Distribution MC Lab SEM III 2012

37
Experiments on d.c. machines 1) Experimentally study different methods of speed control of d.c. shunt motor 2) Experimental determination of magnetization (OCC) and external characteristics of separately-excited d.c. generator 3) Experimental determination of magnetization (OCC) and external characteristics of self-excited d.c. generator 4) Experimentally study Ward Leonard method for controlling speed of separately excited d.c. motor 5) Direct determination of torque speed characteristics and efficiency of d.c. motor using pony brake system 6) Indirect determination of efficiency of d.c. shunt motor using Swinburne’s test 7) Determination of torque speed characteristics using separately exited d.c. generator system 8) Experimental determination of torque speed and efficiency speed characteristics of d.c. series motor using Field’s test 9) Study methods of starting of d.c. series & shunt motors Non-electrical students will do experiment No. 1 & 2 from this set of experiments

Transcript of Distribution MC Lab SEM III 2012

Experiments on d.c. machines

1) Experimentally study different methods of speed

control of d.c. shunt motor

2) Experimental determination of magnetization (OCC)

and external characteristics of separately-excited d.c.

generator

3) Experimental determination of magnetization (OCC)

and external characteristics of self-excited d.c.

generator

4) Experimentally study Ward Leonard method for

controlling speed of separately excited d.c. motor

5) Direct determination of torque speed characteristics

and efficiency of d.c. motor using pony brake system

6) Indirect determination of efficiency of d.c. shunt

motor using Swinburne’s test

7) Determination of torque speed characteristics using

separately exited d.c. generator system

8) Experimental determination of torque speed and

efficiency speed characteristics of d.c. series motor

using Field’s test

9) Study methods of starting of d.c. series & shunt

motors

Non-electrical students will do experiment No. 1 & 2

from this set of experiments

Experiments on transformers

1) Determination of equivalent circuit parameters of a

two winding transformer using open circuit (OC) and

short circuit (SC) tests. Hence predict % regulation at

full load different power factors and % efficiency

2) Perform load test on transformer with different types

of electrical loads. Hence compare experimental and

theoretical values of % regulation

3) Perform polarity and ratio tests on a given single

phase two winding transformer

4) Determine self and mutual inductances of primary

and secondary windings of an iron core two winding

transformer. Derive exact equivalent circuit of

transformer using self- & mutual- inductances and

resistances obtained in the test.

5) Sumpner’s or back-to-back test on two identical

single phase transformers & Heat run test. Hence

compare results with those obtained using OC and

SC tests.

Non-electrical students will do experiment No. 1 & 2

from this set of experiments

B.Tech. EE Pt. II Sem. III (d.c. Machines)

EXPERIMENT No. 1

CHARACTERISTICS OF SEPARATELY EXCITED D.C. GENERATORS

AIM:(a) Experimental determination of magnetizing characteristics (Open Circuit

Characteristics or OCC) of Separately-excited d.c. generators

(b) Perform load tests on d.c. Generators & draw the Load Characteristics of

Separately–excited d.c. Generators.

SPECIFICATIONS OF THE MACHINE: Name Plate rating of machine under test.

APPARATUS REQUIRED:

Apparatus Number Range

d.c. Ammeters 2 1/2A & 30-0-30A

d.c. Voltmeter 1 300V

Tachometer 1

THEORY: Write essential conditions e.m.f generation etc. From your text book

CIRCUIT DIAGRAM:

Separately-excited d.c. Generator:

PROCEDURE: 1) Connect as shown in the Fig 1 for separately excited dc generator.

2) Run the generator at the rated speed.

3) Maintain its speed constant throughout the experiment using motor field resistance.

4) Initially keep the load switch OFF. Whether the generator develops voltage? If ‘Yes’,

why? How can you change the polarity of developed voltage of separately excited d.c.

generator?

5) Vary field current of generator in steps and record the developed voltage.

6) Plot Open circuit voltage versus field current at two different speeds.

7) Adjust the open circuit voltage at 200V at 1000 RPM.

8) Increase electrical load in steps. Note the load current and terminal voltage.

9) Plot the load characteristics of self excited shunt generator at two different speeds.

PLOT GRAPHS:

(a) Magnetizing characteristics:

Field current (on X- axis) Versus Open Circuit Voltage (on Y- axis)

(b) Load Characteristics:

Load current (on X- axis) Vs Terminal Voltage (on Y- axis)

NOTE: Obtain internal characteristics of the d.c. Generator by adding resistance drop to the

external characteristics.

RESULTS: Comment on graphs and torque of motor

DISCUSSIONS:

Effect of speed, armature reaction, field resistance, type of load, type of generators and

commutation etc. Reason of sparking in d.c. machines.

PRECAUTIONS:

While obtaining load characteristics of separately excited dc generator, the armature

terminal should never be shorted. Since field is already connected to dc supply, it will

cause excessive armature current to flow thereby blowing of supply fuses and damaging

armature windings, brushes and commutator.

The prime mover should be stopped only by switching OFF d.c. supply. Attempt to

switch OFF prime mover using starter should be avoided.

The field of d.c. motor need not be open when machine is running.

OBSERVATIONS: - Speed = Constant

Magnetizing Characteristics of dc generators

Self excited d.c. Generator Separately excited d.c. Generator

S.No. Open Circuit

Voltage (V)

Field Current

(A)

S.No. Open Circuit

Voltage (V)

Field Current

(A)

1. 0 1. 0

2. 2.

15. 15

Load Characteristics of Separately excited dc Generators:

Speed = Constant Ifg = Constant

S.No. Terminal

Voltage

Load Current

Output Power

Po=V.A

Speed

N

Motor Torque

60*Po/ (2 N)

(Volt) (Amp) (Watt) (RPM) (N-m)

1. 0

2.

15

Determination of Armature resistance: Draw your own circuit diagram or use digital multi

meter.

B.Tech. EE Pt. II Sem. III (DC Machines)

EXPERIMENT No. 2

CHARACTERISTICS OF SELF EXCITED DC GENERATORS

AIM:(a) Experimental determination of magnetizing characteristics (Open Circuit

Characteristics or OCC) of Self-excited DC generators

(b) Perform load tests on DC Generators & draw the Load Characteristics of

Self-excited DC Generators.

SPECIFICATIONS OF THE MACHINE: Name Plate rating of machine under test.

APPARATUS REQUIRED:

Apparatus Number Range

D.C. Ammeters 2 1/2A & 30-0-30A

D.C. Voltmeter 1 300V

Tachometer 1

THEORY: Write essential conditions of self excitation of dc generator. Define residual

magnetism, critical field resistance and critical speed etc. from your text book

CIRCUIT DIAGRAM:

Self-excited DC Generator:

PROCEDURE: 1) Connect as shown in the Fig 1 for self-excited dc generator.

2) Run the generator at the rated speed using starter of dc motor.

3) Maintain its speed constant throughout the experiment using motor field resistance.

4) Initially keep the load switch OFF. Whether the generator develops voltage? If ‘No’,

why? If ‘Yes’, go to Step No.8

5) Switch OFF the prime mover.

6) Interchange either connections of armature circuit or connections of field circuit of

generator.

7) Repeat Step-2. Whether the generator develops voltage? If ‘Yes’ why?

8) Vary field current of generator in steps and record the developed voltages as in Table 1.

9) Record developed voltage of generator, when generator field is open, i.e., Ifg=0.0

10) Plot Open circuit voltage versus field current.

11) Adjust the open circuit voltage at 200V at 1000 RPM. Increase resistive load in steps.

(Whether the speed falls? If ‘Yes’ why?)

12) Maintain the speed at constant value through out the experiment. Note the load current

and terminal voltage for different loads as in Table 2.

13) Now switch OFF prime mover and put a short circuit across the self excited dc generator.

Run prime mover at required speed and record current (VL=0.0). Plot the load

characteristics of self excited shunt generator. Repeat 1-8 at two different speeds.

.

PLOT GRAPHS:

(a) Magnetizing characteristics of dc Generators:

Field current (on X- axis) Versus Open Circuit Voltage (on Y- axis)

(b) Load Characteristics:

Load current (on X- axis) Vs Terminal Voltage (on Y- axis) for

NOTE: Obtain internal characteristics of the DC Generator by adding resistance drop to the

external characteristics.

RESULTS: Comment on graphs and torque of motor

DISCUSSIONS: Effect of speed, armature reaction, field resistance, residual magnetism, type of

load, type of generators and commutation etc.

PRECAUTIONS:

1) While obtaining load characteristics of separately excited dc generator, the armature terminal

should never be shorted. Since field is already connected to dc supply, it will cause excessive

armature current to flow thereby blowing of supply fuses and damaging armature windings,

brushes and commutator.

OBSERVATIONS: - Speed = Constant

Magnetizing Characteristics of dc generators

Self excited DC Generator

Increasing field current Decreasing field current

S.No. Open Circuit

Voltage (V)

Field Current

(A)

S.No. Open Circuit

Voltage (V)

Field Current

(A)

1. 0 1.

2. 2.

15. 15

Load Characteristics of Self excited dc Generators: Speed = Constant Ifg = Constant

S.No. Terminal

Voltage

Load Current

Output Power

Po=V.A

Speed

N

Motor Torque

60*Po/ (2 N)

(Volt) (Amp) (Watt) (RPM) (N-m)

1. 0

2.

15

16 0

Determination of Armature resistance: Draw your own circuit diagram or use digital multi

meter.

B.Tech. EE Pt. II Sem. III (d.c. Machines)

EXPERIMENT NO 3

EFFICIENCY OF d.c. SERIES MOTOR GENERATOR USING FIELD’S TEST

AIM: - Determination of torque speed characteristics of dc series motor using Field’s Test.

SPECIFICATIONS OF THE MACHINE: Name Plate rating of machine under test.

APPARATUS REQUIRED:

Apparatus Number Range

D.C. Ammeters 2 30-0-30A

D.C. Voltmeter 3 300V

Tachometer 1

Digital Multimeter 1

THEORY:- In Field’s Test, two identical dc series motors are required which are mechanically

as well as electrically coupled. Such a combination of dc series motors is also used in traction, in

which both the machines act as motors; these two machines in traction application can be

connected either in series or in parallel for starting and as well as for speed control. (Is there any

energy saving during starting when all such machines are connected in series /parallel?)

In this test one of the machine acts as a motor, whereas the other machine acts as a loading

generator. Since the field winding resistance of both the series machines is low, the field of the

generator is connected in series with that of motor circuit as shown in Fig 1. (Why?)

Since two identical machines have similar fields and therefore it can be assumed that their iron

losses will be approximately identical.

The motor is started using 2-point resistance type dc series motor starter. This starter is normally

referred as drum controller (because of its shape and rotary switches). Unlike to the shunt motor

starter it is used for starting, speeds control, braking as well as reverse rotation of series motor.

Study its operation. How will you realize its operation using modern contactors or Programming

Logic Controller?

As we know that in order to avoid dangerously high speed, a dc series motor should not be

started and run on no- load. Therefore it must be ensured that generator terminals should be

connected to full electrical load during starting.

Assuming that

V = Supply voltage;

V1= Motor terminal voltage;

I1 =Motor input current

V2 = Generator output voltage;

I2 = Generator current

Let the resistances Ra & Rf are of the armature and field respectively of these two identical

machines.

Power Input to the entire set, P1= V. I1

Power Input to the motor, Pm= V1. I1

Power Output of the generator, Pg= V2. I2

Losses in the entire set = P1 - Pg

Total copper losses for both the machines Pcu = I12

( Ra + 2 Rf ) + I22

Ra

Frictional & windage and core losses of both the machines = P1 - Pg - Pcu

Frictional & windage and core loss of each machines = Po

= (P1 - Pg - Pcu ) / 2

Motor efficiency ( m)= 1- ( Po + I12

( Ra + Rf )) / Pm

Generator input = Pg +Po + I22

Ra + I12Rf

Therefore generator efficiency (g) = Pg / (Pg +Po + I22

Ra + I12Rf )

Torque, T= 60. V2 .I2 / (2*3.14*N) Newton-m

CIRCUIT DIAGRAM:

PROCEDURES:

1) Connect the circuit diagram as shown in Fig 1. Put all the load switches ON. Make sure

that starter is at NULL position.

2) Switch ON the dc power supply and change the position of the starter or Drum

controller.

3) Slowly lower the connected load. Do you observe a rise in speed?

4) Record supply, motor, generator voltages, motor and generator currents and speed.

5) Repeat Steps 1-4 for different positions of Drum controller.

6) Record resistances of armature and field windings.

Measurement of armature resistance: The armature resistance and field resistance both are of

the order of 1 Ohm. See Figure If two load boxes of rating 220V, 0.5, 1.0, 1.5, 2.0 KW load

switches connected in series are to be used as a potential divider for finding dc resistance of

armature. Find the ratings of ammeter and voltmeter. Please note that wrong combinations of

load switches may cause excessive flow of current and can result in damage of series connected

instrument.

Before you switch ON any load switch, calculate the effective resistance and the current which

would actually flow when the supply voltage is 220 Volt.

Change load switches and measure I & V, calculate the resistance as shown in Table 2. Also

measure resistance of armature using Digital multi-meter before starting the experiment and after

you have finished your experiment. Why there is change in resistances of field and armature

circuit.

OBSERVATIONS: (A) Fields Test

Voltage Current Speed Output

Power

Eff.

Torque

S.N

o.

Supply

V

Motor

V1

Gen.

V2

Mot

or

I1

Gen

I2

N PO=V2 I2 (60*PO)/(2*

*N)

M G

1.

2..

12.

(B) Measurement of armature & field resistance:

Armature Resistance Field Resistance

S.No. V I R S.No. V I R

Average Armature Resistance Average Field Resistance

Armature Resistance (DMM) before Test Field resistance (DMM) before test

Armature Resistance (DMM) after Test Field resistance (DMM) after test

PLOT:

(a) Torque – speed characteristics at different positions of drum controller

(b) Motor current, Output power, input power and efficiency (motor & generator both)

versus speed

(c) Transient current during speed reversal.

RESULTS & CONCLUSIONS:

Discuss the shape of characteristics.

What have you learned from this experiment?

What do you conclude from this experiment?

PRECAUTIONS:

B.Tech. EE Pt. II Sem. III (d.c. Machines)

EXPERIMENT 4

SPEED CONTROL OF d.c. SHUNT MOTOR

AIM: - Study different methods of speed control of a dc shunt motor.

SPECIFICATIONS OF THE MACHINE: Name Plate rating of machine under test.

APPARATUS REQUIRED:

Apparatus Number Range

D.C. Ammeters 2 5-0-5A & 1/2A

D.C. Voltmeter 1 300V

Tachometer 1

Digital Multimeter 1

THEORY: The speed of a dc shunt motor mainly depends on field flux or field current and the

voltage appearing across armature of dc motor. This can be understood from the basic equations

of dc motor. The back e.m.f. appearing across the armature can be given as

E b = 2p Z N Φ / 60 (2a)

= K N Φ

and also from circuit point of view,

Eb= V- Ia Ra

Where

2a = total number of parallel path N = speed of operation

2p = total number of poles Ia = armature current

Φ = Flux per pole V = applied voltage

Z = Number of conductors Ra = armature resistance

Combining above equations, we get

Speed N= Eb / K Φ = (V- Ia Ra) / K Φ

Since the flux Φ is directly proportional to field current If , under low saturation of the magnetic

circuit,

N α Eb

&

N α 1 / If

It is also clear that the speed of a dc motor depends on design parameters, which can done only at

design level.

There are basically two experimental methods of speed control of a dc motor.

(a) Armature Voltage Control

(b) Field Flux Control

Note: The resistance Ra shall be rated to carry a current at least 20% of rated motor current

continuously. Its resistance is to be such that Ia.Ra is at least 50% of applied voltage. (why?)

PROCEDURE:

1) Keep maximum armature resistance and minimum field resistance.

2) Field flux control: In the circuit shown the motor is now started with the help of

a starter. Slowly decrease the armature resistance Ra to minimum possible value.

3) Adjust the field current If such that the speed ‘N’ is 20-30% below the rated speed

of the motor.

4) Vary the field current in small steps and note the corresponding speed until the

speed is increased to 15-20% above the rated value.

5) Plot speed N versus field current If (Ra is not changed)

6) For a new value of Ra held constant. Repeat steps 1 to 5..

7) Armature Control: Adjust field current If such that the speed ‘N’ is nearly rated

speed.

8) Increase the resistance Ra in steps and note the corresponding voltage across the

armature Eb and the speed. The speed should go down to 50% rated value.

9) Repeat steps 7-8 for a new value of If held constant.

10) Plot speed N versus Armature voltage Eb (If =constant)

RESULTS & CONCLUSIONS:

Discuss the shape of characteristics.

What have you learned from this experiment?

What do you conclude from this experiment

PRECAUTIONS:

B.Tech. EE Pt. II Sem. III (d.c. Machines)

EXPERIMENT 5

WARD LEONARD CONTROL

AIM: To control the speed of a dc motor using Ward Leonard method

SPECIFICATIONS OF THE MACHINE: Name Plate rating of machine under test.

APPARATUS REQUIRED:

Apparatus Number Range

D.C. Ammeters 2 1/2A & 5-0-5 A

D.C. Voltmeter 1 300V

Tachometer 1

THEORY: Speed of dc motor may be controlled by controlling the field current or by

controlling the voltage applied to the armature of a dc motor. In ward Leonard method, a variable

dc voltage is applied to the armature of the motor for speed control and reversal. Set up for

voltage control is shown in Fig 1. The variable voltage generator is driven by a three phase

induction motor mechanically coupled to the generator. The field winding of the generator is

separately exited using a potentiometer type of regulator, as shown in figure. By connecting one

end of field winding to the mid point of the regulator and the other to the floating contact, field

current can be varied from maximum to zero and also zero to maximum in reverse direction.

Zero fields current will be obtained when floating terminal K is at the mid point of

potentiometer. Moving point K to right makes K at lower potential than C and to the left at

higher potential for the connection shown. Thus a variable voltage can be obtained from the

generator by controlling this potentiometer.

The voltage generated is then applied in the armature of a dc drive motor, whose speed is

to be controlled. The field winding of this motor is connected to a separate dc source. Normally

this excitation is kept constant or changed accordingly. Since the field excitation of the motor is

kept constant and a variable voltage is applied to its armature. Speed of the motor can be

controlled. Further, the motor need not any starter for its starting.

This method of speed control is mostly used for large reversing rolling mill motors, and

for colliery winding, in which much of the energy of motion of the motor and its load can be

recuperated when bringing the speed to zero by causing the variable voltage motor to set as a

generator and feed back energy to the motor generator set.

To stop the motor and its load, the excitation current of the generator is gradually

reduced, so that the e.m.f. induced by the generator falls, and remains below that developed in

the motor armature. Since the motor is still revolving with its fully excited field the motor

current reverses and the motor now sets as a generator. Thus the motor converts its stored energy

of motion into electrical energy and supplies this energy to the generator of the converter set.

The speed of the set increases for a short period and the action of the set is completely reversed

for a brief period and it partly restores energy to the supply system.

CIRCUIT DIAGRAM:-

PROCEDURES:-

1. Connect the set as shown in Figure. Three phase induction motor is supplied through

started.

2. Bring the potentiometer contact ‘K’ close to mid point ‘C’ so that minimum voltage is

applied to the field of the generator.

3. Switch ON ac supply. Run the generator set by starting the induction motor. Note the

speed of the set.

4. Now switch ON dc supply to the generator and motor fields. Close switch S1.

5. Move contact ‘K’ of the potentiometer on one side. Note down the direction of rotation of

the test motor, its speed and exciting current and voltage across armature.

6. Repeat this at intervals by moving contact ‘K’ and record the observations.

7. Now gradually bring the contact ‘K’ close to ‘C’ and then in the other direction. Again

note down the direction of rotation and voltmeter reading corresponding to the speed

from zero to maximum value.

THINGS TO DO:

Draw motor speed versus armature voltage for two different motor field currents.

Draw circuit diagram for changing the polarity of d.c. voltmeter using DPDT.

RESULTS & DISCUSSIONS: Give your own comments on this method. Compare

conventional Ward Leonard control with that of Solid state Ward Leonard control.

PRECAUTIONS: Keep the generator field current at zero, while starting induction motor.

OBSERVATIONS TABLE

Motor field current= ……. Amp

S.No.

Generator Armature Motor

Field current Voltage Current Speed

(Amp) (Volt) (Amp) (RPM)

Positive Generator Field Current

1.

2.

3.

10

S.No.

Generator Armature Motor

Field current Voltage Current Speed

(Amp) (Volt) (Amp) (RPM)

Negative Generator Field Current

1.

2.

3.

10

B.Tech. EE Pt. II Sem. III

EXPERIMENT 6

BRAKE TEST

AIM: Load characteristics of a given D.C. Shunt Motor by brake test (direct load test).

SPECIFICATIONS OF THE MACHINE: Name Plate rating of machine under test.

APPARATUS REQUIRED:

Apparatus Number Range

D.C. Ammeters 2 1/2A & 30-0-30A

D.C. Voltmeter 1 300V

Tachometer 1

THEORY:- Write the determination of efficiency of dc motor using different methods like

(a) Pony brake system

(b) Swinburn’s test

(c) Hopkinson’s test

. Refer your Text Book

CIRCUIT DIAGRAM:-

PROCEDURE:

1) Connect as shown in the diagram.

2) Run the motor at no-load at the rated speed of the motor using a 3-point starter. Slacken

the belts of the spring balance and note the readings in the two spring balances if they are

not zero, it will give the zero error. Note all the readings.

3) Gradually apply the load in steps until the ammeter reads the rated current of the motor.

For each step, note the current, voltage (constant, except the small variations of supply

voltage), speed and reading in the spring balances..

4) Measure the diameter of the pulley in meters and add the thickness of the belt to give the

effective diameter

DRAW GRAPHS:

1) Torque Nm(Y-axis) Vs Output(B.H.P) ( X-axis)

2) Speed RPM(Y-axis) Vs Output(B.H.P) ( X-axis)

3) % Efficiency(Y-axis) Vs Output(B.H.P) ( X-axis)

4) Current (Y-axis) Vs Output(B.H.P) ( X-axis)Amps

DISCUSSIONS: Write results, conclusions & discussions.

PRECAUTIONS:

1. Put water inside the rotating drum to cool it down.

2. In the event of slippage of belt over drum, put little amount of sand to provide friction

between belt and drum.

OBSERVATIONS:

S.No

Line

Current

Line

Voltage

Spring

Balance

Speed Torque Output

B.H.P.

Input

I.H.P

%

Efficiency

(Amp) (Volt) S1 (Kg) S2 (Kg) (RPM) (N-m)

1

2

10

S1, S2 are the spring balance reading (in kg)

CALCULATIONS: (in M. K. S. units)

Torque (T) = (S1 - S2) x (d/2) Kg meter

Since the force exerted on 1 Kg mass is 9.81 N

Torque (T) = (S1 - S2) x (d/2) x 9.81 Nm

Now, B.H.P. = (output) = (2 x 3.14 x N x T) / (60 x 746)

Input power (in horse power) I. H. P. = Voltage x Current /746

% Efficiency = (Output / Input) x 100

B.Tech. EE Pt. II Sem. III (d.c. Machines)

EXPERIMENT 7

SWINBURN’S TEST

AIM: Pre-determination of efficiency by conducting Swinburn’s Test.

SPECIFICATIONS OF THE MACHINE: Name Plate rating of machine under test.

APPARATUS REQUIRED:

Apparatus Number Range

D.C. Ammeters 2 1/2A & 5-0-5A

D.C. Voltmeter 1 300V

Tachometer 1

CIRCUIT DIAGRAM:

PROCEDURE:

1) Connect as shown in the diagram and run the motor with the help of starter at rated speed

of the motor.

2) By pass the armature circuit ammeter at the shunt from high starting current by closing

the switch connected across it.

3) At no-load and rated speed, note the voltage, line current and armature current.

4) Measure the armature and field resistance.

PLOT GRAPHS:

Assumed values of %Efficiency (Y axis) versus Line current (X axis)

Torque versus speed

RESULTS: Comments on graphs, torque of motor.

DISCUSSIONS: Effect of speed, armature reaction, field resistance, type of load and

commutation etc.

PRECAUTIONS:

OBSERVATIONS: At no-load

Line Voltage (Volt)

Field Current (Amp)

Armature Current (Amp)

Armature Resistance (Ohm)

Field resistance (Ohm)

CALCULATIONS:

( . )% 100 100 100

Output Input Losses Input Cons Variable LossesEfficiency

Input Input Input

Let LOI = Line current on no-load & V = applied voltage

The total losses on no-load = LOVI

If fI = Field Current

AOI = armature current under no-load

LO AO fI I I

And if Ar is the armature resistance, the total iron loss+ friction loses is

2

O AO AO AP VI I r

The total constant losses = Losses in field+ fixed losses = t f OP VI P

Now, for an input current LI , the armature current A L f L

f

VI I I I

R

Where resistance of field circuit is fR

The losses in the armature circuit are 2

c A AP I r

( )% 100t cInput P P

EfficiencyInput

( )% 1 100t cP P

EfficiencyInput

OBSERVATION TABLE:

S.NO. Voltage

Assumed

Line

Current

Field

Current

Armature

Current

Constant

Losses

Variable

Losses Input

(Volt) (Amp) (Amp) (Amp) (Watt) (Watt) (Watt)

1.

2

12.

B.Tech. EE Pt. II Sem. III (Transformer)

EXPERIMENT No. 1

OPEN CIRCUIT & SHORT CIRCUIT TESTS

AIM: Perform open circuit and short circuit test on a two winding transformer and hence

determine its equivalent circuit parameter. Calculate its % regulation at full load U.P.F., 0.8

lagging P.F., 0.8 leading P.F. and Z.P.F. lagging, Z.P.F. leading.

SPECIFICATIONS OF TRANSFORMER: Name Plate rating of transformer under test.

Calculate the following quantities:

Full load current referred to primary (high Voltage) side = 1000High

High

KVAI

V

Full load current referred to secondary (Low Voltage) side = 1000Low

Low

KVAI

V

Turn ratio = High

Low

Va

V

Record the resistances of Primary and Secondary windings using Digital Multimeter.

APPARATUS REQUIRED:

Apparatus Number Range Remarks

For Open Circuit Test-from Low Voltage side

Ammeters 1 1/2A > 5-10% of LowI

Voltmeter 1 150- 300V

LPF Wattmeter 1 150 - 300V / 5A > LowV

For Short Circuit Test-from High Voltage side

Ammeters 1 10A > HighI

Voltmeter 1 30V

Wattmeter 1 75V /10A

THEORY: A transformer can be easily analyzed using lumped parameter approach, in which

the transformer is represented by a black-box known as equivalent circuit. If the equivalent

circuit parameter is known then the input output relation can be easily established. A typical

equivalent circuit of transformer is shown in Figure. A transformer works on the basis of the

mutual flux coupling the two coils. In order to minimize the leakage flux the magnetic flux is

allowed to flow in the path provided by the ferromagnetic core. This core is subjected to

alternating magnetization in which the direction of flux alternates at the supply frequency and

due to this certain amount of power is wasted in Hysteresis and eddy current losses.

Using Text Book write:

The e.m.f equation of a two winding transformer.

Derive equivalent circuit of a two winding transformer.

Define core loss-Hysteresis & Eddy current losses.

Define referred value.

DETERMINATION OF EQUIVALENT CIRCUIT PARAMETERS:

At no load the current oI is assumed to flow in the parallel combination of cR and mX or,

o c mI I I

Where real component of current cos oo c o o

c

VI I I

R

Similarly, imaginary component of sin oo m o o

m

VI I I

jX

Input power, 2coso o o o o L cP V I I r P

Where Lr =resistance of low voltage winding and cP = core loss

The input power oP is the wattmeter reading of LPF wattmeter when rated voltage (Low

voltage side) at rated frequency is applied on low voltage side of transformer, keeping high

voltage side open. The e.m.f LE or the voltage appearing across parallel branches of equivalent

circuit is, ( )L o o L LE V I r jx

Where Lx =leakage reactance of low voltage side.

Thus, c c LP I E

If drop in winding is assumed to be zero, then

Core loss = Wattmeter reading or c oP P

e.m.f. =Applied voltage or L oE V

Thus using open circuit test, the two equivalent circuit parameters can be calculated as,

2 2 2cos cos

c cL Lc

c o o c o o

P PE ER

I I I I

sin

L Lm

m o o

E EX

I I

.

These values are referred to low voltage side. The rated copper loss cuP is normally taken as

wattmeter reading scW during short circuit test at rated current HighI , when a feeble voltage scV

is applied on high voltage side of transformer, keeping its low voltage side short circuited.

Transformer’s series equivalent circuit parameters referred to high voltage side are

scexh

sc

WR

I

scexh

sc

VZ

I

Thus, 2 2

exh exh exhX Z R

These parameters are referred from high voltage side. The equivalent circuit parameters referred

to high voltage side are obtained by obtained referred values of &c mR X from high voltage side.

2 / 2

High c Low cI R I R

Thus, 22

/ 2

2 2

HighLowc c c c

High Low

VIR R R a R

I V

Similarly, 22

/ 2

2 2

HighLowm m m m

High Low

VIX X X a X

I V

The parameters &exh exhR X , which are referred to high voltage side remains unchanged. The

approximate equivalent circuit diagram is shown in Figure.

Here, transformer resistance referred to HV side, / 2

exh H L H LR r r r a r

And transformer leakage reactance referred to HV side, / 2

exh H L H LX x x x a x

Similarly equivalent circuit of transformer referred to low voltage side are

/

2

HexL L H L

rR r r r

a

/

2

HexL L H L

xX x x x

a

POWER EFFICIENCY: It is assumed that KVA rating of transformer is S ; Rated Copper loss

or the wattmeter reading under short circuit test is CUP ; Rated Core Loss or wattmeter reading

under Open Circuit test is I oP P . The power efficiency of a two winding transformer operating

at thk fraction of load and at a power factor cos is,

% Power Efficiency,

2

2

2

100 100

cos100

cos

1 100cos

I CU

I CU

I CU

Output Input Losses

Input Input

kS

kS P k P

P k P

kS P k P

Calculate % efficiency at different fraction of load and at two different power factors.

OBERVATIONS:

Resistance of High voltage winding, Hr =

Resistance of low voltage windings, Lr =

Open Circuit Test (High voltage side Open) Short Circuit Test ( Low Voltage side

Shorted)

Voltage Vo Rated LV, 110V Voltage VSC 5-10% of rated voltage

Current Io A Current ISC Rated High Voltage

side current, 6.8A

Wattage Wo W Wattage WSC W

PHASOR DIAGRAMS: Draw to the scale phasor diagrams at ZPF Lag /Lead, UPF and 0.8 P.F.

Lag/Lead.

Use approximate equivalent circuit for V= VHigh; I = IHigh; REX1=Rexh & XEX1=Xexh. Choose a

suitable scale. Measure the length of phasor E in terms of Volt.

% REGULATION Further, calculate the no load voltage E for different power factors.

For a lagging power factor angle, E can be written as,

2 2

1 1 1cos cos sinEX EX EXE V IR IX IR

Thus calculate the % regulation (UP) & (DOWN) using the expressions given below:

%Re ( ) 100E V

gulation UPV

%Re ( ) 100E V

gulation DOWNE

How will you calculate the above for leading power factor? Compare different quantities in

Tabular form..

P.F. No Load Voltage E % Regulation % Power

Efficiency

Calculated Measured UP DOWN

ZPF Lag.

0.8 Lag.

UPF

0.8 Lead

ZPF Lead

TABLE: POWER EFF. At 0.8 P.F. LAGGING & 0.8 P.F. LEADING

PROCEDURE:

Note the name plate rating of transformer, decide the range of instruments.

Connect circuit diagram as shown in Figure for Open circuit Test keeping high voltage

side kept Open (why?). Apply rated voltage corresponding to low voltage winding. Note

, &o o oV I W

Connect circuit diagram as shown in Figure for Short circuit Test keeping low voltage

side short circuited (why?). Apply rated current of high voltage winding. Note

, &sc sc scV I W . Note that very small voltage is required during short circuit test.

Measure the winding resistances &H Lr r of high voltage and low voltage windings

respectively using a Digital Multi meter.

S.No. Fraction Input Constant

loss

Variable

Loss

Total

Loss

%

Efficiency

(W) (W) (W) (W)

1 0

2 0.1

3 0.2

16 1.5

PLOT:

% Efficiency versus fraction of load for two different power factors

% regulation versus load current at unity power factors

PRECAUTIONS:

1. Choose correct rating instruments required for different tests.

2. During short circuit test, apply reduced voltage such that only rated current flows.

Excessive voltage may cause melting of insulation of shorted wire and subsequent fire.

B.Tech. EE Pt. II Sem. III (Transformer)

EXPERIMENT No. 2

LOAD TEST ON A SINGLE PHASE TRANSFORMER

AIM: Perform load test on transformer and plot % regulation, % efficiency versus load current.

SPECIFICATIONS OF TRANSFORMER: Name Plate rating of transformer under test.

COMPUTER PROGRAM: Obtain the equivalent circuit parameters referred to low voltage

side from the earlier experiment. Write a computer program for finding % regulation (UP &

DOWN) and % efficiency at any power factor and any load current.

APPARATUS REQUIRED:

Apparatus Number Range

Ammeters 2 15A & 10A

Voltmeter 2 150- 300V

Wattmeter 1 150 - 300V / 15A

THEORY: The load test on a two winding can be performed from either side. When a

transformer is subjected to electrical loading the voltage on the input side takes place. For a

220V/11V, it is convenient to perform load test from LV side, because the input voltage can be

kept constant during the experiment.

For the primary winding, we have, 1 1 1 1 1 1V I r jI x E

And for secondary winding referred to primary side, / / / / / /

2 2 2 2 2 2E V I r jI x

When an a.c. voltage of magnitude V1 is applied across the primary winding, it results in an

induced e.m.f. equal 1 max 14.44E f N .According to Lenz’s law, this e.m.f. opposes the applied

voltage and results in a flow of current,

1 11

1 1

V EI

r jx

Here it must be clearly understood that the quantity 1x is the leakage reactance of primary

winding. It should not be confused with the quantity 1L , where 1L being the self

inductance of primary winding.

% REGULATION Further, calculate the no load voltage E for different power factors.

For a lagging power factor angle, E can be written as,

2 2

1 1 1cos cos sinEX EX EXE V IR IX IR

Thus calculate the % regulation (UP) & (DOWN) using the expressions given below:

%Re ( ) 100E V

gulation UPV

%Re ( ) 100E V

gulation DOWNE

How will you calculate the above for leading power factor?

POWER EFFICIENCY: It is assumed that KVA rating of transformer is S ; Rated Copper loss

or the wattmeter reading under short circuit test is CUP ; Rated Core Loss or wattmeter reading

under Open Circuit test is I oP P . The power efficiency of a two winding transformer operating

at thk fraction of load and at a power factor cos is,

% Power Efficiency,

2

2

2

100 100

cos100

cos

1 100cos

I CU

I CU

I CU

Output Input Losses

Input Input

kS

kS P k P

P k P

kS P k P

The power efficiency can also be calculated using experimental observation.

% 100O

I

WEfficiency

W

PROCEDURE:

Note the name plate rating of transformer, decide the range of instruments.

Connect circuit diagram as shown in Figure for load test. Choose a resistive load.

Initially do not switch ON any load. Keep the voltage on LV side equal to its rated value.

Slowly change the load switches.

Do you observe any change in input voltage? If yes correct the input voltage using auto

transformer.

Record different voltages, currents ant wattmeter.

Repeat the experiment for capacitive and inductive loads.

OBERVATIONS: Using developed computer program, calculate no load voltage , %

regulations and % efficiency For different input currents and power factor.

S.N

o.

Low

Voltage

side

HV side E.M.

F

P.F. % Efficiency % Regulation

V

I

I

I

W

I

V

o

I

o

W

o

EI L

V

H

V

Experime

nt

Calculati

on

Experime

nt

Calculati

on

1 0

2

3

10

PLOT: For different types of loads, viz resistive, inductive and capacitive.

% Efficiency versus load currents

% regulation versus load current

RESULT & DISCUSSIONS:

Discuss the graphs for different types of loads. Why % regulation is negative for capacitive loads

PRECAUTIONS:

3. Choose correct rating instruments required for different tests.

4. Do not apply high voltage on LV windings.

B.Tech. EE Pt. II Sem. III (Transformer)

EXPERIMENT No. 3

SUMPNER’S TEST OR BACK-TO-BACK TEST

AIM: Perform Sumpner’s Test or back- to-back Test on two identical transformers. Hence plot

& compare % regulation, % efficiency versus load current with those obtained in Open circuit

and Short Circuit Test.

SPECIFICATIONS OF TRANSFORMER: Name Plate rating of transformer under test.

COMPUTER PROGRAM: Obtain the equivalent circuit parameters referred to low voltage

side from the OC & SC Test. Write a computer program for finding % regulation (UP &

DOWN) and % efficiency at any power factor and any load current.

APPARATUS REQUIRED:

Apparatus Number Range

Ammeters 2 15A & 10A

Voltmeter 2 75 Volt & 150- 300V

LPF Wattmeter 1 5A, 75 Volt

Wattmeter 1 150 - 300V / 15A

RTD & DMM 1

THEORY:

It is some times required to conduct heat run test on a transformer for different loadings. A

smaller capacity transformer can be conveniently loaded, but it is very difficult to conduct load

test on a large capacity transformer. For such a case, two transformers are connected back to

back so as that each transformer is having its core loss as well as copper losses. The test is useful

for conducting temperature versus time curve as well as for obtaining equivalent circuit

parameters.

Individual secondary share half this voltage. The current flowing in two secondary windings

induces currents in the respective primary windings. The actual current flowing in the individual

primary is IO+I`2. Since secondary windings are in phase opposition the induced current (I’2 on

primary side gets cancelled and the current and power on circuit connected in parallel primary do

not vary. Although the individual primary windings are carrying full load currents, the source

current is only twice the no load current. The voltmeter and wattmeter on LV side read twice the

voltage shared and twice the Copper loss of individual transformer respectively. Similarly, the

ammeter and LPF wattmeter on HV side read twice the no load current and core loss of

individual transformer, respectively.

PROCEDURE:

1) The essential condition in this test is that the low voltage secondary winding should be in

phase opposition. If this condition is not satisfied, there will be a dead short circuit,

causing extensive damage to the transformer as well as to the supply system.

2) The high voltage primary of the two transformers are connected in parallel and

maintained at rated voltage.

3) The power supplied from this side is basically twice the core loss of individual

transformer plus small ohmic losses. Similarly the current is twice the no-load current of

individual transformer.

4) The secondary are connected in series and its open circuit voltage is observed with a

voltmeter having rating of more than twice the rating of Low voltage windings.

Depending on the polarities of secondary terminals this voltmeter reads either zero

voltage or twice the low voltage rating of the device.

5) If voltmeter reads zero then secondary windings are in phase opposition and polarities are

marked as in Fig 1.

6) If this voltage is a finite voltage approximately twice the low voltage rating then

secondary windings are in series phase addition.

7) If this condition arises then switch off and interchange the terminals of secondary of only

one transformer. Again connect the primary supply and re-check the voltage on series

connected open secondary.

8) When the secondary are in phase opposition, give a reduced voltage on series (phase

opposition) connected secondary such that rated (Low Voltage winding) current flows.

9) On the high voltage side rated voltage need to be applied whereas on the low voltage side

rating current need to flow.

10) Record High voltage side and low voltage side readings of Voltage, Current and Power.

Heat Run Test:

1) Dip a mercury thermometer capable of measuring 100-150 C or a RTD in the

transformer oil.

2) Measure the temperature of oil at time t =0

3) Connect the Back to back connected transformer to the rated voltage on HV side of the

transformer and rated current to the series opposition connected LV secondary.

4) Record the temperature of transformer oil after a time step of about 5 minutes.

5) When temperature of oil is about 100C, switch OFF both the supply.

6) Note the temperature of oil as in Step till transformer oil cools down to about ambient

temperature.

7) Plot Temperature versus time curve during heating and cooling cycles..

8) How will you do this test at different load current?

OBSERVATIONS:

High Voltage Side Low Voltage Side

2 IO VO 2 WO ISC 2 VSC WSC

Amp Volt Watts Amp Volt Watts

RESULTS:

(A) Comparison of Equivalent circuit parameters:

RC XM REX1 XEX1

OC/SC B2B OC/SC B2B OC/SC B2B OC/SC B2B

(B) Comparison of calculated % Regulation & % Efficiency:

P.F. % Regulation % Efficiency

OC/SC Test Back to Back OC/SC Tests Back to Back

ZPF Lag.

0.8 Lag.

UPF

0.8 Lead

ZPF

Lead

(C) POWER EFF. At 0.8 P.F. LAGGING & 0.8 P.F. LEADING

Plot graphs fraction of load current ‘k’ versus % efficiency for different power factors. Compare

the graphs with those obtained using OC/SC tests. Also compare core loss & copper loss.

PRECAUTIONS:

S.No. Fraction Input Constant

loss

Variable

Loss

Total

Loss

%

Efficiency

k (W) (W) (W) (W)

1 0

2 0.1

3 0.2

16 1.5