revolving theory.
$i% t standstill. At standstill, the motor is simply a transformer with its secondary short-
circuited. Therefore, the e+uivalent circuit of single-phase motor at standstill will be as shown in
/ig. $0.#
8/13/2019 Notes-Single Phase Motors
106/156
8/13/2019 Notes-Single Phase Motors
107/156
total magneti>ing reactance. ote that in the e+uivalent circuit, the core loss has been
neglected. *owever, core loss can be represented by an e+uivalent resistance in parallel
with the magneti>ing reactance.
8/13/2019 Notes-Single Phase Motors
108/156
ow f 1 ).)) f f5 b 1 ).)) f b
At standstill, f 1 b. Therefore, f 1 b.
J# E f K b 1 I#Lf K I#Lb
where Lf 1 impedance of forward parallel branch Lb 1
impedance of bac!ward parallel branch
$ii% !otor running. ow consider that the motor is pinning at some speed in the direction
of the forward revolving field, the slip being s. The rotor current produced by the forward field
will have a fre+uency sf where f is the stator fre+uency. Also, the rotor current produced by the
bac!ward field will have a fre+uency of $& s%f. /ig. $0.#0% shows the e+uivalent circuit of asingle-phase induction motor when the rotor is rotating at slip s. It is clear, from the e+uivalent
circuit that under running conditions, f becomes much greater than b because the term (H&2&s
increases very much as s tends towards >ero. 'onversely, M falls because the term (H&2&$& s%
decreases since $& s% tends toward &. 'onse+uently, the forward field increases, increasing the
driving tor+ue while the bac!ward field decreases reducing the opposing tor+ue.
ig.#9.19$
Total impedance of the circuit .is given by5
8/13/2019 Notes-Single Phase Motors
109/156
8/13/2019 Notes-Single Phase Motors
110/156
L
8/13/2019 Notes-Single Phase Motors
111/156
r
8/13/2019 Notes-Single Phase Motors
112/156
1 L
8/13/2019 Notes-Single Phase Motors
113/156
#
8/13/2019 Notes-Single Phase Motors
114/156
K L
8/13/2019 Notes-Single Phase Motors
115/156
f
8/13/2019 Notes-Single Phase Motors
116/156
K L
8/13/2019 Notes-Single Phase Motors
117/156
b
8/13/2019 Notes-Single Phase Motors
118/156
where L
8/13/2019 Notes-Single Phase Motors
119/156
#
8/13/2019 Notes-Single Phase Motors
120/156
1 (
8/13/2019 Notes-Single Phase Motors
121/156
#
8/13/2019 Notes-Single Phase Motors
122/156
K 9
8/13/2019 Notes-Single Phase Motors
123/156
#
8/13/2019 Notes-Single Phase Motors
124/156
f(H . 9 9H .
Lb =&
.m&
.9 . (H 9H .
m& & .+ .& &s &L =
& m&+.+.&s &&
9 (H 9H
m.
&&.
.+ .&
&$& s% &
(H 99H+.+.
&$& s%
&&
I
#
= J
#
2 L
r
9.11 .). Series Motor or ni"ersal Motor
A d.c. series motor will rotate in the same direction regardless of the polarity of the supply. ne
can expect that a d.c. series motor would also operate on a single-phase supply. It is then called
an a.c. series motor. *owever, some changes must be made in a d.c. motor that is to operate
satisfactorily on a.c. supply. The changes effected are"
$i% The entire magnetic circuit is laminated in order to reduce the eddy current loss. *ence
an a.c. series motor re+uires a more expensive construction than a d.c. series motor.
$ii% The series field winding uses as few turns as possible to reduce the reactance of the field
winding to a minimum. This reduces the voltage drop across the field winding.
$iii% A high field flux is obtained by using a low-reluctance magnetic circuit.
$iv% There is considerable spar!ing between the brushes and the commutator when the motor is
used on a.c. supply. It is because the alternating flux establishes high currents in the coils short-
circuited by the brushes. ;hen the short-circuited coils brea! contact from the commutator,
excessive spar!ing is produced. This can be eliminated by using high-resistance leads to
connect the coils to the commutator segments.
)onstruction
8/13/2019 Notes-Single Phase Motors
125/156
8/13/2019 Notes-Single Phase Motors
126/156
The construction of en a.c. series motor is very similar to a d.c. series motor except that above
modifications are incorporated 6See /ig. $0.&%7. Such a motor can be operated either on a.c. or
d.c. supply and the resulting tor+ue-speed curve is about the same in each case. /or this reason,
it is sometimes called a universal motor.
8/13/2019 Notes-Single Phase Motors
127/156
8/13/2019 Notes-Single Phase Motors
128/156
$i% brushes are not connected to supply but are short-circuited 6See /ig.
$0.%7. 'onse+uently, currents are induced in the armature conductors by
transformer action.
$ii% the field structure has non-salient pole
construction.Fy adusting the position of short-circuited brushes on the commutator, the
starting tor+ue can be developed in the motor.
)onstruction
The field of stator winding is wound li!e the main winding of a split-phase motor
and is connected directly to a single-phase source. The armature or rotor is similar
to a d.c. motor armature with drum type winding connected to a commutator $not
shown in the figure%. *owever, the brushes are not connected to supply but are
connected to each other or short-circuited. Short-circuiting the brushes effectively
ma!es the rotor into a type of s+uirrel cage. The maor difficulty with an ordinary
single-phase induction motor is the low starting tor+ue. Fy using a commutator
motor with brushes short-circuited, it is possible to vary the starting tor+ue by
changing the brush axis. It has also better power factor than the conventional
single-phase motor.
ig.#9.21$
Principle of operation
The principle of operation is illustrated in /ig. $0.% which shows a two-pole
repulsion motor with its two short-circuited brushes. The two drawings of /ig.$0.% represent a time at which the field current is increasing in the direction
shown so that the left-hand pole is -pole and the right-hand pole is S-pole at the
instant shown.$i% In /ig. $0. $i%%, the brush axis is parallel to the stator field. ;hen
the stator winding is energi>ed from single-phase supply, e.m.f. is induced in
the armature conductors $rotor% by induction. Fy 8en>Gs law, the direction of
the e.m.f. is such that the magnetic effect of the resulting armature currents
will oppose the increase in flux. The direction of current in armature
conductors will be as shown in /ig. $0. $i%%. ;ith the brush axis in theposition shown in /ig. $0. $i%%, current will flow from brush F to
8/13/2019 Notes-Single Phase Motors
129/156
8/13/2019 Notes-Single Phase Motors
130/156
brush A where it enters the armature and flows bac! to brush F through the
two paths A'F and AF. ;ith brushes set in this position, half of the
armature conductors under the -pole carry current inward and half carry
current outward. The same is true under S-pole. Therefore, as much tor+ue is
developed in one direction as in the other and the armature remainsstationary. The armature will also remain stationary if the brush axis is
perpendicular to the stator field axis. It is because even then net tor+ue is
>ero.
8/13/2019 Notes-Single Phase Motors
131/156
$ii% If the brush axis is at some angle other than = or 0= to the axis of the
stator field, a net tor+ue is developed on the rotor and the rotor accelerates to its
final speed. /ig. $0. $ii%% represents the motor at the same instant as that in /ig.
$0. $i%% but the brushes have been shifted cloc!wise through some angle from
the stator field axis. ow e.m.f. is still induced in the direction indicated in /ig.
$0. $i%% and current flows through the two paths of the armature winding frombrush A to brush F. *owever, because of the new brush positions, the greater part
of the conductors under the -pole carry current in one direction while the greater
part of conductors under S-pole carry current in the opposite direction. ;ith
brushes in the position shown in /ig. $0. $ii%, tor+ue is developed in the
cloc!wise direction and the rotor +uic!ly attains the final speed.
$iii% The direction of rotation of the rotor depends upon the direction in which the
brushes are shifted. If the brushes are shifted in cloc!wise direction from thestator field axis, the net tor+ue acts in the cloc!wise direction and the rotor
accelerates in the cloc!wise direction. If the brushes are shifted in anti-cloc!wise
direction as in /ig. $0.&&%. the armature current under the pole faces is reversed
and the net tor+ue is developed in the anti-cloc!wise direction. Thus a repulsion
motor may be made to rotate in either direction depending upon the direction in
which the brushes are shifted.
$iv% The total armature tor+ue in a repulsion motor can be shown to be
Ta
. sin &
where 1 angle between brush axis and stator field axis /or
maximum tor+ue, &1 0= or 1 )@=
Fig.(9.22)
8/13/2019 Notes-Single Phase Motors
132/156
8/13/2019 Notes-Single Phase Motors
133/156
Thus adusting
8/13/2019 Notes-Single Phase Motors
134/156
8/13/2019 Notes-Single Phase Motors
135/156
to )@
8/13/2019 Notes-Single Phase Motors
136/156
=
8/13/2019 Notes-Single Phase Motors
137/156
at starting, maximum tor+ue can be obtained during the starting period. *owever,
8/13/2019 Notes-Single Phase Motors
138/156
8/13/2019 Notes-Single Phase Motors
139/156
has to be adusted to give a suitable running speed.
8/13/2019 Notes-Single Phase Motors
140/156
)haracteristics
$i% The repulsion motor has characteristics very similar to those of an a.c. series motor i.e.,
it has a high starting tor+ue and a high speed at no load.
$ii% The speed which the repulsion motor develops for any given load will depend upon the
position of the brushes.
$iii% In comparison with other single-phase motors, the repulsion motor has a high starring
tor+ue and relatively low starting current.
9.13 !epulsion-Start Induction-!un Motor
Sometimes the action of a repulsion motor is combined with that of a single-phase induction
motor to produce repulsion-start induction-run motor $also called repulsion-start motor%. The
machine is started as a repulsion motor with a corresponding high starting tor+ue. At some
predetermined speed, a centrifugal device short-circuits the commutator so that the machine
then operates as a single-phase induction motor.
The repulsion-start induction-run motor has the same general construction of a repulsion motor.
The only difference is that in addition to the basic repulsion-motor construction, it is e+uipped
with a centrifugal device fitted on the armature shaft. ;hen the motor reaches C@? of its full
pinning speed, the centrifugal device forces a short-circuiting ring to come in contact with the
inner surface of the commutator. This snort-circuits all the commutator bars. The rotor then
resembles s+uirrel-cage type and the motor runs as a single-phase induction motor. At the same
time, the centrifugal device raises the brushes from the commutator which reduces the wear of
the brushes and commutator as well as ma!es the operation +uiet.
)haracteristics
$i% The starting tor+ue is &.@ to ).@ times the full-load tor+ue and the starting current is
3.C@ times the full-load value.
$ii% ue to their high starting tor+ue, repulsion-motors were used to operate devices such as
refrigerators, pumps, compressors etc.
*owever, they posed a serious problem of maintenance of brushes, commutator arid the
centrifugal device. 'onse+uently, manufacturers have stopped ma!ing them in view of the
development of capacitor motors which are small in si>e, reliable and low-priced.
8/13/2019 Notes-Single Phase Motors
141/156
8/13/2019 Notes-Single Phase Motors
142/156
9.1%
8/13/2019 Notes-Single Phase Motors
143/156
!epulsion-Induction Motor
8/13/2019 Notes-Single Phase Motors
144/156
The repulsion-induction motor produces a high starting tor+ue entirely due to repulsion motor
action. ;hen running, it functions through a combination of induction-motor and repulsion
motor action.
8/13/2019 Notes-Single Phase Motors
145/156
8/13/2019 Notes-Single Phase Motors
146/156
)onstruction
8/13/2019 Notes-Single Phase Motors
147/156
/ig. $0.&3% shows the connections of a )-pole repulsion-induction motor for &3 J operation. It
consists of a stator and a rotor $or armature%.$i% The stator carries a single distributed winding fed from single-phase supply.
$ii% The rotor is provided with two independent windings placed one inside the other. The
inner winding is a s+uirrel-cage winding with rotor bars permanently short-circuited. 4laced over
the s+uirrel cage winding is a repulsion commutator armature winding. The repulsion winding is
connected to a commutator on which ride short-circuited brushes. There is no centrifugal device
and the repulsion winding functions at all times.
ig.#9.23$
peration
$i% ;hen single-phase supply is given to the stator winding, the repulsion winding $i.e.,
outer winding% is active. 'onse+uently, the motor starts as a repulsion motor with a
corresponding high starting tor+ue.
$ii% As the motor speed increases, the current shifts from the outer to inner winding due to
the decreasing impedance of the inner winding with increasing speed. 'onse+uently, at running
speed, the s+uirrel cage winding carries the greater part of rotor current. This shifting of
repulsion-motor action to induction-motor action is thus achieved without any switching
arrangement.
$iii% It may be seen that the motor starts as a repulsion motor. ;hen running, it functions
through a combination of principle of induction and repulsion5 the former being predominant.
)haracteristics
$i% The no-load speed of a repulsion-induction motor is somewhat above the
synchronous speed because of the effect of repulsion winding. *owever,
8/13/2019 Notes-Single Phase Motors
148/156
8/13/2019 Notes-Single Phase Motors
149/156
the speed at full-load is slightly less than the synchronous speed as in an induction motor.
8/13/2019 Notes-Single Phase Motors
150/156
$ii% The speed regulation of the motor is about B?.
$iii% The starting tor+ue is &.&@ to 3 times the full-load tor+ue5 the lower value being for large
motors. The starting current is 3 to ) times the full-load current.
This type of motor is used for applications re+uiring a high starting tor+ue with essentially a
constant running speed. The common si>es are .&@ to @ *.4.
9.1& Single-Phase Synchronous Motors
Jery small single-phase motors have been developed which run at true synchronous speed. They
do not re+uire d.c. excitation for the rotor. Fecause of these characteristics, they are called
unexcited single-phase synchronous motors. The most commonly used types are"
$i% (eluctance motors $ii% *ysteresis motors
The efficiency and tor+ue-developing ability of these motors is low5 The output of most of the
commercial motors is only a few watts.
9.1* !eluctance Motor
It is a single-phase synchronous motor which does not re+uire d.c. excitation to the rotor. Itsoperation is based upon the following principle"
;henever a piece of ferromagnetic material is located in a magnetic field5 a force is exerted on
the material, tending to align the material so that reluctance of the magnetic path that passes
through the material is minimum.
8/13/2019 Notes-Single Phase Motors
151/156
ig.#9.2%$
)onstruction
A reluctance motor $also called synchronous reluctance motor% consists of"
$i% a stator carrying a single-phase winding along with an auxiliary winding to produce a
synchronous-revolving magnetic field.
$ii% a s0uirrel-cage rotor having unsymmetrical magnetic construction. This is achieved
by symmetrically removing some of the teeth from the s+uirrel-cage rotor to produce salient
poles on the rotor. As shown in /ig. $0.&) $i%%, ) sailent poles have been produced on me rotor.
The salient poles created on the rotor must be e+ual to the poles on the stator.
ote that rotor salient poles offer low reductance to the stator flux and, therefore, become
strongly magneti>ed.
peration
$i% ;hen single-phase stator having an auxiliary winding is energi>ed, a synchronously-
revolving field is produced. The motor starts as a standard s+uirrel-cage induction motor andwill accelerate to near its synchronous speed.
$ii% As the rotor approaches synchronous speed, the rotating stator flux will exert reluctance
tor+ue on the rotor poles tending to align the salient-pole axis with the axis of the rotating field.
The rotor assumes a position where its salient poles loc! with the poles of the revolving field
6See /ig. $0.&) $ii%%7. 'onse+uently, the motor will continue to run at the speed of revolving flux
i.e., at the synchronous speed.
$iii% ;hen we apply a mechanical load, the rotor poles fall slightly behind the stator poles,
while continuing to turn at synchronous speed. As the load on the motor is increased, the
mechanical angle between the poles increases progressively. evertheless, magnetic attraction
!eeps the rotor loc!ed to the rotating flux. If the load is increased beyond the amount under
which the reluctance tor+ue can maintain synchronous speed, the rotor drops out of step with
the revolving field. The speed, then, drops to some value at which the slip is sufficient to
develop the necessary tor+ue to drive the load by induction-motor action.
)haracteristics
$i% These motors have poor tor+ue, power factor and efficiency.
$ii% These motors cannot accelerate high-inertia loads to synchronous speed.
$iii% The pull-in and pull-out tor+ues of such motors are wea!.
espite the above drawbac!s, the reluctance motor is cheaper than any other type of
synchronous motor. They are widely used for constant-speed applications such as timing
devices, signalling devices etc.
8/13/2019 Notes-Single Phase Motors
152/156
8/13/2019 Notes-Single Phase Motors
153/156
9.1,
8/13/2019 Notes-Single Phase Motors
154/156
ysteresis Motor
8/13/2019 Notes-Single Phase Motors
155/156
It is a single-phase motor whose operation depends upon the hysteresis effect i.e., magneti>ation
produced in a ferromagnetic material lags behind the magneti>ing force.
)onstruction It consists of"
$i% a stator designed to produce a synchronously-revolving field from a single-phase
supply. This is accomplished by using permanent-split capacitor type construction.
'onse+uently, both the windings $i.e., starting as well as main winding% remain connected in the
circuit during running operation as well as at starting. The value of capacitance is so adusted as
to result in a flux revolving at synchronous speed.
$ii% a rotor consisting of a smooth cylinder of magnetically hard steel, without winding or
teeth.
peration
$i% ;hen the stator is energi>ed from a single-phase supply, a synchronously-revolving
field $assumed in anti-cloc!wise direction% is produced due to split-phase operation.
$ii% The revolving stator flux magneti>es the rotor. ue to hysteresis effect, the axis of
magneti>ation of rotor will lag behind the axis of stator field by hysteresis lag angle a as shown
in /ig. $0.&@%. Thus the rotor and stator poles are loc!ed. If the rotor is stationary, the starting
tor+ue produced is given by"
T . sin as sr
where s 1 stator flux.
r 1 rotor flux. /rom now onwards, the rotor accelerates to synchronous speed with a uniform
tor+ue.
$iii% After reaching synchronism, the motor continues to run at synchronous speed andadusts its tor+ue angle so as to develop the tor+ue re+uired by the load.
)haracteristics
$i% A hysteresis motor can synchroni>e any load which it can accelerate, no matter how
great the inertia. It is because the tor+ue is uniform from standstill to synchronous speed.
$ii% Since the rotor has no teeth or salient poles or winding, a hysteresis motor is inherently
+uiet and produces smooth rotation of the load.
8/13/2019 Notes-Single Phase Motors
156/156
ig.#9.2&$
$iii% The rotor ta!es on the same number of poles as the stator field. Thus by changing the
number of stator poles through pole-changing connections, we can get a set of synchronous
speeds for the motor.
pplications
Recommended