Lebanese University
Faculty of Sciences (I)
Physics and Electronics Department
ELEC 3305 Electric Machines Laboratory Manual
2020-2021
ELEC 3305 Electric Machines Laboratory Manual
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SAFETY PRECAUTIONS
There is high voltage AC and current so, when taking measurements we take care
that the connections are secure. We don't make connections with the power on. We
ensure that the power is turned off after taking the measurements.
Nine rules for safe practice and to avoid electric shocks:
1. Be sure of the conditions of the equipment and the dangers present BEFORE working
2.
3.
4.
5.
on a piece of equipment. Many sportsmen are killed by supposedly unloaded guns;
many technicians are killed by supposedly “dead”. Circuits,
NEVER rely on safety devices such as fuses, relays and interlock systems to protect you.
They may not be working and may fail to protect when most needed.
NEVER remove the grounding prong of a three wire input plug .this eliminates the
grounding feature of the equipment making it a potential shock hazard.
Disorganized mess of connecting leads, components and tools only leads to careless
thinking circuits, shocks and accidents.
DO NOT WORK ON WET FLOOR
Do not work on wet floor or bare footed. Always work on a rubber mate or an insulated
floor.
6. DO NOT WORK ALONE.
It’s just good to have someone around to shot off the power give artificial respiration
and to call a doctor.
7. WORK WITH ONE HAND WHILE WORKING WITH ELCTRIC CIRCUITS
A current in between a hand’s crosses your heart and can be more let than a current
from hand to foot .a wise technician always work with one hand .watch your service
man
8. NEVER TALK TO ANYONE WHILE WORKING
Do not let yourself distracted. Also do not talk to anyone, if he is working on dangerous
equipment. Do not be the cause of an accident.
9. ALWAYS MOVE SLOWLY
When work around electrical circuits. Violent and rapid movements lead to accidental
shock and short circuits.
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Content
1- Transformer
- No-Load Test
- Short-Circuit Test
- 3 Phase transformer
o Star-Star
o Star- Delta
1- DC Machine
- Analysis of separately excited machine
- Analysis of series excited machine
2- AC Machine
Asynchronous machine
aSCIM
bSlip-rings motor
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Report Format
1-Objective/ Objectif
2-Brief Theory/ Brève théorie
3-Equipment/ Matériel
4-Circuit diagram/ Schéma du circuit
5-Tables of readings and calculations/ Tableaux
des mesures et des calculs
6-Sketches of the characteristics/ Schéma des
caractéristiques
7-Results and analysis/ Résultats et analyse
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Transformer
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Single Phase Transformer Testing
I.
INTRODUCTION
The transformer is extremely important as a component in many different type of
electrical circuit from small signal electronic circuit to high-voltage power transmission
system, the most common functions of transformers are:
1. Changing the voltage and current levels in an electrical system.
2.Impedance matching.
3.Electrical isolation.
-The first of these functions is probably best known to the reader and is typified by the
distribution transformer in the near by electric pole that steps down the voltage on the
distribution lines from, say 2300(v) , to the household voltage of 115/230(v).
-The second function is found in many communication circuits and is used for example,
to match a load to a line for improved power transfer and minimization of standing
waves
-The third feature is used to eliminate electromagnetic noise in many types of circuits ,
blocking dc signals and user safety in electrical instruments and appliances.
Transformer are used in circuits of all voltage levels, from the micro volt of some
electronic circuits to the highest voltage used in power systems which, today, is
approximately 750,000V.
1 . 1 Transformer construction and electromagnetic structure.
• The magnetic structure of transformer consists of one or more electrical winding
linked together magnetically by a magnetic circuit or core.
The magnetic circuit of most transformers is constructed of a magnetic material, but nonmagnetic material often called “air cores” are found in some application.
The two basic winding are often called primary and secondar.
• A simple two -winding transformer model is shown below.
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The construction of transformer varies greatly, depending on their application, winding
voltage and current rating and the operating frequencies.
In general, the electromagnetic structure is contained within a housing or case for safety
and protection.
In electronic transformer a viscous insulating material is used and, in many power
transformer, a nonflammable insulating oil called transformer oil is used.
The winding of large power transformer insulation generally use conductors with heavier
than magnet wire insulation.
1.2
Transformer Classification:
1.Transformer used in power distribution and transmission system called power system
transformer.
2.Electronic transformers: transformers of many different types and applications used in
electronic circuits. Sometimes electronic transformer are considered as those
transformers with rating of 300 VA and below.
3. Instrument transformer: transformer used to sense voltage or current in both electronic
circuits and power system often called potential transformer and current transformers.
And used in protection of electrical equipments.
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EXPERIMENT 1
No-Load Test
AIM:
To perform open circuit test on a single phase transformer.
APPARATUS REQUIRED:
1.
2.
3.
4.
5.
6.
Wattmeter
AC/DCvoltmeter
AC/DC ammeter- 0- 10 A
Autotransformer, 220V,10A,50Hz,1-phase
Transformer
Connecting wire
THEORY:
From this test it is possible to determine the equivalent circuit of the
transformer. This test is carried out to determine the no load loss or core loss
and no load current, magnetization current and core loss current.
The open circuit test is performed at rated voltage i.e. On low voltage side of
transformer and secondary side is kept open or connected to a voltmeter.
Since the secondary is open, the current drawn by the primary is no load
current Io measured by ammeter A. The value of Io is very small.
Wattmeter reading W represent the core loss or iron loss for all practical
purpose.
𝑊 = 𝑉1 𝐼0 𝐶𝑜𝑠∅𝑜
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𝐶𝑜𝑠∅0 =
𝐼µ = 𝐼𝑜𝑠𝑖𝑛ɸ𝑜
𝑊
𝑉1 𝐼0
𝐼𝑤 = 𝐼𝑜 𝑐𝑜𝑠ɸ𝑜
The open circuit test parameters can be find out by:
𝑋𝑜 = 𝑉1 / 𝐼µ
𝑅𝑜 = 𝑉1/ 𝐼𝑤
𝑌𝑜 = 𝐼𝑜/ 𝑉1
G0=W/ V12
Bo=√(Yo2 -G02 )
CIRCUIT DIAGRAM:
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PROCEDURE:
1.
2.
3.
4.
5.
6.
Make the connections as per circuit diagram.
Make sure that secondary side of transformer is open .
Keep the variac at zero position before switch on the supply.
Switch on AC supply.
By varying the variac apply full supply voltage i.e. 230 volt to the
primary of the transformer and note the reading of wattmeter,
voltmeter and ammeter.
Keep the variac at zero position and switch off the supply.
OBSERVATION TABLE:
Sl.No
Voltmeter
reading, V
(V1O)
Ammeter
reading,
mA (I1O)
Wattmeter (W)
(P1O)
Voltmeter
reading V
(V2O)
Cos Φ
Pcore
RESULT:
By using the reading calculate the value of following parameter: Ro , Xo , Yo
, Go , Bo , W
PRECAUTIONS:
1.
2.
3.
Experiment should be performed at rated voltage.
Connections should be neat and clean.
Experiment should be performed carefully.
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EXPERIMENT 2
Short-circuit Test
AIM:
To perform short circuit test on single phase transformer.
APPARATUS REQUIRED:
Same as above
THEORY:
This test is carried to determine the following:
• Copper loss at full load
• Equivalent impedance, resistance and reactance
The connection diagram for short circuit test on transformer is shown in the
figure. A voltmeter, wattmeter, and an ammeter are connected in HV side of
the transformer as shown. The voltage at rated frequency is applied to that
HV side with the help of a variac of variable ratio auto transformer.
The LV side of the transformer is short circuited. Now with the help of
variac applied voltage is slowly increased until the ammeter gives reading
equal to the rated current of the HV side. After reaching at rated current of
HV side, all three instruments reading (Voltmeter, Ammeter and Watt-meter
readings) are recorded. The ammeter reading gives the primary equivalent of
full load current IL. As the voltage applied for full load current in short
circuit test on transformer is quite small compared to the rated primary
voltage of the transformer, the core losses in transformer can be taken as
negligible here. Hence wattmeter reading gives the copper loss in the
transformer reading.
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Wattmeter reading Wsc= Isc2× Ro
Req=Wsc/ Isc2
Where Req is the equivalent resistance of the transformer.
Zeq= Vsc/ IL
Where Zeq is the equivalent impedance of the transformer.
Xeq= √(Zeq2-Req2)
Where Zeq is the equivalent reactance of the transformer.
CIRCUIT DIAGRAM:
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PROCEDURE:
1.
2.
3.
4.
Connect the circuit as shown in fig and set up the auto transformer to zero
position.
Switch on the supply and apply the voltage gradually with the secondary
winding terminals short circuit.
By varying the variac supply rated current in the primary winding and note
the reading of wattmeter, voltmeter and ammeter.
Switch off the supply.
OBSERVATION TABLE:
Sl.No
Voltmeter
reading, V
(V1SC)
Ammeter
reading, A
(I1SC)
Wattmeter (W)
(P1SC)
Ammeter
reading, A
(I2SC)
Cos Φ
P1SC
RESULT:
Copper loss in the transformer windings =…….. and Voltage regulation at
pf…… is……….
Resistance (Req) =
;
Reactance (Xeq) =
PRECAUTIONS:
1.
2.
3.
Connections should be neat, tight and clean.
Experiments should be performed carefully.
Rated current should not be exceeded.
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; Impedance (Zeq) =
THREE-PHASE TRANSFORMER CONNECTIONS
A three phase transformer or 3φ transformer can be constructed either by
connecting together three single-phase transformers, thereby forming a socalled three phase transformer bank, or by using one pre-assembled and
balanced three phase transformer which consists of three pairs of single
phase windings mounted onto one single laminated core.
The advantages of building a single three phase transformer is that for the
same kVA rating it will be smaller, cheaper and lighter than three individual
single phase transformers connected together because the copper and iron
core are used more effectively. The methods of connecting the primary and
secondary windings are the same, whether using just one Three Phase
Transformer or three separate Single Phase Transformers.
The primary and secondary of the transformer can be independently
connected either in star or delta. There are four possible connections for a 3phase transformer bank.
1.
2.
3.
4.
Δ – Δ (Delta – Delta) Connection
Υ – Υ (Star – Star) Connection
Δ – Υ (Delta – Star) Connection
Υ – Δ (Star – Delta ) Connection
The choice of connection of three phase transformer depends on the various
factors likes the availability of a neutral connection for grounding protection
or load connections, insulation to ground and voltage stress, availability of
a path for the flow of third harmonics, etc.
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EXPERIMENT 3
THREE-PHASE TRANSFORMER
AIM:
To measure the voltage on the primary and secondary winding of a threephase transformer for different configurations.
APPARATUS REQUIRED:
1. AC/DCvoltmeter- (0-500V)
2. Transformer ( 3Phase)
3. Connecting wire
What’s required:
-
Measure the Line to line voltages on the primary and
secondary sides of a 3 phase transformer for the 2
configurations:
Υ – Υ (Star – Star) Connection
Υ – Δ (Star – Delta ) Connection
-
Find the transformation ratio
Calculate the rated curent of the primary and secondary.
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PROCEDURE:
1.
2.
3.
Connect the circuit as shown in figures.
Take the required measurement.
Switch off the supply.
OBSERVATION TABLE:
Sl. no
Voltmeter Voltmeter
reading(V) reading(A)
Υ–Υ
Υ–Δ
L1L2 (Prim.)
U1U2 (Second.)
L1L3
U1U3
L2L3
U2U3
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AC Machine
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INTRODUCTION on INDUCTION MACHINE
1.1 Development:
In 1889 Testa and Ferraris published a description of methods of producing at Poly-phase
current, and the former exhibited a crude type of three phase motor the Frankfort exhibition
1891. An improved construction, with a distributed stator winding and a cage rotor was then
built in 1893. The slip ring rotor was developed at the turn of the century.
Since then, besides the development and improvement of the cage and slip ring M/C The
inherent limitiation of the induction motors as regards its speed and its power factor have been
the subjects of numerous patents and investigations. The chief variants are the synchronous
induction motor for the improvement of p.f. pole changing windings and cascade connection
for speed control, and the introduction of the commutator for the adjustment of both speed and
pf, these special M/C comprise, however, only a small friction of the total of a.c motors in use:
Probably at least 80%of the world 's ac motors are plain polyphase induction motor, while
many of the reminder are single-phase frictional horse-powers motors of while many thousand
are manufactured for small plant. The general reversibility principle of electromagnetic
machines applies to the induction machine. If its rotor is driven at a suitable speed, the
machine operates as an induction generator. The machine can serves a useful but restricted
role in supply system.
1.2 Construction Of Induction Motor:
1.2.1 General Arrangement
The 3-phase induction motor differs in construction from the dc M/C in certain essentials: its
short air gap, absence of a commutator, speed limitation, simple winding and laminated stator
The stator core must be carried in a shell or housing which provides a means for protecting the
stator, carrying the end covers, bearing and terminal box.
1.2. 2 Frame
The frame of a motor may be cast or fabricated. The former method is still common for small
and miniature motors, but medium size and large machine are almost exclusively fabricated.
1.2.3 Core
The stator and rotor cores are built up of thin sheet of special core steel, insulationis done by
means of paper, varnish or sprayed chin clay, the gap surfaces of the plates have suitable slots
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punched out, either open, semi closed, or completely closed. For small M/C, both the stator
and the rotor punching are complete rings, the waste of material and difficulty of handling
make it necessary to employ sectional plates , suitably keyed to the housing.
1.2.4 winding
For large motor or these for high voltage the stator phases maybe formed by single-layer
concentric coils, but even here, as for most medium-sized machines, the double layer lattice
winding is common. In slip rotors of large size, a bar winding can be used because the choice
of the voltage is usually free. For smaller machine wire-wound rotors can be used with coil
arrangements similar to that of the stator.
1.2.5 Shafts,
The shaft of an induction motor is made in order to keep as small as air gaps as is
mechanically possible . The use of of the ball and the roller bearings makes accurate centering
of the rotor rather simpler than with journal bearing ,and the reduction in over all length is an
advantage .Ball and roller bearing are packed with grease and sealed after manufacture and
can generally be relied upon to work for long periods without attention.
1.3 Speed Control
The plain induction motor is essentially a constant-speed, shunt characteristic machine, its
action being initially connected with its synchronous speed. Its nearly constant speed and low
power factor constitute its chief advantage. Much investigation has made since the invention
of the motor to overcome the restrictions on speed control and pf without sacrificIng the
admirable and valuable simplicity and robust construction of the plain induction motor. Of the
several method in use, some control speed ,while others deliberate affect the power factor
also. The chief method of speed control involves no special prevision for the compensation of
low power factor are:
A) Resistance in rotor circuit
B) Pole changing.
C) Cascade connection and variants of it
D) Change of supply frequency.
1.4 induction motor testing
1.4.1 Objective of tests
A manufacture has his motor tested to obtain a brief 'ream' of the motor characteristics, in
order that its guarantees may be satisfied. As most motor are now produced in stock lines,
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only a few machines may receive any thing like a complete test, others will only have
inspection for electrical and mechanical soundness. Where new types are developed or
changes made in design, a more detailed investigation is fruitful, for the wise designer bases
his judgements on test result.
For the investigation of electromagnetic phenomena and for an insight into the behavior of a
machine, many more or less elaborate test can be applied.
1.4.2 No load lest
One of the most information tests is the no load tests ,which gives the core and pulsation loss,
friction and windages loss, magnetizing current and no load power factor. Further, any
rnechanical unbalance , noise, faulty connection, etc, are revealed. The stator connection are
made to a supply of normal frequency and variable voltage, and instruments are Included to
measure the voltage, input power and current after having been started, the motor is run with
its rotor in the normal running condition, i.e. short-circuited, and with the brushgear raised in
the case of slip ring motors with this equipment. When the motor has run long enough for its
bearing, to show distress if faulty, the applied voltage is raised about 20% over normal, and
input power and current observed.
The slip ring is measured, it is not sufficient to measure the speed , for the slip is very small
and cannot be accurately found from the difference between running and synchronous speed.
The reading are taken at lower values of voltage down to that at which the current starts again
to rise.
1.4.3 Locked rotor test:
This is analogous to the short-circuit test of transformer The rotor is held stationary and shortcircuited under its normal running condition. The test consequently reveals no mechanical
defects, but is of importance as furnishing the short-circuit current and power factor which,
with the no-load current and power factor.
In addition the I2R losses measured by the test are necessary for the estimation of efficiency
by loss -summation.
The stator is supplied with a low voltage of normal frequency, to avoid excessive currents.
The position in which the rotor is clamped may affect the current. If so, the variations are
noted when the rotor is locked in various position and a main position found. Alternatively the
rotor may be allowed to rotate very slowly during the progress of the test. The voltage is
raised in steps, with readings of current and power input, until the current reaches not more
than twice normal. The reading are taking quickly to avoid overheating .
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1.4.4 Measurement of resistance:
The resistance is one of the important parameters of the electrical circuit. This element is an
element which consumed the energy and convert this energy to heat, power loss, which is
undesired form of energy in electrical machine .This element can be calculated by ohm's law
which is a suitable method for measuring resistance in lab.
As we know that the resistance varies with the temperature. So we must keep the machine
working at full load for a suitable period to assure that the temperature is raised to the
corresponding worked temperature to have the suitable value of the measure resistance .
1.4.5 Determination of turns ratio:
When there are two coil or more than one coil, besides each others there will be mutual flux
collected between them if any coil is excited. For example, there are two coils if the first one
is excited by AC voltage, there will be induced emf across the terminal of the second coil .
This emf depends on the ratio between the turns numbers of the coils. So from this principle
we can determine the turns ratio by measuring the voltage across the terminal of both coils.
1.4.6 Load test
In the case of a small motor, an absorption brake or a coupled calibrated generator may be
used to load the machine. The motor is operated on normal voltage and frequency at loads
between zero and 50% or 100% over load, reading being taken of voltage- current in all
phases, total power and slip. When guarantees for pull-out torque or starting torque are to be
furnished, these may also be investigated.
1.4.7 Detremination of torque slip curve
Accurate knowledge of the torque slip characteristic of an induction motor is very essential to
have an idea of its starting torque, max torque, slip at max torque in order to asses its
suitability for a particular application.
This would give information regarding the run-up performance, crawling, etc. While the
normal equivalent circuit may help predicating the characteristics becomes essential for
establishing the validity of the theoretical methods.
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1.5 Applications of Induction machine:
1.5.1 Application of cage motors
Our main subsection are;
1. Motors with normal starting torque and current, having cages with low resistance resistance
and reactance, low full load slip ,good efficiency and power factor, and high pull out torque .
2. Motors with normal starting torque and low starting current ,having larger-rotor reactance
but the same slip and efficiency as (1).The power factor and pull -out torque are less. These
motors may be started by direct switching up to lager sizes than those in (1).
3.Motors with high starting torque and low starting current, using deep bar or double cage
rotors . A 200% starting torque with moderate current is obtained on full voltage. The
efficiency, power factor and pull-out torque are lower than for (1).The motors are useful for
starting against load.
4.Motors with high flux load slip, using a comparatively high - resistance cage, with large
starting torque, low starting current and low efficiency used for drives with heavy starting but
light running duty. If employed for loads with rapidly fluctuating torque, they may be attached
to flywheels for load-peak equalization.
1.5.2 application of slip ring motors
Slip ring motors suitable for heavy, frequent starting and acceleration duty cycle. A high
starting torque is obtained with a low starting current at high power factor. The rotor losses are
mainly in the external resistance, easing the problem of rotor Cooling.
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EXPERIMENT 7
No load and Blocked rotor tests on a 3 phase –
Squirrel Cage Induction Motor (SCIM)
Objective:
To study the characteristics of a 3-phase squirrel cage Induction motor
1- To obtain the performance characteristics of a three-phase induction
motor.
2- To perform Load and no-load test.
Apparatus:
-
Voltmeter
0-10 A A.C. Ammeter
3.0 KW A.C. Wattmeter
2000 rpm A.C. Tachometer
Motor Ratings : Power:3.7 KW Voltage : 415 Volts Current :
7.9 Amps Speed: 1430 rpm
Connection: Star, later delta
Theory:
This test is used to determine the no load current Io, power factor, cosф,
wind age & friction losses, core losses, no load resistance Ro and
magnetizing reactance Xo. The motor is uncoupled from its load and rated
voltage is applied to the stator. Since there is no output, the power supplied
to-the stator is the some of its copper losses, core losses and friction and
wind-age losses.
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Procedure :
No-Ioad Test :
• Connect the circuit as shown in below.
• Push the start button, the voltmeter reads the phase voltage,380V.
• Record the No-load current, voltage, power drawn in each phase and
speed
Wye
Connectio
n
1
2
3
4
5
DeltaConnection
I(A)
VLL(V)
P (W)
N(rpm)
PF
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Load Test
Repeat the above steps for a loaded motor.
For each step of load take reading of all instruments.
Take care not to exceed the rated current of the IM when loaded.
Wye
Connection
1
2
3
4
5
I(A)
VLL(V)
P (W)
N(rpm)
PF
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DeltaConnection
EXPERIMENT 8
Test on a 3 phase – slip ring Induction Motor
Objective
To study the characteristics of a 3-phase slip ring IM
- Reversing the rotation of the IM
-Locked rotor Voltage and current
- control the speed of a slip ring IM
Apparatus
Same as above
Procedure
1- Start the motor, observe the direction of rotation and measure the
speed with the digital handheld tachometer.
2- Turn-off the motor and swap two of the three supply leads at the motor
(e.g L1 on V1, L2 on U1, L3 remains on W1)
Start the motor again; observe the direction of rotation and measure the
speed with the digital handheld tachometer again.
3- Turn-off the motor and make the circuit changes shown below:
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Start the motor and measure the locked rotor voltage.
Turn-off the motor and replace the voltmeter by an ammeter; then repeat the
above experiment, this time to measure rotor current (rotor terminal L and M
are to be short-circuited with one-another for this test)
4- Turn-off the motor, remove the ammeter and shorting bridge the
connect the rotor starter. Test the motor’s startup behaviour with
various settings of the starter resistor. For each new start, measure the
no-load speed with the digital handheld tachometer.
Position 6
n/rpm
5
4
3
2
1
Table 1.1 No-load speeds with various starter resistances
(Position 6: 0 resistance, Position 1: Highest resistance)
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