Notes-Single Phase Motors

8/13/2019 Notes-Single Phase Motors

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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.


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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.


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/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$


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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,


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(

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 #


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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.


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/ig. $0.@% shows the rotor circuits for the forward and bac!ward rotating fluxes.


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&


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1 (


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&


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2&, where (


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&


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is the standstill rotor resistance i.e., r


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&


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is e+ual to


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half the standstill rotor resistance. Similarly, x


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&


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1 9


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&


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2& where 9


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&


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is the standstill rotor reactance. At standstill, s 1 # so that impedances of the two circuits are


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e+ual. Therefore, rotor currents are e+ual i.e., I


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&f


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1 I


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&b


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. *owever, when the rotor rotates, the impedances of the two rotor circuits are une+ual and the

rotor current


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I


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&b


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is higher $and also at a lower power factor% than the rotor current I


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&f


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. 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.


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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.


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$ii


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%


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)apacitor (otors


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-started by two-phase motor action through the use of an auxiliary winding and a capacitor.


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ig.#9.*$


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$iii


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%


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Shaded-pole (otors


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-started by the motion of the magnetic field produced by means of a shading coil around a

portion of the pole structure.


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&&

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 & &


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ote that resultant flux has the same value $i.e.


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m


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% bu


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t


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turned )@


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=


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cloc!wise from position #


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.


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$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$


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At instant 3, wt


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=


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0


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.


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=


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and f


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=


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&&

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#


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9.*


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Split-Phase Induction Motor


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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


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normal speed. The normal speed of the motor is below the synchronous speed and

depends upon the load on the motor.


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)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.#


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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.


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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


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L


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r


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1 L


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#


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K L


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f


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K L


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b


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where L


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#


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1 (


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#


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K 9


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#


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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


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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.


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$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


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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.


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$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)


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Thus adusting


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to )@


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=


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at starting, maximum tor+ue can be obtained during the starting period. *owever,


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has to be adusted to give a suitable running speed.


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)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.


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9.1%


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!epulsion-Induction Motor


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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.


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)onstruction


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/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,


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the speed at full-load is slightly less than the synchronous speed as in an induction motor.


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$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.


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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.


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9.1,


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ysteresis Motor


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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.


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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

Notes-Single Phase Motors

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Transcript of Notes-Single Phase Motors