Emii

Government Engineering College

Kozhikode-5

Electrical Machines Lab ManualAI09 308(P), ME09 307(P)

(Version 1.1.0 - June 25, 2012)

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

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

Type set in LATEX2ε, circuit designs and graphs in XCircuit.Platform : GNU/LinuxOther free softwares used: GNU Emacs, GNU Bash, Gedit, Vim . . . . . .

© 2011, 2012, Some Rights Reserved.

You may get a copy of this work from www.sadiq.tk.

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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)

Our sin ere thanks to everyone who have helped us for the perfe tion of this

Manual, espe ially our lassmates - 2010 AEI bat h.

http://www.sadiq.tk

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CONTENTS

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

2. Load test on 3−φ squirrel cage induction motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3. O.C.C of dc shunt generator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

4. Load test on dc shunt generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5. Load test on dc series motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6. Measurement of coupling coefficient of transformer coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

7. O.C and S.C tests on single phase transformer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

8. Three phase power measurement by two wattmeter method. . . . . . . . . . . . . . . . . . . . . . . . . . .42

9. Calibration of single phase energy meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46

10. Resistance measurement using Wheatstones bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

11. Resistance measurement using Kelvins double bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

12. V-I Characteristics of incandescent lamp and linear resistance . . . . . . . . . . . . . . . . . . . . . . . . 58

13. Open circuit and short circuit test on three phase alternator . . . . . . . . . . . . . . . . . . . . . . . . . . 60

14. No load and blocked rotor tests on 3−φ slip ring induction motor. . . . . . . . . . . . . . . . . . . . .64

3

CONNECTION DIAGRAM OF LOAD TEST

A A

V V

M

C

L

V P1

P2 S2

S1

Load

230V

1 - φ50 HzAC

NL

10A

0-150V

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

0-250VC

S

MI

N

P

120/240V 1KVATRANSFORMER

E

B

MI

Rated current on primary side =1000

120= . . . . . .

Rated current on secondary side =1000

240= . . . . . .

REGULATION AND EFFICIENCY CURVES

Reg

ulat

ion

(%)

Eff

icie

ncy

(%)

Output (watts)

Regulation

Efficiency

Experiment 1 4

Experiment 1

LOAD TEST ON SINGLE PHASE

TRANSFORMER

AIM

To conduct load test on the given single phase transformer at unity power factor and determine

the efficiency and regulation curve.

APPARATUS REQUIRED

1. 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.

THEORY

Regulation 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 load

When 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 − I2x2

Where r2 and x2 are the secondary winding resistance and leakage reactance respectively

and I2 is the secondary load current.

The efficiency of transformer is defined asOutput

Input× 100.

As the load current increases the power output increases. The iron loss remains constant

from no load to full load. The copper loss increases as the square of the load current. Thus the

efficiency curve starts from zero, increases to a maximum value(When iron loss = Cu loss) and

thereafter starts 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 power×100= . . . . . . . . .

Regulation =VNL − VL

VNL×100 = . . . . . . . . .

6

PROCEDURE

Connections are made as shown in the circuit diagram. The supply is switched on keeping

the autotransformer in the minimum position and at no load. Adjust the autotransformer to

get the rated voltage of the transformer. The readings of all the meters are noted down. The

secondary voltage at no load is also noted down. This value is VNL. A small load is added

on the secondary side and the meter readings are again noted. The experiment is repeated

for different values of load current till the current on the primary side equals the rated value.

The load is then reduced to zero, the autotransformer is brought back to the zero position and

the supply is switched off. The readings are then tabulated as shown and the regulation and

efficiency curves are plotted.

RESULT

Conducted load test on the given 1-φ transformer and plotted the regulation and efficiency

curves.

VIVA QUESTIONS

1. 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 -

φ50

Hz

AC

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 - 3φ

Synchronous speed =120f

P

=120× 50

4

= 1500 rpm

Experiment 2 8

Experiment 2

LOAD TEST ON 3−φ SQUIRREL CAGE

INDUCTION MOTOR

AIM

To conduct load test on the given 3-φ squirrel cage induction motor and plot the performance

characteristics.

APPARATUS REQUIRED

1. 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

THEORY

A squirrel cage induction motor essentially consists of a stator and a rotor. The stator is a

hollow cylindrical structure with slots on the inner periphery and carries a three phase winding.

The winding can be connected in star or delta and is connected across a 3-φ 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 both ends by means of end rings.

When a 3-φ supply is given to a 3-φ winding displaced by 120 in space, a magnetic field

of constant magnitude but rotating at synchronous speed is produced. This flux links with

the stationary rotor, thus inducing an emf in it. As the rotor circuit is closed, a current flows

through it. The direction of the induced current is such as to oppose the cause producing it.

The cause is the relative motion between the stator magnetic field and the rotor. So the rotor

starts rotating in the same direction as the stator magnetic field and tries to catch up with

it. But practically it is never able to do so. Because if it does so, there would be no relative

motion, no emf 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 the synchronous speed.

The term slip is of importance in an induction motor and is defined as

%slip =Ns −N

Ns× 100

Where,

Ns - Synchronous speed =120× f

PN - rotor speed

f - frequency

P - No. of poles of the machine

An induction motor can never operate at s=0. It always operates between s=0 and s=1(starting).

The performance characteristics are plots of efficiency, torque, speed, slip, pf and line cur-

rent versus output.

Current and torque increases with increase in output. The induction motor is essentially

a constant speed motor. However speed reduces gradually with increase in output and slip

increases gradually with increase in output. The pf is low at low loads and increases with

increase in output. The efficiency increases with increase in output, reaches a peak value and

then gradually drops with further increase in output.

PROCEDURE

The load on the motor is completely removed by loosening the brake drum. The motor is to

be always started and stopped at no load, The supply is switched on and the motor is started

using a Direct On Line Starter (DOL Starter).

The readings of the voltmeter, ammeter, wattmeters and spring balance are noted down.

The speed is measured using a tachometer. The load is then increased in steps, each time noting

down all the above readings. The experiment is repeated for different values of load currents

till the rated current 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


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(cosφ) =(W1 +W2)√

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

Output power=2πNT

60= . . . . . . . . .

Efficiency =Output

Input= . . . . . . . . .

Performance Characteristics

Efficiency

Output (watts)

Slip pf T N η

Torque

Speedpf

Slip

(%)

(rpm

)

(Nm

)

(%)

12

At low loads,(when pf < 0.5) one of the wattmeters read negative, in such cases, the supply

is switched off and the connections to the M and L terminals of the wattmeter are interchanged.

The meter now reads positive, 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 plotted.

RESULT

Conducted load test on the given 3-φ squirrel cage induction motor and plotted the performance

characteristics.

VIVA QUESTIONS

1. 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-φ 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-φ 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 in-

duction machine?

13

CONNECTION DIAGRAM

220VDC

+

-

FL A

20A

Rh1

600Ω2A

F1

F2

A1

A2

S

Rh2

0-2AMC

+

-

0-300VMC

-

+ F2

F1

A1

A2

20A

3 POINT STARTER

V

A

GM

600Ω2A

V+

-

0-30VMC

MACHINE DETAILSMOTOR GENERATOR

3.5 KW 3.5 KWSpeed - 1500rpm Speed - 1500rpm

volts - 220V volts - 220VAmps - 18.6 A Amps - 16 AWinding - shunt Winding - shunt

Field - 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 rpm

We have E ∝ N at same flux or field current

E1

E2=

N1

N2⇒ E2 =

(

N2

N1

)

E1

Where,N2 is 1000 rpm

N1 → Rated speed = 1500 rpm

Experiment 3 14

Experiment 3

O.C.C OF DC SHUNT GENERATOR

AIM

To 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 REQUIRED

1. Voltmeter MC (0-300V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

2. Voltmeter MC (0-30V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.

3. Ammeter MC (0-2A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

4. Rheostat 600Ω, 2A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 nos.

5. Tachometer

THEORY

The 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 current is zero there is some residual magnetism

present in the poles. Hence there is a small voltage generated even at zero field current, which

is called the residual voltage. As the field current is increased, Eo also increases and the curve

traced is almost a straight line. As If is further increased the poles start getting saturated, the

straight line relation no longer holds good and the curve bends and becomes almost horizontal.

Critical resistance

It is that value of resistance in the field circuit at which the generator will just excite(or voltage

build up begins). If the resistance is higher, the machine will fail to build up voltage. It is given

by the slope of the tangent drawn to the linear portion of the magnetization curve from the

origin.

15

MEASUREMENT OF SHUNT FIELD RESISTANCE

220VDC

100Ω,2.8A 0-2A MC

0-250VMC

F1

F2

+

-

+ -A

V

+

-

OBSERVATION

Sl No. V I Rf

16

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. That is, The coils should be so connected that the flux generated

by the field current aids the residual flux.

3. When excited at no load, the shunt field circuit resistance should be less than

the critical resistance.

Critical speed

It is that value of speed at which the given shunt field resistance represents the critical resistance.

It is determined as follows. For the same value of If determine E1 and E2 from the field

resistance lines. ThenE1

E2=

N1

Nc⇒ Nc =

(

E2

E1

)

N1

Where,

Nc is the Critical speed

PROCEDURE

Connections are made as shown in the diagram. The motor field rheostat (Rh1) is kept in

minimum position, the generator field rheostat (Rh2) in maximum position and switch ‘S’ is

kept open at starting. Supply is switched on. The starter handle is gradually moved to cut off

the starter resistance. The rheostat Rh1 is varied till the speed equals the rated speed of the

machine. With ‘S’ open, the residual voltage is measured using the smaller range voltmeter.

Switch ‘S’ is then closed. Rheostat Rh2 is then decreased in steps, each time noting down

the voltmeter and ammeter readings. The process is repeated till the voltage equals 120% of

the rated voltage of the machine. [If Eo does not increase, it means that the machine is not

building up voltage. The field terminals F1 and F2 are interchanged and the process is repeated]

Rheostat Rh2 and Rh1 are brought back to the original position and the supply is then switched

off.

17

OPEN CIRCUIT CHARACTERISTICS

Eo

E1

E2

residual

If1 If (A)

Critical Fieldresistance

lineO.C.C at rated

speed

O.C.C at 1000 rpm

Given shunt fieldresistance line

voltage

(V)

Critical resistance at rated speed,

Rc =E1

If1= . . . . . . . . .

Critical speed of the Machine,

Nc =

(

E2

E1

)

N1 = . . . . . . . . .

18

Measurement of field resistance

Connections are made as shown in the diagram 2. For different values of voltages determine

the current. The ratio gives the field resistance. The O.C.C and field resistance line is drawn

and the critical speed of the machine is determined.

RESULT

No load test was conducted on the given d.c shunt generator and the O.C.C was plotted.

Critical resistance at rated speed = . . . . . . . . .

Critical speed of the machine = . . . . . . . . .

VIVA QUESTIONS

1. 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 determined?

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

Rh1

600Ω2A

A1

A2

Rh2

0-30

0VM

C

MC0-2A

+

-

+

-

600Ω2A

F1

F2 Load

200VDC

20A

3 Point Starter

A1

A2

A0-20A MC

+ -

MACHINE DETAILS

MOTOR 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

50Ω5A

+ -

A1

A2

0-5A MC

0-10VMC

+

-

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

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

Experiment 4 20

Experiment 4

LOAD TEST ON D.C SHUNT

GENERATOR

AIM

To conduct load test on the given D.C shunt generator and plot the external and internal

characteristics.

APPARATUS REQUIRED

1. Voltmeter MC (0-300V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

2. Voltmeter MC (0-10V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.

3. Ammeter MC (0-2A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

4. Ammeter MC (0-5A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

5. Rheostat 600Ω, 2A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 nos.

6. Tachometer

THEORY

Load characteristics of the machine can be broadly classified into:-

1) External characteristics

2) Internal Characteristics

External Characteristics(V vs IL)

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 shown in 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 in terminal 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 V.

2. As the load current increases, Ia increases, armature reaction effect also increases. Due

to demagnetizing effect of armature reaction, the induced emf Eg decreases, thereby

decreasing V.

3. Due to reasons (1) and (2), the terminal voltage decreases, which in turn reduces the field

current Ish, thereby decreasing Eg causing further decrease in V.

21

OBSERVATION - LOAD TEST

Sl V IL Ish Ia Eg

no. (volts) (A) (A) (A) (V )


Terminal Voltage (V) = . . . . . . . . .V

Load Current (IL) = . . . . . . . . . A

Shunt Field Current (Ish) = . . . . . . . . . A

Armature Current (IA) = IL + Ish = . . . . . . . . . A

Generated emf (Eg) = V + IaRa = . . . . . . . . .

Internal and External Characteristics

V/Eg

Drop DueTo ArmatureResistance

Drop due to armatureReaction Effect

IL Ia (A)

Internal

ExternalCharacteristics

CharacteristicsEg vs Ia

V vs IL

/

(V)

22

Internal Characteristics [Eg vs Ia]

It is a plot of the internally generated emf (Eg) and armature current (Ia). It is a curve similarto the external characteristics and lies above it.

Eg = V + IaRa

& Ia = IL + Ish

PROCEDURE

Connections are made as shown in the diagram. rheostat Rh1 is kept in minimum position andRh2 in maximum position. Switch S2 is kept open. Supply is switched on and the motor isstarted using a 3-point starter. The motor field rheostat Rh1 is varied till the speed equals therated speed of the motor. The generator field rheostat Rh2 is varied till the voltmeter reads therated voltage of the machine. Switch S2 is then closed. The load on the generator is increased.The readings of the voltmeter and ammeters are noted down. The experiment is repeated fordifferent values of load current till the rated current of the generator is reached. During theexperiment, 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 originalposition and the machine is switched off.

Measurement of Ra

Connections are made as shown in the diagram. Keeping the rheostat in the minimum outputvoltage position, supply is switched on. The rheostat is then varied in steps and the voltmeterand ammeter readings are noted. The ratio gives the armature resistance.

The readings are then tabulated as shown. The external and internal characteristics arethen plotted.

RESULT

Conducted load test on the given DC shunt generator and plotted the external and internalcharacteristics.

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?

23

CONNECTION DIAGRAM

+

-

V

A

20A

F1 F2

L A

+

-

A1

A2

S1S2

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 = . . . . . . . . .

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 readings, S1 = . . . . . . . . . S2 = . . . . . . . . .

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

Torque(T ) = 9.8 (S1 − S2)R = . . . . . . . . . Where R is the radius

of brake drum

Output power =2πNT

60= . . . . . . . . .

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

Efficiency =Output power

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

Experiment 5 24

Experiment 5

LOAD TEST ON DC SERIES MOTOR

AIM

To conduct load test on the given d.c series motor and plot the performance characteristics.

APPARATUS REQUIRED

1. Voltmeter (0-250)V MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

2. Ammeter (0-20)A MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

3. Tachometer - to measure speed

THEORY

In a series motor, the field winding is connected in series with the armature winding. Thus the

same current flows through the field and armature windings.

Electrical characteristics(T vs Ia) :- It shows the variation of torque with the armature

current.We have

T ∝ φIa where φ is the flux/pole

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

Thus

T ∝ I2a

However after magnetic saturation φ remains almost constant, Hence T ∝ Ia

Thus 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 speed with torque.

We have N ∝ Eb

φ∝ 1

φas Eb is almost constant where Eb is back emf

In a series motor φ ∝ Ia

∴ N ∝ 1

Ia

That is, as Ia increases, Speed decreases.

The same pattern is followed in the N -T characteristics. The curve traced 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).

25

Performance CharacteristicsT η N

O

Torque

Efficiency

Speed

Output (watts)

(rpm

)(%

)(N

m)

Electrical Characteristics Mechanical Characteristics

T

Ia (A)

N

T (Nm)

(rpm)(Nm)

26

Performance characteristics shows the variation of speed, torque and efficiency with change

in output.

PROCEDURE

Connections are made as shown in the connection diagram. A small load is applied to the

motor by tightening the brake drum. The motor should never be started at no load. Supply is

switched on and the motor is started using a 2-point starter. The voltage, current, speed and

spring balance readings are noted down. The experiment is repeated 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.

RESULT

Load test was conducted on the given DC series motor and the performance, electrical and

mechanical characteristics are plotted.

VIVA QUESTIONS

1. 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 characteristics.

5. What is the condition for maximum efficiency in a d.c motor?

6. What are the different losses occurring in a d.c machine?


8. Give some applications of d.c series motor.

27

CONNECTION DIAGRAM

FLUX AIDING

P

N

A

V1V2

V3

1A

NL

B

C

Aut

oT

rans

form

er

0-500mAMI

0-250VMI

0-15

0V

S2

S1P2

P1

230/115V, 3kVATransformer

230V

50Hz

1 - φAC

MI

0-250VMI

E

V1 ⋍ V2 + V3

FLUX OPPOSING

P

N

A

V1V2

V3

1A

NL

B

C

Aut

oT

rans

form

er

0-500mA MI

0-250VMI

0-15

0VS2

S1

P2

P1

230/115V, 3kVATransformer

230V

50Hz

1 - φAC

MI

0-250VMI

E

V1 ⋍ V2 − V3

MEASUREMENT OF RESISTANCE OF COILS

V

A

0-20V

50Ω5A

0-10VMC

0-5A MC

P1

P2

S1

S2

+

-

+ -

Experiment 6 28

Experiment 6

MEASUREMENT OF COUPLINGCOEFFICIENT OF TRANSFORMER

COILS

AIM

To determine the self inductance, mutual inductance and coupling coefficient of the given trans-

former windings.

APPARATUS REQUIRED

1. Transformer 230/115V, 1KVA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

2. Voltmeter 0-250V MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 nos.

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.

PRINCIPLE

The property of a coil due to which it opposes any change of current through it is known as self

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

L =Nφ

I=

NNIµ0µrA

Il=

N2

S(H)

Mutual inductance is the ability of one coil to produce an emf in a nearby coil by induction

when the current in the first coil changes. The coefficient of mutual inductance(M) is defined

as the Weber turns in one coil due to ampere current in the other coil.

M =N2φ1

I1=

N2N1I1I1S

=N1N2

S

29

OBSERVATIONS

Measurement of impedance of coils

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

Flux aiding

Flux opposing

Measurement of resistance of coils

Sl. No. V (v) I(A) R(Ω)

1

2

3

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

30

Consider two coupled coils A and B

L1 =N2

1

S

L2 =N2

2

S

Flux produced in A due to current I1 is φ1 =N1I1S

Let a fraction k1 of this link with the second coil

ie, φ2 = k1φ1

Then

M =(k1φ1N2)

I1=

(k1N1N2)

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

Flux produced in B due to current I2 is φ2 =N2I2S

Suppose a fraction k2 of this links with A

ie, φ1 = k2φ2

M =(k2φ2N1)

I2=

(k2N1N2)

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

from (1) and (2)

M2 =(k1k2N

21N

22 )

SS= k1k2

N21

S

N22

S

M2 = k1k2L1L2 or k =M

(√L1L2)

where k =√k1k2

The constant k is called the coefficient of coupling and may be defined as the ratio of mutual

inductance actually present between the two coils to the maximum possible value. If the flux

due to one coil completely links the other then k = 1. If the flux of one coil does not link the

other coil at all then k = 0.

31

CALCULATION

For coils connected in series with fluxes aiding each other

Total impedance of coils ZA =V1

I= . . . . . . . . .

Reactance of coils XA =√

Z2A −R2 = . . . . . . . . . Where, R = r1 + r2

is the total resistance

Inductance LA =XA

2πf= . . . . . . . . . of both windings

For coils connected in series with fluxes opposing

Total impedance of coils ZB =V1

I= . . . . . . . . .

Reactance of coils XB =√

Z2B −R2 = . . . . . . . . .

Inductance LB =XB

2πf= . . . . . . . . .

When coils are connected with flux aiding each other

Total inductance, LA = L1 + L2 + 2M (1)

When coils are connected with flux opposing each other

Total inductance, LB = L1 + L2 − 2M (2)

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

4(

L1

L2

)

=

(

N1

N2

)2

=

(

240

120

)2

= 4

L1 = 4L2

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

5= . . . . . . . . .

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

Coupling coefficient k =M√L1L2

= . . . . . . . . .

32

PROCEDURE

Measurement of impedance of coils

Connections are made as shown in the first figure. Supply is switched on with autotransformer in

the minimum position. The autotransformer is adjusted to get the rated voltage in voltmeter1.

The corresponding readings in all meters are noted down. In this case the fluxes produced by

both the coils are additive in nature (ie, V1 = V2 + V3).

Next the connections of the second coil are reversed. The fluxes produced 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 as with the additive polarity. This is done to maintain the same flux in

both the cases. The readings of all meters are noted down and tabulated as shown.

Measurement of resistance

Connections are made as in the third figure. For different values of voltages the readings of

both meters are noted down and tabulated.

RESULT

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

VIVA QUESTIONS

1. 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?


33

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

120/240V, 1KVATransformer

150V, 2A, lpf

230V

50Hz1 - φ

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

120/240V, 1KVATransformer

75V, 5A, upf

230V1 - φ50HzAC

Rated Current =Rated KV A

Rated V oltage on h.v side=

1000

240= . . . . . . . . .

OBSERVATIONVSC ISC WSC

Experiment 7 34

Experiment 7

O.C AND S.C TESTS ON SINGLE PHASE

TRANSFORMER

AIM

To conduct open circuit and short circuit tests on the given 120/240 V, 1KVA transformer and

predetermine the following:-

1. Equivalent circuit as referred to l.v side

2. Equivalent circuit a referred to h.v side

3. Efficiency curve at 0.8 pf

4. Regulation curve at 1/2 Full load

INSTRUMENTS REQUIRED

1. 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 of the transformer. It is conducted to determine

the core loss(iron loss or no load loss). The low voltage side of the transformer is supplied at

rated voltage with the h.v side left open. The current, voltage and power on the 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 be neglected and the power loss read by the wattmeter is the core

loss of the transformer. Since the flux linking with the core is constant at all loads, the core

loss remains same for all loads. The parameters R0 and X0 (the shunt branch) are determined

using this test.

35

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

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

W0 = V0I0 cosφ0 cosφ0 =W0

V0I0= . . . . . . . . .

sinφ0 = . . . . . . . . .Iw = I0 cosφ0 = . . . . . . . . .Iµ = I0 sinφ0 = . . . . . . . . .

Core loss component resistance as referred to l.v side R0 =V0

Iw= . . . . . . . . .

Magnetising reactance as referred to l.v side X0 =V0

Iµ= . . . . . . . . .

The parameters R0 and X0 as referred to the h.v side areR′

0 = R0 × k2 = . . . . . . . . .

X ′0 = X0 × k2 = . . . . . . . . . where k =

N2

N1=

E2

E1=

240

120

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

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

Total eqvt. wdg. resistance as referred to h.v side R02 =WSC

ISC= . . . . . . . . .

Total eqvt. impedance as referred to to h.v side Z02 =VSC

ISC= . . . . . . . . .

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

Z202− R2

02 =

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

120V 240VR0

X0

R01X01

Z'L

I1 I'2I0

=

= =

=ZL

R'0 X'0

R02X02

I1 I'2I0

=

= =

=

36

Short Circuit test

The short circuit test is conducted to determine the full load copper loss and the equivalent

resistance and leakage reactance referred to the winding in which the test is conducted. The test

is conducted on the h.v side with the l.v side short circuited by a thick conductor. A low voltage

just enough to circulate the rated current of the transformer is supplied to the transformer. The

voltage supplied is usually only 5-10% of the normal supply voltage and so the flux linking with

the core is small. Thus core losses can be neglected and the wattmeter reading gives the full

load Cu loss of the transformer.

PROCEDURE

Open Circuit test

Connections are made as shown in the connection diagram 1. The h.v side is left open. The

supply is switched on with the autotransformer in the minimum position. The autotransformer is

gradually varied till the voltmeter reads the rated voltage of the primary side of the transformer.

The corresponding ammeter and wattmeter readings are noted 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 autotransformer in the minimum position. The autotransformer is grad-

ually varied till 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

)

37

Calculation of efficiency :-

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

η =Power Output

Power Input=

x× F.L(V A)× cosφ

x× F.L(V A)× cosφ+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 cosφ = . . . . . . . . .

Power output = x× F.L(V A)× cosφ = . . . . . . . . .

Power input = Power Output + Wi + x2WCu = . . . . . . . . .

Efficiency =Power Output

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

Efficiency curveη

FLO/P1/21/4 3/4

FL FL FL

38

The corresponding voltmeter and wattmeter readings are noted down and 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 tabulated. 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 the secondary terminal voltage with the

primary voltage held constant) is then calculated using the approximate formula at various

power factors and half the full load, Regulation curve is then plotted.

39

Calculation of Regulation

Regulation at any load and p.f is given by

% Reg =I2R02 cosφ± I2X02 sinφ

Eo2

‘+’ for lagging

‘-’ for leading

Where I2 is the current at any load and= xI2FL

Wherex → Fraction of full loadI2FL→ full load current on secondary sideE0

2 → 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

40

RESULT

O.C and S.C tests were conducted on the given 1-φ transformer and predetermined the regula-

tion and efficiency curves.

VIVA QUESTIONS

1. 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?

41

CONNECTION DIAGRAM

400V

3 -

φ50

Hz

AC

5A

5A

5A

R

Y

B

N

E2

E3

A

V 0-600V

M

C

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

600V, 5A, upf

M

C V

L

3−φInductive

Load0-10A

50Ω, 5A

50Ω, 5A

50Ω, 5A

E1

B1

C1

C2

B2

B3

C3NL

PHASOR DIAGRAM

30φ

30

φ

IB

IR

IY

VY

VB

VBY

VRY

VR

φ

Experiment 8 42

Experiment 8

THREE PHASE POWERMEASUREMENT BY TWOWATTMETER METHOD

AIM

To measure the power factor and power consumed by a 3-φ RL load using two wattmeter

method.

APPARATUS REQUIRED

1. Voltmeter (0-600V) MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

2. Ammeter (0-5A) MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

3. Wattmeter 600V, 5A, upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 nos.

4. Rheostat 50Ω, 5A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 nos.

5. 3-φ Inductive load (0-10A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

6. 3-φ Autotransformer (0-415V, 10A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

PRINCIPLE

In two wattmeter method the current coils of two watt meters are connected in two phases and

the potential coils between the corresponding phase and the third phase. It can be proved that

the sum of the wattmeter readings gives the total power.

From the phasor diagram

Reading of Wattmeter 1, W1 = VRY IR cos(30 + φ)

Reading of Wattmeter 2, W2 = VBY IB cos(30− φ)

W1 +W2= VRY IR(cos 30 cosφ− sin 30 sinφ) + VBY IB(cos 30 cosφ+ sin 30 sinφ)

= VRY IR cos 30 cosφ+ VBY IB(cos 30 cosφ)

Assuming balanced load

VRY = VBY = VBR = VL

& IR = IB = IY = IL

Where VL and IL are the line values of voltage and current.

= VLIL cos 30 cosφ+ VLIL cos 30 cosφ

= 2VLIL

(√32

)

cosφ

=√3VLIL cosφ

43

OBSERVATIONS

Sl.Case

voltage CurrentWattmeter

PowerPhase Power

reading angle factorNo. V(V ) I(A) W1 W2 P(W ) φ cosφ

1 W1 and W2 read +ve

2W1 reads +ve, W2 readszero

3W1 reads +ve, W2 reads-ve


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

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

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

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

Total power P = W1 +W2 = . . . . . . . . .

Phase angle φ = tan−1

[√3(W1 −W2)

W1 +W2

]

= . . . . . . . . .

Power factor = cosφ = . . . . . . . . .

44

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

= [2 cos 30 cosφ]VLIL=√3 cosφVLIL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (1)

W1 −W2 = VLIL cos(30− φ)− VLIL cos(30 + φ)= [−2 sin 30 sinφ]VLIL= − sinφVLIL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2)

From (1) and (2)

tanφ =(W1 −W2)/VLIL

(W1 +W2)/√3VLIL

=√3(W1 −W2)

(W1 +W2)

When pf is unity, φ = 0 and W1 = W2.When 1.0 > pf > 0.5, 0 < φ < 60 and both W1 and W2 read positive.When pf = 0.5 , φ = 60 and W1 = 0, hence W2 alone reads the total power.When 0.5 > pf > 0, 60 < φ < 90, W1 reads negative and W2 positive

PROCEDURE

Connections are done as shown in the figure. The resistance is kept in the maximum positionand the inductive load is set to minimum. The supply is switched on with the autotransformerin the minimum position.

The autotransformer is adjusted to get rated voltage in the voltmeter. The load is purelyresistive, the power factor is nearly unity and both wattmeters read positive. Readings are takencorresponding to this condition. The inductive load is then increased till one of the wattmeterbecomes zero. This corresponds to a pf of 0.5. Again all readings are noted. On furtherincreasing the inductive load one of the wattmeters starts deflecting in the negative direction.This indicates that the power factor of the circuit is less that 0.5. The supply is now switchedoff and the pressure coil or current coil (ie, C & V or M & L) connections of the wattmeterreading negative is interchanged. Supply is switched on and the readings corresponding tothis condition are noted. The reading of the wattmeter whose terminals are interchanged is berecorded as negative. The power factor and power are calculated using the formula given.

RESULT

Power consumed by a 3-φ RL load is measured using two wattmeter method.

VIVA QUESTIONS

1. What is the expression for power in a 3-φ circuit?2. Derive the expression for power factor in terms of the wattmeter readings.3. What are the other methods of measuring 3-φ power.4. What does a zero reading in one of the wattmeters signify?

45

CONNECTION DIAGRAM

230V1 - φ50Hz

AC

P

N

A

V

5A

B

CNL E

0-250VMI

M

C

L

V

0-5A MI 250V, 5A, upfP.C

C.C

P1

P2

N1

N2

Energy

K=240V, 5A

Meter

LAMPLOAD

ERROR CURVE

Line Current

% E

rror

Experiment 9 46

Experiment 9

CALIBRATION OF SINGLE PHASE

ENERGY METER

AIM

To calibrate the given 1-φ energy meter at unity power factor by direct loading.

APPARATUS REQUIRED

1. 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-φ Autotransformer (0-270V, 13A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

6. Stop watch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

7. Connecting wires

8. Lamp load

PRINCIPLE

An energy meter is an instrument used to measure electrical energy. It keeps a record of the

total energy consumed in a circuit during a particular period. It is an integrating type of

instrument. It essentially consists of two electromagnets called the shunt magnet and series

magnet. A coil having a large number of turns of fine wire is fitted on the shunt magnet called

the pressure coil and is connected across the supply mains. The series electromagnet is wound

with a few turns of heavy gauge wire called the current coil and is connected in series with the

load. An aluminium disc is mounted on a vertical spindle and is free to rotate between the two

magnets. The reaction between the magnetic fields setup by the two electromagnets and eddy

currents set up a driving torque in the disc and the disc starts rotating. The average torque

thus produced is proportional to the true power of the circuit.

47

OBSERVATIONS(direct loading)

Sl. no. V(v) I(A) W(w) T(s) T.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 (IR) =5

k= . . . . . . . . .

Wattmeter reading (W ) = . . . . . . . . .

True energy for ‘t’ seconds (TR) = W × t = . . . . . . . . .

% Error =I.R− T.R

T.R× 100 = . . . . . . . . .

48

Calibration involves comparing the energy measured by an energy meter with a standard

instrument. The standard chosen here is a wattmeter. Since the wattmeter measures only the

power, it has to be multiplied with time to get the energy reading. The readings are then

compared to find the 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 same supply at rated voltage. Energy meters of

high rating when tested by direct loading would involve large amount of power. Such meters

are thus tested using phantom loading, wherein the pressure coil is supplied from rated supply

and current coil circuit from a separate low voltage supply.

PROCEDURE

Connections are made as shown in the connection diagram. The supply is switched on, keeping

the autotransformer in the minimum position. The autotransformer is then varied to get the

rated voltage. The lamp load is then switched on and the ammeter adjusted for a small value

of current. The corresponding readings of voltmeter, ammeter and wattmeter are noted down.

The time for five revolutions of the energy meter disc is also noted. The experiment is repeated

in steps adding loads till the rated current of the energy meter is reached. The true energy and

indicated energy is evaluated and the error found out. The error curves are then plotted as

shown.

RESULT

The given energy meter is calibrated by direct loading at upf and the error curve plotted.

VIVA QUESTIONS

1. 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 energy meter work?

4. What is the disadvantage of direct loading method?

5. How are the meters selected for this experiment?

49

WHEATSTONES BRIDGE

P Q

SR

GI1

I2

PORTABLE FORM OF WHEATSTONES BRIDGE

1

.1

.01

.001

10

100 1000

RATIO

GALVO

1

2

3

4

5 6

7

8

9

10 1

4

5 6

7

8

9

10

2

3

1

4

5 6

7

8

9

10

2

3

1

4

5 6

7

8

9

10

2

3

x10

x1000 x100

x1

EXT.BATT.

SERIESARM

EXT

INT

CONTROLSENSITIVITYMIN MAX

B G X

GALV

0-30V

10V

50Ω,5Α

G

V

EXT

INTBG

M1000

M100

M10

R1 R2

Experiment 10 50

Experiment 10

RESISTANCE MEASUREMENT USING

WHEATSTONES BRIDGE

AIM

(a) To measure the resistance of given voltmeter (0-30V) using Wheatstones bridge.

(b) To draw the circuit for extending the range of the given voltmeter (0-30V) to read up

to 300V.

APPARATUS REQUIRED

1. Wheatstones bridge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.

2. Voltmeter (0-30V) MC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

3. Galvanometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

4. Rheostat 50Ω, 5A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.

5. D.C source (0-30V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

PRINCIPLE

This is the best and most common method of measuring medium resistance (from 1Ω - 0.1

MΩ). The general circuit arrangement is shown in the figure. P and Q are two known and

fixed resistances. S is a known variable resistance and R is the unknown resistance. When the

bridge is balanced, no current flows through the galvanometer, then

I1 × 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 adjusted using decade dials of x1, x10, x100

and x1000. R can be found out using the above formula.

51

OBSERVATIONS

Sl. Unknown P

Q

S1× S2× S3× S1× S = S1 + S2XΩ

Mean

no. resistance 1000Ω 100Ω 10Ω 1Ω +S3 + S4 X

Voltmeter

(0− 30V )

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

EXTENSION OF INSTRUMENT RANGE

V

300V

0-30V

VV

RS RV

VT

Let Rv be the resistance of the voltmeter.

Rs be the resistance to be connected in series.

Vv be the range of the given voltmeter.

VT be the range to which the extension is to be made.

The value of the resistance to be connected in series to extend the range is

Rs =

[(

VT

Vv

)

− 1

]

Rv = (m− 1)Rv

where ‘m’ =VT

Vv, is the multiplying factor.

52

PROCEDURE

The given voltmeter(unknown resistance) is connected to the terminal marked R1 and R2 on thebridge. The toggle switches are adjusted for external battery and galvanometer. An externalbattery is connected to terminals BB′ through a rheostat. A galvanometer is connected to theterminals marked Galvo. on the bridge. The P/Q ratio (range selector) is suitably selected.The resistance ‘S’ is varied by varying the four decade resistances (one at a time starting fromthe highest range) till null deflection is observed in the galvanometer, when the ‘B’ and ‘G’keys are pressed. Adjustments are made till null deflection is obtained, The reading of the‘Range selector’ and the four dials of the variable resistance ‘S’ are noted. The readings aretabulated as shown. The experiment is repeated for different values of range selector(P/Q ratio).

Extension of range of meter:- First the resistance of the given voltmeter Rv is measuredusing Wheatstones bridge. To extend the range of given voltmeter a resistance Rs is connectedin series with voltmeter as shown in the figure.

Since they are in series, current is the same through voltmeter and series resistance.

I =Vv

Rv=

(VT − Vv)

Rs

Rs = (VT − Vv)Rv

Vv=

[(

VT

Vv

)

− 1

]

Rv = (m− 1)Rv

where ‘m’ is the multiplying factor =VT

Vv.

RESULT

1) Resistance of given volt meter is . . . . . . Ω2) Resistance to be connected in series to extend its range to 300V is . . . . . . Ω

VIVA QUESTIONS

1. What is the range of resistances that can be measured using a wheatstonesbridge?

2. Why can’t a wheatstones bridge be used for measurement of small value ofresistance?

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

53

QP

p q

SR

R1 E

r

G

KELVINS DOUBLE BRIDGE

FINALINITIAL

PRESS KEYCURRENT SWITCH

DCSOURCE

GALV.

0-2.

5A

10V 45Ω,5Α

+C1

+P1

-C2

-P2

X.00002Ω/DIV

G

A

FOR.OFF

REV.

x1x10

x100

x.1

x.01

1 0 10

9

8

765

4

3

2

100

10 0

10

20

Experiment 11 54

Experiment 11

RESISTANCE MEASUREMENT USING

KELVINS DOUBLE BRIDGE

AIM

a) 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 REQUIRED

1. 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. Rheostat 45Ω,5A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.

PRINCIPLE

This method is the best available for precise measurement of low resistances(less than 1Ω). In

the figure ‘R’ is the low resistance to be measured and ‘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

variable. These are connected to form two sets of ratio arms, which are used for range selection.

The ratio Q/P is kept same as q/p ratio along with ‘S’ being varied till null deflection of the

galvanometer is obtained.

ThenR

S=

Q

P=

q

por R =

Q

P× S

55

OBSERVATIONS

Sl. Unknownremarks

Range S1 S2 × 10−4 S = S1 + S2 X mean

No. resistance multiplier mΩ Ω mΩ mΩ mΩ

1Ammeter direct

+leads reverse

2Leads direct

alone 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 ammeter

To 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 ammeter

RSH the resistance to be connected in shunt

IM the range of the given ammeter

IT the range to which the extension is to be made

The value of resistance to be connected in shunt to extend the range of the given ammeter

to (0-25A)

= RSH =(IM ×RA)

(IT − IM )=

RA(

ITIM− 1

) =RA

m− 1

where ‘m’ is the multiplying factor =ITIM

56

PROCEDURE

Connections are made as shown in the figure. Choose a suitable range multiplier . Set the

current switch in forward position. Press the galvanometer initial key first and adjust main

dial and slide wire to get null deflection in the galvanometer. Then press the galvanometer

final key and check whether the galvanometer reads null deflection. If not, adjust the dial

readings to get null deflection. The readings of the main dial and 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 adjusted to get null deflection and the readings are

noted again. The mean of the two is taken as the correct value. This is done to eliminate errors

due to thermal effect. The ammeter is then disconnected and the resistance of the connecting

leads alone is measured using the same method. The experiment is repeated with different

values of range multiplier. The readings are tabulated as shown.

Resistance of ammeter = (Resistance of ammeter + leads) - (Resistance of leads alone)

Extension of range

To 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 ×RSH

RSH =

(

IMISH

)

×RA =(IM ×RA)

(IT − IM )=

RA(

ITIM− 1

) =RA

m− 1

where ‘m’ is the multiplying factor =ITIM

RESULT

1) Resistance of given ammeter( 0− 2.5A) = . . . . . . Ω

2) Resistance to be connected in shunt to extend its range to (0− 25A) = . . . . . . Ω

VIVA QUESTIONS

1. 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?

57

CIRCUIT DIAGRAM FOR LINEAR RESISTANCE

220VDC

2AA

V 0-250V

0-1A

+

1.2A1000Ω

1.7A300Ω

2A

-

+ -+

-

CIRCUIT DIAGRAM FOR INCANDESCENT LAMP

220VDC

2AA

V 0-250V

0-1A

+

1.7A300Ω

2A

-

+ -

LAMP

+

-

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

Experiment 12 58

Experiment 12

V-I CHARACTERISTICS OFINCANDESCENT LAMP AND LINEAR

RESISTANCE

AIM

To determine the V-I Characteristics of linear resistance and incandescent lamp.

APPARATUS REQUIRED

Rheostat - 300Ω, 1.7A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.Voltmeter - 0-300V MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.Ammeter - 0-2A MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.Incandescent lamp - 240V, 100W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.Rheostat - 1000Ω, 1.2A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

PRINCIPLE

The resistance of a material is practically a constant at constant temperature, so for the linearresistance, according to ohm’s law the current flowing through the circuit is directly proportionalto the voltage applied.

ie, I ∝ VV = IR

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

Here R is a constant therefore we get a linear relationship between voltage and current. Butin the case of incandescent lamp, large amount of heat is produced so there is a considerablechange in the resistance thus as the voltage increases we get a non linear relationship betweenvoltage and current.

PROCEDURE

The connections are made as per the circuit diagram. The linear resistance is connected inthe circuit first. Keeping the potential divider in the minimum output voltage positions, thesupply is switched on. The rheostat(300Ω, 1.7A) is adjusted to get different voltages till therated voltage is reached and corresponding current readings are noted down.

The experiment is repeated by connecting incandescent lamp in place of the rheostat. TheV-I characteristics of linear resistance and incandescent lamp are plotted.

RESULT

V-I characteristic of linear resistance and incandescent lamp are plotted.

59

CIRCUIT DIAGRAM - OCC & SCC

+

-

V

A

M

L F A15A

F1

F2

Rh1

300Ω1.7A

A1

A2

0-300V

0-10A

220VDC

15A

A

+

-

2A

2A

Rh2

1000Ω 1.2A

F1 F2

B

Y

R

0-2A

R

YB

MI

MI

N

S1

S2

+ -

MC

S3

GS

D.C motor alternator

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

+5A50Ω

5A

-

+ -

R

N

MC

MC

+

-

OBSERVATIONS AND CALCULATIONS

O.C TESTIfVOC

S.C TESTIaIf

Measurement of RaV I Ra

Experiment 13 60

Experiment 13

OPEN CIRCUIT AND SHORT CIRCUITTEST ON A THREE PHASE

ALTERNATOR

AIM

To conduct open circuit and short circuit tests on a three phase alternator and predeterminethe regulation curve by emf method at half load and full load.

APPARATUS REQUIRED

Voltmeter - 0-300V, MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.- 0-10V, PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

Ammeter - 0-10A, MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.- 0-2A, PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.- 0-5A, PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

Rheostat - 300Ω, 1.7A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.- 1000Ω, 1.2A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.- 50Ω, 5A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

PRINCIPLE

As 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 rated value to zero, speed and field currentremaining constant

%Reg =E − V

V× 100

Where E - Generated emf

V - Terminal voltageFor small machines the regulation may be found by direct loading. For large machines the

voltage regulation is predetermined by using indirect methods like emf method, mmf method,Potier and ASA methods All these methods require open circuit characteristics and short circuitcharacteristics.

The open circuit characteristics [also called open circuit saturation curve or magnetizationcurve] is a plot of no load terminal voltage versus field excitation with the machine running atrated speed. Under these conditions the induced voltage is directly proportional to the flux.The shape of curve is therefore a typical B-H curve or magnetization curve. The short circuitcharacteristics is a plot between armature current and field excitation with a symmetrical shortcircuit applied across the terminals. Under these conditions current in the armature winding

61

Regulation curve O.C.C and S.C.C of3φ alternator

%Reg

0.8 0.8 0.6 0.4

(upf)

0.2 0

(lag)p.f

0.2 0.4 0.6p.f

(lead)

0

(full load)

(half load)

%Reg

S.C testO.C testI a

(

A)

VO

C (

V)

If (A)

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

VPH =VL√3= . . . . . . . . .

Ra(dc) = . . . . . . . . .Effective value Ra = 1.6×Ra(dc) = . . . . . . . . .From graph Zs = VOC/ISC = . . . . . . . . .

∴ Xs =√

Z2s −R2

a = . . . . . . . . .

Sample Calculation

Eo =

(V cosφ+ IRa)2 + (V sinφ± IXS)

21/2

‘-ve’ for leading‘+ve’ for lagging

% regulation =Eo − V

V× 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)

62

wholly depends on the internal impedance consisting of synchronous reactance Xsand the wind-ing resistance Ra.

Now Ra being small compared to Xs the pf under short circuit condition is zero powerfactor lagging and therefore the armature reaction mmf is almost wholly demagnetizing.

The short circuit characteristics is a straight line. From O.C.C & S.C.C the synchronousimpedance is evaluated as follows.

For any value of excitation or field current If , if VOC is the open circuit voltage & ISC is theshort circuit current, then synchronous impedance Zs=VOC/ISC . The value of Zs is calculatedfor the unsaturated region. For the computation of regulation, it is convenient to take Zs atsuch a value of excitation which give rise to Vph[normal voltage per phase]on open circuit. Thearmature resistance is measured using ammeter-voltmeter method. Under working conditionsthe effective value of Ra is increased due to skin effect and temperature effect. The effectivevalue of Ra is generally taken as 1.6 times the d.c value.Synchronous reactance per phase Xs =

√

Z2a −R2

a Ω per phase.

Eo =√

(V cosφ+ IRa)2 − (V sinφ± IXs)2

where +ve sign for lagging power factor and -ve for leading. Now percentage regulation foreach case is computed as

% Regulation =Eo − V

V× 100

PROCEDURE

O.C test

Connections are made as shown in the connection diagram. Switches S3 and S2 are kept in theopen position. The motor field rheostat Rh1 is kept in minimum position and the alternatorfield rheostat Rh2 in the maximum position. Supply is switched on by closing switch S1. Thedc motor is started using the 3-point starter. The motor field rheostat Rh1 is varied till thespeed becomes equal to the rated speed. Switch S2 is closed. Rh2 is varied in steps and the fieldcurrent and voltmeter reading are noted down. The experiment is repeated for different valuesof field current till the voltmeter reading shows 120% of the rated voltage of the alternator.Rheostat Rh2 is brought back to the maximum resistance position.

S.C test

Switch S3 is closed and rheostat Rh2 is varied till the ammeter reading in the alternator (A2)reads the rated current of the machine. The corresponding value of field current is noted down.

Armature resistance is 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.

RESULT

The open circuit and short circuit test was conducted on the given 3-φ alternator and theregulation curves for half load & full load are plotted.

63

CONNECTION DIAGRAM - NO LOAD TEST

400V

3 -

φ50

Hz

AC

10A

10A

10A

R

Y

B

E2

E3

A

V 0-500V

M

C

L0-5A

600V,5A,lpf

600V,5A,lpf

M

C V

L

E1

B1

C1

C2

B2

B3

C3

STATOR

R

Y BR3

R2

R1

ROTOR

V

BLOCKED ROTOR TEST

400V

3 -

φ50

Hz

AC

10A

10A

10A

R

Y

B

E2

E3

A

V 0-250V

M

C

L0-10A 250V,10A,upf

250V,10A,upf

M

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 - 1440rpmPhase 3-φH.P - 5.0

Experiment 14 64

Experiment 14

NO LOAD AND BLOCKED ROTORTESTS ON 3 PHASE SLIP RING

INDUCTION MOTOR

AIM

To perform no load and blocked rotor test on a three phase slip ring induction motor and

determine the equivalent circuit.

APPARATUS REQUIRED

Voltmeter - (0-500V) MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

- (0-250V) MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

- (0-30V) PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

Ammeter - (0-5A) MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

- (0-10A) MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

- (0-10A)PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

Wattmeter - 500V, 5A, lpf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 nos.

- 250V, 10A, upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 nos.

Rheostat - 9Ω, 8.5A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

Autotransformer

PRINCIPLE

Slip ring motors are always started with full line voltage applied across the stator terminals. The

value of starting current is adjusted by introducing a variable resistance in the rotor circuit.The

controlling resistance is in the form of resistances connected in star. The resistance is gradually

cut out of the rotor circuit as the motor gathers speed.

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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

+5A50Ω

5A

-

+ -R

BY-

+

For finding stator resistance, Rs :

No. V (V ) I(A) Rs

R

R

R

R(meas)

R(meas) =R× 2R

3R=

2

3R

Rs/ph(dc) =2

3R(meas)

Rs/ph(ac) = 1.6× 2

3×R(meas) = . . . . . . . . .

66

By introducing the rotor resistance, the rotor current is reduced at starting and the starting

torque is increased the latter due to improvement in power factor.

No load test:-

If the motor is run at rated voltage and frequency without any mechanical load, it will draw

power necessary to supply the no load losses. The no load current will have two components.

The active component and the magnetizing component, the former being very small as the no

load losses are small. The power factor at no load is therefore very low. The no load power

factor is always less than 0.5 and hence at no load one of the wattmeter at input side reads

negative.

The no load input W0 to the stator consists of

1. Small stator copper loss

2. Core losses

3. The loss due to friction and windage.

The rotor copper loss can be neglected, since slip is small at no load.

Blocked rotor test :-

The stator is supplied with a low voltage of rated frequency just sufficient to circulate rated

current through the stator with the rotor blocked and short circuited. The power input, current

and the voltage applied are noted down.

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 applied voltage is only a small percentage of the

normal voltage. Again since the rotor is at stand still the mechanical losses are absent. Hence

the blocked rotor input can be taken as approximately equal to the copper losses.

67

From no load test:-

V0 = . . . . . . . . . I0 = . . . . . . . . . W0 = . . . . . . . . .V0/ph = V0 = . . . . . . . . .

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

phase current(I0/ph) =IL√3

= . . . . . . . . .

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

cosφ0 =W0√3V0I0

= . . . . . . . . .

∴ φ0 = . . . . . . . . .sinφ0 = . . . . . . . . .

R0/ph =V0/ph

I0/ph cosφ0= . . . . . . . . .

X0/ph =V0/ph

I0/ph sinφ0= . . . . . . . . .

From blocked rotor test:-

VSC = . . . . . . . . . ISC = . . . . . . . . . WSC = . . . . . . . . .VSC/ph = VSC = . . . . . . . . .

WSC/ph =WSC

3= . . . . . . . . .

ISC/ph =ISC√3

= . . . . . . . . .

(Total winding resistance as

referred to the stator side)

R01 =WSC/ph

I2SC/ph= . . . . . . . . .

Z01 =VSC/ph

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

(Total leakage reactance as re-

ferred to the stator side)

X01 =√

Z201−R2

01= . . . . . . . . .

(Rotor resistance as referred

to the stater side)

R′2 = R01 −RS(eff)

= . . . . . . . . .

(Electrical equivalent of the

mechanical load)

RL = R′2

[

1− s

s

]

R0 X0

R01X01

=

= =

=V/ph= RL

68

PROCEDURE

No load test:-

Connections are made as shown in the diagram for no load test. Brake drum is made free torotate by loosening the belt. The autotransformer is placed in zero position. Then the supplyis switched on and the auto transformer is adjusted to supply the rated voltage to the machine.The handle of the starter is rotated to cut out the rotor resistance. Readings 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 brakedrum. The auto transformer is set to the zero voltage position. Then the three phase supplyis switched on. By adjusting the autotransformer, the ammeter reading is made equal to ratedcurrent of the machine. Readings of the two wattmeters, voltmeter and the ammeter are notedand tabulated.

Measurement of stator resistance:-

Connections are done for the stator resistance measurements. It is measured using the voltmeter-

ammeter method. The measured value is2

3Rph as the machine is ∆ connected. Thus Rph =

1.5Rmeas. Rs(eff) is taken as 1.6 times Rph to account for skin effect and heating effect.

RESULT

No load and blocked rotor tests were conducted on the given three phase slip ring inductionmotor and the equivalent circuit parameters were determined.

69

Emii

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