20201019-AC Machines
20201019-AC Machines
20201019-AC Machines
AC Motors
Synchronous Induction
Wound Synchronous Induction
Rotor
Capacitor
Wound rotor Start
Synchronous
Capacitor
Run
Capacitor
Start & Run
o Outer frame
It is the outer body of the motor. Its main function is to support the stator core and to
protect the inner parts of the machine.
For small machines, the outer frame is casted, but for the large machine, it is fabricated.
Outer Frame
Stator Core
Stator
Slots
Terminal Box
Stator
Winding
Base
o Stator Core
The core of the stator carries three phase windings which are usually supplied from a
three-phase supply system.
The stator core is built of high-grade silicon steel stampings.
Its main function is to carry the alternating magnetic field which produces hysteresis
and eddy current losses.
The stampings are fixed to the stator frame. Each stamping are insulated from the other
with a thin varnish layer.
o Stator windings
The stator windings are housed in stator slots with double layer winding.
These windings are distributed and are mostly short pitched.
The short-pitched and distributed windings are effective to limit the magnitudes of the
harmonics in the airgap flux. Sometimes, integral slot windings are also used.
When rotor rotates at that time the air gap reluctance is different at different point. So,
this pulsating reluctance produces pulsating exciting current, irregular torque, noise etc,
to reduce this effect large number of stator slots are selected.
But by using large number of slots results increase the manufacturing cost. So, that the
number of rotor slots and stator slots are selected different and rotor slots keep skew to
get uniform reluctance in the air gap.
The air gap should be selected as small as possible to reduce magnetizing current
required to set up air gap flux.
Anyway, the stator of the motor is wound for a definite number of poles, depending on
the speed of the motor.
If the number of poles is greater, the speed of the motor will be less and if the number of
poles is less than the speed will be high.
The windings may be connected in start or delta.
As the relationship between the speed and the pole of the motor is given as
1 120𝑓
𝑁𝑆 ∝ 𝑜𝑟 𝑁𝑆 =
𝑃 𝑃
(b) Rotor
The rotor is also built of thin laminations of the same material as the stator.
The laminated cylindrical core is mounted directly on the shaft.
These laminations are slotted on the outer side to receive the conductors. There are two
types of rotor:
Shaft
Winding
Conductors
Shaft
Brushes
Phase-II
m
1200
Phase-I O 1200
0 1 2 3 4
1200
Phase-III
Figure 1.5 (a) Phasor representation Figure 1.5 (b)
r
600
r 3
2
60 0
2 3 3 2
1 600
1
r
600
r
2
9
4 m
3
2m
2
9
4 m
3
2m
2
9
4 m
3
2m
2
9
4 m
3
2m
Working Principle
For simplicity, consider one conductor on the stationary rotor as shown in Figure 1.10(a).
This conductor be subject to the rotating magnetic field produced when a three phase
supply is connected to the three phase winding of the stator.
Consider the rotation of the magnetic field be clockwise.
A magnetic field moving clockwise has the effect as a conductor moving anticlockwise in
a stationary field.
According to Faraday’s law of electromagnetic induction, emf will be produced in the
conductor.
Rotor
Stator
Force as
Rotor
conductor
By completing the rotor circuit either using end rings or external resistances the
induced emf causes current to flow in the conductor.
By using right hand rule we can determine the direction of induced current in the
conductor.
By using right hand rule the direction of the induced current is outwards (shown as dot)
in Figure 1.10 (b).
The current in the rotor conductor produces its own magnetic field as shown inFigure
1.10 (c).
We know that when a current carrying conductor put in a magnetic field a force is
produced. This force is produced on the rotor conductor.
The direction of this force can be calculated by using left-hand rule as shown in Figure
1.10(d).
It is seen that the force acting on the conductor is in the same direction as the direction
of the rotating magnetic field.
The rotor conductor is in a slot on the circumference of the rotor, the force acts in a
tangential direction to the rotor and develops a torque in a rotor.
Similarly, torque produces in all the rotor conductors.
Since, the rotor is free to move then it rotates in the same direction as the rotating
magnetic field. Thus, three phase induction motor is self-starting motor.
sE2
R2 sX
2
2
E2
2
R2
s X 2
2
I0 I2
Iw I
V R0 R2
X0
S
E1 E2
Here,
R1 stator winding resis tan ce
X1 stator winding reac tan ce
R0 the core loss component
X 0 the magnetizing reac tan ce of the winding
R2 / s the power of the rotor, which includes output mechanical power
and copper loss of rotor
If we draw the circuit with referred to the stator then the circuit will look like
I0 I2
Iw I
V R0 R2 (1 s)
X0
s
E1
R1 X1 X 2' R2 '
I 1 A
I0 I2
Iw I
V R0 R2 ( 1 s )
X0 E2 E1
S
This has been done as the voltage drop between the stator resistance and reactance is
less and there is not much difference between the supply voltage and the induced
voltage. However, this is not appropriate due to following reasons:
The magnetic circuit of induction motor has an air gap so exciting current is larger
compared to transformer so exact equivalent circuit should be used.
The rotor and stator inductance is larger in induction motor.
In induction motor, we use distributed windings.
This model can be used if approximate analysis has to be done for large motors. For
smaller motors, we cannot use this.
o Power Relation of Equivalent Circuit
Input power to stator- 3 VI1cosƟ. Where, Vis the stator voltage applied. I1 is the current
drawn by the stator winding,cosƟis the power factor.
Rotor input = Power input- Stator copper and iron losses.
Rotor Copper loss = Slip × power input to the rotor.
Developed Power = (1 - s) × Rotor input power.
I0 I2
Iw I
V1 R0 X0 E1 Motor sE2 sX2
V1
E1
I1
I2'
Iw I0
1
I
2
I2R2
I2
sE 2
Stable Region
T
Unstable Region
TFL C
TST B
O Slip
s = sm s=1
S=0 (N = 0)
(N = Ns)
s
N s (5th N
S5 harmonic
)
,where N is rotor speed
N s (5thharmonic )
N
sN
5
N
5s
N
s N (1 s)
s
5
Ns
5
1 5(1- s)
6 - 5s
Ns
Here represents fifth harmonic field rotating opposite to the rotor.
5
The frequency of rotor currents induced by fifth harmonic rotating field is
2( fr ( fifth harmonic) )
2
f (6 5s)
5P 5P
N
s (6 5s)
5
Now, speed of fifth harmonic rotor field with respect to stator
Negative sign is used before Ns / 5 6 - 5s which indicates 5th harmonic field rotates
opposite to rotor movement.
Fundamental
Stable
T
Resultant
TL
s
Slip
5th Harmonic
ns/5 ns/7 0
7th Harmonic
1.2 Slip 6/7
1
Stable
Torque
Crawling Speed
Now, 5th harmonic currents will have phase difference of 5 ⨯ 120 = 600° =2 ⨯ 360 120
N
= 120°. Hence the revolving speed set up will be in reverse direction with speed s .
5
The small amount of 5 harmonic torque produces breaking action and can be
th
neglected.
Construction
Construction wise a LIM is similar to three phase induction motor in more ways than
one as it has been shown in the Figure 1.36 below.
A C
Rotor
Stator
B
Rotor
Stator Coil
Stator
Stator
Coil
Rotor
Stator
Working of LIM
When the primary of an LIM is excited by a balanced three phase power supply, a
traveling flux is induced in the primary instead of rotating 3 φ flux, which will travel
along the entire length of the primary.
Electric current is induced into the aluminum conductors or the secondary due to the
relative motion between the traveling flux and the conductors.
This induced current interacts with the traveling flux wave to produce linear force or
thrust F.
If the secondary is fixed and the primary is free to move, the force will move the
primary in the direction of the force, resulting in the required rectilinear motion.
When supply is given, the synchronous speed of the field is given by the equation:
120 f
Ns
p
Where, f is supply frequency in Hz, and p = number of poles, ns is the synchronous
speed of the rotation of magnetic field in rps.
The developed field will results in a linear traveling field, the velocity of which is given
by the equation,
Vs 2 f m / Sec
Where, Vs is velocity of the linear traveling field, and is the pole pitch.
For a slip of s, the speed of the LIM is given by,
V (1 s)Vs
o Applications of LIM
Automatic sliding doors in electric trains.
Mechanical handling equipment, such as propulsion of a train of tubs along a certain
route.
Metallic conveyor belts.
Pumping of liquid metal, material handling in cranes, etc.
o Advantages of LIM
It has no rotating part so it gives low mechanical loss.
It is simple and rugged in construction.
It is more efficient compare to conventional motor.
It has lower initial cost.
It gives higher speed.
*****************
Torque
Basic Principle of motor is to convert electrical power into mechanical power. So, that
induced torque (electromechanical torque or developed torque) in induction motor
depends on the rotor current, rotor power factor and rotating flux.
The torque is given by,
Tind I2 cos2
Where,
Rotating flux
I2 Rotor current per phase
cos2 Rotor power factor
Now, rotor emf per phase at s tan dstill,
E2 2
Tind E2 I2cos2
or
ind 2
Z s Z s
sE R
kE2 2 2
2 2
R2 sX 2 R2 sX 2
2 2
2
Tind 2 N m .................................................................(1.1)
ksE2R
R2 sX 2
2 2
kR2
Tst (Assuming Supply voltage V is constant )
R2 X22 2
R 2R
dT 1
s
k =0 2 2
dR2 R2 X2 R2 X 2 2 2
2 2 2
2 R2 X 2
1
2
2 2R 2
R2 X 2
2
2 2
R 2 X 2 2R 2
2 2 2
R2 X 2
2 2
R2 X 2
Generally the rotor resistance is not more than 1 to 2 % of its leakage reactance for
higher efficiency.
To get the high starting torque, extra resistance is added in the rotor circuit at the
starting time and cut slowly as motor get speed.
The torque developed by the motor during running condition is called running torque.
sR2 R2 R2
R s X2
2 2 or 2
or
R2 2 is Zero.
R 2
2 2 2 sX 2 X s 2R X
s s
2
2 2
Therefore, induced torque becomes maximum when rotor resistance per phase is equal
to the rotor reactance per phase under running condition.
The above equation 1.3shows that the torque is independent of the rotor resistance.
If smax is the value of slip at which the torque is obtained, then s max
R2
X2
T kR E 2 2sX 2R X
st
2 2
2
2 2 2
T R X
2
ksE 2
R2X 2
max 2 2 2 2 2
max 2
2
2 R2 X 2 1
X2 X 2 2 X 2
The slip at which maximum torque occurs is given as smax ,
smax R2
X
2
sR2 2 X 2
R22 (sX2)2
2
Tst 2
kR
2 2E
R2 X 2 2
Since, E2V
2
kR2 V
Tst 22
R22 X 2 2
Where k2 is the another constant
Tst V2 2
w
V R0 R
X 2
0 S
E1 E2
Here,
R1 stator winding resis tan ce
X1 stator winding reac tan ce
R0 the core loss component
X 0 the magnetizing reac tan ce of the winding
R2 / s the power of the rotor, which includes output mechanical power
and copper loss of rotor
If we draw the circuit with referred to the stator then the circuit will look like
I0 I2
Iw I
V R0 R2 (1 s)
X0
s
E1
R1 X1 X 2' R2 '
I 1 A
I0 I2
Iw I
V R0 R2 ( 1 s )
X0 E2 E1
S
This has been done as the voltage drop between the stator resistance and reactance is
less and there is not much difference between the supply voltage and the induced
voltage. However, this is not appropriate due to following reasons:
The magnetic circuit of induction motor has an air gap so exciting current is larger
compared to transformer so exact equivalent circuit should be used.
The rotor and stator inductance is larger in induction motor.
In induction motor, we use distributed windings.
This model can be used if approximate analysis has to be done for large motors. For
smaller motors, we cannot use this.
o Power Relation of Equivalent Circuit
Input power to stator- 3 VI1cosƟ. Where, Vis the stator voltage applied. I1 is the current
drawn by the stator winding,cosƟis the power factor.
Rotor input = Power input- Stator copper and iron losses.
Rotor Copper loss = Slip × power input to the rotor.
Developed Power = (1 - s) × Rotor input power.
W1
M L
A
C V
Rated
3-Ph 3-Ph
AC Induction
Supply Motor
M L
C V
W2
Generally, the power factor of the induction motor under no-load condition is less than
0.5 at that time one wattmeter shows negative reading.
After reverse the terminal of current coil of wattmeter and then take the reading of
wattmeter.
In this test the following parameters can be calculated.
V0 Line voltage
I0 = Line current
Pi = Core loss
Pcu = Copper loss
Pfw = Friction and windage loss
P1, P2 Re adings of wattmeter at no load
P1+P2 = 3V0 I0 cos0
Pconst
A
Pfw
O VRated
W1
M L
A
C V
3-Phase 3-Ph
AC Induction
supply
Motor
Blocked
Rotor
M L
C V
W2
P1 P2
R
3 Is ph
01 2
X 01 Z 012 R 012
X 01
X1 X 2 '
2
R2 ' R01 R1
Torque - Slip Characteristics of Induction Motor
The torque slip curve gives the information about the variation of torque with the slip.
The slip is defined as the ratio of difference of synchronous speed and actual rotor
speed to the synchronous speed of the machine.
The variation of slip can be obtained with the variation of speed that is when speed
varies the slip will also vary and the torque corresponding to that speed will also vary.
Braking Generating
Mode
Mode
o Motoring Mode
o Generating Mode:
In this mode of operation induction motor runs above the synchronous speed and it
should be driven by a prime mover.
The stator winding is connected to a three phase supply in which it supplies electrical
energy.
Actually, in this case, the torque and slip both are negative so the motor receives
mechanical energy and delivers electrical energy.
An Induction motor is not much used as generator because it requires reactive power
for its operation.
That is, reactive power should be supplied from outside and if it runs below the
synchronous speed by any means, it consumes electrical energy rather than giving it at
the output. So, as far as possible, induction generators are generally avoided.
o Braking Mode:
In the braking mode, the two leads or the polarity of the supply voltage is changed so
that the motor starts to rotate in the reverse direction and as a result the motor stops.
This method of braking is known as plugging.
This method is used when it is required to stop the motor within a very short period of
time. The kinetic energy stored in the revolving load is dissipated as heat.
Also, motor is still receiving power from the stator which is also dissipated as heat. So as
a result of which motor develops heat energy.
If load which the motor drives accelerates the motor in the same direction as the motor
is rotating, the speed of the motor may increase more than synchronous speed.
In this case, it acts as an induction generator which supplies electrical energy to the
mains which tends to slow down the motor to its synchronous speed, in this case the
motor stops. This type of breaking principle is called dynamic or regenerative braking.
PAG PConv
Air-Gap Power
Pout Loadm
Pin 3VL I L cos ind
m
PStray
Pcu (rotor ) Pfw
Pi
Pcu ( stator )
Circle Diagram
The circle diagram of an induction motor is very useful to study its performance under
all operating conditions.
The “Circle Diagram” means that it is the figure or curve which is drawn as a circular
shape. As we know, the diagrammatic representation is easier to understand and
remember compared to theoretical and mathematical descriptions.
Resistance Test
o
By voltmeter-ammeter method determine per phase equivalent stator resistance, R1.
If the machine is wound rotor type, find the equivalent rotor resistance R2′ also after
measuring rotor resistance and required transformations are applied.
T max
P max
S
Slip=1
P C’
Rotor copper
loss
K
PE
Efficiency of the machine at the operating point P,
PD
Power factor of the machine at operating point P,is cos
EF
Slip of the machine at the operating point P, s
PF
Starting torque at rated voltage (in syn. watts) = SK
To find the operating points corresponding to maximum power and maximum torque,
draw tangents to the circle diagram parallel to the output line and torque line
Induction generator:
Contactor
STOP (C) (P)
a
START
b
O/L Relay
O/L Relay
NC
Contact
Stator
Squirrel
Cage rotor
Sometimes fuses are also provided for short circuit protection in the circuit.
The DOL starter is simple and cheap.
Rotor
3-Phase
Stator
Auto Transformer
Start
Run
Rotor
O/L
Relay
Stator Run
Switch
S
Start
Rotor
Slip Rings
Star
connected
Rotor
Winding
Star
Connected
Rheostat
Smoothing
Reactor
Bridge
Rectifier Inverter
o Kramer’scascade system
In Kramer’s cascade, the slip-ring induction motor is started using rotor resistance
starter.
By changing the direction of phase rotation, the resistance of the rotor circuit is varied
and thus speed of the slip ring motor is controlled.
When machine is running, the rotor circuit EMF is rectified and connected to a
separately excited DC motor. The DC motor is connected to the main shaft of
induction motor by means of gears. By varying the field current of DC motor, the
speed of shaft can be varied in sub synchronous region.
DC Rotary
M
Motor Converter
Main
Motor
Slip rings
of
Main
Motor
Figure 1.28Kramer’s cascade system
Very large motors above 4000 kW such as steel rolling mills use such type of speed
control.
The main advantage of this method is that a smooth speed control is possible. Similarly
wide range of speed control is possible.
Another advantage of the system is that the design of a rotary converter is practically
independent of the speed control required.
Similarly if rotary converter is overexcited, it draws leading current and thus power
factor improvement is also possible along with the necessary speed control.
Scherbius
M Machine
Main
Motor
Starting
Resiatance
Energy efficient motors are specially designed to increase the efficiency of induction
motor.
Improving maintenance
Most of the motors are made from the silicon steel (de-carbonized cold rolled silicon
steel).
So, after used long time period the electrical property of core does not change
considerably but due to poor maintenance motor loss its efficiency.
For example, by using improper lubrication and friction motor reduce its efficiency.
An induction generator usually draws its excitation power from an electrical grid. Because of this, induction generators
cannot usually black start a de-energized distribution system. Sometimes, however, they are self-excited by using
phase-correcting capacitors.
Principle of operation
An induction generator produces electrical power when its rotor is turned faster than the synchronous speed. For a
typical four-pole motor (two pairs of poles on stator) operating on a 60 Hz electrical grid, the synchronous speed is
1800 rotations per minute (rpm). The same four-pole motor operating on a 50 Hz grid will have a synchronous speed of
1500 RPM. The motor normally turns slightly slower than the synchronous speed; the difference between synchronous
and operating speed is called "slip" and is usually expressed as per cent of the synchronous speed. For example, a motor
operating at 1450 RPM that has a synchronous speed of 1500 RPM is running at a slip of +3.3%.
In normal motor operation, the stator flux rotation is faster than the rotor rotation. This causes the stator flux to induce
rotor currents, which create a rotor flux with magnetic polarity opposite to stator. In this way, the rotor is dragged along
behind stator flux, with the currents in the rotor induced at the slip frequency.
In generator operation, a prime mover (turbine or engine) drives the rotor above the synchronous speed (negative slip).
The stator flux still induces currents in the rotor, but since the opposing rotor flux is now cutting the stator coils, an
active current is produced in stator coils and the motor now operates as a generator, sending power back to the
electrical grid.
Excitation
Equivalent circuit of induction generator
An induction machine requires an externally-supplied armature current. Because the rotor field always lags behind
the stator field, the induction machine always consumes reactive power, regardless of whether it is operating as a
generator or a motor.
A source of excitation current for magnetizing flux (reactive power) for the stator is still required, to induce rotor
current. This can be supplied from the electrical grid or, once it starts producing power, from the generator itself. The
generating mode for induction motors is complicated by the need to excite the rotor, which begins with only residual
An induction machine can be started by charging the capacitors, with a DC source, while the generator is turning
typically at or above generating speeds. Once the DC source is removed the capacitors will provide the magnetization
current required to begin producing voltage.
An induction machine that has recently been operating may also spontaneously produce voltage and current due to
residual magnetism left in the core .
Active power
Active power delivered to the line is proportional to slip above the synchronous speed. Full rated power of the
generator is reached at very small slip values (motor dependent, typically 3%). At synchronous speed of 1800 rpm,
generator will produce no power. When the driving speed is increased to 1860 rpm (typical example), full output power
is produced. If the prime mover is unable to produce enough power to fully drive the generator, speed will remain
somewhere between 1800 and 1860 rpm range.
Required capacitance
A capacitor bank must supply reactive power to the motor when used in stand-alone mode. The reactive power supplied
should be equal or greater than the reactive power that the machine normally draws when operating as a motor.
The basic fundamental of induction generators is the conversion from mechanical energy to electrical energy. This
requires an external torque applied to the rotor to turn it faster than the synchronous speed. However, indefinitely
increasing torque doesn't lead to an indefinite increase in power generation. The rotating magnetic field torque excited
from the armature works to counter the motion of the rotor and prevent over speed because of induced motion in the
opposite direction. As the speed of the motor increases the counter torque reaches a max value of torque (breakdown
torque) that it can operate until before the operating conditions become unstable. Ideally, induction generators work
best in the stable region between the no-load condition and maximum torque region.
Rated current
The maximum power that can be produced by an induction motor operated as a generator is limited by the rated current
of the machine's windings.
In induction generators, the reactive power required to establish the air gap magnetic flux is provided by a capacitor
bank connected to the machine in case of stand-alone system and in case of grid connection it draws reactive power
from the grid to maintain its air gap flux. For a grid-connected system, frequency and voltage at the machine will be
dictated by the electric grid, since it is very small compared to the whole system. For stand-alone systems, frequency
and voltage are complex function of machine parameters, capacitance used for excitation, and load value and type.
Uses
Induction generators are often used in wind turbines and some micro hydro installations due to their ability to produce
useful power at varying rotor speeds. Induction generators are mechanically and electrically simpler than other
generator types. They are also more rugged, requiring no brushes or commutators.
Limitations
An induction generator connected to a capacitor system can generate sufficient reactive power to operate on its own.
When the load current exceeds the capability of the generator to supply both magnetization reactive power and load
power the generator will immediately cease to produce power. The load must be removed and the induction generator
restarted with either a DC source, or if present, residual magnetism in the core.[2]
Induction generators are particularly suitable for wind generating stations as in this case speed is always a variable
factor. Unlike synchronous motors, induction generators are load-dependent and cannot be used alone for grid
frequency control.
Example application
As an example, consider the use of a 10 hp, 1760 r/min, 440 V, three-phase induction motor as an asynchronous
generator. The full-load current of the motor is 10 A and the full-load power factor is 0.8.
For a machine to run as an asynchronous generator, capacitor bank must supply minimum 4567 / 3 phases
= 1523 VAR per phase. Voltage per capacitor is 440 V because capacitors are connected in delta.
Typically, slip should be similar to full-load value when machine is running as motor, but
negative (generator operation):
*****************
Introduction
We use the single-phase power system more widely than three phase system for domestic purposes,
commercial purposes and some extent in industrial uses. Because, the single-phase system is more
economical than a three-phase system and the power requirement in most of the houses, shops, offices are
small, which can be easily met by a single phase system.
The single phase motors are simple in construction, cheap in cost, reliable and easy to repair and maintain.
Due to all these advantages, the single phase motor finds its application in vacuum cleaners, fans, washing
machines, centrifugal pumps, blowers, washing machines, etc.
The stator of the single-phase induction motor has laminated stamping to reduce eddy current losses on its
periphery. The slots are provided on its stamping to carry stator or main winding. Stampings are made up of
silicon steel to reduce the hysteresis losses. When we apply a single phase AC supply to the stator winding,
1. Firstly, the single-phase induction motors are mostly provided with concentric coils. We can easily
adjust the number of turns per coil can with the help of concentric coils. The mmf distribution is
almost sinusoidal.
2. Except for shaded pole motor, the asynchronous motor has two stator windings namely the main
winding and the auxiliary winding. These two windings are placed in space quadrature to each other.
Rotor of Single Phase Induction Motor
The construction of the rotor of the single-phase induction motor is similar to the squirrel cage three-phase
induction motor. The rotor is cylindrical and has slots all over its periphery. The slots are not made parallel to
each other but are a little bit skewed as the skewing prevents magnetic locking of stator and rotor teeth and
makes the working of induction motor more smooth and quieter (i.e. less noisy).
The squirrel cage rotor consists of aluminum, brass or copper bars. These aluminum or copper bars are called
rotor conductors and placed in the slots on the periphery of the rotor. The copper or aluminum rings
permanently short the rotor conductors called the end rings.
To provide mechanical strength, these rotor conductors are braced to the end ring and hence form a complete
closed circuit resembling a cage and hence got its name as squirrel cage induction motor. As end rings
permanently short the bars, the rotor electrical resistance is very small and it is not possible to add external
resistance as the bars get permanently shorted. The absence of slip ring and brushes make the construction
of single phase induction motor very simple and robust.
According to double field revolving theory, we can resolve any alternating quantity into two components.
Each component has a magnitude equal to the half of the maximum magnitude of the alternating quantity,
and both these components rotate in the opposite direction to each other. For example – a flux, φ can be
resolved into two components
Each of these components rotates in the opposite direction i. e if one φ m/2 is rotating in a clockwise direction
then the other φm / 2 rotates in an anticlockwise direction.When we apply a single phase AC supply to the
stator winding of single phase induction motor, it produces its flux of magnitude, φ m. According to the double
field revolving theory, this alternating flux, φ m is divided into two components of magnitude φ m/2. Each of
these components will rotate in the opposite direction, with the synchronous speed, N s.Let us call these two
components of flux as forwarding component of flux, φ f and the backward component of flux, φb. The
resultant of these two components of flux at any instant of time gives the value of instantaneous stator flux at
From the above topic, we can easily conclude that the single-phase induction motors are not self-starting
because the produced stator flux is alternating in nature and at the starting, the two components of this flux
cancel each other and hence there is no net torque. The solution to this problem is that if we make the stator
flux rotating type, rather than alternating type, which rotates in one particular direction only. Then the
induction motor will become self-starting.
P.GANESH,EEE Department AC Machines 62
Now for producing this rotating magnetic field, we require two alternating flux, having some phase
difference angle between them. When these two fluxes interact with each other, they will produce a resultant
flux. This resultant flux is rotating in nature and rotates in space in one particular direction only.
Once the motor starts running, we can remove the additional flux. The motor will continue to run under the
influence of the main flux only. Depending upon the methods for making asynchronous motor as Self
Starting Motor, there are mainly four types of single phase induction motor namely,
1. Split phase induction motor,
2. Capacitor start inductor motor,
3. Capacitor start capacitor run induction motor,
4. Shaded pole induction motor.
5. Permanent split capacitor motor or single value capacitor motor.
1. Single phase induction motors are simple in construction, reliable and economical for small power
rating as compared to three phase induction motors.
2. The electrical power factor of single phase induction motors is low as compared to three phase
induction motors.
3. For the same size, the single-phase induction motors develop about 50% of the output as that of three
phase induction motors.
4. The starting torque is also low for asynchronous motors/single phase induction motor.
5. The efficiency of single phase induction motors is less compared to that of three phase induction
motors.
Single phase induction motors are simple, robust, reliable and cheaper for small ratings. They are available
up to 1 KW rating.Single-phase motors do not have a unique rotating magnetic field like multi-phase
motors. The field alternates (reverses polarity) between pole pairs and can be viewed as two fields
rotating in opposite directions. They require a secondary magnetic field that causes the rotor to move in
a specific direction. After starting, the alternating stator field is in relative rotation with the rotor. Several
methods are commonly used:
Shaded-pole motor
A common single-phase motor is the shaded-pole motor and is used in devices requiring low
starting torque, such as electric fans, small pumps, or small household appliances. In this motor, small
single-turn copper "shading coils" create the moving magnetic field. Part of each pole is encircled by a
copper coil or strap; the induced current in the strap opposes the change of flux through the coil. This
causes a time lag in the flux passing through the shading coil, so that the maximum field intensity moves
INTRODUCTION
An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating
current.[2] For reasons of cost and simplicity, most alternators use a rotating magnetic field with a
stationary armature.[3] Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In
principle, any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines
driven by automotive and other internal combustion engines.
An alternator that uses a permanent magnet for its magnetic field is called a magneto. Alternators in power
stations driven by steam turbines are called turbo-alternators. Large 50 or 60 Hz three-phase alternators in power plants
generate most of the world's electric power, which is distributed by electric power grids.[4]
Alternating current generating systems were known in simple forms from the discovery of the magnetic induction of
electric current in the 1830s. Rotating generators naturally produced alternating current but, since there was little use
for it, it was normally converted into direct current via the addition of a commutator in the generator. The early
machines were developed by pioneers such as Michael Faraday and Hippolyte Pixii. Faraday developed the "rotating
rectangle", whose operation was heteropolar – each active conductor passed successively through regions where the
magnetic field was in opposite directions.[9] Lord Kelvin and Sebastian Ferranti also developed early alternators,
producing frequencies between 100 and 300 Hz..
Principle of operation
A conductor moving relative to a magnetic field develops an electromotive force (EMF) in it (Faraday's Law). This
EMF reverses its polarity when it moves under magnetic poles of opposite polarity. Typically, a rotating magnet, called
the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts
across the conductors, generating an induced EMF (electromotive force), as the mechanical input causes the rotor to
turn.
The rotating magnetic field induces an AC voltage in the stator windings. Since the currents in the stator windings vary
in step with the position of the rotor, an alternator is a synchronous generator. [3]
The rotor's magnetic field may be produced by permanent magnets, or by a field coil electromagnet. Automotive
alternators use a rotor winding which allows control of the alternator's generated voltage by varying the current in the
rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor, but are
restricted in size, due to the cost of the magnet material. Since the permanent magnet field is constant, the terminal
voltage varies directly with the speed of the generator. Brushless AC generators are usually larger than those used in
automotive applications.
An automatic voltage control device controls the field current to keep output voltage constant. If the output voltage
from the stationary armature coils drops due to an increase in demand, more current is fed into the rotating field coils
through the voltage regulator (VR). This increases the magnetic field around the field coils which induces a greater
voltage in the armature coils. Thus, the output voltage is brought back up to its original value.
Alternators used in central power stations also control the field current to regulate reactive power and to help stabilize
the power system against the effects of momentary faults. Often there are three sets of stator windings, physically offset
P.GANESH,EEE Department AC Machines 69
so that the rotating magnetic field produces a three phase current, displaced by one-third of a period with respect to
each other. [17]
Synchronous speeds
One cycle of alternating current is produced each time a pair of field poles passes over a point on the stationary
winding. The relation between speed and frequency is where is the frequency in Hz (cycles per second). is the number
of poles (2, 4, 6, …) and is the rotational speed in revolutions per minute (RPM). Very old descriptions of alternating
current systems sometimes give the frequency in terms of alternations per minute, counting each half-cycle as
one alternation; so 12,000 alternations per minute corresponds to 100 Hz.
The output frequency of an alternator depends on the number of poles and the rotational speed. The speed
corresponding to a particular frequency is called the synchronous speed for that frequency. This table[18] gives some
examples:
Rotation speed (RPM), giving…
Poles
50 Hz 60 Hz 400 Hz
Classifications
Alternators may be classified by method of excitation, number of phases, the type of rotation, cooling method, and their
application.
By excitation
There are two main ways to produce the magnetic field used in the alternators, by using permanent magnets which
create their own persistent magnetic field or by using field coils. The alternators that use permanent magnets are
specifically called magnetos.
In other alternators, wound field coils form an electromagnet to produce the rotating magnetic field.
A device that uses permanent magnets to produce alternating current is called a permanent magnet alternator (PMA). A
permanent magnet generator (PMG) may produce either alternating current, or direct current if it has a commutator.
Direct-connected direct-current (DC) generator
This method of excitation consists of a smaller direct-current (DC) generator fixed on the same shaft with the
alternator. The DC generator generates a small amount of electricity just enough to excite the field coils of the
P.GANESH,EEE Department AC Machines 70
connected alternator to generate electricity. A variation of this system is a type of alternator which uses direct current
from the battery for initial excitation upon start-up, after which the alternator becomes self-excited.[19]
Transformation and rectification
This method depends on residual magnetism retained in the iron core to generate weak magnetic field which would
allow a weak voltage to be generated. This voltage is used to excite the field coils for the alternator to generate stronger
voltage as part of its build up process. After the initial AC voltage buildup, the field is supplied with rectified
voltage from the alternator.[19]
Brushless alternators
A brushless alternator is composed of two alternators built end-to-end on one shaft. Until 1966, alternators used brushes
with rotating field.[20] With advancement in semiconductor technology, brushless alternators are possible. Smaller
brushless alternators may look like one unit but the two parts are readily identifiable on the large versions. The larger of
the two sections is the main alternator and the smaller one is the exciter. The exciter has stationary field coils and a
rotating armature (power coils). The main alternator uses the opposite configuration with a rotating field and stationary
armature. A bridge rectifier, called the rotating rectifier assembly, is mounted on the rotor. Neither brushes nor slip
rings are used, which reduces the number of wearing parts. The main alternator has a rotating field as described above
and a stationary armature (power generation windings).
Varying the amount of current through the stationary exciter field coils varies the 3-phase output from the exciter. This
output is rectified by a rotating rectifier assembly, mounted on the rotor, and the resultant DC supplies the rotating field
of the main alternator and hence alternator output. The result of all this is that a small DC exciter current indirectly
controls the output of the main alternator.
By number of phases
Another way to classify alternators is by the number of phases of their output voltage. The output can be single phase,
or polyphase. Three-phase alternators are the most common, but polyphase alternators can be two phase, six phase, or
more.[19]
By rotating part
The revolving part of alternators can be the armature or the magnetic field. The revolving armature type has the
armature wound on the rotor, where the winding moves through a stationary magnetic field. The revolving armature
type is not often used.[19] The revolving field type has magnetic field on the rotor to rotate through a stationary armature
winding. The advantage is that then the rotor circuit carries much less power than the armature circuit, making the slip
ring connections smaller and less costly; only two contacts are needed for the direct-current rotor, whereas often a rotor
winding has three phases and multiple sections which would each require a slip-ring connection. The stationary
armature can be wound for any convenient medium voltage level, up to tens of thousands of volts; manufacture of slip
ring connections for more than a few thousand volts is costly and inconvenient.
Cooling methods
Many alternators are cooled by ambient air, forced through the enclosure by an attached fan on the same shaft that
drives the alternator. In vehicles such as transit buses, a heavy demand on the electrical system may require a large
alternator to be oil-cooled. [22] In marine applications water-cooling is also used. Expensive automobiles may use water-
cooled alternators to meet high electrical system demands.
Specific applications
P.GANESH,EEE Department AC Machines 71
Electric generators
Most power generation stations use synchronous machines as their generators. Connection of these generators to the
utility grid requires synchronization conditions to be met.[23]
Automotive alternators
Alternators are used in modern automobiles to charge the battery and to power the electrical system when its engine is
running.
Until the 1960s, automobiles used DC dynamo generators with commutators. With the availability of affordable silicon
diode rectifiers, alternators were used instead.
Diesel electric locomotive alternators
In later diesel electric locomotives and diesel electric multiple units, the prime mover turns an alternator which provides
electricity for the traction motors (AC or DC).
The traction alternator usually incorporates integral silicon diode rectifiers to provide the traction motors with up to
1200 volts DC (DC traction, which is used directly) or the common inverter bus (AC traction, which is first inverted
from dc to three-phase ac).
The first diesel electric locomotives, and many of those still in service, use DC generators as, before silicon power
electronics, it was easier to control the speed of DC traction motors. Most of these had two generators: one to generate
the excitation current for a larger main generator.
Optionally, the generator also supplies head end power (HEP) or power for electric train heating. The HEP option
requires a constant engine speed, typically 900 RPM for a 480 V 60 Hz HEP application, even when the locomotive is
not moving.
Marine alternators
Marine alternators used in yachts are similar to automotive alternators, with appropriate adaptations to the salt-water
environment. Marine alternators are designed to be explosion proof so that brush sparking will not ignite explosive gas
mixtures in an engine room environment. They may be 12 or 24 volt depending on the type of system installed. Larger
marine diesels may have two or more alternators to cope with the heavy electrical demand of a modern yacht. On single
alternator circuits, the power may be split between the engine starting battery and the domestic or house battery (or
batteries) by use of a split-charge diode (battery isolator) or a voltage-sensitive relay.
Radio alternators
High frequency alternators of the variable-reluctance type were applied commercially to radio transmission in the low-
frequency radio bands. These were used for transmission of Morse code and, experimentally, for transmission of voice
and music. In the Alexanderson alternator, both the field winding and armature winding are stationary, and current is
induced in the armature by virtue of the changing magnetic reluctance of the rotor (which has no windings or current
carrying parts). Such machines were made to produce radio frequency current for radio transmissions, although the
efficiency was low.
If the rectangular turn rotates in clockwise direction against axis a-b as shown in the below figure, then after
completing 90 degrees rotation the conductor sides AB and CD comes in front of the S-pole and N-pole
respectively. Thus, now we can say that the conductor tangential motion is perpendicular to magnetic flux
lines from north to south pole.
So, here rate of flux cutting by the conductor is maximum and induces current in the conductor, the direction
of the induced current can be determined using Fleming’s right hand rule. Thus, we can say that current will
pass from A to B and from C to D. If the conductor is rotated in a clockwise direction for another 90 degrees,
then it will come to a vertical position as shown in the below figure.
Now, the position of conductor and magnetic flux lines are parallel to each other and thus, no flux is cutting
and no current will be induced in the conductor. Then, while the conductor rotates from clockwise for
another 90 degrees, then rectangular turn comes to a horizontal position as shown in the below figure. Such
that, the conductors AB and CD are under the N-pole and S-pole respectively. By applying Fleming’s right
hand rule, current induces in conductor AB from point B to A and current induces in a conductor CD from
point D to C.
P.GANESH,EEE Department AC Machines 73
So, the direction of current can be indicated as A – D – C – B and direction of current for the previous
horizontal position of rectangular turn is A – B – C – D. If the turn is again rotated towards vertical position,
then the induced current again reduces to zero. Thus, for one complete revolution of rectangular turn the
current in the conductor reaches to maximum & reduces to zero and then in the opposite direction it reaches
to maximum & again reaches to zero. Hence, one complete revolution of rectangular turn produces one full
sine wave of current induced in the conductor which can be termed as the generation of alternating current by
rotating a turn inside a magnetic field.
Now, if we consider a practical synchronous generator, then field magnets rotate between the stationary
armature conductors. The synchronous generator rotor and shaft or turbine blades are mechanically coupled
to each other and rotates at synchronous speed. Thus, the magnetic flux cutting produces an induced emf
which causes the current flow in armature conductors. Thus, for each winding the current flows in one
direction for the first half cycle and current flows in the other direction for the second half cycle with a time
lag of 120 degrees (as they displaced by 120 degrees). Hence, the output power of synchronous generator can
be shown as below figure.
Do you want to know more about synchronous generators and are you interested in designing electronics
projects? Feel free to share your views, ideas, suggestions, queries, and comments in the comment section
below.
EMF Equation of a Synchronous Generator
The generator which runs at a synchronous speed is known as the synchronous generator. The synchronous
generator converts the mechanical power into electrical energy for the grid.The Derivation of EMF Equation of a
synchronous generator is given below.
Let,
P be the number of poles
ϕ is Flux per pole in Webers
N is the speed in revolution per minute (r.p.m)
f be the frequency in Hertz
Zph is the number of conductors connected in series per phase
Tph is the number of turns connected in series per phase
Kc is the coil span factor
Kd is the distribution factor
Flux cut by each conductor during one revolution is given as Pϕ Weber. Time taken to complete one
revolution is given by 60/N sec
Average EMF induced per conductor will be given by the equation shown below
The average EMF equation is derived with the following assumptions given below.
Coils have got the full pitch.
All the conductors are concentrated in one stator slot.
Root mean square (R.M.S) value of the EMF induced per phase is given by the equation shown below.
Eph = Average value x form factor
Therefore,
If the coil span factor Kc and the distribution factor Kd , are taken into consideration than the Actual EMF
induced per phase is given as
Equation (1) shown above is the EMF equation of the Synchronous Generator.
Coil Span Factor
The Coil Span Factor is defined as the ratio of the induced emf in a coil when the winding is short
pitched to the induced emf in the same coil when the winding is full pitched.
Distribution Factor
Distribution factor is defined as the ratio of induced EMF in the coil group when the winding is
distributed in a number of slots to the induced EMF in the coil group when the winding is concentrated
in one slot.
Armature Reaction in Alternator or Synchronous Generator
Every rotating electrical machine works based on Faraday’s law. Every electrical machine requires a
magnetic field and a coil (Known as armature) with a relative motion between them. In case of an alternator,
we supply electricity to pole to produce magnetic field and output power is taken from the armature. Due to
relative motion between field and armature, the conductor of armatures cut the flux of magnetic field and
hence there would be changing flux linkage with these armature conductors. According to Faraday’s law of
electromagnetic induction there would be an emf induced in the armature. Thus, as soon as the load is
connected with armature terminals, there is a current flowing in the armature coil.
As soon as current starts flowing through the armature conductor there is one reverse effect of this current on
the main field flux of the alternator (or synchronous generator). This reverse effect is referred as armature
The electromechanical energy conversion takes place through magnetic field as a medium. Due to relative
motion between armature conductors and the main field, an emf is induced in the armature windings whose
magnitude depends upon the relative speed and as well as the magnetic flux. Due to armature reaction, flux is
reduced or distorted, the net emf induced is also affected and hence the performance of the machine
degrades.
Armature Reaction in Alternator
In an alternator like all other synchronous machines, the effect of armature reaction depends on the power
factor i.e the phase relationship between the terminal voltage and armature current.
Reactive power (lagging) is the magnetic field energy, so if the generator supplies a lagging load, this implies
that it is supplying magnetic energy to the load. Since this power comes from excitation of synchronous
machine, the net reactive power gets reduced in the generator.
Hence, the armature reaction is demagnetizing. Similarly, the armature reaction has magnetizing effect when
the generator supplies a leading load (as leading load takes the leading VAR) and in return gives lagging
VAR (magnetic energy) to the generator. In case of purely resistive load, the armature reaction is cross
magnetizing only.
The armature reaction of alternator or synchronous generator, depends upon the phase angle between, stator
armature current and induced voltage across the armature winding of alternator.
The phase difference between these two quantities, i.e. Armature current and voltage may vary from –
90oto+90oIfthisangleisθ,then,
To understand actual effect of this angle on armature reaction of alternator, we will consider three standard
cases,
When θ = 0
When θ = 90o
When θ = – 90o
Armature Reaction of Alternator at Unity Power Factor
At unity power factor, the angle between armature current I and induced emf E, is zero. That means,
armature current and induced emf are in same phase. But we know theoretically that emf induced in the
P.GANESH,EEE Department AC Machines 76
armature is due to changing main field flux, linked with the armature conductor.
As the field is excited by DC, the main field flux is constant in respect to field magnets, but it would be
alternating in respect of armature as there is a relative motion between field and armature in the alternator. If
Hence, from these above equations (1) and (2) it is clear that the angle between, φ f and induced emf E will be
90o.
Now, armature flux φa is proportional to armature current I. Hence, armature flux φa is in phase with armature
current I.
Again at unity electrical power factor I and E are in same phase. So, at unity power factor, φa is phase with E.
So at this condition, armature flux is in phase with induced emf E and field flux is in quadrature with E.
Hence, armature flux φa is in quadrature with main field flux φ f.
As this two fluxes are perpendicular to each other, the armature reaction of the alternator at unity power
factor is purely distorting or cross-magnetizing type.
As the armature flux pushes the main field flux perpendicularly, distribution of main field flux under a pole
face does not remain uniformly distributed. The flux density under the trailing pole tips increases somewhat
while under the leading pole tips it decreases.
Armature Reaction of Alternator at Lagging Zero Power Factor
At lagging zero electrical power factor, the armature current lags by 90 o to induced emf in the armature.
As the emf induced in the armature coil due to main field flux thus the emf leads the main field flux by 90o.
From equation (1) we get, the field flux,
1.TheswitchSisopened.
2. The alternator is made to rotate using prime mover at synchronous speed and same speed is
maintainedconstantthroughoutthetest.
3. The excitation value is changed using a potential divider, from zero up to the rated value in a definite
number of steps. The open circuit EMF is measured with the help of voltmeter. The
readingsaretabulated.
4. A graph of If and (Voc)ph i.e. field current and open circuit voltage per phase is plotted to some scale.
This is open circuit characteristics.
2.Zero power factor test:
To conduct zero power factor test, the switch S is kept closed. Due to this, a purely inductive load
gets connected to an alternator through an ammeter. A purely inductive load has a power factor of cos
90° i.e. zero lagging hence the test is called zero power factor test.
The machine speed is maintained constant at its synchronous value. The load current delivered by
an alternator to purely inductive load is maintained constant at its rated full load value by varying
excitation and by adjusting variable inductance of the inductive bad. Note that, due to purely inductive
load, an alternator will always operate at zero power factor lagging.
Key Point: In this test, there is no need to obtain a number of points to obtain the curve. Only two points
are enough to construct a curve called zero power factor saturation curve.
The below is the graph of terminal voltage against excitation when delivering full load zero
power factor current. One point for this curve is zero terminal voltage (short circuit condition) and the
field current required to deliver full load short circuit armature current. While other point field current
required to obtain rated terminal voltage while delivering rated full load armature current. With the help
of these two points, the zero power factor saturation curve can be obtained as
1. Plot open circuit characteristics on a graph paper as shown in the below figure.
2. Plot the excitation corresponding to zero terminal voltage i.e. short circuit full zero power
3. Draw the tangent to O.C.C. through origin which is line OB as shown dotted in below figure. This is
called air line.
4. Draw the horizontal line PQ parallel and equal to OA.
5. From point, Q draw the line parallel to the air line which intersects O.C.C. at point R. Join RQ and
join PR. The triangle PQR is called Potier triangle.
6. From point R, drop a perpendicular on PQ to meet at point S.
7. The zero power factor full load saturation curve is now be constructed by moving triangle PQR so
that R remains always on OCC and line PQ always remains horizontal. The dotted triangle is shown in
the above figure. It must be noted that the Potier triangle once obtained is constant for a given armature
current and hence can be transferred as it is.
8.Though point A, draw a line parallel to PR meeting OCC at point B. From B, draw a perpendicular on
OA to meet it at point C. Triangles OAB and PQR are similar triangles.
9. The perpendicular RS gives the voltage drop due to the armature leakage reactance i.e. IXL
10. The length PS gives field current necessary to overcome the demagnetising effect of armature
reaction at full load.
11. The length SQ represents field current required to induce an EMF for balancing leakage reactance
drop RS. These values can be obtained from any Potier triangle such as OAB, PQR and so on.
So armature leakage reactance can be obtained as,
Because of the assumption of unsaturated magnetic circuit the regulation computed by this method will
be less than the actual and hence this method of regulation is called optimistic method.
The two components of total field m.m.f. which are FO and FAR are indicated in O.C.C. (open circuit
characteristics) and S.C.C. (short circuit characteristics) as shown in the Figure.
Zero lagging p.f. : As long as power factor is zero lagging, the armature reaction is completely
demagnetising. Hence the resultant FR is the algebraic sum of the two components FO and FAR. Field
m.m.f. is not only required to produce rated terminal voltage but also required to overcome completely
demagnetising armature reaction effect.
OA = FO
AB = FAR cross magnetising
OB=FR=FO=FAR.
2 3,000 3,600
4 1,500 1,800
6 1,000 1,200
8 750 900
10 600 720
12 500 600
14 429 514
16 375 450
18 333 400
20 300 360
22 273 327
24 250 300
26 231 277
28 214 257
30 200
The axis along the axis of the rotor is called the direct or the d axis. The axis perpendicular to d axis is
known as the quadrature or q axis. The direct axis flux path involves two small air gaps and is the path
of the minimum reluctance. The path shown in the above figure by ϕq has two large air gaps and is the
path of the maximum reluctance.
The rotor flux BR is shown vertically upwards as shown in the figure below.
As, Rd < Rq, the direct axis component of MMF Fd produces more flux than the quadrature axis
component of the MMF. The fluxes of the direct and quadrature axis produce a voltage in the windings
of the stator by armature reaction.
Let,
Ead be the direct axis component of the armature reaction voltage.
Eaq be the quadrature axis component of the armature reaction voltage.
Since each armature reaction voltage is directly proportional to its stator current and lags behind by 90
degrees angles. Therefore, armature reaction voltages can be written as shown below.
The voltage E’ is equal to the sum of the terminal voltage V and the voltage drops in the resistance and
leakage reactance of the armature. The equation is written as
The armature current is divided into two components; one is the phase with the excitation voltage E f and
the other is in phase quadrature to it.
If
Iq is the axis component of Ia in phase with Ef.
Id is the d axis Ia lagging Ef by 90 degrees.
Therefore,
Let,
The reactance Xd is called the direct axis synchronous reactance, and the reactance Xq is called
the quadrature axis synchronous reactance.
Combining the equations (9) (10) and (11), we get the equations shown below.
The equation (12) shown above is the final voltage equation for a salient pole synchronous generator.
P.GANESH,EEE Department AC Machines 89
Parallel Operation of Alternator
Alternator is really an AC generator. In alternator, an EMF is induced in the stator (stationary wire) with
the influence of rotating magnetic field (rotor) due to Faraday’s law of induction. Due to the
synchronous speed of rotation of field poles, it is also known as synchronous generator.
Here, we can discuss about parallel operation of alternator. When the AC power systems are
interconnected for efficiency, the alternators should also have to be connected in parallel. There will be
more than two alternators connected in parallel in generating stations.
Condition for Parallel Operation of Alternator
There are some conditions to be satisfied for parallel operation of the alternator. Before entering into
that, we should understand some terms which are as follows.
The process of connecting two alternators or an alternator and an infinite bus bar system in parallel is
known as synchronizing.
Running machine is the machine which carries the load.
Incoming machine is the alternator or machine which has to be connected in parallel with the system.
The conditions to be satisfied are
The phase sequence of the incoming machine voltage and the bus bar voltage should be identical.
The RMS line voltage (terminal voltage) of the bus bar or already running machine and the incoming
machine should be the same.The phase angle of the two systems should be equal.
The frequency of the two terminal voltages (incoming machine and the bus bar) should be nearly the
same. Large power transients will occur when frequencies are not nearly equal.
Departure from the above conditions will result in the formation of power surges and current. It also
results in unwanted electro-mechanical oscillation of rotor which leads to the damage of equipment.
General Procedure for Paralleling Alternators
The figure below shows an alternator (generator 2) being paralleled with a running power system
(generator 1). These two machines are about to synchronize for supplying power to a load. Generator 2
is about to parallel with the help of a switch, S1. This switch should never be closed without satisfying
the above conditions.
To make the terminal voltages equal. This can be done by adjusting the terminal voltage of
incoming machine by changing the field current and make it equal to the line voltage of running system
using voltmeters.
There are two methods to check the phase sequence of the machines. They are as follows
First one is using a Synchroscope. It is not actually check the phase sequence but it is used to measure
the difference in phase angles.
Second method is three lamp method (Figure 2). Here we can see three light bulbs are connected to the
Next, we have to check and verify the incoming and running system frequency. It should be nearly the
same. This can be done by inspecting the frequency of dimming and brightening of lamps.
When the frequencies are nearly equal, the two voltages (incoming alternator and running system) will
alter the phase gradually. These changes can be observed and the switch, S1 can be made closed when
the phase angles are equal.
Advantages of Parallel Operating Alternators
When there is maintenance or an inspection, one machine can be taken out from service and the other
alternators can keep up for the continuity of supply.
Load supply can be increased.
During light loads, more than one alternator can be shut down while the other will operate in nearly full
load.
High efficiency.
The operating cost is reduced.
Ensures the protection of supply and enables cost-effective generation.
The generation cost is reduced.
Breaking down of a generator does not cause any interruption in the supply.
Reliability of the whole power system increases.
If is the number of pole pairs (rarely, planes of commutation) instead, simply divide both formulas
by 2.
Examples
A single-phase, 4-pole (2-pole-pair) synchronous motor is operating at an AC supply frequency of
50 Hz. The number of pole-pairs is 2, so the synchronous speed is:
A three-phase, 12-pole (6-pole-pair) synchronous motor is operating at an AC supply frequency of
60 Hz. The number of pole-pairs is 6, so the synchronous speed is:
Construction
The principal components of a synchronous motor are the stator and the rotor. The stator of synchronous
motor and stator of induction motor are similar in construction. With the wound-rotor synchronous
doubly fed electric machine as the exception, the stator frame contains wrapper plate. Circumferential
ribs and keybars are attached to the wrapper plate. To carry the weight of the machine, frame
mounts and footings are required. When the field winding is excited by DC excitation, brushes and slip
rings are required to connect to the excitation supply. The field winding can also be excited by a
brushless exciter. Cylindrical, round rotors, (also known as non salient pole rotor) are used for up to six
poles. In some machines or when a large number of poles are needed, a salient pole rotor is used. The
construction of synchronous motor is similar to that of a synchronous alternator.
Operation
The operation of a synchronous motor is due to the interaction of the magnetic fields of the stator and
the rotor. Its stator winding, which consists of a 3 phase winding, is provided with a 3 phase supply, and
the rotor is provided with a DC supply. The 3 phase stator winding carrying 3 phase currents produces 3
phase rotating magnetic flux (and therefore a rotating magnetic field). The rotor locks in with the
rotating magnetic field and rotates along with it. Once the rotor field locks in with the rotating magnetic
field, the motor is said to be in synchronization. A single-phase (or two-phase derived from single
phase) stator winding is possible, but in this case the direction of rotation is not defined and the machine
may start in either direction unless prevented from doing so by the starting arrangements.
Once the motor is in operation, the speed of the motor is dependent only on the supply frequency. When
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the motor load is increased beyond the breakdown load, the motor falls out of synchronization and the
field winding no longer follows the rotating magnetic field. Since the motor cannot produce
(synchronous) torque if it falls out of synchronization, practical synchronous motors have a partial or
complete squirrel-cage damper (amortisseur) winding to stabilize operation and facilitate starting.
Because this winding is smaller than that of an equivalent induction motor and can overheat on long
operation, and because large slip-frequency voltages are induced in the rotor excitation winding,
synchronous motor protection devices sense this condition and interrupt the power supply (out of step
protection).
Starting methods
Above a certain size, synchronous motors are not self-starting motors. This property is due to the inertia
of the rotor; it cannot instantly follow the rotation of the magnetic field of the stator. Since a
synchronous motor produces no inherent average torque at standstill, it cannot accelerate to synchronous
speed without some supplemental mechanism.
Large motors operating on commercial power frequency include a squirrel-cage induction winding
which provides sufficient torque for acceleration and which also serves to damp oscillations in motor
speed in operation.[2] Once the rotor nears the synchronous speed, the field winding is excited, and the
motor pulls into synchronization. Very large motor systems may include a "pony" motor that accelerates
the unloaded synchronous machine before load is applied. Motors that are electronically controlled can
be accelerated from zero speed by changing the frequency of the stator current.
Very small synchronous motors are commonly used in line-powered electric mechanical clocks or timers
that use the power line frequency to run the gear mechanism at the correct speed. Such small
synchronous motors are able to start without assistance if the moment of inertia of the rotor and its
mechanical load is sufficiently small [because the motor] will be accelerated from slip speed up to
synchronous speed during an accelerating half cycle of the reluctance torque." Single-phase synchronous
motors such as in electric wall clocks can freely rotate in either direction unlike a shaded-pole type.
See Shaded-pole synchronous motor for how consistent starting direction is obtained.
The operational economics is an important parameter to address different motor starting
methods. Accordingly, the excitation of the rotor is a possible way to solve the motor starting issue. In
additions, modern proposed starting methods for large synchronous machines includes repetitive polarity
inversion of the rotor poles during startup.
Applications, special properties, and advantages
Use as synchronous condenser
By varying the excitation of a synchronous motor, it can be made to operate at lagging, leading and
unity power factor. Excitation at which the power factor is unity is termed normal excitation
voltage.[31] The magnitude of current at this excitation is minimum.[31] Excitation voltage more than
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normal excitation is called over excitation voltage, excitation voltage less than normal excitation is
called under excitation.[31] When the motor is over excited, the back emf will be greater than the motor
terminal voltage. This causes a demagnetizing effect due to armature reaction.
The V curve of a synchronous machine shows armature current as a function of field current. With
increasing field current armature current at first decreases, then reaches a minimum, then increases. The
minimum point is also the point at which power factor is unity.
This ability to selectively control power factor can be exploited for power factor correction of the power
system to which the motor is connected. Since most power systems of any significant size have a net
lagging power factor, the presence of overexcited synchronous motors moves the system's net power
factor closer to unity, improving efficiency. Such power-factor correction is usually a side effect of
motors already present in the system to provide mechanical work, although motors can be run without
mechanical load simply to provide power-factor correction. In large industrial plants such as factories
the interaction between synchronous motors and other, lagging, loads may be an explicit consideration
in the plant's electrical design
Steady state stability limit
where, is the torque
is the torque angle
is the maximum torque
here,
When load is applied, torque angle increases. When = 90° the torque will be maximum. If
load is applied further then the motor will lose its synchronism, since motor torque will be less than load
torque. The maximum load torque that can be applied to a motor without losing its synchronism is called
steady state stability limit of a synchronous motor.
Other
Synchronous motors are especially useful in applications requiring precise speed and/or position control.
Speed is independent of the load over the operating range of the motor.
Speed and position may be accurately controlled using open loop controls; e.g., stepper motors.
Low-power applications include positioning machines, where high precision is required,
and robot actuators.
They will hold their position when a DC current is applied to both the stator and the rotor windings.
A clock driven by a synchronous motor is in principle as accurate as the line frequency of its power
source. (Although small frequency drifts will occur over any given several hours, grid operators actively
adjust line frequency in later periods to compensate, thereby keeping motor-driven clocks accurate;
see Utility frequency#Stability.)
MODULE-IV
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1 .Explain the constructional details of the alternator used in thermal station.
2. Explain in detail why the air gap of synchronous machine is always greater than induction machine
3 . Explain with neat circuit diagram predetermination of voltage regulation of alternator by potier
triangle method.
4. Explain the need and condition for paralleling the alternator.
5. In a 1500KVA ,3.3KV,50HZ 3-phase star connected alternator, a field current of 60A produces a
short circuit current of 350A and open circuit voltage of 1.5KV (line to line) calculate the voltage
regulation at full load 0.8PF lag and 0.9pf lead .The armature resistance per phase is 0.4ohm.
6. Two similar 4000KVA alternator operating in parallel. the governor of the first machines is such that
frequency drops from 50HZ at no load to 47HZ at full load and the second is at 50HZ at no load and
48.5 HZ at full load.
(a) How they will share a load of 6000KVA
(b)How much max UPF load can they carry
MODULE-V
1.Explain the principle of operation of synchronous motor and why they are not self starting?
2.Explain the different methods of starting of synchronous motors.
3.What is the hunting and explain its effects and remedies in details
4.Explain the effect of increased load with constant excitation in a synchronous motor.
5.Draw and explain V and inverted V curves of a synchronous motor
6.Explain the effect of load in synchronous motor with phasor diagram