SINGLE PHASE INDUCTION

MOTORS
 Introduction
 The motors which work on single phase ac
supply are called single phase
induction motors.

 The power rating of these motors are very

small. Some of them are of fractional
horse power.
 Are used in applications like small toys,
small fans, hair driers etc.
Types of Single Phase
Motors
Single
Phase
Motors
Inducti Univers
on al Repulsio Synchronou
n Motors s Motors
Motor motors
Split Repulsion Reluctance
Phase Start motor
Type Induction-
run Motor
Capacitor Hysteresi
Type Repulsion s Motor
Induction
Shaded Motor
Pole Type
 Construction
 Single phase induction motors has
 One rotating (rotor) and

 Other stationary (stator)

.
 Single Phase Induction Motor
 As the name suggests, this type of motor has only
one stator winding (main winding) and operates
with a single-phase power supply.
 In all single-phase induction motors, the rotor is the
squirrel cage type.
 The single-phase induction motor is not self-starting.
 When the motor is connected to a single-phase power
supply, the main winding carries an alternating
current. This current produces a pulsating magnetic
field. Due to induction, the rotor is energized.
 As the main magnetic field is pulsating, the torque
necessary for the motor rotation is not generated. This
will cause the rotor to vibrate, but not to rotate. Hence,
the single phase induction motor is required to
Single Phase Induction
Motor
 The single-phase induction motor operation can
be described by two methods:
– Double revolving field theory; and
– Cross-field theory.
 Double field revolving theory
 A single-phase ac current supplies the main
winding that produces a pulsating magnetic field.

 Mathematically, the pulsating field could be

divided into two fields, which are rotating in
opposite directions.

 The interaction between the fields and the

current induced in the rotor bars generates
opposing torque.

 According to this theory any alternating quantity

can be resolved into two rotating components
which rotate in opposite direction and each
 Double field revolving theory

 Under conditions, with only

the main field energized
the motor will not start.
 However, if an external
torque moves the motor in
any direction, the motor will
begin to rotate.
 The pulsating filed is
divided a forward and
reverse rotating field
 Motor is started in the
direction of forward rotating
field this generates small
 Starting torque
 The single-phase motor starting torque is
zero because of the pulsating single-
phase magnetic flux.
 The starting of the motor requires the
generation of a rotating magnetic flux
similar to the rotating flux in a three-
phase motor.
 Two perpendicular coils that have currents
90° out of- phase can generate the
necessary rotating magnetic fields which
start the motor.
 Therefore, single-phase motors are built
 The phase shift is achieved by connecting
– a resistance,
– an inductance, or
– a capacitance in series with the starting
winding.

 Most frequently used is a capacitor to generate

the starting torque.
 Split-Phase AC Induction Motor

 The split-phase motor is also known as an

induction start/induction run motor.
 It has two windings: a start and a main winding.
 The start winding is made with smaller gauge
wire and fewer turns, relative to the main winding
to create more resistance, thus putting the start
winding’s field at a different angle than that of the
main winding which causes the motor to start
rotating.
 The main winding, which is of a heavier wire,
keeps the motor running the rest of the time.
Split-Phase AC Induction Motor
 Split-Phase AC Induction Motor

 The starting torque is 1.5 to 2 times of the full

load torque with a starting current of 6 to 8 times
the full load current.

 Since the starting winding is made up of fine

wires, the current density is high and the winding
heats up quickly. If the starting period exceeds 5
sec. , the winding may burn out unless the motor
is protected by built-in thermal relay.
 These are constant speed motors, speed
variation is 2-5% from no load to full load.
 Applications for split-phase motors include small
grinders, small fans and blowers and other low
 Capacitor Start AC Induction
Motor
 This is a modified split-phase motor with a
capacitor in series with the start winding to
provide a start “boost.”
 Like the split-phase motor, the capacitor start
motor also has a centrifugal switch which
disconnects the start winding and the
capacitor when the motor reaches about 75%
of the rated speed.
 Since the capacitor is in series with the start
circuit, it creates more starting torque, typically
200% to 400% of the rated torque.
Capacitor Start AC Induction Motor
 Capacitor Start AC
IM

 And the starting current, is much lower than the

split-phase due to the larger wire in the start
circuit.

 They are used in a wide range of belt-drive

applications like small conveyors, large
blowers and pumps, as well as many direct-
drive or geared applications.
 Permanent Split Capacitor (Capacitor
Run) AC IM

A permanent split capacitor (PSC) motor has a run

type capacitor permanently connected in series
with the start winding.
 The run capacitor must be designed for continuous use,

it cannot provide the starting boost of a starting

capacitor.
Permanent Split Capacitor (Capacitor
Run) AC IM
 The typical starting torque of the PSC
motor is low, from 30% to 150% of the
rated torque.
 PSC motors have low starting current,
u s u al l y le s s t h a n 2 0 0 % o f t h e r a t e d
current, making them excellent for
applications with high on/off cycle
rates
Permanent Split Capacitor (Capacitor
Run) AC IM
The PSC motors have several advantages:

 The motor design can easily be altered for

use with speed controllers.
 They can also be designed for optimum
efficiency and High-Power Factor (PF) at
the rated load.
 They’re considered to be the most
reliable of the single-phase motors, mainly
because no centrifugal starting switch is
required.
 Applications of permanent split
capacitor (PSC)

 Permanent split-capacitor motors have a wide

variety of applications depending on the design.

 These include fans, blowers with low starting

torque, washing machines, oil burners, small
machine tools etc.

 Power rating of these motors lies between

60W and 250W
Capacitor Start/Capacitor Run AC
Induction Motor

 This motor has a start type capacitor in

s e r i e s w i t h t h e a u x i l i a r y w i n d i n g l i ke t h e
capacitor start motor for high starting torque.
 Like a PSC motor, it also has a run type capacitor
that is in series with the auxiliary winding after
the start capacitor is switched out of the circuit.
This allows high overload torque.
 This type of motor can be designed for lower full-
load currents and higher efficiency.
 This motor is costly due to start and run
capacitors and centrifugal switch.
Capacitor Start/Capacitor Run AC Induction
Motor
Capacitor Start/Capacitor Run AC
Induction Motor
 It is able to handle applications demanding for
any other kind of single-phase motor.
 These include woodworking machinery, air
compressors, high-pressure water pumps,
vacuum pumps and other high torque
applications requiring 1 to 10 hp.
 Shaded-Pole AC Induction
Motor

 Shaded-pole motors have only one main winding

and no start winding.
 Starting is by means of a design that rings a
continuous copper loop around a small portion of
each of the motor poles.

 This “shades” that portion of the pole, causing the

magnetic field in the shaded area to lag behind the
field in the unshaded area.

 The reaction of the two fields gets the shaft rotating.

Shaded-Pole AC Induction Motor
 Advantages

 Because the shaded-pole motor lacks a start

w i n d i n g , s ta rti n g s w i tc h o r c a p a c i t o r , i t i s
electrically simple and inexpensive.

 Also, the speed can be controlled merely by

varying voltage, or through a multi-tap winding.
Mechanically, the shaded-pole motor
construction allows high-volume production.

 In fact, these are usually considered as

“disposable” motors, meaning they are much
cheaper to replace than to repair.
 Disadvantages

 It’s low starting torque is typically 25% to

75% of the rated torque.
 It is a high slip motor with a running speed
7% to 10% below the synchronous speed.
 Generally, efficiency of this motor type is
very low (below 20%).
 A series ac motor is the same electrically as a dc
series motor but construction differs slightly.
 Special metals, laminations, and windings are used
which reduce losses caused by eddy currents,
hysteresis, and high reactance.
 Dc power can be used to drive an ac series motor
efficiently, but the opposite is not true.
 The characteristics of a series ac motor are
similar to those of a series dc motor. It is a
varying-speed machine.
 It has low speeds for large loads and high
speeds for light loads.
Universal motor

 The motors which can be used with a

single phase AC source as well as a DC
source of supply and voltages are called
as Universal Motor. It is also known as
Single Phase Series Motor.
 A universal motor is a commutation
type motor.
Construction of the
universal motor
 The construction of the universal motor is
same as that of the series motor.
 In order to minimize the problem of
commutation, high resistance brushes with
increased brush area are used.
 To reduce Eddy current losses the stator core
and yoke are laminated.
 The Universal motor is simple and less costly.
 It is used usually for rating not greater than
7 5 0 W .
Characteristic of Universal
motor
 The characteristic of Universal motor is
similar to that of the DC series motor.
 When operating from an AC supply, the
series motor develops less torque.
 By interchanging connections of the fields
with respect to the armature, the direction
of rotation can be altered.
 Universal motor
 The direction of the
d e v e l o p e d t o rq u e w i l l
remain positive, and
direction of the rotation
will be as it was before.

 The nature of the torque

will be pulsating, and the
frequency will be twice
that of line frequency as
shown in the waveform.
Universal motor
Thus, a Universal motor can work on both AC and
DC. However, a series motor which is mainly
designed for DC operation if works on single
phase AC supply suffers from the following
drawbacks.
 The efficiency becomes low because of hysteresis

and eddy current losses.

 The power factor is low due to the large

reactance of the field and the armature windings.

 The sparking at the brushes is in excess.
Universal motor
In order to overcome the following drawbacks,
certain modifications are made in a DC series
motor so that it can work even on the AC current.
They are as follows:-
 The field core is made up of the material having a
low hysteresis loss. It is laminated to reduce the
eddy current loss.
 The area of the field poles is increased to reduce
the flux density. As a result, the iron loss and the
reactive voltage drop are reduced.
 To g e t t h e re q u i re d t o rq u e t h e n u m b e r o f
conductors in the armature is increased.
Universal motor
 A compensating winding is used for reducing
the effect of the armature reaction and
improving the commutation process.
 The winding is placed in the stator slots as
shown in the figure below.
 Universal motor
 The series motor with the compensated
winding is shown in the figure below.

The winding is put in the

stator slot. The axis of
compensating winding is
90 degrees with the main
field axis.

The compensating winding

is connected in series with
both the armature and the
field, hence, it is called
Conductively compensated.
 Advantages &Disadvantage

Advantages
 High starting torque
 Very compact design if high running speeds are
used.

Disadvantage
 Requires maintenance
 and short life problems caused by the
commutator.

As a result, such motors are usually used in AC

devices such as food mixers and power tools
which are used only intermittently, and often
 Motor that can be used with a single- phase ac source
as well as a dc source of supply voltages are called
universal motor.
 T h e s t a t o r a n d ro t o r w i n d i n g s o f t h e m o t o r a re
connected in series through the rotor commutator.
 The universal motor is also known as an AC series
motor or an AC commutator motor.
Universal
Motor

MOTORS 5
 Series-connected
 Rotor and Stator are
connected in series
 Operates on either
 ac or dc

MOTORS 6
ECE 441 7
The Commutator Bar and Brushes are a
switch that reverses the current in the
armature coil as it rotates
ECE 441 9
 Flux
“Bunchin
g”

Force on conductor B
is upwards, rotation
is counter-clockwise
(CCW)
+Positive ECE 441 1
0
 Flux
“Bunchin
g”

Force on Conductor B
is upwards, rotation
is counter-clockwise
-Negative Half-
Cycle- (CCW)
ECE 441 1
1
 REPULSION
MOTORS
Repulsion motors are classified under
single phase motors. In magnetic
repulsion motors the stator windings are
connected directly to the ac power
supply and the rotor is connected to
commutator and brush assembly, very
similar to that of DC armature.

Repulsion Motor
 REPULSION MOTORS

Repulsion motors are based on the

p r i n c i p l e o f re p u l s i o n b e t w e e n t w o
magnetic fields. Consider a 2-pole motor
with a vertical magnetic axis. The
armature is connected to a commutator
a n d b r u s h e s . T h e b r u s h e s a re s h o r t
circuited using a low-resistance jumper.
When alternating current is supplied to
the field (stator) winding, it induces an
electromotive force (emf) in the armature.
 C REPULSION MOTORS

 The direction of
alternating current is such
that it creates a north pole
at the top and a south pole
at the bottom. The
direction of induced emf is
given by Lenz's law,
according to which the
direction of induced emf
opposes the cause
producing it. The induced
emf induces current in the
 Application of Repulsion
motors :
• high speed
lift

• fans &
pumps
 Application of Repulsion
motors :
• Hoists
• Mining
Equipment

• Air
Compressors
 Applications of Single Phase
IM

 The low initial cost suits the shaded-pole motors to

low horsepower or light duty applications.

 Its Largest use is in multi-speed fans for household

use.

 But the low torque, low efficiency and less

sturdy mechanical features make shaded-pole
motors impractical for most industrial or commercial
use, where higher cycle rates or continuous duty are
the norm.
Equivalent Circuit of a
Single Phase Induction
Motor
Flowchart of Single phase
Induction Motor
 The Design Procedure

The main specifications for a single phase induction motor for design

purposes are:-

1. Rated output in W or K.W.

2. Rated Voltage V

3. Rated current A

4. Rated speed r.p.m.

5. Frequency HZ

6. Poles, P

7. Pull out torque Nm

8. Starting torque Nm
10.Power-factor %

11. Motor – type : split phase

a) Resistance start induction run (low starting torque)

b) Capacitor start induction run (medium starting torque)

c) Capacitor start capacitor run (High starting torque)

I. One capacitor

II. Two capacitor

 Output Equation

The output equation relates the desired output characteristics of

t h e i n d u c t i o n m o t o r t o t h e m a c h i n e ’ s m a i n d e t e rm i n i n g

specifications to which the motor should be designed based on.

F = Flux per Pole

Kw=Winding Factor

f=Frequency

V=Rated voltage

I=Full load current in the main winding, A

Tm=Number of turns of the main winding

P=Number of poles
D=stator bore diameter, m

L=Stator core length, m

τP=Pole Pitch

ns=Synchronous speed, r.p.s

Bav=Average value of flux density in the air gap, Wb/m2 (Specific

magnetic loading

ac= Ampere-conductor per meter of arm. Periphery, ac/m

(specific electric loading

η=Full load efficiency

Cos F= Full load power factor

The KVA rating of a single phase induction motor is given by;

KVA= V I*10-3 --------------------------(1)

V=4.44 K W f Tph

=Bav L ( D/p)

ac= (2Tph Iph)/ ( D)

f= ns P/2

Substituting for the value of V in equation (1); then

KVA= 4.44 K w f T ph I*10 -3

Substituting for the values of f, and Tph

D π
2
KVA =4.44 KW ( ns P/2) (Bav L) ( )*10-3

= (1.11 2 KW Bav ac*10-3) D2 Lns

Again this can be expressed as;
KVA=CO D2 Lns
Where;
Co=1.11 π2 KW Bav ac*10-3
 Design of Main Dimensions

h. × 0.746
=
X. . X .

To separate D and L from

D2L
L= pole pitch ( 0.6 to 2)
Choice of Specific Loadings

The value of Bav lies between 0.35 – 0.55 Tesla.

The value of ac varies between 5000 – 15000 ampere

conductors per meter.
DESIGN OF
STATOR
For sinusoidal distribution the number of turns of each coil are calculated as;

2
Xö逷 7 − 9; ö XX ½ Xö逷 = ö × 90 = 0.2225
14

4
Xö逷 6 − 10; ö XX ½ Xö逷 = ö × 90 = 0.4339
14

6
Xö逷 (5 − 11); ö XX ½ Xö逷 = ö × 90 = 0.6235
14

8
Xö逷 (4 − 12); ö XX ½ Xö逷 = ö × 90 = 0.7818
14

10
Xö逷 (3 − 13); ö XX ½ Xö逷 = ö × 90 = 0.9009
14

12
Xö逷 (2 − 14); ö XX ½ Xö逷 = ö × 90 = 0.9749
14
 14
Xö逷 (1 − 15); ö XX ½ Xö逷 = ö × 90 = 0.5000
14
= 4.4375
Percentage of turns in coil 7-9

0.2225
× 100 = 5.014
4.4375
Percentage of turns in coil 6-10

0.4339
× 100 = 9.778
4.4375
Percentage of turns in coil 5-11

0.6235
× 100 = 14.051
4.4375
Percentage of turns in coil 4-12

0.7818
× 100 = 17.618
4.4375
Percentage of turns in coil 3-13

0.9009
× 100 = 20.302
4.4375
Percentage of turns in coil 2-14

0.9749
× 100 = 21.9696
4.4375

Percentage of turns in coil 1-15

0.500
× 100 = 11.2676
4.4375
The turns in each coil will be
Xö逷 7 − 9 = (0.05014 × 48)
= 2.406 ≈2
Xö逷 6 − 10 = (0.09778 × 48)
= 4.693 ≈5
Xö逷 5 − 11 = (0.1405 × 48)
= 6.744 ≈7
Xö逷4 − 12 = (0.17618 × 48)
= 8.456 ≈8
Xö逷 3 − 13 = (0.20302 × 48)
= 9.745 ≈ 10
Xö逷 2 − 14 = (0.219696 × 48)
= 10.545 ≈ 11
Xö逷 1 − 15 = (0.112676 × 48)
= 5.40 ≈5
Total 48
Amended value of = 2 × 48 = 96
The winding factor is calculated as;

(2 × 0.2225) + (5 × 0.4339) + (7 × 0.6235) + (8 × 0.7818)

+ (10 × 0.9009) + (11 × 0.9749) + (5 × 0.500)
96

= 0.3695 ≈ 0.4
 (1) Running Winding ( Main Winding)

The stator windings of single phase induction motors are

concentric type .There are usually 3 or more coils per pole each
having same or different number of turns.

=
4.44  X∅
(2) Conductor size

Main winding full current is given by

. × 746
=
. . X φ
Watt/r.p. 3.6 7.2 12 18
s
X. . X 9.5 12 15.5 18

(3) Area of running winding

δ
Am=
(4) Number of Stator Slots

The number of stator slots per pole is usually

between 9 to 12.The no. of slots should be divisible
by the number of poles so that a balanced ,regular
winding can be used .A large number of slots
reduces the leakage reactance by reducing the slot
and zigzag leakage .A large number of slots also
reduces troubles due to field harmonics .It also
reduces the intensity of magnetic noise increasing
the pitch of noise components .

A small number of slots reduces the cost of winding

and gives a better space factor .
(5) Size of Stator Slots

All the stator slots do not have the same number of

conductors and some contain both running winding
and starting winding conductors. The starting winding
has a small cross sectional area and its effect upon the
size of slot is small. The running winding coil with
largest number of turns will determine the size of slot.
The ratio of insulated conductor area to slot area
should never exceed 0.5.
Suppose Zs is the total number of conductors per slot
and d1 mm is the diameter of insulated conductor.

Area required for insulated conductors =

êp ú 2
Zs ´ê úd1
ë4 û
 é1 ù êp ú 2
Minimum slot area required =ê úZs ´ê úd1
ë0.5 û ë4 û

The slot area provided in the stamping is calculated

by ,multiplying the mean width by the depth of slot.
ê ú
The average slot width Ws (av) = êp ( D . d ss ) ú0Wts
ë Ss û

dss= depth of stator slot

Wts= width of stator tooth
Ss- Number of stator slots

Area of each slot = Ws(av) x dss

(6) Stator Teeth

The stator tooth density can be generally from 1.4 to

1.7 Wb/m2 .
For general purpose machine a flux density of 1.45
Wb/m2 is taken while for high torque machines it is
1.8 Wb/m2 .

Net iron length Li= 0.95 L

Flux density in the stator teeth =

fm
B ts 6
( S s / p ) ´W ts ´ Li
(7) Stator Core

The flux density in the stator core should not exceed

1.5 Wb/m2.It lies between 0.9 to 1.4 Wb /m2.

Flux in the stator core

f
f c 6 m
2
Flux density in the stator core
=
f m
B 6
2 ´ d cs ´ L i
c s

dcs= depth of stator core

(8) Length of mean turn
The length of mean turn for each of the coils per pole of
a concentric winding .

8.4 ( + )
 = × 逷X r +2
DESIGN OF
ROTOR
(1)Number of Rotor Slots
The number of rotor slots is so chosen that there is
no noise producing combinations. The number of
rotor slots are divisible by number of poles. The
number of rotor slots S r differ from S s by 20 % or
more .Die cast aluminum rotor construction is used.

(2)Area of Rotor Bars

The cage rotor winding may be either of copper
bars and end rings are of cast aluminum.
Manufacturing is cheaper with cast aluminium.With
cast rotors ,the joints between bars and end rings
are eliminated.

Total stator copper section for main winding Am = 2

Tm am mm2
The total rotor copper section is 0.5 to 0.8 of total
Total cross section of rotor bars Ar= Sr ab
Where ab= area of each bar,mm2

For
Copper
A ,
r
6 0 . 5 to 0 . 8
Am
For Aluminum,

A r
6 1 to 1 . 6
A m
(3) Area of End Rings

Ie = (Sr*Ib)/πp
Area of each end ring Ae = Ie /δe
mm2
Area of the rotor bars can be
calculated as
Ab = Ib /δb mm2
I S rIb S r a bd
a e 6 e
6 6 b
d e ppde ppde
0 . 32 A d
6 r
´ b
p d e
 Ib= current in each bard
=b a b

dbd e = current density in bars and end

rings
If we take, db 6 de

Area of each end

0 . 32 A
6 r

ring p
(4) Rotor Resistance
It should be as low as possible to keep the rotor
copper loss minimum and to maintain high
efficiency ,high full load speed and minimum
temperature rise. In single phase motors ,rotor
resistance effects the maximum torque for a given
flux and a high value of pull out torque is obtained
with rotor resistance.
For split phase motors = 0.45 to 0.55
For capacitor start motors = 0.45 to 0.8

(5)Flux density
Rotor Teeth in the stator teeth =

fm
B tr 6
( S s / p ) ´W tr ´ Li
(6) Rotor Core
Flux density in the stator core

f
B 6 m

2 ´ d ´ L
c r
cr i

dcs= depth of stator core

(7) Length of the air gap

L ö ö r X
= 0.2 + 2 
 Calculation of Operating
Characteristics
(1)Mmf for Air Gap

The flux produced by stator mmf is passes through the

following parts :
(a)Air gap (2) stator teeth (c) stator core (d) rotor
teeth ( e) rotor core

The value of flux density at 60 ° from interpolar axis

is 1.57 times Bav for single phase machines
B60 = 1.57 Bav

Mmf required for air gap AT 60 = 800,000 Bg 60 K g lg

(2)Iron loss
The iron loss in stator teeth is found by calculating
their flux densities and weights .
The total iron loss for induction motors is 1.5 to 2.5
times the sum of stator tooth and core loss due to
fundamental frequency flux.

(3) Friction and Windage loss

The bearing friction and windage loss will depend
upon the type of bearing to be used whether ball
bearings or sleeve bearings .For sleeve bearings and
a speed of 1500 rpm ,it is usually from 4 to 8 % of the
watt output.

r L mtm T m
(4) Main winding Resistance
6
rsm am
 (5) Stator Resistance

L mts ´ T s
rs 6 r
as
(6) Rotor Resistance

L mtr ´ Tr
rr 6 r
ar
The value of rotor resistance referred to running
winding ;
é Lb 2 Ds ù
rrm 6 8Tm K wm .
2
rê K ring ú
ëS r a b p p a e
2
û
 Design of Starting Winding for Split
phase Motors
The starting winding is designed for maximum
torque per ampere of starting current .In order that
starting winding can produce a revolving field the
flux set up by it must be out of phase with flux set
up by the main winding. With resistance split phase
motors the required resistance is obtained by using
a small section wire i.e. about 25 % of main winding.
The phase angle between starting winding current
and the line voltage should be about 0.4 of main
winding.