Government Engineering College

Kozhikode-5

Electrical Machines Lab Manual(Version 1.0.04 - 08-08-2011)

Prepared by:- Mohammed Sadik(2010 AEI batch) Ranjith A.R (2010 AEI batch) Nikhil Narayanan (2010 AEI batch)

Guided by :- Smt. Sangeetha K(Associate Professor(EEE))

Verified by:- Smt. Sangeetha K(Associate Professor(EEE))

Type set in LATEX2 by Mohammed Sadik .P.K

Circuit Designs & Graphs in XCircuit by Mohammed Sadik .P.K

AEI logo in XCircuit by Mohammed Sadik,Ranjith AR,Mansoor M

Platform : GNU/Linux(Ubuntu)

Other Open Source softwares used: Bash,Gedit,Vim,Inkscape,Gimp. . . . . .

This work is licensed under :Creative Commons Attribution-NonCommercial-ShareAlike 2.5 India License.To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/2.5/in/or send a letter to Creative Commons, 444 Castro Street, Suite 900, Mountain View,California, 94041, USA.

Thanks to:-

1. Smt. Sangeetha K(Associate Professor(EEE))2. Sri. Asokan (Assistant Professor(EEE))3. Dr. Reena P(Associate Professor(ECE))4. Najmudheen P.K (Cherumukku)5. Jazeel M (Kolappuram)6. Selil C.P(MES Kuttippuram-EC)7. Mansoor M (GECK-2008 AEI batch)8. Sajith P.P (GECK-2010 AEI batch)9. Misthah K.M (AWH Kozhikode-EEE)10. Abhijit Navale (Mumbai)11. Midhun M (GECK-2010 AEI batch)12. Dijeesh K (GECK-2010 AEI batch)13. Shaaz Khalid A.K (GECK-2010 AEI batch)14. Mufeed Aslam (GECK-2010 AEI batch)15. Musthafa M (GECK-2010 AEI batch)

Our sincere thanks to everyone who have helped us for the perfection of this

Manual, especially our classmates - 2010 AEI batch.

CONTENTS

1. Load test on single phase transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2. Load test on 3 squirrel cage induction motor . . . . . . . . . . . . . . . . . . . 83. O.C.C of dc shunt generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4. Load test on d.c shunt generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5. Load test on dc series motor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

6. Measurement of coupling coefficient of transformer coils . . . . . . . . . . 32

7. O.C and S.C tests on single phase transformer . . . . . . . . . . . . . . . . . . . . 38

8. Three phase power measurement by two wattmeter method . . . . . . . 46

9. Calibration of single phase energy meter . . . . . . . . . . . . . . . . . . . . . . . . . . 52

10. Resistance measurement using Wheatstones bridge. . . . . . . . . . . . . .56

11. Resistance measurement using Kelvins double bridge . . . . . . . . . . . . 60

12. V-I Characteristics of incandescent lamp and linear resistance . . . 64

13. Open circuit and short circuit test on a three phase alternator . . . 68

14. No load & blocked rotor tests on 3 slip ring induction motor . . . 74

License Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

meet me at https://launchpad.net/sadiq

3

CONNECTION DIAGRAM OF LOAD TEST

A A

V V

M

C

L

V P1

P2 S2

S1

Load

230V1 - 50 HzAC

NL

10A

0-150V

0-10A MI 150V,10A,upf 0-5A MI

0-250VC

S

MI

N

P

115/230V 1KVATRANSFORMER

E

B

MI

Rated current on Primary side = 1000115 =

Regulation and efficiency curves

Reg

ulat

ion

Effic

ienc

y

Output

Regulation

Efficiency

Experiment 1 4

Experiment 1

LOAD TEST ON SINGLE PHASE

TRANSFORMER

AIMTo conduct load test on the given single phase transformer at unity

power factor and determine the efficiency and regulation curve.

APPARATUS REQUIRED1. Voltmeter 0-250V MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.2. Voltmeter 0-150V MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.3. Ammeter 0-10A MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.4. Ammeter 0-5A MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.5. Wattmeter 150V,10A,upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.6. Autotransformer(cont. variable) 0-270V,10A . . . . . . . . . . . . . . . . . . 1 no.

THEORYRegulation of a transformer is defined as the drop in terminal voltage of

a transformer expressed as a percentage of the no-load terminal voltage.

%Regulation =Vno load Von load

Vno loadWhen a purely resistive load is connected across the secondary, the

transformer will be working at unity power factor.

Terminal voltage V = Induced emf [E2] I2r2 I2x2Where r2 and x2 are the secondary winding resistance and leakage re-

actance respectively and I2 is the secondary load currentAs the load current increases the power output increases. The iron loss

remains constant from no load to full load. The copper loss increases asthe square of the load current. Thus the efficiency curve starts from zero,increases to a maximum value(When iron loss = Cu loss) and thereafterstarts decreasing.

5

OBSERVATION

Sl. V1 I1 W1 V2 I2 Output Efficiency RegulationNo. (volts) (Amp) (watts) (volts) (Amp) (watts) (%) (%)

V2 at no load = VNL =

Sample Calculation (set no . . . . . . )

Primary Voltage (V1) = . . . . . . . . .

Primary Current (I1) = . . . . . . . . .

Wattmeter Reading (W1) = Input power = . . . . . . . . .

Secondary Voltage (V2) = . . . . . . . . .

Secondary Current (I2) = . . . . . . . . .

Output = I2V2 = . . . . . . . . .

Efficiency =Output power

Input power100 = . . . . . . . . .

Regulation =VNL VLVNL

100 = . . . . . . . . .

6

PROCEDUREConnections are made as shown in the circuit diagram. The supply is

switched on keeping the autotransformer in the minimum position and atno load. Adjust the autotransformer to get the rated voltage of the trans-former. The readings of all the meters are noted down. The secondaryvoltage is also noted down. This value is VNL. A small load is added on thesecondary side and the meter readings are again noted. The experimentis repeated for different values of current till the current on the primaryside equals the rated value. The load is then reduced to zero, the auto-transformer brought back to the zero position and the supply is switchedoff. The readings are then tabulated as shown.

RESULTConducted load test on the given 1- transformer and plotted the reg-

ulation and efficiency curves.

VIVA QUESTIONS1. What do you understand by regulation of a transformer?2. What are the other methods of testing transformers?3. What is the disadvantage of testing a transformer using load test?4. Is a high or low value of regulation preferred for a transformer? Give

reasons.5. What are the reasons for the drop in terminal voltage as the secondary

current is increased?

7

CONNECTION DIAGRAM

400V

3 -

50H

zA

C

10A

D.O.LSTARTER

0-600VMI

0-10A MI 600V,10A,upf

C

M

V

L

IM

600V,10A,upf

10A

10A M

C V

L

V

AR

Y

B

B

Y

RS1 S2

MACHINE DETAILS

Voltage V - 415 VCurrent I - 7.5APower - 5HPConnection - Speed(rpm) - 1440Phase - 3

Synchronous speed =120fP

= 120 504 = 1500 rpm

Experiment 2 8

Experiment 2

LOAD TEST ON 3 SQUIRREL CAGEINDUCTION MOTOR

AIMTo conduct load test on the given 3- squirrel cage induction motor and

plot the performance characteristics.

APPARATUS REQUIRED1. Voltmeter 0-600V MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.2. Ammeter 0-10A MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.3. Wattmeter 600V,10A,upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 nos.4. Tachometer - to measure speed

THEORYA squirrel cage induction motor essentially consists of a stator and a

rotor. The stator is a hollow cylindrical structure with slots on the in-ner periphery and carries a three phase winding. The winding can beconnected in star or delta and is connected across a 3- supply.The rotor is also a cylindrical structure with slots on the outer periphery.

The slots carry thick Al or Cu bars. These bars are short circuited at bothends by means of end rings.When a 3- supply is given to a 3 winding displaced by 120 in space,

a magnetic field of constant magnitude but rotating at synchronous speedis produced. This flux links with the stationary rotor, thus inducing anemf in it. As the rotor circuit is closed, a current flows through it. Thedirection of the induced current is such as to oppose the cause producingit. The cause is the relative motion between the stator magnetic field andthe rotor. So the rotor starts rotating in the same direction as the statormagnetic field and tries to catch up with it. But practically it is neverable to do so. Because if it does so, there would be no relative motion, noemf and hence no torque.

9

OBSERVATIONS

Sl. V I W1 W2 S1 S2 N T Output Input slip pf EfficiencyNo. (volts) (Amp) (watts) (watts) (Kg) (Kg) (rpm) (Nm) (watts) (watts) (%)

10

Thus an induction motor always runs at a speed slightly less than thesynchronous speed.The term slip is of importance in an induction motor and is defined as

%slip =Ns NNs

100

Where,

Ns - Synchronous speed =120 f

PN - rotor speedf - frequencyP - no. of poles of the machine

An induction motor can never operate at s = 0. It always operates betweens = 0 and s = 1(starting).The performance characteristics are plots of efficiency, torque, speed,

slip, pf and line current versus output.Current and torque increases with increase in output. The induction

motor is essentially a constant speed motor. However speed reduces grad-ually with increase in output and slip increases gradually with increase inoutput. The p.f is low at low loads and increases with increase in output.The efficiency increases with increase in output, reaches a peak value andthen gradually drops with further increase in output.

PROCEDUREThe load on the motor is completely removed by loosening the brake

drum. The motor is to be always started and stopped at no load, Thesupply is switched on and the motor is started using a Direct On LineStarter (DOL Starter).The readings of the voltmeter, ammeter, wattmeters and spring balance

are noted down. The speed is measured using a tachometer. The load isthen increased in steps, each time noting down all the above readings. Theexperiment is repeated for different values of load currents till the ratedcurrent of the machine is reached.During the experiment, the machine may get heated up. It is cooled by

pouring some water into the brake drum.

11

Sample Calculation (set no . . . . . . )

Voltage (V ) = . . . . . . . . .Current (I) = . . . . . . . . .Wattmeter Reading 1 (W1) = . . . . . . . . .Wattmeter Reading 2 (W2) = . . . . . . . . .Spring balance Readings S1 = . . . . . . . . . S2 = . . . . . . . . .Speed (N) = . . . . . . . . .Torque (T ) = (S1 S2)Rg Where,

R = Radius of brakedrum = . . . . . . . . .g = 9.8 m/s2

Synchronous speed =120 f

P =120 50

4 = 1500 rpm

slip(%) =(Ns N)

Ns 100 = . . . . . . . . .

Input power = (W1 +W2) = . . . . . . . . .

powerfactor(cos) =(W1 +W2)

3V I= . . . . . . . . .

Output power= 2piNT60 = . . . . . . . . .

Efficiency =OutputInput = . . . . . . . . .

Performance Characteristics

Efficiency

Output

Slip pf T N

Torque

Speedpf

Slip

12

At low loads,(when the pf is < 0.5) one of the wattmeters read negative,in such cases, the supply is switched off and the connections to the M andL terminals of the wattmeter are interchanged. The meter now readspositive, but it is to be recorded as negative.The load on the machine is removed completely and the supply is

switched off.The readings are tabulated and the performance characteristics are plot-

ted.

RESULTConducted load test on the given 3- squirrel cage induction motor and

plotted the performance characteristics.

VIVA QUESTIONS1. How are the meter ratings selected for this experiment?2. Why does one of the wattmeters read -ve at starting?3. What is slip in an induction motor?4. What are the two types of 3- induction motors and what is the

difference between the two?5. What is the value of slip at starting?6. What are the advantages and disadvantages of squirrel cage induction

motor?7. What is the condition for maximum torque in an induction motor?8. What are the different losses in an induction motor?9. Give some applications of 3- squirrel cage induction motor?10. Explain a typical Torque-slip characteristic.11. What is the effect of increased rotor resistance on the performance

of an induction machine?

13

CONNECTION DIAGRAM

220VDC

+

-

FL A

20A

Rh16002A

F1

F2

A1

A2

SRh2

0-2AMC

+

-

0-300VMC

-

+ F2

F1

A1

A2

20A

3 POINT STARTER

V

A

GM

6002A

V+

-

0-30VMC

MACHINE DETAILSMOTOR GENERATOR

3.5 KW 3.5 KWSpeed - 1500rpm Speed - 1500rpmvolts - 220V volts - 220VAmps - 18.6 A Amps - 16 AWinding - shunt Winding - shuntField - 220V,0.46A Field - 220V,0.46A

OBSERVATION

Sl No. Field current Eo at rated speed Eo at 1000 rpm

To determine O.C.C at 1000 rpmWe have E N at same flux or field current

E1E2

=N1N2

E2 =(N2N1

)E1

Where N2 is 1000 rpmand N1 Rated speed = 1500 rpm

Experiment 3 14

Experiment 3

O.C.C OF DC SHUNT GENERATOR

AIMTo conduct no load test on the given d.c shunt generator and determine

the following:-1. Open circuit characteristics at rated speed.2. Predetermine the O.C.C at 1000 rpm.3. The critical field resistance at rated speed.4. The critical speed of the machine.

APPARATUS REQUIRED1. Voltmeter MC (0-300V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.2. Voltmeter MC (0-30V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.3. Ammeter MC (0-2A). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.4. Rheostat 600,2A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 nos.5. Tachometer

THEORYThe O.C.C is a curve showing the relationship between the no load emf

generated and the shunt field current (Eo and If). Even when the field cur-rent is zero there is some residual magnetism present in the poles. Hencethere is a small voltage generated even at zero field current, which is calledthe residual voltage. As the field current is increased, Eo also increasesand the curve traced is almost a straight line. As If is further increasedthe poles start getting saturated and the straight line relation no longerholds good and the curve bends and becomes almost horizontal.

Critical Resistance:- It is that value of resistance in the field circuitat which the generator will just excite (or voltage build up begins). If theresistance is higher, the machine will fail to build up voltage.

15

MEASUREMENT OF SHUNT FIELD RESISTANCE

220VDC

100,2.8A 0-2A MC

0-250VMC

F1

F2

+

-

+ -A

V

+

-

OBSERVATION

Sl No. V I Rf

16

It is given by the slope of the tangent drawn to the linear portion of themagnetization curve from the origin.

Conditions for Voltage build up in a d.c shunt generator:-1. There should be some residual magnetism in the poles.2. For the given direction of rotation, the shunt field coils should be

properly connected ie, The coils should be so connected that the fluxgenerated by the field current aids the residual flux.

3. When excited at no load, the shunt field circuit resistance should beless than the critical resistance.

Critical Speed :- It is that value of speed at which the given shunt fieldresistance represents the critical resistance. It is determined as follows.For the same value of If determine E1 and E2 from the field resistancelines. Then

E1E2

=N1Nc

Nc =(E2E1

)N1

Where,Nc is the Critical speed

PROCEDUREConnections are made as shown in the diagram. The motor field rheostat

(Rh1) is kept in minimum position, the generator field rheostat (Rh2) inmaximum position and switch S is kept open at starting. Supply isswitched on. The starter handle is gradually moved to cut off the starterresistance. The rheostat Rh1 is varied till the speed equals the rated speedof the machine. With S open, the residual voltage is measured using thesmaller range voltmeter. Switch S is then closed. Rheostat Rh2 is thendecreased in steps, each time noting down the voltmeter and ammeterreadings. The process is repeated till the voltage equals 120% of the ratedvoltage of the machine. [If Eo does not increase, it means that the machineis not building up voltage. The field terminals F1 and F2 are interchangedand the process is repeated] Rheostat Rh2 and Rh1 are brought back tothe original position and the supply is then switched off.

17

OPEN CIRCUIT CHARACTERISTICS

Eo

E1

E2

residualIf1 If

Critical Fieldresistance

line O.C.C at ratedspeed

O.C.C at 1000 rpm

Given shunt fieldresistance line

{voltage

Critical resistance at rated speed,

Rc =E1If1

= . . . . . . . . .

Critical speed of the Machine,

Nc =

(E2E1

)N1 = . . . . . . . . .

18

Measurement of field resistance

Connections are made as shown in the diagram 2. For different valuesof voltages determine the current. The ratio gives the field resistance. TheO.C.C and field resistance line is drawn and the critical speed of the ma-chine is determined.

RESULTNo load test was conducted on the given d.c shunt generator andthe O.C.C was plotted.Critical resistance at rated speed = . . . . . . . . .Critical speed of the machine = . . . . . . . . .

VIVA QUESTIONS1. What is the need for starter in a d.c motor?2. How does a 3-point starter function?3. Why is Rh1 kept in minimum position at starting?4. Why is Rh2 kept in maximum position at start up?5. What is residual voltage? How is it measured?6. What is critical resistance? How can it be measured?7. What are the conditions necessary for voltage build up in a d.c shunt

generator?8. What is critical speed?9. Explain the shape of the O.C.C

19

LOAD TEST

+

- S1 S2

V

A

GM

L F A20A

F1

F2

Rh1600

2AA1

A2

Rh2

0-30

0VM

C

MC0-2A

+

-

+

-

6002A

F1

F2 Load

200VDC

20A

3 Point Starter

A1

A2

MACHINE DETAILSMOTOR GENERATOR

3.5 KW 3.5 KWSpeed - 1500rpm Speed - 1500rpmvolts - 220V volts - 220VAmps - 18.6 A Amps - 16 A

Winding - shunt Winding - shunt

Measurement of Armature Resistance

A

V G20VDC 505A

+-

A1

A2

0-5A MC

0-10VMC

+

-

Sl Voltage V Current I Resistance RNo. (volts) (Amperes) (ohms)

Armature Resistance Ra = . . . . . . . . .

Experiment 4 20

Experiment 4

LOAD TEST ON D.C SHUNTGENERATOR

AIMTo conduct load test on the given D.C shunt generator and plot the

external and internal characteristics.

APPARATUS REQUIRED1. Voltmeter MC (0-300)V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.2. Voltmeter MC (0-30)V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.3. Ammeter MC (0-2)A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.4. Rheostat 600,2A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 nos.5. Tachometer

THEORYLoad characteristics of the machine can be broadly classified into1) External characteristics2) Internal Characteristics

External Characteristics(V vs I2) :-It is a curve showing the variation in terminal voltage of the generator

as the load on the generator is increased. The characteristics are as shownin the figure.At no load, the terminal voltage of the generator is at its rated value.

As the load current is increased the terminal voltage drops. The drop interminal voltage is due to the following reasons.1) For a generator V = Eg IaRa, as the load current increases, Ia

increases, IaRa drop increases, thus decreasing the terminal voltage V2) As the load current increases, Ia increases, armature reaction effect

also increases. Due to demagnetizing effect of armature reaction, the in-duced emf Eg decreases, thereby decreasing V .

21

OBSERVATION - LOAD TEST

Sl V IL Ish Ia Egno. (volts) (A) (A) (A) (V )

Sample Calculation (set no . . . . . . )

Terminal Voltage (V ) = . . . . . . . . . voltsLoad Current (IL) = . . . . . . . . .AShunt Field Current (Ish) = . . . . . . . . .AArmature Current (IA) = IL + Ish = . . . . . . . . .AGenerated emf (Eg) = V + IaRa

Internal and External Characteristics

V/ Eg

Drop DueTo ArmatureResistance

Drop due to armatureReaction Effect

IL Ia

Internal

ExternalCharacteristics

CharacteristicsEg vs Ia

V vs IL

/

22

3)Due to reasons (1) and (2), the terminal voltage decreases, which inturn reduces the field current Ish, thereby decreasing Eg causing furtherdecrease in V .Internal Characteristics [EgvsIa] :- It is a plot of the internally gen-

erated emf (Eg) and armature current (Ia). It is a curve similar to theexternal characteristics and lies above it.

Eg = V + IaRa

& Ia = IL + Ish

PROCEDUREConnections are made as shown in the diagram. rheostat Rh1 is kept in

minimum position and Rh2 in maximum position. Switch S2 is kept open.Supply is switched on and the motor is started using a 3-point starter. Themotor field rheostat Rh1 is varied till the speed equals the rated speed ofthe motor. The generator field rheostat Rh2 is varied till the voltmeterreads the rated voltage of the machine. Switch S2 is then closed. Theload on the generator is increased. The readings of the voltmeter andammeters are noted down. The experiment is repeated for different valuesof load current till the rated current of the generator is reached. Duringthe experiment, the speed is to be maintained constant at the rated value.The load is then switched off completely, the rheostats are brought back

to the original position and the machine is switched off.

Measurement of RaConnections are made as shown in the diagram. Keeping the rheostat

in the maximum position, supply is switched on. The resistance is thenincreased in steps and the voltmeter and ammeter readings are noted. Theratio gives the armature resistance.The readings are then tabulated as shown. The external and internal

characteristics are then plotted.

RESULTConducted load test on the given DC shunt generator and plotted the

external and internal characteristics.

23

24

VIVA QUESTIONS

1. What is the need for starter with a d.c motor?2. How does a 3-point starter function?3. Why is Rh1 kept in minimum position at starting?4. Why is Rh2 kept in maximum position at starting?5. Why does the terminal voltage of a generator decrease with increase

in load?6. How are the meter ratings selected for this experiment?7. What are the different losses in a d.c generator?8. What is the condition for maximum efficiency in a d.c machine?9. What is armature reaction? How does it effect the functioning of

the machine?

25

CONNECTION DIAGRAM

+

-

V

A

20A

F1 F2

L A

+

-

A1

A2

S1 S2

BRAKE DRUM

20A

2 Point Starter

0-300V MC220V

DC

+ -

0-20A

Machine Details

HP - 5Volts - 230 VAmp - 17 Aspeed - 1500 rpm

Radius of brakedrum, R = . . . . . . . . .

Experiment 5 26

Experiment 5

LOAD TEST ON DC SERIES MOTOR

AIMTo conduct load test on the given d.c series motor and plot the perfor-

mance characteristics.

APPARATUS REQUIRED1. Voltmeter (0-250)V MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.2. Ammeter (0-20)A MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.3. Tachometer - to measure speed

THEORYIn a series motor, the field winding is connected in series with the arma-

ture winding. Thus the same current flows through the field and armaturewindings.

Electrical characteristics(T1Ia) :- It shows the variation of torquewith the armature current.We have

T Ia where is the flux/pole

IaIa (as Ia up to the point of magnetic saturation)

Thus T I2a

27

OBSERVATIONS

Sl. V I S1 S2 Speed Torque Output Input EfficiencyNo. volts Amp. Kg Kg rpm N m watts watts

Sample Calculation ( set no. . . . . . . )

Voltmeter reading (v) = . . . . . . . . .

Current (I) = . . . . . . . . .

Spring balance reaadings S1 = . . . . . . . . . S2 = . . . . . . . . .

Speed(N) = . . . . . . . . .

Torque(T ) = 9.8 (S1 S2)R = . . . . . . . . . Where R is the radiusof brake drum

Output power =2piNT

60= . . . . . . . . .

Input power = V.I = . . . . . . . . .

Efficiency =Output power

Input power= . . . . . . . . .

28

However after magnetic saturation remains almost constant, HenceT IaThus the curve is a parabola up to magnetic saturation and shows a

linear variation after the point.Mechanical Characteristics(N1 vs T ):- It shows the variation of speedwith torque.

We have N Eb 1

as Eb is almost constant where Eb is

back emfIn a series motor Ia

N 1Ia

(ie) as Ia increases, Speed decreases.

The same pattern is followed in the N -T characteristics. The curvetraced is a rectangular hyperbola.A series motor should never be started at no load. At no load, Ia is very

small, hence the speed of the motor becomes dangerously high(as N 1Ia ).Performance characteristics shows the variation of speed, torque and

efficiency with change in output.

PROCEDUREConnections are made as shown in the connection diagram. A small load

is applied to the motor by tightening the brake drum. Supply is switchedon and the motor is started using a 2-point starter. The voltage, current,speed and spring balance readings are noted down. The experiment isrepeated for different loads till the rated current of the machine is reached.During the experiment when the machine gets heated up, it is cooled

by pouring water into the brake-drum.The load is then reduced till the current reaches a small value and the

supply is switched off.

29

Performance CharacteristicsT N

O

Torque

Efficiency

Speed

Output

Electrical Characteristics Mechanical Characteristics

T

Ia

N

T

30

RESULTLoad test was conducted on the given DC series motor and the perfor-

mance, electrical and mechanical characteristics are plotted.

VIVA QUESTIONS1. What is the precaution to be taken when working with a d.c series

motor?2. What is the need for starter with a d.c motor?3. How does a 2-point starter function?4. Explain the shape of the electrical and mechanical characteristics5. What is the condition for maximum efficiency in a d.c motor?6. What are the different losses occurring in a d.c machine?7. How are the meter ratings selected for this experiment?8. Give some applications of d.c series motor.

31

CONNECTION DIAGRAMFLUX AIDING

P

N

A

V1 V2V3

1A

NL

B

C

Aut

oTr

ansf

orm

er

0-500mAMI

0-250VMI

0-15

0V

S2

S1P2

P1

240/120V,3kVATransformer

230V50Hz1 - AC

MI

0-250VMI

E

V1 w V2 + V3FLUX OPPOSING

P

N

A

V1 V2V3

1A

NL

B

C

Aut

oTr

ansf

orm

er

0-500mA MI

0-250VMI

0-15

0VS2

S1

P2

P1

240/120V,3kVATransformer

230V50Hz1 - AC

MI

0-250VMI

E

V1 w V2 V3MEASUREMENT OF RESISTANCE OF COILS

V

A

0-20V505A 0-10V

MC

0-5A MC

P1

P2

S1

S2

+

-

+ -

Experiment 6 32

Experiment 6

MEASUREMENT OF COUPLINGCOEFFICIENT OF TRANSFORMER

COILS

AIMTo determine the self inductance, mutual inductance and coupling co-

efficient of the given transformer windings.

APPARATUS REQUIRED1. Transformer 240/120V,1kVA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.2. Voltmeter 0-250V MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 no.3. Voltmeter 0-150V MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.4. Voltmeter 0-10V MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.5. Ammeter 0-500mA MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.6. Ammeter 0-5A MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.7. Autotransformer 0-270v,10A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

PRINCIPLEThe property of a coil due to which it opposes any change of current

through it is know as self inductance. The coefficient of self induction(L)is defined as Weber turns per ampere in the coil.

L =N

I=NNI0rA

Il=N 2

S(H)

Mutual inductance is the ability of one coil to produce an emf in anearby coil by induction when the current in the first coil changes. Thecoefficient of mutual inductance (M) is defined as the Weber turns in onecoil due to ampere current in the other coil.

M =N21I1

=N2N1I1I1S

=N1N2S

33

OBSERVATIONS

Measurement of impedance of coils

Condition V1(v) V2(v) V3(v) I(A) Z() L(H)

Flux aiding

Flux opposing

Measurement of resistance of coils

Sl. No. V (v) I(A) R()

1

2

3

Winding Resistance R = r1 + r2 = . . . . . . . . .

34

Consider two coupled coils A and B

L1 =N 21S

L2 =N 22S

Flux produced in A due to current I1 is 1 =N1I1S

Let a fraction k1 of this link with the second coil

ie, 2 = k11

Then

M =(k11N2)

I1=

(k1N1N2)

S. . . . . . . . . . . . . . . . . . . . . . . . (1)

Flux produced in B due to current I2 is 2 =N2I2S

Suppose a fraction k2 of this links with A

ie, 1 = k22

M =(k22N1)

I2=

(k2N1N2)

S. . . . . . . . . . . . . . . . . . . . . . . . (2)

from (1) and (2)

M 2 =(k1k2N

21N

22 )

SS= k1k2

N 21S

N 22S

M 2 = k1k2L1L2 or k =M

(L1L2)

where k =k1k2

The constant k is called the coefficient of coupling and may be definedas the ratio of mutual inductance actually present between the two coils tothe maximum possible value. If the flux due to one coil completely linksthe other then k = 1. If the flux of one coil does not link the other coil atall then k = 0.

35

CALCULATION

For coils connected in series with fluxes aiding each other

Total impedance of coils ZA =V1I

= . . . . . . . . .

Reactance of coils XA =Z2A R2 = . . . . . . . . . Where, R = r1 + r2

is the total resistance

Inductance LA =XA2pif

= . . . . . . . . . of both windings

For coils connected in series with fluxes opposing

Total impedance of coils ZB =V1I

= . . . . . . . . .

Reactance of coils XB =Z2B R2 = . . . . . . . . .

Inductance LB =XB2pif

= . . . . . . . . .

When coils are connected with flux aiding each otherTotal inductance, LA = L1 + L2 + 2M (1)

When coils are connected with flux opposing each otherTotal inductance, LB = L1 + L2 2M (2)

subtracting (2) from (1), M =(LA LB)

4(L1L2

)=

(N1N2

)2=

(240

120

)2= 4

L1 = 4L2

Substituting in (1), LA = 5L2 + 2M or L2 =LA 2M

5= . . . . . . . . .

L1 = 4L2 = . . . . . . . . .

Coupling coefficient k =ML1L2

= . . . . . . . . .

36

PROCEDUREMeasurement of impedance of coilsConnections are made as shown in the first figure. Supply is switched

on with autotransformer in the minimum position. The autotransformeris adjusted to get the rated voltage in voltmeter1. The correspondingreadings in all meters are noted down. In this case the fluxes produced byboth the coils are additive in nature ( ie V1 = V2 + V3).Next the connections of the second coil are reversed. The fluxes pro-

duced by the two coils are now in subtractive polarity(ie, V1 = V2 V3).The autotransformer is adjusted so as to get the same reading in V2 aswith the additive polarity. This is done to maintain the same flux in boththe cases. The readings of all meters are noted down and tabulated asshown.

Measurement of ResistanceConnections are made as in the third figure. For different values of volt-

ages the readings of both meters are noted down and tabulated.

RESULTThe coupling coefficient of the given transformer windings is . . . . . . . . . .

VIVA QUESTIONS1. What is meant by coupling coefficient of a transformer? What are

the limiting values?2. Why is the voltage V2 maintained constant in the second case?3. How are the meter ratings selected for this experiment?

37

CONNECTION DIAGRAM(OPEN CIRCUIT TEST)

P

N

A

V

2A

B

CNL E

0-150VMI

M

C

L

V

l.v h.v

P

0-2A MI

115/230V,1kVATransformer

150V,2A,lpf

230V

50Hz1 - AC

OBSERVATION

Vo Io Wo

CONNECTION DIAGRAM(SHORT CIRCUIT TEST)

P

N

A

V

5A

B

CNL E

0-50VMI

M

C

L

V

l.vh.v

0-5A MI

115/230V,1kVATransformer

75V,5A,upf

230V1 - 50HzAC

Rated Current =Rated KV A

Rated V oltage on h.v side=

1000

230= . . . . . . . . .

OBSERVATION

VSC ISC WSC

Experiment 7 38

Experiment 7

O.C AND S.C TESTS ON SINGLE PHASETRANSFORMER

AIMTo conduct open circuit and short circuit tests on the given 115/230 V,

1kVA transformer and predetermine the following:-1. Equivalent circuit as referred to l.v side2. Equivalent circuit a referred to h.v side3. Efficiency curve at 0.8 pf4. Regulation curve at 1/2 Full load

INSTRUMENTS REQUIRED1. Voltmeter 0-150V MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.2. Voltmeter 0-50V MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.3. Ammeter 0-2.5A MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.4. Ammeter 0-5A MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.5. Wattmeter 150V,2.5A,lpf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.6. Wattmeter 75V,5A,upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.7. Autotransformer 0-270V,10A

PRINCIPLE

Open circuit test:- This test is usually conducted on the l.v side ofthe transformer. It is conducted to determine the core loss ( iron loss or noload loss). The low voltage side of the transformer is supplied with ratedvoltage with the h.v side left open. The current, voltage and power onthe input side is noted. Since the no-load primary current is small (2-10% of the rated current) the copper losses in the primary winding can beneglected and the power loss read by the wattmeter is the core loss of thetransformer. Since the flux linking with the core is constant at all loads,

39

From O.C test (l.v side) :-

V0 = . . . . . . . . . I0 = . . . . . . . . . W0 = . . . . . . . . .

W0 = V0I0 cos0 cos0 =W0V0I0

= . . . . . . . . .

sin0 = . . . . . . . . .Iw = I0 cos0 = . . . . . . . . .I = I0 sin0 = . . . . . . . . .

Core loss component resistance as refered to l.v sideR0 =V0Iw

= . . . . . . . . .

Magentising reactance as refered to l.v side X0 =V0I

= . . . . . . . . .

The parameters R0 and X0 as referred to the h.v side areR0 = R0 k2 = . . . . . . . . .X 0 = X0 k2 = . . . . . . . . . where k =

N2N1

=E2E1

=230

115

From S.C test (h.v side) :-

VSC = . . . . . . . . . ISC = . . . . . . . . . WSC = . . . . . . . . .

Total eqvt. wdg resistance as refered to h.v side R02 =WSCISC

= . . . . . . . . .

Total eqvt. impedance as refered to to h.v side Z02 =VSCISC

= . . . . . . . . .

Total eqvt. leakage reactance referred to h.v side X02 =Z202 R202 =

The parameters R02, Z02, and X02 as referred to l.v side areR01 = R02|K2 = . . . . . . . . .X01 = X02|K2 = . . . . . . . . .Z01 = Z02|K2 = . . . . . . . . .

EQUIVALENT CIRCUITas referred to l.v side as referred to h.v side

115V 230VR0 X0

R01 X01

Z'L

I1 I'2I0

=

= =

=ZL

R'0 X'0

R02 X02

I1 I'2I0

=

= =

=

40

the core loss remains same for all loads. The parameters R0 and X0 (theshunt branch) are determined using this test.

Short Circuit Test :- The short circuit test is conducted to deter-mine the full load copper loss and the equivalent resistance and leakagereactance referred to the winding in which the test is conducted. The testis conducted on the h.v side with the l.v side short circuited by a thickconductor. A low voltage just enough to circulate the rated current of thetransformer is supplied to the transformer. The voltage supplied is usuallyonly 5-10% of the normal supply voltage and so the flux linking with thecore is small. Thus core losses can be neglected and the wattmeter readinggives the full load Cu loss of the transformer.

PROCEDURE

Open Circuit Test:- Connections are made as shown in the connectiondiagram1. The h.v side is left open. The supply is switched on with theautotransformer in the minimum position. The autotransformer is gradu-ally varied till the voltmeter reads the rated voltage of the primary side ofthe transformer. The corresponding ammeter and wattmeter readings arenoted down and tabulated as shown.

Short Circuit test:- Connections are made as shown in the diagram 2.The l.v side is short circuited. Supply is switched on with the autotrans-former in the minimum position. The autotransformer is gradually variedtill the ammeter reads the rated current of the transformer on the h.v side.

(Rated Current =

Rated volt Amperes of transformer

Rated voltage on h.v side

)

41

Calculation of Efficiency :-

Efficiency of the transformer at any load and p.f is given by

=Power Output

Power Input=

x F.L(V A) cosx F.L(V A) cos+Wi + x2WCu

Where,Wi - is the core lossWCu - full load Cu lossx - is the fraction of full load

x Output Wi x2WCu Input Efficiency

1/4

1/2

3/4

1

Sample Calculation(set no. . . . . . . )

x = . . . . . . . . . F.L(V A) = . . . . . . . . .p.f cos = . . . . . . . . .Power output = x F.L(V A) cos = . . . . . . . . .Power input = Power Output +Wi + x

2WCu = . . . . . . . . .

Efficiency =Power Output

Power Input= . . . . . . . . .

Efficiency curve

FLO/P1/21/4 3/4

FL FL FL

42

The corresponding voltmeter and wattmeter readings are noted downand tabulated as shown.Using the readings obtained from the two tests, the equivalent circuit

as referred to the l.v side and h.v side are drawn.The efficiency at various fractions of full load are calculated and tabu-

lated. The efficiency curve is then plotted.Regulation of the transformer (which gives the variation of the secondary

terminal voltage from no load to full load expressed as a percentage of thesecondary terminal voltage with the primary voltage held constant) is thencalculated using the approximate formula at various power factors and halfthe full load, Regulation curve is then plotted.

43

Calculation of Regulation

Regulation at any load and p.f is given by

%Reg =I2R02 cos I2X02 sin

Eo2+ for lagging

- for leading

Where I2 is the current at any load and

= xI2FL

Where x Fraction of full loadI2FL full load current on secondary sideE02 rated voltage on secondary side

lagging upf leading 0 0.2 0.4 0.6 0.8 1 0.8 0.6 0.4 0.2 0

Sample calculation . . . . . .(for one lead and one lag case)

Regulation Curve

%Reg

0 0.2 0.4 0.6 0.8 0.8 0.6 0.4 0.2 0

(upf)

p.f(lead) (lag)

%Reg

p.f

44

RESULT

O.C and S.C tests were conducted on the given 1- transformer andpredetermined the regulation and efficiency curves.

VIVA QUESTIONS1. How are the meter ratings selected for O.C and S.C tests?2. Why is the O.C test conducted on the l.v side of the transformer and

S.C test on h.v side?3. What are the losses measured in an O.C test?4. What are the losses measured in an S.C test?5. What is the condition for maximum efficiency in a transformer?6. What is meant by regulation of a transformer?7. Is a high or low value of regulation preferred? Why?8. How can the parameters on one side of the transformer be transferred

to the other side?

45

CONNECTION DIAGRAM

400V

3 -

50H

zA

C

5A

5A

5A

R

Y

B

N

E2

E3

A

V 0-600V

M

C

L0-5A MI 600V,5A,upf

600V,5A,upfM

C V

L

3Inductive

Load1-10A

50,5A

50,5A

50,5A

E1

B1

C1

C2

B2

B3

C3NL

PHASOR DIAGRAM

30

30

IB

IR

IY

VY

VB

VBY

VRY

VR

Experiment 8 46

Experiment 8

THREE PHASE POWERMEASUREMENT BY TWOWATTMETER METHOD

AIMTo measure the power factor and power consumed by a 3- RL load

using two wattmeter method.

APPARATUS REQUIRED1. Voltmeter (0-500V) MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.2. Ammeter (0-5A) MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 nos.3. Wattmeter 600V,5A,upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 nos.4. Rheostat 100,5A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 nos.5. 3- Inductive load (0-10A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.6. 3- Autotransformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

PRINCIPLEIn the two wattmeter method current coils of two watt meters are con-

nected in two phases and the potential coils between the correspondingphase and the third phase. It can be proved that the sum of the wattmeterreadings gives the total power.From the phasor diagramReading of Wattmeter 1,W1 = VRY IR cos(30 + )Reading of Watt meter 2,W2 = VBY IB cos(30 )

W1+W2 = VRY IR(cos 30 cossin 30 sin)+VBY IB(cos 30 cos+sin 30 sin)= VRY IR cos 30 cos+ VBY IB(cos 30 cos)

Assuming balanced loadVRY = VBY = VBR = VL

& IR = IB = IY = ILwhere VL and IL are the line values of voltage and current

47

OBSERVATIONS

Sl.Case

voltage CurrentWattmeter

PowerPhase Power

Reading Angle factorNo. V(V ) I(A) W1 W2 P(W ) cos

1 W1 and W2 read+ve

2 W1 reads +ve, W2reads zero

3 W1 reads +ve, W2reads -ve

Sample Calculation (set no . . . . . . )

Voltage V = . . . . . . . . .

Current I = . . . . . . . . .

Wattmeter reading W1 = . . . . . . . . .

Wattmeter reading W2 = . . . . . . . . .

Total Power P = W1 +W2 = . . . . . . . . .

Phase angle = tan1[

3(W1 W2)W1 +W2

]= . . . . . . . . .

Power factor = cos = . . . . . . . . .

48

= VLIL cos 30 cos+ VLIL cos 30 cos

= 2VLIL

(32

)cos

=3VLIL cos

For leading pf (When the load is capacitive)W1 = VLIL cos(30 )W2 = VLIL cos(30 + ) = Power in a 3 circuitW1 +W2 = VLIL cos(30 ) + VLIL cos(30 + )

= [2 cos 30 cos]VLIL=3 cosVLIL

W1 W2 = VLIL cos(30 ) VLIL cos(30 + )= [2 sin 30 sin]VLIL= sinVLIL

tan =(W1 W2)/VLIL

(W1 +W2)/3VLIL

=3(W1 W2)(W1 +W2)

When pf is unity, = 0 and W1 = W2When 1.0 > pf > 0.5,0 < < 60 both W1 and W2 read positiveWhen pf = 0.5 , = 60 and W1 = 0, hence W2 alone reads the total

powerWhen 0.5 > pf > 0, 60 < < 90

W1 reads negative and W2 positive

PROCEDUREConnections are done as shown in the figure. The resistance is kept in

the maximum position and the inductive load is set to minimum. Thesupply is switched on with the autotransformer in the minimum position.The autotransformer is adjusted to get rated voltage in the voltmeter.

The load is purely resistive, the power factor is nearly unity and bothwattmeters read positive. Readings are taken corresponding to this con-dition. The inductive load is then increased till one of the wattmeterbecomes zero. This corresponds to a pf of 0.5. Again all readings arenoted. On further increasing the inductive load one of the wattmetersstarts deflecting in the negative direction. This indicates that the powerfactor of the circuit is less that 0.5. The supply is now switched off andthe pressure coil(or current coil ie, C & V or M & L) connections of the

49

50

wattmeter reading negative is interchanged. Supply is switched on andthe readings corresponding to this condition are noted. The reading ofthe wattmeter whose terminals are interchanged is be recorded asnegative. The power factor and power are calculated using the formulagiven.

RESULTPower consumed by a 3- RL load is measured using two wattmeter

method.

VIVA QUESTIONS1. What is the expression for power in a 3- circuit?2. Derive the expression for power factor in terms of the wattmeter

readings.3. What are the other methods of measuring 3- power.4. What does a zero reading in one of the wattmeters signify?

51

CONNECTION DIAGRAM

230V1 - 50HzAC

P

N

A

V

5A

B

CNL E

0-250VMI

M

C

L

V

0-5A MI 250V,5A,upf P.C

C.C

P1P2

N1N2

Energy

K=240V,5A

Meter

ERROR CURVE

LINE CURRENT

% E

RRO

R

Experiment 9 52

Experiment 9

CALIBRATION OF SINGLE PHASEENERGY METER

AIMTo calibrate the given 1- energy meter at unity power factor by direct

loading.

APPARATUS REQUIRED1. Energy meter 240V,5A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.2. Wattmeter 250V,5A,upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.3. Ammeter (0-5A) MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.4. Voltmeter (0-250V) MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.5. 1- Autotransformer (0-270V,13A) . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.6. Stop watch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.7. Connecting wires8. Lamp load

PRINCIPLEAn energy meter is an instrument used to measure electrical energy. It

keeps a record of the total energy consumed in a circuit during a particularperiod. It is an integrating type of instrument. It essentially consists oftwo electromagnets called the shunt magnet and series magnet. A coilhaving a large number of turns of fine wire is fitted on the shunt magnetcalled the pressure coil and is connected across the supply mains. Theseries electromagnet is wound with a few turns of heavy gauge wire calledthe current coil and is connected in series with the load. An aluminiumdisc is mounted on a vertical spindle and is free to rotate between thetwo magnets. The reaction between the magnetic fields setup by the twoelectromagnets and eddy currents set up a driving torque in the disc andthe disc starts rotating. The average torque thus produced is proportionalto the true power of the circuit.

53

OBSERVATIONS(direct loading)Sl. no. V(v) I(A) W(w) T(s) A.R(kwh) I.R(kwh) % Error

Sample Calculation(set no. . . . . . . )

Energy meter constant k = . . . . . . . . .Voltmeter Reading (V ) = . . . . . . . . .Ammeter Reading (I) = . . . . . . . . .Time for 5 rev. of energy meter disc (t) = . . . . . . . . .

Indicated energy for 1 revolution of energy meter disc =1

k= . . . . . . . . .

Indicated energy for 5 revolution of energy meter disc =5

k= . . . . . . . . .

Wattmeter Reading (W ) = . . . . . . . . .True energy for t seconds (TR) = W t = . . . . . . . . .% Error =

I.R T.RT.R

100 = . . . . . . . . .

54

Calibration involves comparing the energy measured by an energy meterwith a standard instrument. The standard chosen here is a wattmeter.Since the wattmeter measures only the power, it has to be multiplied withtime to get the energy reading. The readings are then compared to findthe error in the energy meter.Calibration can be done either by direct loading or phantom loading. In

direct loading both the current and pressure coils are fed from the samesupply at rated voltage. Energy meters of high rating when tested by di-rect loading would involve large amount of power. Such meters are thustested using phantom loading.

PROCEDUREConnections are made as shown in the connection diagram. The supply

is switched on, keeping the autotransformer in the minimum position. Theautotransformer is then varied to get the rated voltage. The lamp loadis then switched on and the ammeter adjusted for a small value of cur-rent. The corresponding readings of voltmeter, ammeter and wattmeterare noted down. The time for five revolutions of the energy meter disc isalso noted. The experiment is repeated in steps adding loads till the ratedcurrent of the energy meter is reached. The true energy and indicatedenergy is evaluated and the error found out. The error curves are thenplotted as shown.

RESULTThe given energy meter is calibrated by direct loading at upf and the

error curve plotted

VIVA QUESTIONS1. What is meant by calibration of the energy meter?2. What is the standard used for calibration of energy meter?3. How does an induction type energymeter work?4. What is the disadvantage of direct loading method?5. How are the meters selected for this experiment?

55

WHEATSTONES BRIDGE

P Q

SR

GI1

I2

1

.1

.01

.001

10100 1000

RATIO

GALVO

1

2

3

4

5 67

8

9

10 1

4

5 67

8

9

10

2

3

1

4

5 67

8

9

10

2

3

1

4

5 67

8

9

10

2

3

x10

x1000 x100

x1

EXT.BATT.

SERIESARM

EXT

INT

CONTROLSENSITIVITYMIN MAX

B G X

GALV

0-30V

10V

50,5G

V

EXT

INTBG

M1000

M100

M10

R1 R2

PORTABLE FORM OF WHEATSTONES BRIDGE

Experiment 10 56

Experiment 10

RESISTANCE MEASUREMENT USINGWHEATSTONES BRIDGE

AIM(a) To measure the resistance of given voltmeter (0-30V) using Wheat-

stones Bridge.(b) To draw the circuit for extending the range of the given voltmeter

(0-30V) to read up to 300V.

APPARATUS REQUIRED1. Wheatstones Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.2. Voltmeter (0-30V) MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.3. Galvanometer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.4. High Resistance box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.5. D.C source (0-30V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.

PRINCIPLEThis is the best and most common method of measuring medium resis-

tance (from 1 - 0.1 M). The general circuit arrangement is shown inthe figure. P and Q are two known and fixed resistances. S is a knownvariable resistance and R is the unknown resistance. When the bridge isbalanced, no current flows through the galvanometer, thenI1 P = I2 R

I1 Q = I2 S, ie,(P

Q

)=

(R

S

)or R =

(P

Q

) S

The unknown resistance can then be determined.In the portable form of the Wheatstones Bridge the ratio(P/Q) can be

set to values of 1,10,100,1000. The standard resistance S can be adjustedusing decade dials of x1,x10,x100 and x1000. R can be found out usingthe above formula.

57

OBSERVATIONS

Sl. Unknown P

Q

S1 S2 S3 S1 S = S1 + S2 X Meanno. resistance 1000 100 10 1 +S3 + S4 X

Voltmeter(0 30V )

Resistance of the given voltmeter is . . . . . . . . .

EXTENSION OF INSTRUMENT RANGE

V

300V

0-30VVV

RS RV

VT

Let Rv be the resistance of the voltmeterRs the resistance to be connected in seriesVv the range of the given voltmeterVT the range to which the extension is to be made

The value of the resistance to be connected in series to extend the rangeis

Rs =

[(VTVv

) 1

]Rv

58

PROCEDURE

The given voltmeter (unknown resistance) is connected to the terminalmarked R1 and R2 on the bridge. The toggle switches are adjusted forexternal battery and galvanometer. An external battery is connected toterminals BB through a rheostat. A galvanometer is connected to theterminals marked Galvo. on the bridge. The P/Q ratio (range selec-tor) is suitably selected. The resistance S is varied by varying the fourdecade resistances (one at a time starting from the highest range) till nulldeflection is observed in the galvanometer, when the B and G keys arepressed. Adjustments are made till null deflection is obtained, The readingof the Range selector and the four dials of the variable resistance S arenoted. The readings are tabulated as shown. The experiment is repeatedfor different values of range selecter(P/Q ratio)

Extension of range of meter:- First the resistance of the given volt-meter Rv is measured using Wheatstones bridge. To extend the range ofgiven voltmeter a resistance Rs is connected in series with voltmeter asshown in the figure.Since they are in series, current is the same through voltmeter and series

resistance.

I =VvRv

=(VT Vv)

Rs

Rs = (VT Vv)RvVv

=

[(VTVv

) 1

]Rv

RESULT1) Resistance of given volt meter is . . . . . . .. 2) Resistance to be connected in series to extend its range to 300V

is . . . . . . . . .

VIVA QUESTIONS1. What is the range of resistances that can be measured using a

wheatstones bridge?2. Why cant a wheatstones bridge be used for measurement of small

value of resistance?3. How can a low range voltmeter be used for measurement of high

voltages?

59

QP

p qSR

R1 E

r

G

KELVINS DOUBLE BRIDGE

FINALINITIAL

PRESS KEYCURRENT SWITCH

DCSOURCE

GALV.

0-2.

5A

10V 45,5

+C1

+P1

-C2

-P2

X.00002/DIV

G

A

FOR.OFF

REV.

x1x10

x100

x.1

x.01

1 0 1098

765

4

3

2

100

10 010

20

Experiment 11 60

Experiment 11

RESISTANCE MEASUREMENT USINGKELVIN DOUBLE BRIDGE

AIMa) To measure the resistance of the given ammeter(0-2.5A) using Kelvins

double bridge.b) To draw the circuit for extension of range of the meter to read up to

25A.

APPARATUS REQUIRED1. Kelvins Double Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.2. D.C source (0-30V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.3. Ammeter (0-2.5A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.4. Galvanometer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.5. High Resistance box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.6. Rheostat 45,5A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.

PRINCIPLEThis method is best available for the precise measurement of low resis-

tances(less than 1) in the figure R is the low resistance to be measuredand S is a standard variable resistance of the same order of magnitude,P,Q, p and q are four non-inductive resistances, one pair of which are vari-able. These are connected to form two sets of ratio arms, which are usedfor range selection. The ratio Q/P is kept same as q/p ratio along withS being varied till null deflection of the galvanometer is obtained.

ThenR

S=Q

P=q

por R =

Q

P S

61

OBSERVATIONS

Sl. Unknownremarks

Range S1 S2 104 S = S1 + S2 X meanNo. resistance multiplier m m m m

1Ammeter direct+leads reverse

2Leads directalone reverse

Resistance of ammeter + leads = . . . . . . . . .Resistance of leads alone = . . . . . . . . .Resistance of ammeter alone = . . . . . . . . .Resistance of the given ammeter (0-2.5A) = . . . . . . . . .

Extension of range of ammeterTo extend the range of ammeter a resistance is connected in shunt as

shown

RSH

ARAIM

IT=25A 2.5A

Let RA be the resistance of the ammeterRSH the resistance to be connected in shuntIM the range of the given ammeterIT the range to which the extension is to be made

The value of resistance to be connected in shunt to extend the range ofthe given ammeter to (0-25A)

= RSH =(IM RA)(IT IM)

62

PROCEDURE

Connections are made as shown in the figure. Choose a suitable rangemultiplier . Set the current switch in forward position. Press the gal-vanometer initial key first and adjust main dial and slide wire to getnull deflection in the galvanometer. Then press the galvanometer finalkey and check whether the galvanometer reads null deflection. If not, ad-just the dial readings to get null deflection. The readings of the main dialand slide wire are noted down.The current switch is then put to the reverse position. This reverses

the direction of current in circuit. The main dial and slide wire are ad-justed to get null deflection and the readings are noted again. The meanof the two is taken as the correct value. This is done to eliminate errorsdue to thermal effect. The ammeter is then disconnected and the resis-tance of the connecting leads alone is measured using the same method.The experiment is repeated with different values of range multiplier. Thereadings are tabulated as shown.Resistance of ammeter = Resistance of ammeter + leads - Resistance of

leads alone

Extension of RangeTo extend the range of ammeter a resistance is connected in shunt as

shown. Since both are in parallel, voltage across both is the same.

IM RA = ISH RSHRSH =

(IMISH

)RA = (IM RA)

(IT IM)RESULT1) Resistance of given ammeter( 0- 2.5A) = . . . . . . . . . 2) Resistance to be connected in shunt to extend its range to (0-25A)

= . . . . . . . . .

VIVA QUESTIONS1. How does a Kelvins double bridge differ from a wheatstones bridge?2. What is the range of resistances that can be measured using a Kelvins

double bridge?3. How can a low range ammeter be used for measurement of larger

values of currents?

63

CIRCUIT DIAGRAM FOR LINEAR RESISTANCE

220VDC

2AA

V 0-250V

0-1A

+

1.2A1000

1.7A300

2A

-

+ -+

-

CIRCUIT DIAGRAM FOR INCANDESCENT LAMP

220VDC

2AA

V 0-250V

0-1A

+

1.7A300

2A

-

+ -

LAMP

+

-

Experiment 12 64

Experiment 12

V-I CHARACTERISTICS OFINCANDESCENT LAMP AND LINEAR

RESISTANCE

AIMTo determine the V-I Characteristics of linear resistance and incandes-

cent lamp.

APPARATUS REQUIREDRheostat - 300, 1.7AVoltmeter - 0-300V MCAmmeter - 0-2A MCIncandescent lamp - 240V, 100WRheostat - 1000, 1.2A

PRINCIPLEThe resistance of a material is practically a constant at constant tem-

perature, so for the linear resistance, according to ohms law the currentflowing through the circuit is directly proportional to the voltage applied.

ie,I VV = IR

where,V is voltage applied, I the current, R the resistance

Here R is a constant therefore we get a linear relationship betweenvoltage and current. But in the case of incandescent lamp, large amountof heat is produced so there is a considerable change in the resistance thusas the voltage increases we get a non linear relationship between voltage& current.

65

OBSERVATION AND CALCULATION

Linear Resistance Incandescent lampSl. Voltage Current Resistance Voltage Currentno. V I R=V/I V A

V-I CHARACTERISTICS

I

VLinear

ResistanceIncandescent

Lamp

66

PROCEDUREThe connections are made as per the circuit diagram. The linear resis-

tance is connected in the circuit first. Keeping the potential devider inthe minimum output voltage positions, the supply is switched on. Therheostat(300, 1.7A) is adjusted to get different voltages till the ratedvoltage is reached and corresponding current readings are noted down.The experiment is repeated by connecting incandscent lamp in place of

the rheostat. The V-I characteristics of linear resistance and incandescentlamp are plotted.

RESULTV-I characteristic of linear resistance and the incandescent lamp are

plotted.

67

CIRCUIT DIAGRAM- OCC & SCC

+

-

VA

M

L F A15A

F1

F2

Rh1300

1.7AA1

A2

0-300V

0-10A

220VDC

15A

A

+

-

2A

2A

Rh2

1000 1.2A

F1 F2B

Y

R

0-2A

R

YB

MI

MI

N

S1

S2+ -

MC

S3

D.C motor alternater

V -230V V -415VI-17A I-5A

H.P -3.5 KV A-3.5rpm-1500 rpm-1500

Conn.-Star

MEASUREMENT OF ARMATURE RESISTANCE

20VDC

5AA

V0-10V

0-5A

+

5A50

5A

-

+ -

R

NMC

MC+

-

Experiment 13 68

Experiment 13

OPEN CIRCUIT AND SHORT CIRCUITTEST ON A THREE PHASE

ALTERNATOR

AIMTo conduct open circuit and short circuit tests on a three phase alter-

nator and predetermine the regulation curve by emf method at half loadand full load.

APPARATUS REQUIREDVoltmeter - 0-300V,MI

- 0-10V, PMMC.Ammeter - 0-10A,MI

- 0-2A,PMMC- 0-5A,PMMC.

Rheostat - 300,1.7A- 1000,1.2A- 50,5A

PRINCIPLEAs the load on the alternator is varied the terminal voltage also varies.

This is due to1.Voltage drop due to armature resistance IR.2.Voltage drop due to armature reactance IXL.3.Voltage due to armature reaction effect.The voltage regulation of a synchronous generator is defined as the rise

in voltage at the terminals when the load is reduced from full load ratedvalue to zero, speed and field current remaining constant

%reg =E VV

100Where E - Generated emf

V - Terminal voltage

69

OBSERVATIONS AND CALCULATIONS

O.C TESTIfVOC

S.C TESTIaIf

Measurement of RaV I Ra

Regulation curve

%Reg

0 0.2 0.4 0.6 0.8 0.8 0.6 0.4 0.2 0

(upf)

p.f(lead) (lag)

%Reg

p.f

(half load)

(full load)

Line voltage VL = . . . . . . . . .

VPH =VL3= . . . . . . . . .

Ra(dc) = . . . . . . . . .Effective value Ra = 1.6Ra(dc) = . . . . . . . . .

From graph Zs =VOC/ISC = . . . . . . . . .

Xs =Z2s R2a = . . . . . . . . .

70

For small machines the regulation may be found by direct loading. Forlarge machines the voltage regulation is predetermined by using indirectmethods like emf method, mmf method, Potier and ASA methods Allthese methods require open circuit characteristics and short circuit char-acteristics.The open circuit characteristics [also called open circuit saturation curve

or magnetization curve] is a plot of no load terminal voltage versus fieldexcitation with the machine running at rated speed. Under these condi-tions the induced voltage is directly proportional to the flux. The shape ofcurve is therefore a typical B-H curve or magnetization curve. The shortcircuit characteristics is a plot between armature current and field excita-tion with a symmetrical short circuit applied across the terminals. Underthis conditions current in the armature winding wholly depends on the in-ternal impedance consisting of synchronus reactance Xs and the windingresistance Ra.Now Ra being small compared to Xs the pf under short circuit condition

is zero power factor lagging and therefore the armature reaction mmf isalmost wholly demagnetizing.The short circuit characteristics is a straight line. From O.C.C & S.C.C

the synchronous impedance is evaluated.For any value of excitation if VOC is the open circuit volage & ISC is

the short circuit current, then synchronous impedance Zs=VOC/ISC . Thevalue of Zs is calculated for the unsaturated region. For the computationof regulation, it is convenient to take Zs at such a value of excitation whichgive rise to Vph[normal voltage per phase]on open circuit. The armatureresistance is measured using ammeter,voltmeter method. Under workingconditions the effective value of Ra is increased due to skin effect andtemperature effect. The effective value of Ra is generally taken as 1.6times the d.c value.

71

O.C.C and S.C.C of 3 alternator

S.C testO.C test

I a VO

C

If

Isc

Voc

If1

Sample Calculation

Eo ={(V cos+ IRa)

2 + (V sin IXS)2}1/2

-ve for leading+ve for lagging

% regulation =Eo VV

100

lagging leading 0 0.2 0.4 0.6 0.8 upf 0.8 0.6 0.4 0.2 0

Reg.(FL)

Reg.(HL)

72

Synchronous reactance per phase Xs =Z2a R2a per phase. Eo =

(V cos+ IRa)2 (V sin IXs)2where +ve sign for lagging power factor and -ve for leading. Now per-

centage regulation for each case is computed as

% regulation =Eo VV

100

PROCEDURE

O.C testConnections are made as shown in the connection diagram. Switches

S3 and S2 are kept in the open position. The motor field rheostat Rh1is kept in minimum position and the alternator field rheostat Rh2 in themaximum position. Supply is switched on by closing switch S1. The dcmotor is started using the 3-point starter. The motor field rheostat Rh1 isvaried till the speed becomes equal to the rated speed. Switch S2 is closed.Rh2 is varied in steps and the field current and voltmeter reading are noteddown. The experiment is repeated for different values of field current tillthe voltmeter reading shows 120% of the rated voltage of the alternator.Rheostat Rh2 is brought back to the maximum resistance position.

S.C testSwitch S3 is closed and rheostat Rh2 is varied till the ammeter read-

ing in the alternator (A2) reads the rated current of the machine. Thecorresponding value of field current is noted down.Armature resistance was found by voltmeter-ammeter method.The regulation is then determined at various power factors for half and

full loads and the regulation curve is plotted.

RESULTThe open circuit and short circuit test was conducted on alternator and

O.C.C and S.C.C were conducted. The graph is plotted.

73

CONNECTION DIAGRAM - NO LOAD TEST

400V

3 -

50H

zA

C

10A

10A

10A

R

Y

B

E2

E3

A

V 0-500V

M

C

L0-5A 600V,5A,lpf

600V,5A,lpfM

C V

L

E1

B1

C1

C2

B2

B3

C3

STATOR

R

Y BR3

R2

R1

ROTOR

V

BLOCKED ROTOR TEST

400V

3 -

50H

zA

C

10A

10A

10A

R

Y

B

E2

E3

A

V 0-250V

M

C

L0-10A 250V,10A,upf

250V,10A,upfM

C V

L

E1

B1

C1

C2

B2

B3

C3

STATOR

R

Y BR3 R2

R1

ROTOR

BLOCKEDROTOR

S1 S2

V

Machine Details

Voltage - 415VCurrent - 7.5Aspeed -1440rpm

3-H.P -5.0

Experiment 14 74

Experiment 14

NO LOAD AND BLOCKED ROTORTESTS ON 3 PHASE SLIP RING

INDUCTION MOTOR

AIMTo perform no load and blocked rotor test on a three phase slip ring

induction motor and determine the equivalent circuit.

APPARATUS REQUIREDVoltmeter - (0-500V) MI

- (0-250V) MI- (0-30V) PMMC.

Ammeter - (0-5A) MI- (0-10A) MI- (0-10A)PMMC

Wattmeter - 500V,5A,lpf- 250V,10A,upf

Rheostat - 9,8.5AAutotransformer

PRINCIPLESlip ring motors are always started with full line voltage applied across

the stator terminals. The value of starting current is adjusted by inducinga variable resistance in the rotor circuit. The controlling resistance is inthe form of resistances connected in star. The resistance is gradually cutout of the rotor circuit as the motor gathers speed.

75

OBSERVATIONS AND CALCULATIONS

No load test:

V0(V ) I0(A) W1 W2 W0=W1 +W2

Blocked rotor test:

VSC(V ) ISC(A) W1(w) W2(w) WSC=W1 +W2

MEASUREMENT OF STATOR RESISTANCE

220VDC

5AA

V0-20V

0-5A

+

5A50

5A

-

+ - R

BY-

+

For finding stator resistance; Rs :

No. V (V ) I(A) Rs

Rs(dc) = . . . . . . . . . Rseff =

32 1.6Rs(dc)= . . . . . . . . .

76

By introducing the rotor resistance, the rotor current is reduced at start-ing and the starting torque is increased the latter due to the improvementin power factor.

No load test:-If the motor is run at rated voltage and frequency without any me-

chanical load, it will draw power to supply the no load losses. The noload current will have two components. The active components and themagnetizing component, the former being very small as the no load lossesare small. The power factor at no load is therefore very low. The noload power factor is always less than 0.5 and hence at no load one of thewattmeter at input side reads negative.The no load input W0 to the stator consists of1. Small stator copper loss2. Core losses3. The loss due to friction and windage.

The rotor copper loss can be neglected. Since slip is small under no load.

Blocked rotor test :-The stator is supplied with a low voltage of rated frequency and with the

rotor blocked and short circuited, so that full load current flows throughthe stator. The power input, current and the voltage applied are noteddown.The power input during the blocked rotor test is wholly consumed in

the stator and rotor copper losses. The core loss is low because the appliedvoltage is only a small percentage of the normal voltage. Again since therotor is at stand still the mechanical losses are absent. Hence the blockedrotor input can be taken as approximately equal to the copper losses.

77

No load test:-

V0 = . . . . . . . . . I0 = . . . . . . . . . W0 = . . . . . . . . .

V0/ph = V0 = . . . . . . . . .

Line current(IL) =I0 = . . . . . . . . .

phase current(I0/ph) =IL3

= . . . . . . . . .

Power consumed =W0 = . . . . . . . . .

cos0 =W03V0I0

= . . . . . . . . .

0 = . . . . . . . . .

sin0 = . . . . . . . . .

R0/ph =V0/ph

I0/ph cos0

= . . . . . . . . .

X0/ph =V0/ph

I0/ph sin0

= . . . . . . . . .

78

PROCEDURE

No load test:-The connections are made as shown in the diagram for no load test.

Brake drum is made free to rotate by loosening the belt. The autotrans-former is placed in zero position. Then the supply is switched on and theauto transformer is adjusted to supply the rated voltage to the machine.The handle of the starter is rotated to cut out the rotor resistance. Read-ings of the wattmeters, voltmeter and ammeter are noted and tabulated.

Blocked rotor test:-Connections are made as shown. The rotor is blocked by tightening the

belt on the brake drum. The auto transformer is set to the zero voltageposition. Then the three phase supply is switched on. By adjusting theautotransformer, the ammeter reading is made equal to rated current ofthe machine. Readings of the two wattmeters, voltmeter and the ammeterare noted and tabulated.

Measurement of stator resistance :-Connections are done for the stator resistance measurements. It is mea-

sured using the voltmeter-ammeter method.

79

From blocked rotor test:-

VSC = . . . . . . . . . ISC = . . . . . . . . . WSC = . . . . . . . . .

VSC/ph = VSC = . . . . . . . . .

WSC/ph =WSC3

= . . . . . . . . .

ISC/ph =ISC3

= . . . . . . . . .

R01 =WSC/ph

I2SC/ph= . . . . . . . . . (Total winding resistance as referred

to the stator side)

Z01 =VSC/ph

ISC/ph= . . . . . . . . .

X01 =Z201 R201 = . . . . . . . . . (Total leakage reactance as referred

to the stator side)R2 = R01 RS(eff) = . . . . . . . . . (Rotor resistance as referred to the

stater side)

RL = R2

[1 ss

](Electrical equivalent of the mechanical load)

R0 X0

R01 X01

=

= =

=

V/ph= RL

80

RESULT

No load and blocked rotor tests were conducted on the given three phaseslip ring induction motor and the equivalent circuit parameters were de-termined.

81

License information

Currently this Manual is licensed under Creative Commons Attribution-NonCommercial-ShareAlike 2.5 India (CC BY-NC-SA 2.5).But later on Im intending to release the LATEX source of this book in

any GNU General Public License(GPL) compatible licenses (visit this:http://www.gnu.org/licenses/license-list.html). I will be releasing the sourceafter some standardisation of LATEX codes (at my spare time), anyway nowyou can get the vector images (Xcircuit editable postscript images) at mysite linked at https://launchpad.net/sadiq . If you cant find anythingthere, you might also request them via my email (Im happy to do so :))at sadiqpkp at gmail dot com .

82