electric machines lab i-manual_1st cycle

7/29/2019 Electric Machines Lab I-Manual_1st Cycle

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Electric Machines Lab IEEE 221

OBJECTIVES:

The objectives of laboratory experiments are:

i. To understand the principle of operation of all the electrical machines to supplement the theorylearned in the class room.

ii. Each electrical equipment is made to meet certain specifications. To verify whether theirspecifications are met, this is usually undertaken by a consumer or an authority lib Bureau of

Indian Standards.

iii. To gain experience in selecting appropriate instruments.SAFETY:

You are doing experiments with electric supply. It may cause even a fatal accident. To avoid this please

keep in mind the followings:i. When in DOUBT DO NOT DO IT is the first and foremost safety condition!

ii. You have to wear shoes compulsorily and stand on mats made by insulating materials toelectrically isolate your body from the earth.

iii. Dont wear loose neck chains or bangles.iv. You have to tuck in your shirts or wear an overcoat.v. Tighten the hair.

vi. While adjusting the rheostats, as far as possible use only one hand and keep other hand on yourback.

ATTENDANCE:

If you are absent for a lab class, then you have lost several things to learn. Laboratory should be treatedas temple, which will decide your career. So, dont fail to make your presence with your record

notebook having completed experiments, observation with completed experiments, that days

experiment particulars with required knowledge about it and stationeries.

MAKING CONNECIONS:

i. Get circuit diagram approval from your staff in charge.ii. Go to the respective worktable and start to give connections as per the circuit diagram from the

source side.iii. Make series connections first. Then make parallel connections like voltmeter, pressure coil etc.iv. Dont use meter terminals as junctions.v. Meters should be positioned properly so that they can be read conveniently without any parallax

error.

vi. Check the rheostat positions and connections. Check whether they have been connectedproperly or not.

vii. Before switching on the power, get circuit connection approval from the staff in charge .


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OBSERVATION:

i. Note all the required readings in their respective tables.ii. During the load test on motors, if the pointers of spring balances vibrate, then arrest the vibration

gently and take readings.

iii. If brake load is applied to the motor, then pour the water in the brake drum to avoid heating of belt.iv.

At the time of overloading, readings should be taken quickly.

CALCULATION:

i. Get verification of your observation and formulae for calculation from your staff in charge.ii. Calculate the required quantities by suitable formulae and tabulate them with units.

iii. Draw the necessary graphs and write the result and inference.

GENERAL PRECAUTIONS WHILE WORKING IN ELECTRICAL

LABORATORIES:

1. Understand the equipment to be tested and apparatus to be used.2. Select proper type (AC or DC) and range of meters. Moving coil meters should be used for DC

measurements. Moving iron meters should be used for AC measurements.

3. Do not touch live terminals.4. Use suitable wires (thin wires for parallel connection and thick wires for series connection).5. Make all the connections tight.6. Do not leave loose wires (Wires not connected).7. Get the connections checked before switching on the supply.8. Never exceed the permissible values of current, voltage and speed of any machine, apparatus,

load etc. For load test, one can load the machine up to 110% of its rated Capacity. Fuse rating

should not exceed 110% of the rated current for load test.

9. Switch on or off the load gradually and not suddenly.10.Under the load conditions, UPF wattmeters should be used for power measurement because of

high power factor.

11.Under no load conditions, LPF wattmeters should be used for power measurement because oflow power factor.

12.Following are the notations used in the manual:a. A1, A2Armature terminals.b. F1,F2 or E1,E2Shunt field terminals.c. D1, D2 or S1, S2Series field terminals.d. L, E, AStarter terminalse. SPSTSingle Pole Single Throw switch.f. DPSTDouble Pole Single Throw switch.g. TPSTTriple Pole Single Throw switch.h. MCMoving Coil instrument.


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i. MIMoving Iron instrument.j. +, -DC supply terminals.k. P, NSingle phase AC supply terminals.l. R, Y, B, NThree phase AC supply terminals.

Marking System (Total Marks: 100):

Evaluation Components are:

1. Observation : 15 Marks (Internal Component) The student needs to prepare a write up about the experiment in the observation page(s)

(in Record Sheets) before coming to the lab. The circuit diagram and the model graphsshould be on the left side of the observation. Neatness, correctness and proper

formatting will be considered as factors during the evaluation.

The faculty in charge can conduct a short viva related to the procedure of theexperiment. This short viva carries 5 mark weight-age

2. Viva : 20 Marks ( Internal Component): The student needs to prepare the theory related to the experiment before coming to thelab. Viva will be asked either before doing the experiment or during the experiment or

after completing the experiment. Questions related to previous experiments done or any

general topic related to electric machines could be asked for viva.3. Conduction: 15 Marks ( Internal Component):

Proper connections to the machine, measuring instruments and the mounting of theinstruments etc will be considered for evaluation.

The following factors also will be considered for evaluation in this section:The way experiment is conductedReading are takenThe way rheostat is usedThe way spring loading is done etc...4. Result and Inference: 10 Marks (Internal Component)

Once the experiment is over, the student needs to enter the experimentreadings/associated data in the record sheets. Once the data entry and the necessary

graphs are sketched, the student should write the necessary result and inference. In theinference section students should write the reason behind the obtained results and

graphs.

The students should submit the record and get their record corrected on the day in whichthe experiment was done and before leaving the lab. Neatness, writing scale in graphsheets, writing units etc carries weight-age.

5. Term paper: 20 Marks ( Internal Component)6. End Semester Examination (20 Marks) ( External Component)


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List of Experiments (1st

cycle only):

Sl.

No.

Category Title of the Experiments

1. DC M/C a. Open circuit characteristics of DC shunt generator.b. Load test on DC shunt generator.

2. DC M/C Load test on DC series motor.

3. DC M/C a. Load test on DC shunt motor.b. Load test on DC compound motor.

4. Transformer Load test on 1- transformer.

5. Transformer Open circuit and short circuit on 1- transformer.

6. Transformer Separation of iron loss in 1- transformer.


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EX.NO:

DATE :

OPEN CIRCUIT CHARACTERISTIC OF DC SHUNT GENERATOR

AIM:

To obtain experimentally the open circuit characteristics of DC shunt generator at rated speed

and hence

i) To deduce the characteristic at two third of rated speed.ii) To predetermine the shunt field circuit resistance required to build up rated EMF at rated

speed, critical field resistance and critical speed.

APPARATUS USED:

(To find out from machine ratings)

THEORY:

In a DC generator, the expression for the voltage generated in the armature winding on no load can be

written as

EG= (ZN/60)*(P/A) volts

-flux per pole in webers which depends on field current

Z-number of armature conductors-constant for a given machine

P-number of polesA- number of parallel paths

N-speed in rpm

Open circuit characteristic is called magnetisation characteristic since it gives the variation of

flux per pole versus field current. At constant speed, EG is directly proportional to the flux per pole and

hence field current until the onset of saturation. So we get open circuit characteristic as a straight line

passing through origin. During saturation, a large change in field current is required to produce even a

small change in generated voltage.

Critical field resistance is the resistance of the shunt field winding beyond which the machine

will fail to excite. Critical field resistance is obtained by drawing a tangential line through origin for theinitial portion of the open circuit characteristic curve (Since the armature is open circuited) and finding

its slope.

Critical speed is the speed for which the given value of shunt field resistance represents the

critical field resistance. For self excited generator, there will be some EMF even when the field current

is zero due to residual magnetism.


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PROCEDURE:

Connections are made as per circuit diagram. Keeping the motor field rheostat at minimum

position, the generator field rheostat at maximum position and SPST open, the DC supply is given to

the motor. The starter handle is gradually brought from off to on position so that the resistance in thearmature circuit is cut down slowly and completely. Motor is brought to any desired speed using the

motor field rheostat. With the SPST switch open, the emf due to residual magnetism is noted down.Then SPST switch is closed and the field excitation of the generator is increased in steps with the help

of generator field rheostat and at each step generated emf is noted down. The procedure is continued

until the built up voltage of generator reaches about 110% of the rated voltage. The readings are

tabulated. Throughout the experiment, the speed must be maintained constant.

DATA OBSERVED:

Rated speed:

FIELD CURRENT

IF A

INDUCED EMF at rated

speed for increasing IF

EG V

To Deduce OCC at any other speed

( Let the new speed be two-third of rated speed i.e., N2 = (2/3)Nrated)

INDUCED EMF at rated

speed for increasing IF

EG1 V

DEDUCED EMF

EG2 V

GRAPHS DRAWN:

1. OCC at rated speed

2. OCC at two thirds rated speed.


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DATA PROCESSED:

1. A tangent is drawn to the initial linear portion of the OCC passing through the origin taken atrated speed.

Critical field resistance = slope of the tangent.

2. To obtain the OCC at two-thirds rated speed:At any given field current, emf induced is proportional to the speed.

EG1/EG2 = Nrated/N2.

Therefore EG2 = (N2/Nrated) EG1.

3. Field circuit resistance required to build up rated emf is calculated from the OCC drawn at ratedspeed.

4. Critical speed at the given field current:Join the point of rated emf on the OCC drawn at rated speed to the origin. This is the field

resistance line to get rated emf at rated speed.Rcritical/Rfield = Nrated/Ncritical

Therefore Ncritical = Nrated (Rfield/ Rcritical)

RESULT:

1. Critical field resistance at rated speed =

2. Field Circuit resistance required to build up rated emf at rated speed =

3. Critical speed at the rated field current =


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CIRCUIT DIAGRAM:

OPEN CIRCUIT CHARACTERISTIC:

NAME PLATE DETAILS:

MOTOR: GENERATOR:

Capacity : Capacity :

Voltage : Voltage :

Current : Current :

RPM : RPM :

Excitation : Excitation :

Type : Type :


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EX.NO:

DATE:

LOAD TEST ON DC SHUNT GENERATOR

AIM:

To obtain the internal and external characteristics of a DC shunt generator.

PRECAUTIONS:

1. While starting the DC motor, starter handle should be moved from OFF to ONPosition slowly and steadily.

2. While measuring armature resistance, the armature of the machine should bestationary.

APPARATUS USED:

(To find out from machine ratings)

THEORY:

If a DC shunt generator, after building up to its rated voltage is loaded, its terminal voltage will

drop. This drop increases as load increases. There are three reasons of voltage drop in DC shunt

generator (i) Armature reaction (ii) armature resistance (iii) Drop in field current due to reduction in

terminal voltage. The different types of load characteristics are

(i)Internal or total characteristic (EG VS IA): It gives the relationship between the EMF actually induced

in the armature after allowing for the demagnetizing effect of armature reaction and the armature

current.

(ii)External or performance characteristic (VT VS IL ): It is drawn between the load voltage taking into

account both the armature reaction and armature resistance and load current. The drop in voltage is so

less that it can be considered as a constant voltage machine.

PROCEDURE:

On no load keeping the field rheostat of motor at minimum, the field rheostat of the DCgenerator at maximum and SPST switch open, DC supply is given to the motor. The DC motor is

started with help of the 3 point starter and it is adjusted from OFF to ON position slowly and steadily.

After the motor attains its rated speed, SPST switch is closed and the field rheostat of the DC generator

is adjusted so that the generator terminal voltage is built up to the rated value on no load.Load side DPST is closed. Maintaining the speed of the motor-generator set constant at the

rated speed of generator, load on the generator is increased gradually in proper steps from no load to

110% of full load. At each step, the terminal voltage and the load current are noted down. Throughout

the experiment, the field resistance must be maintained constant at no load value.

To find RA by V-I method:

The armature resistance of the generator is determined by V-I method.


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Connections are made as per the circuit diagram. The load is increased in steps and in each step the

corresponding armature current and voltage across the armature are noted down.

DATA OBSERVED AND PROCESSED:

Rated speed = ------rpm Field resistance=--------ohms.

LOAD CURRENT IL

A

FIELD CURRENT IFA

ARMATURE CURRENT

IA = IL + IF A

TERMINAL VOLTAGE

VT V

INDUCED VOLTAGE

EG V

To find RA by V-I method:

VOLTAGE ACROSS

ARMATURE

Va V

ARMATURE CURRENT

Ia A

ARMATURE RESISTANCE

RA = Va/Ia

TO FIND RA:

SAMPLE CALCULATION:

IA = IL + IF


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INDUCED EMF EG = VT + IARA

GRAPHS DRAWN:

1. Internal characteristics: EG VS IA2. External characteristics: VT VS IL

RESULT:

The internal and external characteristics were shown in the graph.

INFERENCE:

CIRCUIT DIAGRAM:

TO FIND INTERNAL AND EXTERNAL CHARACTERISTICS:


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TO FIND RA:


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Experiment No: LOAD TEST ON DC SERIES MOTOR

Date:

AIM:

To conduct load test on the DC Series Motor and to plot its performance characteristics.

APPARATUS REQUIRED:

No Apparatus Range Type Quantity

1. Ammeter ( 010 ) A MC 1

2. Voltmeter ( 0- 300 ) V MC 1

3. Digital Tachometer 1

PRECAUTION:

DC Series Motor SHOULD BE started with some initial load.

THEORY:

Load Test or Brake Test is the direct method of finding the efficiency of the DC motors of

smaller capacity. To determine directly the efficiency, the motor is loaded directly by means of Brakedrum - spring balance arrangement. The various performance characteristics of the DC Series motor

can also be obtained from the Load Test.

While discussing the DC motor characteristics, the following two relations should be kept in

mind. Speed of the DC motor is given by

Where

Eb = Back EMF of the motor

= V- Ia Ra

The torque of the machine is given by

Where

Ia is armature current

Flux of the machine


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The flux of the machine is directly proportional to field current which also depends on the

load. From the above discussion,

Speed of the DC series motor is given by

For series motor, the field current is equal to armature current. So flux of the machine is

directly proportional to armature current Ia. So, when motor is loaded, field current (armature current in

case of DC Series motor) increases with increase in load causing the flux to increase thereby reducingthe speed. At high loads, flux becomes constant due to saturation and hence the speed remains nearly

constant.

The torque of the DC Series motor is given by

For DC Series motor, Flux is proportional to the armature current. So the torque is

proportional to the square of armature current. After saturation, flux becomes constant and hencetorque is linearly proportional to the armature current. As starting torque is higher for this motor, it is

preferred in traction.

CIRCUIT DIAGRAM

NAME PLATE DETAILS:

Capacity:

Rated Voltage:

Rated Current:

Rated Speed:


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PROCEDURE:

1. The connections are made as per the circuit diagram2. The DC Series Motor should be started with some initial load. So keep some initial load on the

motor before starting.

3.

Change the starter handle of Two Point Starter from OFF to ON Position slowly and steadily.

4. Load the motor using the Brake drum loading arrangement up to 110% of the rated current ofthe motor. Then in decremental steps reduce the load conveniently.

5. At each load observe and tabulate the followinga) Line currentb) Supply voltagec) Speedd) Spring balance readings.

FORMULAE USED:

1. Power Input

2. Torque

Where

R = Radius of the Brake Drum in metre (m).

3. Power Output

4. Efficiency

Where

N = Speed of the motor in rpm

T= Torque in Nm

GRAPHS DRAWN:

I) Performance Characteristics:

1. Po Vs Torque

2. Po Vs Speed

3. Po Vs Efficiency

4. Po Vs Line Current

II) Mechanical Characteristic:

5. Speed Vs Torque


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DATA OBSERVED AND PROCESSED: Radius of the Brake Drum: R = _________ m

No.

Line

Voltage

VL (

V )

Line

Current

IL (

A )

Spring Balance

Reading

Total Force

Acting

( S1~ S2 )

kg

Speed

N(rpm)

Torque

T (Nm)

Power

Input Pi

(kW)

Power

Output Po

(kW)

Efficiency

(%)S 1 (kg) S 2 (kg)


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MODEL GRAPHS

(I) Performance Characteristics

(II) SpeedTorque Characteristics

TABULATION:

Loading

Line

Current

( A )

Speed

(rpm)

Torque

(Nm)

Efficiency

(%)

FL

th

FL

th

FL

FL

RESULTS:


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Experiment No: LOAD TEST ON DC SHUNT MOTOR

Date:

AIM:

To conduct load test on the DC Shunt Motor and to plot its performance characteristics.

APPARATUS REQUIRED:

No Apparatus Range Type Quantity

1. Ammeter ( 010 ) A MC 1

2. Voltmeter ( 0- 300 ) V MC 1

3. Rheostat 230 /1.7 A 1

THEORY:

Load Test or Brake Test is the direct method of finding the efficiency of the DC motors of smallercapacity. To determine directly the efficiency, the motor is loaded directly by means of Brake drum - spring

balance arrangement. The various performance characteristics of the DC Series motor can also be obtained from

the Load Test.

While discussing the DC motor characteristics, the following two relations should be kept in mind.

Speed of the DC motor is given by

Where

Eb = Back EMF of the motor

= V- Ia Ra

The torque of the machine is given by

Where

Ia is armature current

Flux of the machine

The flux of the machine is directly proportional to field current which also depends on the load.

From the above discussion, Speed of the DC shunt motor is given by


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For DC shunt motor, the flux is proportional to field current which is almost constant. When motor is

loaded, armature current increases with increase in load. This causing an increase in armature drop I aRa.Generally armature resistance Ra is quite small. So IaRa is quite small compared to applied voltage V. But, thiscauses small drop in the speed on loading. So, it is a constant speed motor which is used for constant speedapplications such as lathes, belts drives.

The torque of the DC Shunt motor is given by

For DC Shunt motor, Flux is constant. So, torque is proportional to the armature current and we willget a straight line for torque-armature current characteristic passing through origin.

CIRCUIT DIAGRAM

NAME PLATE DETAILS:

Capacity:

Rated Voltage:

Rated Current:

Rated Speed:

PROCEDURE:

1. On no load keeping the resistance in field rheostat of motor at minimum position, DC supply is given tothe motor.

2. The DC shunt motor is started with help of the 3 point starter and it is adjusted from OFF to ON positionslowly and steadily.


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3. On no load, line voltage, line current, spring balance readings and speed are noted down.4. The motor is loaded directly with the help of brake drum loading arrangement.5. For each load, observe and tabulate the following

a. Line currentb. Supply voltagec. Speedd. Spring balance readings.

6. The procedure is repeated until the line current is 110% of the rated value.FORMULAE USED:

1. Power Input

2. Torque

Where, R = Radius of the Brake Drum in metre (m).

3. Power Output

4. Efficiency

Where, N = Speed of the motor in rpm

T= Torque in Nm

GRAPHS DRAWN:

I) Performance Characteristics:1. Po Vs Torque

2. Po Vs Speed

3. Po Vs Efficiency

4. Po Vs Line Current

II) Mechanical Characteristic:

5. Speed Vs Torque


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DATA OBSERVED AND PROCESSED: Radius of the Brake Drum: R = _________ m

No.

Line

Voltage

VL (

V )

Line

Current

IL (

A )

Spring Balance

Reading

Total Force

Acting

( S1~ S2 )

kg

Speed

N(rpm)

Torque

T (Nm)

Power

Input Pi

(kW)

Power

Output Po

(kW)

Efficiency

(%)S 1 (kg) S 2 (kg)


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MODEL GRAPHS

(i) Performance Characteristics

(ii) SpeedTorque Characteristics

TABULATION:

Loading

Line

Current (

A )

Speed (rpm) Torque (Nm) Efficiency (%)

th FL

th FL

th FL

FL

RESULTS:


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EX.NO:

DATE : LOAD TEST ON DC COMPOUND MOTOR

AIM:

To conduct load test on the DC Compound motor and to plot its performance

characteristics.

APPARATUS REQUIRED:

THEORY:

A DC compound motor has two field windings: shunt and series field. The series field

is decided by the motor current and this mmf aids or opposes the shunt field depending upon

whether it is cumulative or differential type. The characteristic between output and speed willlie between those of the shunt and series motors. As the compound motor is loaded, the motor

current increases and current through series field also increases. In cumulative type, due to

increase in series flux, the net flux increases. So, speed decreases as N (Eb/)

PROCEDURE:

On no load, keeping the resistance in shunt field rheostat of motor at minimum, DC

supply is given to the motor. The DC motor is started with help of the 3 point starter and it is

adjusted from OFF to ON position slowly and steadily. On no load, line voltage, line current,

spring balance readings and speed are noted down. The motor is loaded directly with the help

of brake drum loading arrangement in steps and for each load, the spring balance readings,

the line voltage, line current and speed are noted down. The procedure is repeated until theline current is 110% of the rated value.

SAMPLE CALCULATION:

Power input Pi = VL IL x10-3 kW

Torque T = (S1S2) x r x 9.81 Nm,

where r is the radius of the brake drum in m.

Power output Po = [2NT/60] x 10-3 kW

% efficiency = (Po/Pi ) x 100


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LINE VOLTAGE

VL V

LINE CURRENTIL A

SPRING BALANCE READING

S1 kg

SPRING BALANCE READING

S2 kg

TOTAL FORCE ACTING

(S1S2) kg

SPEED

N rpm

POWER INPUT

Pi Kw

TORQUE

T Nm

POWER OUTPUT

PO kW

EFFICIENCY

%

GRAPHS DRAWN:

I) PERFORMANCE CHARACTERISTICS:

1. Po Vs TORQUE

2. Po Vs SPEED

3. Po Vs EFFICIENCY

4. Po Vs LINE CURRENT

II) MECHANICAL CHARACTERISTIC:

5. SPEED Vs TORQUE


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MECHANICAL CHARACTERISTIC: PERFORMANCE CHARATERISTICS

RESULT:

Load test is conducted on the DC Compound motor and the following results are

obtained .

LOAD LINE CURRENT

A

SPEED

rpm

TORQUE

Nm

EFFICIENCY

%

FL

FL

FL

FULL LOAD


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NAME PLATE DETAILS:

Capacity :

Voltage :

Current :

RPM :

Excitation :

Type :


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Experiment No.: LOAD TEST ON SINGLE-PHASE TRANSFORMER

Date :

AIM:

a. To determine the transformation ratio of the given single phase transformer fordifferent input voltages.

b. To plot the variation of iron loss with different input voltagesc. To conduct the load test on the given single phase transformer and to

determine its performance characteristics.

APPARATUS REQUIRED:

SerialNo.

Apparatus Range and Type Quantity

1 Voltmeter (0-300)V, (MI) 2

2 Ammeter (0-10)A, (MI) 2

3 Watt meter 300V 10A, UPF 1

4 Variable Resistive Load 1

5 Single phase variac 250V,8A 1

6 Single Phase Transformer 230 V /230 V 1

THEORY:

The transformation ratio of the given single phase transformer is defined as the ratio

of secondary voltage to the primary voltage. When it is on no load, the copper losses will be

very less. So the input power can be taken as approximately equal to iron loss. This iron loss

varies with input voltage. When the transformer is loaded, the secondary terminal voltage

changes and load dependent losses will be present in its windings. The change in terminal

voltage depends on the magnitude and the phase of the current (or load impedance). Thecopper losses vary as the square of the winding currents. The voltage applied across the

primary winding should be kept constant throughout the test. Voltage regulation of

transformer is defined as the change in secondary voltage from no load to full load expressed

as the percentage of either no load voltage or load voltage, keeping the primary voltage

constant.

PROCEDURE:

Connections are made as per the circuit diagram. The single phase AC supply is

switched ON, keeping no load on the secondary side of the transformer and single phase

variac at zero voltage position. By adjusting the variac, the voltage is applied to the primaryside of the transformer in steps. At each step, the voltmeter, ammeter and wattmeter readings

are noted. This is done up to its rated primary voltage of transformer. The DPST on the load

side is closed and the transformer is loaded in steps up to 110% of the rated secondary


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current. At each load, corresponding input current, load current, primary voltage, secondary

voltage and input power are noted down. The primary voltage should be kept constant

throughout the experiment.

CIRCUIT DIAGRAM:


TRANSFORMER PRIMARY VOLTAGE = ----------V

TRANSFORMER SECONDARY NO-LOAD VOLTAGE = ----------V

On no load:

MF for primary watt meter = ------ Primary winding resistance R1 =------------ohms

Sl

No

Primary

Voltage

V1

Primary

Current

I1

Input

Power

(Pin)

( Kw)

Copper

Loss

I12R

(Kw)

Iron Loss

(PinI12R)

(kw)

Secondary

Voltage

V2

On load:

MF for primary wattmeter = ------ MF for secondary wattmeter = -------


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Sl

No

Primary

Voltage

(Volts)

Primary

Current

I1 (A)

Input

Power

P1 (Kw)

Secondary

Voltage

V2

Secondary

Current

I2

Output

Power

P2

%

Efficiency

%

Regulation

SAMPLE CALCULATIONS:

TRANSFORMATION RATIO = SECONDARY VOLTAGE/PRIMARY VOLTAGE

Power input Pin = ----------kW

Power output, Pout = V2 I2 x 10-3 = -----kW

% efficiency = (Pout/Pin) x 100%

% voltage regulation = [(V2 (NL)V2 (L)) / V2 (NL)] x 100

MODEL GRAPH:

RESULT:

LOAD %EFFICIENCY %REGULATION

FULL LOAD

FL

FL

FL


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EX.NO:

DATE

OPEN CIRCUIT AND SHORT CIRCUIT TESTS ON SINGLE PHASE TRANSFORMER

AIM:

To conduct open circuit and short circuit tests on given single phase transformer and

to (i) determine its equivalent circuit parameters (ii) predetermine its performance

characteristics

APPARATUS REQUIRED:

THEORY:

Efficiency and regulation of transformer are determined either by direct loading orfrom calculations based on its equivalent circuit parameters which can be determined from

open circuit and short circuit tests (indirect method). In second method, efficiency and

regulation are predetermined without actually loading the transformer. Power consumption is

less in the second method. So, it is mostly applied for large size transformers. Open circuit

test is used to determine the shunt branch parameters and short circuit test is used to find out

series branch parameters. Open circuit test gives core loss as copper loss is negligible and

short circuit test gives copper losses as core loss which depends on the applied voltages is

negligible.

PROCEDURE:

OPEN CIRCUIT TEST:Connections are made as per the circuit diagram. The high voltage side of the

transformer is kept open and the single phase variac is kept at zero voltage position. The

single phase AC supply is switched ON. Then the rated voltage is applied to the low voltage

side of the transformer by adjusting the variac and corresponding primary current and the

power input are noted down.

SHORT CIRCUIT TEST:

Connections are made as per the circuit diagram. The low voltage side of the

transformer is shorted and the single phase variac is kept at zero voltage position. The single

phase AC supply is switched ON. Reduced voltage is applied to high voltage side of the

transformer by using the variac such that rated current is circulated through the winding andthe applied voltage and power input are noted down.

DATA OBSERVED:

OPEN CIRCUIT TEST: ON HV SIDE

INPUT VOLTAGE, VO = -------- V

INPUT CURRENT, IO = -------- A

INPUT POWER (iron loss), PO = -------- W

SHORT CIRCUIT TEST: ON LV SIDE

INPUT VOLTAGE, VSC = -------- V

INPUT CURRENT, ISC = -------- A


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INPUT POWER (copper loss at full load), PSC = -------- W

Wattmeter constant (wattmeter multiplication factor)

= ( voltage setting)(current setting)(PF of wattmeter)/ full scale deflection

MF for OC test = -------- MF for SC test = --------

DATA PROCESSED:

FROM OPEN CIRCUIT TEST :( Referred to LV side, since readings were taken on

LV side)

Vo Io Coso = Po

Coso = Po/Vo Io

Zo = Vo/ Io

Il = Io Coso

Im = Io Sino

Ro = Vo/ Il

Xo = Vo/ Im

FROM SHORT CIRCUIT TEST :( Referred to HV side, since readings were taken

on HV side)

Psc = Isc2 Req

Req

= Psc/ Isc2

Zeq = Vsc/Isc

Xeq = (Zeq2Req

2)1/2

1) TO DRAW EQUIVALENT CIRCUIT :

K = ( Vrated-h v/ Vrated-l v)

a) REFERRED TO HV SIDE

RO' = RO K2

XO' = XO K2

b) REFERRED TO LV SIDE

Xeq' = Xeq/K2

Req' = Req/K2

PREDETERMINATION OF PERFORMANCE CHARACTERISTICS:

Assuming any load power factor & for any fraction of full load, efficiency and regulation can

be predetermined as follows :-

2) EFFICIENCY :

Iron loss, Po = ----------x 10-3kW Rated load, Qrated = ----------- kVA


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LOAD POWER

FACTOR, Cos1 0.8

FRACTIONAL

LOAD, x

0.2 0.4 0.6 0.8 1 1.25 0.2 0.4 0.6 0.8 1 1.25

COPPER LOSS

W

TOTAL LOSSES

W

OUTPUT POWER

W

INPUT POWER

W

% EFFICIENCY

3) VOLTAGE REGULATION at RATED kVA (i.e., x = 1):

LOAD POWER FACTOR

Cos ZERO 0.2 0.4 0.6 0.8 1.0

Req Cos Xeq Sin %REGULATION,LAGGINGPF

%REGULATION,LEADINGPF

SAMPLE CALCULATIONS: Reading number : ---------

PRE-DETERMINATION OF EFFICIENCY AND REGULATION:

Load Power factor = ----------

Fraction of full load, x = ----------

Copper loss at any load Pc= x2 Psc = ---------kW

Total loss PL= Po + Pc = ---------kW

Output power,Pout = x .(QratedCos) = ---------kW

Input power, Pin = Pout + PL = ---------kW


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% efficiency = {x .(QratedCos) /[x.(QratedCos) + Po + x2 Psc] } x 100 = ---------

% regulation = (xI/V)[Req Cos Xeq Sin] x 100 = --------

+ for lagging and - for leading

where, I = rated current (full load current) I and V referred to HV side

V = rated voltage since Req and Xeq are referred

at HV side.

4) MAXIMUM EFFICIENCY :

Fraction of full load at which maximum efficiency occurs,

Xm = (Po/PSC)1/2

Load at which maximum efficiency occurs = Xm(Qrated )

To find Maximum efficiency, substitute x =Xm in the efficiency formula.

Maximum efficiency at UPF = %

Maximum efficiency at 0.8 PF = %

5) POWER FACTOR AT ZERO REGULATION :

For zero regulation, Req Cos = Xeq Sin

Tan = Req/Xeq

= Tan 1 ( Req/Xeq)

Hence power factor at zero regulation = Cos (Tan-1(Req/Xeq)) leading.

GRAPHS DRAWN:

1. %Efficiency at UPF and at 0.8 PF Vs Output in kW.

2. %Voltage regulation Vs Powerfactor.


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RESULT:

1. Maximum efficiency of transformer at UPF = .. %2. Maximum efficiency of transformer at 0.8 PF = %3. Load power factor corresponding to zero regulation = ---------- leading4. Load kVA corresponding to maximum efficiency = kVA5. Voltage regulation at rated output & at UPF = .. %

CIRCUIT DIAGRAM:


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NAME PLATE DETALIS

KVA=

PH=

RATED VOLTAGE=

RATED CURRENT=


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EX.NO:

DATE :

SEPARATION OF IRON LOSSES IN A SINGLE PHASE

TRANSFORMER

AIM:

To separate the iron losses of a single phase transformer into the components of hysteresisloss and eddy current loss, at various frequencies.

APPARATUS REQUIRED:

PRECAUTION:

1. Motor field rheostat should be kept at minimum resistance position and motor armature rheostatshould be kept at maximum resistance position.

2. The alternator field rheostat should be kept at maximum resistance position.3. Motor should be run in anti clock wise direction.THEORY:

Iron losses in a transformer = Hysteresis loss + Eddy current loss

Hysteresis loss Bm1.6

f

Eddy current loss Bm2f

2

Therefore, if Bm is maintained constant ,Iron losses can be expressed as Pi = A.f + B.f2

(or) Pi/f = A + B.f

Bm-flux density in the core in Wb/sq.m.

f-frequency in Hz

This is the equation of a straight line. So if the iron losses are measured at different frequencies, graphcan be plotted with Pi /f in the y axis and the frequency in x-axis. Slope and y intercept of theresultant straight line will give the constant B and A respectively. But A and B will remain constant atall frequencies only if the Flux density is kept constant. The flux density can be kept constant by

keeping E/f as constant as the emf equation of transformer is given by

E1 = 4.44 f Tph (or) E1/f = 4.44 Tph

Where Tphnumber of turns per phase.

PROCEDURE:

1. Circuit connections are given as per circuit diagram.2. The prime mover is started with the help of the three point starter and is made to run at rated

speed.3. By varying the alternator field rheostat, gradually the rated primary voltage is applied to the

transformer keeping its secondary side open.4. By adjusting the speed of prime mover the required frequency is obtained and corresponding

readings are noted.


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5. Frequency is calculated corresponding to this new speed ( f = PN / 120 ). Now the value of Eis determined by calculation, so as to keep the value of E/f constant as before. Alternator fieldis excited to get this new emf. The corresponding wattmeter reading is noted. The procedureis repeated for different speeds.

6. The prime mover is switched off using DPST switch after bringing all the rheostats to initialposition.

7. From the tabulated readings, iron loss is separated as eddy current loss and hysteresis loss byusing respective formulae.

Graph drawn:

Pi / f Vs f Hysteresis loss Vs f and Eddy current loss Vs f in the same graph.

DATA OBSERVED & PROCESSED:

S.No. Induced

voltage, E

(volts)

Frequency,

f

(Hz)

Speed, N

(rpm)

Iron loss,

Pi

watts E / f Pi/f

A.f

Hysteresis

loss,

watts

B.f2

Eddy current

loss,

watts

1.

2.3.4.5.6.

FORMULAE USED:

1) Frequency (f ) = Ns P / 120PNo of poles

Nssynchronous speed in rpm.

2) Hysteresis loss (Wh) = A x f in Watts where Aconstant.

3) Eddy current loss (We) = B x f^ 2 in watts

4) Iron loss or core loss (Wi) = (We + Wh) in watts

Wi = A x f + B x f ^2

Wi /f = A + Bf

Here the constant A from the origin to the print distance at where the line cuts the y-axis in the graphdrawn between (Wi/f) and frequency (f). The constant B = (Wi / f) / f.


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RESULT:

Thus separation of no load losses in single-phase transformer is done and hence eddy current andhysteresis losses of a single phase transformer are found out at various frequencies.

SEPARATION OF IRON LOSSES OF SINGLE PHASE TRANSFORMER:

electric machines lab i-manual_1st cycle

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