Experiment # 7

Load Test of a Three Phase Induction Motor

Equipment:

1. Terminal Board
2. Measuring unit
3. 3-phase Induction motor
4. DC generator
5. Brake Control Unit

Theoretical Background:

Induction Motor:

Principle:

In AC motors the rotor does not receive the electrical power by
conduction but by induction in the same way as in case of secondary of a
2-winding transformer receives its power from primary. Thats why such
motors are called as induction motors. In fact the induction motor can be
treated as rotating transformer i.e. one in which primary winding is
stationary but the secondary is free to rotate.
Where a poly phase electrical supply is available, the three-phase (or
poly phase) AC induction motor is commonly used, especially for higher-
powered motors. The phase differences between the three phases of the
poly phase electrical supply create a rotating electromagnetic field in the
motor. Through electromagnetic induction, the rotating magnetic field
induces a current in the conductors in the rotor, which in turn sets up a
counterbalancing magnetic field that causes the rotor to turn in the direction
the field is rotating. The rotor must always rotate slower than the rotating
magnetic field produced by the poly phase electrical supply; otherwise, no
counterbalancing field will be produced in the rotor.



Construction:
An induction motor essentially consist of two main parts
(a) Stator
(b) Rotor
(a) Stator:
The stator is the outer body of the motor which houses the driven
windings on an iron core. In a single speed three phase motor design, the
standard stator has three windings, while a single phase motor typically has
two windings. The stator core is made up of a stack of round pre-punched
laminations pressed into a frame which may be made of aluminum or cast
iron.

The laminations are basically round with a round hole inside through
which the rotor is positioned. The inner surface of the stator is made up of a
number of deep slots or grooves right around the stator.

It is into these slots that the windings are positioned. The
arrangement of the windings or coils within the stator determines the
number of poles that the motor has.
A standard bar magnet has two poles, generally known as North and
South. Likewise, an electromagnet also has a North and a South pole.

As the induction motor Stator is essentially like one or more
electromagnets depending on the stator windings, it also has poles in
multiples of two. i.e. 2 pole, 4 pole, 6 pole etc. Greater the no of poles,
greater the speed and vice versa. The stator winding ,when supplied with 3
phase currents ,produces a magnetic flux which is for constant magnitude
but revolves at synchronous speed (given by Ns=120f/p).This revolving
magnetic flux induces an emf in the rotor by mutual induction.

The winding configuration, slot configuration and lamination steel all
have an effect on the performance of the motor. The voltage rating of the
motor is determined by the number of turns on the stator and the power
rating of the motor is determined by the losses which comprise copper loss
and iron loss, and the ability of the motor to dissipate the heat generated by
these losses. The stator design determines the rated speed of the motor
and most of the full load, full speed characteristics.



(b) Rotor:

The Rotor comprises a cylinder made up of round laminations
pressed onto the motor shaft, and a number of short-circuited windings.
The rotor windings are made up of rotor bars passed through the rotor,
from one end to the other, around the surface of the rotor. The bars
protrude beyond the rotor and are connected together by a shorting ring at
each end. The bars are usually made of aluminum or copper, but
sometimes made of brass.

The position relative to the surface of the rotor, shape, cross sectional
area and material of the bars determine the rotor characteristics.
Essentially, the rotor windings exhibit inductance and resistance, and these
characteristics can effectively be dependent on the frequency of the current
flowing in the rotor. A bar with a large cross sectional area will exhibit a low
resistance, while a bar of a small cross sectional area will exhibit a high
resistance.

Likewise a copper bar will have a low resistance compared to a brass
bar of equal proportions. Positioning the bar deeper into the rotor,
increases the amount of iron around the bar, and consequently increases
the inductance exhibited by the rotor. The impedance of the bar is made up
of both resistance and inductance, and so two bars of equal dimensions will
exhibit different A.C. impedance depending on their position relative to the
surface of the rotor. A thin bar which is inserted radialy into the rotor, with
one edge near the surface of the rotor and the other edge towards the
shaft, will effectively change in resistance as the frequency of the current
changes. This is because the A.C. impedance of the outer portion of the
bar is lower than the inner impedance at high frequencies lifting the
effective impedance of the bar relative to the impedance of the bar at low
frequencies where the impedance of both edges of the bar will be lower
and almost equal. The rotor design determines the starting characteristics.

Types of Rotor:

(a) Squirrel cage rotor
(b) 2-phase Wound or Wound rotor


(a) Squirrel Cage Rotor:
Most common AC motors use the squirrel cage rotor, which will be
found in virtually all domestic and light industrial alternating current motors.
The squirrel cage takes its name from its shape - a ring at either end of the
rotor, with bars connecting the rings running the length of the rotor. It is
typically cast aluminum or copper poured between the iron laminates of the
rotor, and usually only the end rings will be visible. The vast majority of the
rotor currents will flow through the bars rather than the higher-resistance
and usually varnished laminates. Very low voltages at very high currents
are typical in the bars and end rings; high efficiency motors will often use
cast copper in order to reduce the resistance in the rotor.
In operation, the squirrel cage motor may be viewed as a transformer
with a rotating secondary - when the rotor is not rotating in sync with the
magnetic field, large rotor currents are induced; the large rotor currents
magnetize the rotor and interact with the stator's magnetic fields to bring
the rotor into synchronization with the stator's field. An unloaded squirrel
cage motor at synchronous speed will consume electrical power only to
maintain rotor speed against friction and resistance losses; as the
mechanical load increases, so will the electrical load - the electrical load is
inherently related to the mechanical load. This is similar to a transformer,
where the primary's electrical load is related to the secondary's electrical
load.

This is why, as an example, a squirrel cage blower motor may cause
the lights in a home to dim as it starts, but doesn't dim the lights when its
fan-belt (and therefore mechanical load) is removed. Furthermore, a stalled
squirrel cage motor (overloaded or with a jammed shaft) will consume
current limited only by circuit resistance as it attempts to start. Unless
something else limits the current (or cuts it off completely) overheating and
destruction of the winding insulation is the likely outcome.
Virtually every washing machine, dishwasher, standalone fan, record
player, etc. uses some variant of a squirrel cage motor.
(b) Wound Rotor:
An alternate design, called the wound rotor, is used when variable
speed is required. In this case, the rotor has the same number of poles as
the stator and the windings are made of wire, connected to slip rings on the
shaft. Carbon brushes connect the slip rings to an external controller such
as a variable resistor that allows changing the motor's slip rate. In certain
high-power variable speed wound-rotor drives, the slip-frequency energy is
captured, rectified and returned to the power supply through an inverter.
Compared to squirrel cage rotors, wound rotor motors are expensive and
require maintenance of the slip rings and brushes, but they were the
standard form for variable speed control before the advent of compact
power electronic devices. Transistorized inverters with variable frequency
drive can now be used for speed control, and wound rotor motors are
becoming less common. (Transistorized inverter drives also allow the
more-efficient three-phase motors to be used when only single-phase
mains current is available, but this is never used in household appliances,
because it can cause electrical interference and because of high power
requirements.)
This type of motor is becoming more common in traction applications such
as locomotives, where it is known as the asynchronous traction motor.
The speed of the AC motor is determined primarily by the frequency of the
AC supply and the number of poles in the stator winding, according to the
relation:

N
s
= 120f / p

Where

N
s
= Synchronous speed, in revolutions per minute
F = AC power frequency
p = Number of poles per phase winding

Slip:
Actual RPM for an induction motor will be less than this calculated
synchronous speed by an amount known as slip, that increases with the
torque produced. With no load, the speed will be very close to
synchronous. When loaded, standard motors have between 2-3% slip,
special motors may have up to 7% slip, and a class of motors known as
torque motors are rated to operate at 100% slip (0 rpm/full stall).

The slip of the AC motor is calculated by:

s = (N
s
N
r
) / N
s


Where

N
r
= Rotational speed, in revolutions per minute.
s = Normalized Slip, 0 to 1.

Of all AC machines the poly phase induction motor is the one which is
extensively used for various kinds of industrial drives. It has the followings
main advantages and disadvantages

Production of Rotating Magnetic Field:

The fundamental principle of operation of AC machine operation is
that if a 3-phase set of currents, each of equal magnitude and difference by
120 (a balanced 3-phase system) flows in a 3-phase winding, then it will
produce a rotating magnetic field of constant magnitude.

Conditions for Rotating Magnetic Field:

To produce a rotating magnetic field, the following conditions must be met:
1. The supply must be poly phase
2. There must be angular displacement between the axis of the coils
for the a, b, and c phases
Production of Torque:

The 3-phase stator windings set up a rotating magnetic field. The flux
produced passes through the air-gap, sweeps past the rotor surface and so
cuts the rotor conductors which are yet (at startup) stationary. Due to the
relative speed between the rotating flux and the stationary conductors, an
emf is induced in the latter according to Faradays law of electromagnetic
induction and this is dynamically induced emf. The frequency of the
induced emf is the same as the supply frequency. Its magnitude is
proportional to the relative velocity between the flux and the conductors
and its direction is given by Flemings Right-hand rule. Since the rotor
conductors form a closed circuit, rotor current is produced whose direction
as given by Lenzs law is such as to oppose the cause producing it. In this
case, the cause which produces the rotor current is the relative velocity
between the rotating flux and the stationary conductors. Hence to reduce
the relative speed, the rotor starts to rotate in the same direction as that of
the flux and tries to catch up with it.

Percentage Slip:

In practice, the rotor never succeeds in catching up with the stator
field. If it really did so, then there would be no relative speed between the
two, hence no rotor emf, no rotor current and therefore no torque to
maintain rotation. That is why the rotor runs at a speed which is always less
than the speed of the stator field. The difference in speed depends on the
load on the motor.

The difference between the synchronous speed N
S
and the actual
speed N
r
of the rotor divided by the synchronous speed is called the
percentage slip. Mathematically,
100 %

=
S
r S
N
N N
Slip




Losses in an Induction Motor:

The power input to an induction motor, P
in
, is in the form of three-
phase electric voltages and currents. The first loss in the machine is I
2
R
losses in the stator windings (the stator copper loss). Then some amount of
power is lost as hysteresis and eddy currents in the stator (stator core
losses). The power remaining at this point is transferred to the rotor of the
machine across the air gap between the stator and the rotor. This power is
called the air-gap power of the machine. After the power is transferred to
the rotor, some of it is lost as I
2
R losses (the rotor copper loss), and the rest
is converted from electrical to mechanical form. Finally, friction and
windage losses and stray losses are subtracted. The remaining power is
the output of the motor, P
out
.

The core losses in the induction motor come partially from the stator
circuit and partially from the rotor circuit. Since an induction motor normally
operates at a speed near synchronous speed, the relative motion of the
magnetic fields over the rotor surface is quite slow, and the rotor core
losses are very tiny compared to the stator core losses.

The higher the speed of an induction motor, the higher its friction,
windage and stray losses. On the other hand, the higher the speed of the
motor, the lower will be its core losses. Therefore, these three categories of
losses are sometimes lumped together and called rotational losses. The
total rotational losses of a motor are often considered to be constant with
changing speed, since the component losses change in opposite directions
with change in speed


Advantage:

It has very simple and extremely rugged, almost unbreakable
construction (especially squirrel cage type).
Its cost is low and is very reliable.
It has sufficiently high efficiency.
It requires minimum of maintenance.
It starts up from rest and needs no extra starting motor and has not
to be synchronized .its starting arrangement is simple especially for
squirrel cage type motor.

Disadvantages:

Its speed cannot be varied without sacrificing some of its efficiency.
Just like a dc shunt motor its speed decreases with increase in load.
Its starting torque is somewhat inferior to that of the dc shunt motor.

Block Diagram:



Procedure:

Set the apparatus as shown in the block diagram.

Set the rated line voltage, V
L
, and for each different value note the
values of line current, I
L
, the total true 3-phase power, W
T
, and the
total reactive power, Q, using measuring unit when you are applying
load on the DC Generator using Brake Control Unit. For P and Q
measurement refer to Appendix of Manual.

Measure the total power using the concept of 2-wattmeter method
(Use appendix for 2-wattmeter method) for 3-phase measurement.

Note the values of torque, T, the output power, P
out
, and the motor
speed, N
r
, are measured from the brake control unit (BCU) for each
value.

Calculate the values of power factor, cos , % slip and the efficiency,
from the formulas given.

Plot the graphs as mentioned








Observations: Synchronous Speed = N
s
= _______ rpm

S.No.
Line to
Line
Voltage
V
L


(V)
Line
Current
I
L



(A)
Total
Power
W
T



(W)
Reactive
Power
Q


(VAR)
Power
Factor
cos
Rotor
Speed
N
r


(rpm)
%
Slip
Torque



(N.m)
Output
Power
P
out



(W)
Efficiency,



(%)
1
2
3
4
5
6
7
8
9


Formulas


100 %

=
S
r S
N
N N
Slip
L L
T
I V
W
3
cos = 100 =
in
out
P
P













Graphs:

1. Plot output versus efficiency
2. Plot reactive power versus efficiency
3. Plot speed versus efficiency
4. Plot power factor versus efficiency
5. Plot torque versus efficiency
6. Plot torque slip curve
































Scale
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Scale
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Comments:

What is the purpose of load test of induction motor?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
________________________________________

Why one of the watt meters gives negative, zero and then positive reading
as the load is increased gradually.
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
________________________________________
Is it possible for both watt meters to give same reading, if possible then
why?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
________________________________________
What would you do with W
1
if its reading becomes negative?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
________________________________________
What is power factor of induction motor at no load and why?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
________________________________________


Why does power factor increases with load?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
________________________________________

What type of motor was used in experiment, slip ring or squirrel?
______________________________________________________________________
______________________________________________________________________
_________________________________

What type of motor was used in experiment, slip ring or squirrel?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
________________________________________
Is induction motor a variable speed motor?
______________________________________________________________________
______________________________________________________________________
_________________________________
Why does motor damage due to over loading.
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
________________________________________
Can power factor of induction motor be leading?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
________________________________________
















Marks Obtained = _________
Date = __________________

Signature = ______________
EXPERIMENT NO 8


TO RUN AN INDUCTION MOTOR AS INDUCTION GENERATOR


BLOCK DIAGRAM:












APPARATUS:

Induction motor
DC Machine
Break control unit
Measuring unit
Power supply


PROCEDURE:

First run DC motor (separately excited) and note the direction of rotation of the motor.
Connect ac motor separately and run it and note its direction of rotation.
If it is same as that of the DC motor then procedure further otherwise first make the
direction of rotation of both same by changing the phase sequence.
Start both of them together. Measuring unit will show the values of voltage, current, real
power and reactive power.
At this point the speed is less than 1500 rpm. Now increase the speed of the motor so that
it becomes equal to the rated speed that is 1500 rpm.
At this speed the motor is in floating condition i.e when slip becomes zero the machines
would be in floating condition. Again note the values of current, voltage, active and
reactive power for the floating condition.
Now increase the speed further from 1500 rpm. At this point the motor becomes
generator and the values of voltage, current active and reactive power become negative.
Note down these readings in the table.

Ind
Motor
DC
M/C
M.U BCU
Power
supply


OBSERVATIONS





































State of
machine
V I W
1
W
2
W
T
p.f Q
Ind. Motor
Floating
Ind.
Generator

EXPERIMENT # 9

CONSTRUCTION OF V-CURVES OF A SYNCHRONOUS MOTOR

THEORETICAL BACKGROUND

SYNCHRONOUS MOTOR
Synchronous motors are AC motors that have a field circuit
supplied by an external DC source. They convert AC electrical power to mechanical power. It is
electrically identical to an alternator or AC generator. Some characteristics of the synchronous
motor are:
It runs either at synchronous speed or not at all i.e. while running it maintains a constant
speed. The only way to change its speed is to vary the supply frequency.
It is not self-starting. It has to be run up to synchronous or near synchronous speed by
some means before it can be synchronized to the supply.
It is capable of being operated under a wide range of power factors both lagging and
leading.

PRODUCTION OF TORQUE:
In a synchronous motor, a three-phase set of stator currents
produces a rotating magnetic field, B
S
. The field current, I
F
of the motor produces a steady-state
magnetic field, B
R
. Therefore, there are two magnetic fields present in the machine, and the rotor
field will tend to line up with the stator field, just as two bar magnets will tend to line up if
placed near each other. Since the stator magnetic field is rotating, the rotor magnetic field will
constantly try o catch up. Larger the angle between the two magnetic fields, the greater the
torque on the rotor of the motor. The basic principle of a synchronous motor operation is that the
rotor chases the rotating stator magnetic field around in a circle, never catching up with it.

SPEED OF SYNCHRONOUS MOTOR:
The rotor (which is initially unexcited) is speeded up to
synchronous or near synchronous speed by some arrangements and then excited by the DC
source. The moment this synchronously rotating rotor is excited, it is magnetically locked into
position with the stator i.e. the rotor poles are engaged with the stator poles and both run
synchronously in the same direction. It is because of this interlocking of stator and rotor poles
that the motor has either to run synchronously or not at all. The synchronous speed is given by
the usual relation:
P
f
N
S
120
=
However, this engagement is not very rigid. As the load on the motor is increased, the rotor
progressively tends to fall back in phase by some angle but it still continues to run
synchronously.

V-CURVES:
The V-curves of a synchronous motor show how armature current varies with its
field current when motor input is kept constant. These are obtained by plotting armature
current while motor input is kept constant and are so called because of their shape. There is a
family of such curves, each corresponding to a definite power intake.
To draw these curves experimentally, the motor is run from constant voltage and constant
frequency bus bars. Power input to motor is kept constant at a definite value. Next, the field
current is increased in small steps and corresponding armature currents are noted. When plotted,
we get a V-curve for a particular constant motor input. Similar curves are drawn by keeping
motor input constant at different values.

EFFECT OF FIELD CURRENT CHANGES:
Considering a synchronous motor in which the
mechanical load is constant. When the field current is increased, the magnitude of the back emf,
E
A
in the motor increases, but does not affect the real power supplied by the motor. The power
supplied by the motor changes only when the shaft load torque changes. Since a change in
armature current, I
A
, does not affect the shaft speed and since the load attached to the shaft is
unchanged, the real power supplied is unchanged. The terminal voltage is also kept constant by
the power source supplying the motor.
Therefore, as the value of E
A
increases, the magnitude of I
A
first decreases and then increases
again. At low E
A
, the armature current is lagging and the motor is an inductive load. It is
therefore consuming reactive power Q. As the field current is increased, E
A
increases and the
armature current eventually lines up with the voltage and the motor is purely resistive. As the
field current is further increased, the armature current becomes leading, and the motor becomes a
capacitive load. So now it consumes negative reactive power Q or alternatively supplying
reactive power to the system.

A plot of I
A
versus I
F
for a synchronous motor is as
shown in Figure-1 above. For each curve, the minimum armature current occurs at unity power
factor, when only real power is being supplied to the motor. At any other point on the curve,
some reactive power is being supplied to or by the motor as well. For field currents less than the
value giving minimum I
A
, the armature current is lagging, consuming Q. In this situation, the
motor is said to be underexcited. For field currents greater than the value giving minimum I
A
,
the armature current is leading, supplying Q to the power system as a capacitor would. This case
is for an overexcited motor.Therefore, by controlling the field current of a synchronous motor,
the reactive power supplied to or consumed by the power system can be controlled.

Also, as explained above that an overexcited motor
can be run with leading power factor, this property renders it extremely important in phase
advancing purposes in industrial loads driven by induction motors and lighting and heating loads
supplied through transformers. Both transformers and induction motors draw lagging currents
from the line. Especially on light loads, the power drawn by them has a large reactive component
and the power factor has a very low value. This reactive power results in losses in many ways.
By using synchronous motors in conjunction with induction motors or transformers, the lagging
reactive power required by the latter is supplied locally by the leading reactive component taken
by the former, thereby relieving the line and generators of much of the reactive component.
When used in this way, the synchronous motor is called a synchronous capacitor because it
draws leading current from the line.


BLOCK DIAGRAM




APPARATUS:

Terminal Board
Measuring unit
Synchronous motor
variable DC supply
DC generator
Break control unit
PROCEDURE
Set up the apparatus as shown in Figure.
Give a constant voltage and constant frequency supply to the motor (using infinite bus-
bars).
Vary the field current and note the corresponding value of armature current for each
value.
Calculate the power factor using equation (1) below.
Plot a graph of field current against armature current which is the V-curve.
Repeat the above steps for different loads.


OBSERVATIONS

V = 220V


At no load With medium load With full Load
Ia If Ia If Ia If













DRAW THE V-CURVES HERE