Ac machine windings

 AC machines differ from DC machines by having their armature

windings almost always located on the stator while their field
windings are located on the rotor.
 A set of three-phase ac voltages is induced into the stator armature
windings of an ac machine by the rotating magnetic field from the
rotor field windings (generator action).
 Conversely, a set of three-phase currents flowing in the stator
armature windings produces a rotating magnetic field within the
stator.
 This magnetic field interacts with the rotor magnetic field to
produce the torque in the machine (motor action).
 AC Machines

Synchronous Asynchronous
Machines Machines
(Induction Machine)
Synchronous Synchronous
Generator Motor Induction Induction
Used as motors as Generator Motor
A primary Most widely
well as power factor Due to lack of a
used electrical
source of separate field
compensators motors in both
electrical excitation, these
(synchronous domestic and
energy machines are rarely
condensers) industrial
used as generators.
applications
Synchronous machines are AC machines that have a field
circuit supplied by an external DC source.
The machine which converts mechanical power into AC
electrical power is called a Synchronous Generator or
Alternator.
However, if the same machine can be operated as a motor is
known as Synchronous Motor.
It most commonly used in hydropower plant.
Asynchronous or induction machine The asynchronous machine is
the electrical machine most widely used as a motor, it is extensively
used in several industry applications and in the rail transport, for
instance.
However, this machine has also a large use as a generator in electric
power plants, particularly in the renewables.
 Induction motor
Three-phase induction motors are the most common and
frequently encountered machines in industry
simple design, rugged, low-price, easy maintenance
 wide range of power ratings: fractional horsepower to 10 MW
 run essentially as constant speed from zero to full load
 speed is power source frequency dependent
• not easy to have variable speed control
• requires a variable-frequency power-electronic drive for
optimal speed control
 Construction
An induction motor has two main parts
a stationary stator
 consisting of a steel frame that supports a hollow, cylindrical core
 core, constructed from stacked laminations, having a number of
evenly spaced slots, providing the space for the stator winding
A revolving rotor
 composed of punched laminations, stacked to create a series
of rotor slots, providing space for the rotor winding
conventional 3-phase windings made of insulated wire
(wound rotor) » similar to the winding on the stator.
aluminum bus bars shorted together at the ends by two
aluminum rings, forming a squirrel-cage shaped circuit
(squirrel-cage)
Two basic design types depending on the rotor design
 squirrel-cage
wound-rotor
 Induction motor types according to rotor construction:
Squirrel cage type
The most common type of IM
Rotor winding is composed of copper bars embedded in the rotor slots
and shorted at both end by end rings
Simple, low cost, robust, low maintenance
Wound rotor type
 Rotor winding is wound by wires, the winding terminals can be
connected to external circuits through slip rings and brushes.
 Easy to control speed, more expensive.

A slip ring is an electromechanical device that allows the transmission

of power and electrical signals from a stationary to a rotating structure.
 External resistor are mainly used for start up
 Principle of operation
 This rotating magnetic field cuts the rotor windings and produces an
induced voltage in the rotor windings
 Due to the fact that the rotor windings are short circuited, for both
squirrel cage and wound-rotor, and induced current flows in the rotor
windings
The rotor current produces another magnetic field
A torque is produced as a result of the interaction of those two
magnetic fields

where ind is the induced torque,BR and BS are the magnetic flux
densities of the rotor and the stator respectively
 Ac machine winding
 The Armature winding of a machine is defined as an
arrangement of conductors' design to produce emfs by
relative motion in a magnetic field.
 Electrical machines employ groups of conductors
distributed in slots over the periphery of the armature.
 The groups of conductors are connected in various
types of series-parallel combination to form Armature
winding.
 The conductors connected in series so as to increase
the voltage rating.
 They are connected in parallel to increase the current
rating.
 Common Terminologies associated with ac windings

 Conductor:
– The active length of a wire or strip in the slot.
 Turn:
– A turn consists of two conductors separated from each
other by a pole pitch or nearly so, and connected in
series as shown in fig.(a)
– The conductors forming a turn are kept a pole pitch
apart in order that the emf in two are additive to produce
maximum resultant emf. N S

Conductor

Conductor

Pole-pitch

a) Single turn coil

 Coil: A coil may consist of a single turn or may consist of many turns,
placed in almost similar magnetic position, connected in series.
 Coil-Side: A coil consists of two coil sides, which are placed in two
different slots, which are almost a pole pitch apart.
 The group of conductors on one side of the coil form one coil side
while the conductors on the other side of the coil situated a pole pitch
(or approximately a pole pitch apart) forms the second coil side.
N S N S

Conductor

Conductor

Coil side

a) Single turn coil b) 3 turns coil

 The connections joining the conductors form the end connectors
or in the mass, the overhang or end winding.
 When the coil sides forming a coil are spaced exactly one pole
pitch a part they are said to be of full-pitch.
 However, the coil span may be less than a pole pitch, in which
case the coil is described as short pitched or chorded.
Overhang
C

B D
Coil-sides

Pole-pitch

Single turn coil

 POLE – PITCH
It is the distance between the centres of pole
faces of two adjacent poles is called pole pitch.

Pole pitch = 180 Phase angle

SLOT PITCH:
It is the phase angle between two adjustment slots

COIL SPAN OR COIL PITCH

It is the distance between two coil sides of a coil
 Full Pitch and Short Pitch Winding

Full Pitch Winding

If the coil span is equal to pole pitch then the winding is called Full Pitch Winding

Coil Span = Pole Pitch

e1 V e2 V

Short Pitch Winding

If the coil span is less than Pole

Pitch is called Short pitch
winding
e2 V

e1 V
Advantages of Short Chorded winding or Chorded Pitch Winding

1.Copper is saved
2.Mechanical strength of the coil is increased
3.Induced EMF in improved

Slot Angle : The angular displacement between any two

adjacent poles in electrical degree
Slot angle (β) = 180
(Number of slots / Pole)
 TYPES OF AC MACHINES WINDINGS
 They are two basic physical types for
the windings. They deal differently
with the mechanical problem for
arranging coils in sequence around
the armature.
 The two types are:
1. Single layer winding and
2. Double layer winding
 Single- layer winding
• One coil-side occupies the total slot area
• Used only in small ac machines
 SINGLE LAYER WINDING…

 Fig (a) below shows an

arrangement for a single layer
winding.
 In this type of winding
arrangement one coil side of a
coil occupies the whole of the
slot.
(a)
 Single layer winding are not
used for machine having
commutator. Coil
side
 Single layer winding allow the
use of semi-closed and closed
types of slots.
Semi-closed Open slot
slot
 Double- layer winding

• Coil-sides in two layers

• Double-layer winding is more common used above
about 5kW machines
 DOUBLE LAYER WINDING
 The double layer winding have
identical coils with one coil side of
each coil lying in top half of the slot
and the other coil side in bottom
half of another slot exactly or
approximately one pole pitch. Fig
(a)
 Each layer may contain more than
one coil side in case large numbers (a)
of coils are required (fig c).
 Figure (c) shows the arrangement
Coil
wherein there are 8 coil sides per Top coil side
(top layer)
sides Top
layer
slot. Open slots are frequently used
to house double layer windings. Bottom coil side
Bottom
layer
(Bottom layer)

(b) (c)
The advantages of double-layer winding over single layer
winding:
a. Easier to manufacture and lower cost of the coils
b. Fractional-slot winding can be used
c. Chorded-winding is possible
d. Lower-leakage reactance and therefore , better performance of the
machine
e. Better emf waveform in case of generators
NUMBER OF PHASES AND PHASES SPREAD
 An ac winding, meant to be user for a 'm' phase
system, should produce emfs of equal
magnitude in all the phase.
 These emfs should have identical waveforms
and equal frequency.
 Their displacement in time should be y =2/m
electrical radians.
 This is obtained by having similar pole phase
groups (a pole phase group is defined as a
group of coils of a phase under one pole) and
arranging the groups to have an effective
displacement of y =2/m electrical radians in
space.
 Consider the case of a 12-slot armature A
having 2 poles and wound for three 12
1
2
phases as show in fig below (a). If the flux e1
density wave shape is considered 11 e12 e2
e3
3

sinusoidal, the emfs of the conductors in C e11

the slots can be represented as a phasors 10 e10 e4 4

displaced from each other by an e9 e5
(electrical) angle, as shown in Fig(a). 9 e8
e7
e6 5
P 
s   radian  30 8
S 6 7
6

 If the winding is divided into three groups B

(one for each phase) spread over two
pole pitches, the electrical displacement Fig.(a) e3
e4

in space between the groups is 2/3 e2

electrical radian or 1200 electrical. EA

 Each phase is located in four consecutive EC

e1

slots and so the phase spread is 4 x 30 0 = e12 1200

1200 electrical. e11

1200
e5

 If the conductors in the slots are e10 e9 1200

connected as per the phasor diagram fig e6

(b) , the summation of conductors emfs e7

would give three emfs displaced 1200 in
time following a phase sequence of ABC EB e8

in time. The space sequence is also ABC. Fig.(b)

 A
 Let the winding be split up into six 600 phase
B’
groups spread over two pole pitches as shown in 12
1
2
fig (a). e1
11 e12 e2
 Conductors of phase A are placed in slots, 1,2 e3
3
C’
e11
and 7,8.
10 e10 e4 4
 Conductors of phase B are placed in slots 5,6 and
e9 e5
11,12. C
9 e8 e6 5
e7
 Conductors of phase C are placed in slots 3,4 and
9,10. 8 6
7 B
 Conductors in slot 7,8 are return conductors for
conductors in slots 1,2. A’

 Conductors in slots 11,12 are return conductors

for conductors in slots 5,6. Fig.(a)
-e8
 Conductors in slots 3,4 are return conductors for EA
conductor in slots 9,10. -e7

 If the conductors were connected as represented e2

by the phasor diagram (b), we would still get three

equal emfs displaced by 1200 in time following a e1

phase sequence ABC.

e5
 The space sequence being A C B A C B. Thus it EC
e10 e9
e6
is clear that with six 600 phase groups (three 600 -e4 -e3
-e11
groups per pole) spread over pole pitches, it is
possible to obtain three equal emfs displaced EB -e12

1200 in time.
Fig.(b)
 TYPES OF SINGEL LAYER WINDINGES
 The three most common types of single layer windings
are
1. Concentric windings ( Unequal coil span)
2. Chain windings (equal coil span)
3. Mush windings (equal coil span)