Ee2259 - Electrical Machines

SSN COLLEGE OF ENGINEERING

KALAVAKKAM- 603 110

LAB MANUAL

EE2259

ELECTRICAL MACHINES – I LABORATORY

IV Sem EEE ( D.C Machines & Transformer experiments)

Prepared By

N.PANDIARAJAN. B.E.,M.Sc(Engg),M.B.A.

Associate Professor

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

2

SSN College of Engineering, Kalavakkam

Department of Electrical and Electronics Engineering

EE2259 – Electrical Machines-1 Laboratory

IV Semester - Electrical and Electronics Engineering

Duration: Dec 2009 to April 2010

INDEX

CYCLE-I

(Fill in the order by which done)

Ex.

No. Date Experiment Name

Page

No

Staff

signature

1.

2.

3.

4.

5.

6.

7.

CYCLE-II

8.

9.

10.

11.

12.

13

14.

3

General Instructions to students for EEE Laboratory courses

SAFETY: 1 You are doing experiments with the help of electrical power. You have to be very

careful. You must clearly know the supply system to your worktable in particular and

the entire laboratory in general.

2 Incase of any wrong observation, you have to IMMEDIATELY SWITCH OFF supply

to the worktable.

3 You have to tuck in your shirts and you have to wear a overcoat.

4 Wearing loose garments in side the lab. is strictly prohibited.

5 You have to wear shoes compulsorily.

ATTENDANCE:

1 Every time you come to the laboratory class, you have to come with your record note

book, observation notebook, calculators etc.

2 You have to attend the lab. at the stroke of the bell in the laboratory.

3 You have to give your attendance. You have to submit your records. You have to show

the day's experiment's circuit diagram and get it signed. You have to occupy the

respective worktable of the machine. Collect required meters etc as per indent slip.

MAKING CONNECTIONS:

1 Start giving connections as per the circuit diagram from one side of the circuit.

2 Series circuits are to be only given first, with the help of power wires.

3 Make parallel connections, namely voltmeters last using silk wires.

4 Terminals of the meters should not be used as junction points. There should not be any

loose connection. Thoroughly check the connections and keep re rheostat positions

suitable for starting. Also keep all meters in such a way to read the readings

conveniently. Call staff to check the connections.

DOING EXPERIMENTS: 1 Start the experiment as per the procedure. First check for the direction of rotation in

case of machines and deflection of meters.

2 If any of the things are wrong, then SWITCH OFF and modify the connections. Inform

to the staff and then START.

OBSERVATION:

1 Enter all readings in the tabulation. Note down the multiplication factor of any meter

immediately. .

2 During load test on motors, the needle of the spring balances may be vibrating.

3 Arrest gently the vibration and take the reading.

4 At the time of over loading, the readings are to be taken quickly.

5 Pour water in the brake drum at the time of loading.

CALCULATION:

1 Calculate all required quantities and enter in the tabulation. Units are VERY, VERY

PORTANT.Draw the necessary graphs. Write the result. Show it to the staff for getting

signature.

RECORD:

1 As the name Implies, it is a record: PERMANENT RECORD for reference. Write

neatly; Draw circuit diagrams neatly and label correctly.

2 Enter readings in the tabulation.

3 UNITS are to be written for various quantities.

4 Draw Graph. Complete the record before you come for next lab class.

5 Bring the record for submission during next lab class.

4

Additional Instructions

Avoid wearing any loose metallic rings, straps or bangles, as they are likely to prove dangerous at times.

Before entering into the laboratory class, you must be well prepared for the experiment that you are going to do on that day.

You must bring the related textbook, which may deal with the relevant experiment.

Get the circuit diagram approved with correct meter & fuse ratings

Get the reading verified. Then inform the technician so that supply to the worktable can be switched off.

You must get the observation note corrected within two days from the date of completion of experiment. Write the answer for all the discussion questions in the observation note. If not, marks for concerned observation will be proportionately reduced.

If you miss any practical class due to unavoidable reasons, intimate the staff in charge and do the missed experiment in the repetition class.

Such of those students who fail to put in a minimum of 75% attendance in the laboratory class will run the risk of not being allowed for the University Practical Examination. They will have to repeat the lab course in subsequent semester after paying prescribed fee.

Avoid wearing any loose metallic rings, straps or bangles, as they are likely to prove dangerous at times.

Girls should put their plait inside their overcoat

Acquire a good knowledge of the surrounding of your worktable. Know where the various live points are situated in your table.

In case of any unwanted things happening, immediately switch off the mains in the worktable. The same must be done when there is a power break during the experiment being carried out.

Avoid carrying too many instruments at the same time.

Avoid using water hydrant for electrical fires.

5

How to fix rating? 1.GENERAL:

1 We have to fix meters and provide fuse for the experiment. Identify the correct

machine/transformer on which experiment is to be conducted. Notedown the complete

NAMEPLATE DETAILS including Name of the company, Serial No, etc of the

machine / transformer in your observation notebook.

2 MOVING COIL METERS are to be used for measuring dc quantities.

3 MOVING IRON METERS are to be used for measuring ac quantities.

2. LOAD EXPERIMENTS:

1 In this case, we have to load upto 125% of rated capacity. By seeing the Nameplate

details, ammeters, voltmeters can be selected. It may be MC or MI meters. Normally

rated voltages are to be measured (0-300V) MC for dc and (0-300V) MI for single

phase ac and (0-600V) MI voltmeters for 3 phase ac may be used.

2 Power measurement using wattmeters must be carefully done. Under load conditions,

powerfactor will be high; henceUPF wattmeters may be used, with suitable voltage and

current level.

3 Under no-load conditions, Power factor will be poor; hence LPF wattmeters with

correct voltage and current levels may be selected.

4 In the case of transformers, kVA rating will be known from the Name plate details. For

eg: 1 KVA; 200/100V 1 phase ac transformer has the rated current in the primary ,

Ip = 200 =5A;

and In the Secondary, Is = 100 =10A;

• Hence ammeters for Primary and Secondary in this case may be (0- 5A)MI and (0-

10A)MI respectively are used when load experiments are conducted.

3. NO LOAD EXPERIMENTS:

• Voltmeters may be selected suitably. Since there is no load, the ammeter may be of

lower range. Normally, no-load current will be around 10% and 15% of rated current in

the case of machines and transformers respectively. According to the no-load current

value arrived, appropriate ammeter may be used.

4. LOAD:

There is no difficulty in the case of motors, because mechanical (brake down) loads may be

employed.But in the case of generators and transformers, power rating and voltage level of the

loads are to be specified. Normally water loads, resistive loads and lamp loads are available.

By seeing the nameplate of the machine, the voltage and power of the load is to be specified. .

5. FUSE: Fuse is an important element in an electric circuit. It is a protective element. Fuse is to be

provided in order to protect machines and meters. Fuse rating in any case should not exceed

rated current of the machines in the case of load experiments and not exceed the ammeter

range in the case of no-load experiments.

6

Duties of a student

1. Attend the classes regularly. Class may be missed only under exceptional

circumstances beyond one’s control.

2. Take class notes in separate notebook for each subject.

3. Write down all that written by the teacher on the board. In addition write down some

points told by the teacher orally. This will keep your attention focussed towards the

subject and will help you to appreciate the teacher’s lecture better.

4. Refer your class notes while reading from a book.

5. Purchase at least one textbook written by a standard author or as suggested by your

teacher or as prescribed by the University. Cheap books, books having large number of

solved problems etc. can be additional source material, but these cannot substitute a

textbook. Cost of a book is much lower than the cost of marks and knowledge.

6. Take the unit tests seriously by giving your best performance. Your performance in

tests will be an indicator towards your future performance in the university

examinations.

7. Do a lot of writing work at home. Writing is remembering. Remembering through

writing is faster than remembering through reading. In addition writing will help you to

remember the spellings, improve your grammar and writing speed. Formulae and their

derivations can be learned through writing only. Solve a large number of unsolved and

also solved problems independently.

8. Take your studies seriously. Get very good marks in the University examinations

because these are a must for a good placement. As only brilliant students are admitted

in this college, a little hard work may lead you to get University Ranks. Getting a

University Rank will be an achievement that will deliver you fruits for the lifetime in

several different ways. This is MUST for good placement also.

9. Do not hesitate to consult your teacher in case of any difficulty in understanding any

topic. Do not accumulate your doubts.

Basic duties to be completed to get the permission to do the experiment 1) All the members in the groups have to go through the experiment and

Complete the given circuit diagram(s) with fuse rating, instrument ratings,

Apparatus required tables and all other required things.

2) Each student has to answer the following questions compulsorily.

i) What is the aim of the exercise?

ii) What is the procedure to do the exercise?

iii) What are the precations?

iv) Two or three qtns given at each exercise.

Those who are unable to meet out the above requirement, need not be allowed to

do the exercise.

.

7

SSN COLLEGE OF ENGINEERING

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

LABORATORY PRACTICE

SAFETY RULES

1 SAFETY is of paramount importance in the Electrical Engineering Laboratories.

2 Electricity NEVER EXECUSES careless persons. So, exercise enough

care and attention in handling electrical equipment and follow safety practices in the

laboratory.(Electricity is a good servant but a bad master).

3 Avoid direct contact with any voltage source and power line voltages.

(Otherwise, any such contact may subject you to electrical shock)

4. Wear rubber-soled shoes.

(To insulate you from earth so that even if you accidentally contact a live point,

current will not flow through your body to earth and hence you will be protected from

electrical shock)

5. Wear laboratory-coat and avoid loose clothing.

(Loose clothing may get caught on an equipment/instrument and this may lead

to an accident particularly if the equipment happens to be a rotating machine)

6. Girl students should have their hair tucked under their coat or have it in a knot.

7. Do not wear any metallic rings, bangles, bracelets, wristwatches and neck chains.

(When you move your hand/body, such conducting items may create a short circuit

or may touch a live point and thereby subject you to electrical shock)

8. Be certain that your hands are dry and that you are not standing on wet floor.

(Wet parts of the body reduce the contact resistance thereby increasing the severity

of the shock)

9. Ensure that the power is OFF before you start connecting up the circuit.

(Otherwise you will be touching the live parts in the circuit)

10. Get your circuit diagram approved by the staff member and connect up the circuit

strictly as per the approved circuit diagram.

11. Check power chords for any sign of damage and be certain that the chords use safety

plugs and do not defeat the safety feature of these plugs by using ungrounded plugs.

12. When using connection leads, check for any insulation damage in the leads and avoid

such defective leads.

13. Do not defeat any safety devices such as fuse or circuit breaker by shorting across it.

Safety devices protect YOU and your equipment.

8

14. Switch on the power to your circuit and equipment only after getting them checked

Up and approved by the staff member.

15. Take the measurement with one hand in your pocket.

(To avoid shock in case you accidentally touch two points at different potentials

with your two hands)

16. Do not make any change in the connection without the approval of the staff member.

17. In case you notice any abnormal condition in your circuit ( like insulation heating up,

resistor heating up etc ), switch off the power to your circuit immediately and

inform the staff member.

18. Keep hot soldering iron in the holder when not in use.

19. After completing the experiment show your readings to the staff member and switch off

the power to your circuit after getting approval from the staff member.

20. While performing load-tests in the Electrical Machines Laboratory using the

brake-drums:

(i) Avoid the brake-drum from getting too hot by putting just enough water

into the brake-drum at intervals; use the plastic bottle with

a nozzle (available in the laboratory ) to pour the water.(When the drum

gets too hot, it will burn out the braking belts)

(ii) Do not stand in front of the brake-drum when the supply to the load-test

circuit is switched off. (Otherwise, the hot water in the brake-drum will

splash out on you)

(iii) After completing the load-test, suck out the water in the brake-drum using

the plastic bottle with nozzle and then dry off the drum with a sponge

which is available in the laboratory.(The water, if allowed to remain in the

brake-drum, will corrode it)

21. Determine the correct rating of the fuse/s to be connected in the circuit after

understanding correctly the type of the experiment to be performed:

no-load test or full-load test, the maximum current expected in the circuit and

accordingly use that fuse-rating.(While an over-rated fuse will damage the

equipment and other instruments like ammeters and watt-meters in case of over load,

an under-rated fuse may not allow one even to start the experiment)

22. At the time of starting a motor, the ammeter connected in the armature circuit

overshoots, as the starting current is around 5 times the full load rating of the

motor. Moving coil ammeters being very delicate, may get damaged due to

high starting current. A switch has been provided on such meters to disconnect

the moving coil of the meter during starting. This switch should be closed after

the motor attains full speed. Moving iron ammeters and current coils of wattmeters

are not so delicate and hence these can stand short time overload due to high

starting current. No such switch is therefore provided on these meters. Moving

iron meters are cheaper and more rugged compared to moving coil meters.

21. Moving iron meters can be used for both a.c. and d.c. measurement.

22. Moving coil instruments are however more sensitive and more accurate as compared

9

to their moving iron counterparts and these can be used for d.c. measurements only.

Good features of moving coil instruments are not of much consequence for you as

other sources of errors in the experiments are many times more than those caused

by these meters.

23. Some students have been found to damage meters by mishandling in the following

ways:

i. Keeping unnecessary material like books, lab records, unused meters etc.

causing meters to fall down the table.

ii. Putting pressure on the meter (specially glass) while making connections or

while talking or listening somebody.

iii. Switching on the circuit without getting the connections checked by the staff.

STUDENTS ARE STRICTLY WARNED THAT FULL COST OF THE METER WILL

BE RECOVERED FROM THE INDIVIDUAL WHO HAS DAMAGED IT IN SUCH A

MANNER.

10

SSN College of Engineering, Kalavakkam

Department of Electrical and Electronics Engineering

EE2259 – Electrical Machines-1 Laboratory

IV Semester - Electrical and Electronics Engineering

INDEX

Ex.

No. Experiment Name

Page

No

1. O.C.C & Load Characteristics of D.C Separately

Excited Shunt Generator.

11

2. O.C.C & Load Characteristics of D.C Self Excited

Shunt Generator. 19

3. Load Test on D.C. Compound Generator

27

4. Load test on D.C. Shunt Motor 33

5. Speed Control of D.C.Shunt Motor 39

6. Load Test on D.C Series Motor 43

7. Load Test on D.C. Compound Motor 49

8. Load Test on Single Phase Transformer 55

9. OC , SC tests on Single Phase Transformer 61

10. Swinburne’s Test 69

11. Sumner’s Test 73

12. Hopkinson’s Test 79

13 Separation Of Losses In Single Phase Transformer 85

11

Ex.No: Date:

1. Open Circuit Characteristics (O.C.C) &

Load Characteristics of D.C Separately Excited Shunt Generator. (P.325 & 333 of text book)

AIM: I. To determine the no load magnetization or open circuit characteristic separately

excited dc shunt generator, compare it with self excited one, and hence

a. To deduce the O.C.C. for various speed, 1000 and 1250 rpm.

b. To determine the critical field resistance and critical speed.

II. To determine the external and internal (load) characteristics of the separately excited

DC shunt generator by actually loading the machine.

APPARA TUS REQUIRED:

S.No Name of the apparatus Range Type Quantity

1

2

3

4

5

6

7

8

THEORY: (i) O.C.C

In any D.C generator generated emf is d1rectly proportional to the exciting field current

i.e. flux. O.C.C shows the relation between the no-load generated emf in the armature, 'Eo'

and the field or exciting current 'If’ at a given fixed speed.

It should be noted that OCC for a higher speed would be above this curve and for a

lower speed, would be below it.

(ii) Load Charactristics:

Let us consider a dc generator giving it’s rated no load voltage 'Eo' for a certain

constant field current. If there were no armature reaction and armature voltage drop, then this

voltage would have remained constant as shown in modal graph by the dotted horizontal line I.

But when the generator is loaded, the voltage falls due to these two causes, thereby

giving slightly drooping characteristics. If we subtract from Eo the value of voltage drops due

to armature reaction for different loads, then we get the value of E, the emf actually induced

in the armature under load conditions. Curve II is plotted in this way and is known as the

INTERNAL CHARACTERISTIC.

If we subtract from E the armature drop laRa, we get terminal voltage V. The reverse

procedure may also be adopted to obtain Internal characteristic (ie) adding laRa to V. Curve III

represents the EXTERNAL CHARACTERISTIC.

Comparison of self and separately excited generator gen characteristics: Since the field current depends on terminal voltage in self excited generator, it’s load

characteristics would be more drooping than the other one. In O.C.C. there is no much

appreciable difference between the two types.

12

Circuit Diagram:

R2

Name Plate Details:

Fuse Rating Calculation: D.C. Shunt Motor: O.C.C D.C Generator: O.C.C

Load Test: Load Test:

Observation: O.C.C Test

E0 At different speeds Sl.no If

(Amps)

E 0

(Volts) 750rpm 1500rpm

D.C. Shunt Motor D.C Shunt Generator

Capacity: Capacity:

Voltage: Voltage:

Current: Current:

Speed: Speed:

Field Current: Field Current:

+

D

P

S

T

S

AL

V M

A

AA

-

220 V

D.C.

L F A

Af

SPSTS

G VRL

+ -

D

P

S

T

S

+

- z

zz

+

-

z

zz

3 Point Starter

A

AA

13

Range fixing:

The current drawn by the shunt motor on no-load is 15 to 20% of full load current.

The current drawn by the shunt motor on load must not exceed 120% of full load

current.

Motor Field circuit rheostat rating is _____ Ω ; ____ A (the current rating should be

slightly higher than the rated current)

Generator load Current IL = _______ A

∴ The range of ammeter AL is (0- )A

The rated field current is _____ A

∴ The range of ammeter Af is (0- )A

Generator Field circuit rheostat rating is _____ Ω ; ____ A (the current rating should

be slightly higher than the rated current and ohmic value should be as high as possible).

Rated voltage of Generator V = _______ Volts

∴ The range of voltmeter V is (0- ) Volts

PROCEDURE:

O.C.C.Test: 1. Connections are given as shown in the circuit diagram.

2. After the connections are checked, Keeping R1 minimum resistance position and R2 in

maximum resistance position, the supply switch is closed.

3. The motor is started with the help of a dc 3 point starter.

4. The field rheostat R1 of the motor is adjusted to make it run at the rated speed.

Keeping the SPST switch open, Emf generated due to residual magnetism is noted

from voltmeter. (when .field current is zero.)

5. Then SPST switch is closed. By varying the field rheostat R2 in the field circuit of

generator, different values of generated emf Eo and field current If are noted from

voltmeter and ammeter and tabulated.

Load Test:

1. Keeping the generator side DPST open, the field rheostat in the generator side is

adjusted for the rated voltage of the generator which is seen in the voltmeter.

2. Now the DPST switch is closed and the resistive load is put up on the generator step by

step. The terminal voltage, armature and load current values are noted down for each

step from the respective meters.

3. Note that while taking each set of readings, the field current is maintained constant as

that for rated voltage [because due to heating, shunt field resistance is increased]

Measurement of Ra and Rsh : 1. Connections are given as shown in the circuit diagram.

2. Keep the resistances in maximum position and close the supply switch and take

Minimum three readings.

14

Load Test:

Speed: .............. RPM; No load voltage: ............. volts

Sl.

No.

Terminal

voltage

Load current

IL = Ia If Eg = V + IaRa

Model Graphs:

E0

Draw Rc line, such that it is tangent to the initial portion of O.C.C. at rated speed and

passes through origin.

The value of critical field resistance, Rc = the slope of Rc line

Critical speed, Nc = speedRatedNhereNAC

BCRR −× ;

Rsh Line

Eg

If

Rc line

O.C.C. Line

V, Eg

IL, Ia

V vs IL

Eg vs Ia

C

B

A

15

Model Calculation:

O.C.C

E0 ∝ N

So, for different speeds, O.C.C. can be deduced from the O.C.C.at rated speed.

N1 = E1

N2 E2

Load test:

For separate excitation Ia = IL

So, induced emf on load, Eg = V + IaRa

GRAPH: Open circuit characteristic is drawn by taking field current 'If' along x axis and generated

voltage Eo along y axis.

To find critical field resistance: From the origin a tangent is drawn to OCC at the linear

portion. The slope of the tangent will give the critical field resistance.

Calculate E0 at different speeds and draw O.C.C. for diff speeds in a separate graph.

External characteristic is drawn taking load current 'IL' along x axis and terminal voltage

'V' along yaxis.

Internal characteristic is drawn by adding laRa drop to external characteristic curve, 'la'

along x axis and 'E' along y axis.

16

Measurement of Rsh of Generator

Measurement of Ra for Generator

Observation:

A + -

V

+

-

D

P

S

T

S

+

-

220 V

D.C

Z

ZZ

A + -

V G +

-

A

AA

D

P

S

T

S

+

-

RPS

Measurement of Ra:

Sl.

No.

V

(Volts)

I

(Amps)

Ra = V/I

(Ohms)

Measurement of Rsh:

Sl.

No.

V

(Volts)

I

(Amps)

Rsh = V/I

(Ohms)

17

Result:

QUESTIONS:

1. What is the principle of operation of DC generator?

2. Where the field winding is placed in DC generator? Why?

3. List out the factors involved in voltage build up of a DC shunt generator.

4. Explain briefly the function of a commutator in a DC machine. Can commutator action

be performed by a solid state device.

5. Define and explain critical field circuit resistance and critical speed of DC shunt

generator.

6. What is armature reaction?

7. What is internal & external characteristics of DC shunt generator?

8. What are the applications of DC shunt generator?

REFERENCES: 1. I.J.Nagrath & D.P.Kothari “ Electrical Machines.”

2. Albert E. Clayton & H.N. Honcock “ The performance and Design of Direct Current

Machines.”

3. K. Murugash Kumar “ D.C. Machines & Transformers”

18

Circuit Diagrams:

:

Name Plate Details:

Fuse Rating Calculation: D.C. Shunt Motor: D.C Generator:

O.C.C O.C.C


Observation: O.C.C Test

E0 At different speeds Sl.no If

(Amps)

E 0

(Volts) 750rpm 1500rpm

D.C. Shunt Motor D.C Shunt Generator

Capacity: Capacity:

Voltage: Voltage:

Current: Current:

Speed: Speed:


+

D

P

S

T

S

AL

V M

A

AA

-

220 V

D.C.

L F A

SPSTS

G VRL

+ -

D

P

S

T

S

+

- z

zz

3 Point Starter

Af

A

AA +

-

z

zz

19

Ex.No :

Date:

2. Open Circuit Characteristics (O.C.C) &

Load Characteristics of D.C Self Excited Shunt Generator.

(P.325 to 337 of text book) AIM: I. To determine the no load magnetization or open circuit characteristic self

excited dc shunt generator and hence,

a. To deduce the O.C.C. for various speed, 1000 and 1250 rpm.

c. To determine the critical field resistance and critical speed.

II. To determine the external and internal (load) characteristics of the self excited

DC shunt generator by actually loading the machine.

APPARA TUS REQUIRED:


1

2

3

4

5

6

7

8

THEORY: (i) O.C.C & (ii) Load Charactristics:

Refer the expiment “separately excited shunt generator”

Comparison of self and separately excited generator gen characteristics: Since the field current depends on terminal voltage in self excited generator, it’s load

characteristics would be more drooping than the other one. In O.C.C. there is no much

appreciable difference between the two types.

Range fixing:

The current drawn by the shunt motor on no-load is 15 to 20% of full load current.


current.










∴ The range of voltmeter V is (0-

20

Load Test:

Speed: .............. RPM; No load voltage: ............. volts

Sl.

No.

Terminal

voltage

Load current

IL If Ia =IL + If Eg = V + IaRa

Model Graphs:

Draw Rc line, such that it is tangent to the initial portion of O.C.C. at rated speed and

passes through origin.

The value of critical field resistance, Rc = the slope of Rc line

Critical speed, Nc = speedRatedNhereNAC

BCRR −× ;

Rsh Line

E0

If

Rc line

O.C.C. Line

V, Eg

IL, Ia

V vs IL

Eg vs Ia

C

B

A

21

PROCEDURE:

O.C.C.Test:

1. Connections are given as shown in the circuit diagram.

2. After the connections are checked, Keeping R1 minimum resistance position and R2 in

maximum resistance position, the supply switch is closed.

3. The motor is started with the help of a dc 3 point starter.

4. The field rheostat R1 of the motor is adjusted to make it run at the rated speed.

Keeping the SPST switch open, Emf generated due to residual magnetism is noted

from voltmeter. (when .field current is zero.)

5. Then SPST switch is closed. By varying the field rheostat R2 in the generator side,

different values of generated emf Eo and field current If are noted from voltmeter and

ammeter and tabulated.

Load Test:

1. Keeping the generator side DPST open, the field rheostat in the generator side is

adjusted for the rated voltage of the generator which is seen in the voltmeter.

2. Now the DPST switch is closed and the resistive load is put up on the generator step by

step. The terminal voltage, armature and load current values are noted down for each

step from the respective meters.

3. Note that while taking each set of readings, the field current is maintained constant as

that for rated voltage [because due to heating, shunt field resistance is increased]

Measurement of Ra and Rsh : 1. Connections are given as shown in the circuit diagram.

2. Keep the resistances in maximum position and close the supply switch and take

Minimum three readings.

22

Measurement of Rsh of Generator :

Measurement of Ra for Generator:

Observation:

A + -

V

+

-

A

A

A

3

P

+

-

220 V

D.C

Z

ZZ

A + -

V G +

-

A

AA

D

P

S

T

S

+

-

RPS

Measurement of Ra:

Sl.

No.

V

(Volts)

I

(Amps)

Ra = V/I

(Ohms)

Measurement of Rsh:

Sl.

No.

V

(Volts)

I

(Amps)

Rsh = V/I

(Ohms)

23

Model Calculation:

O.C.C

E0 ∝ N

So, for different speeds, O.C.C. can be deduced from the O.C.C.at rated speed.

N1 = E1

N2 E2

Load test:

For separate excitation Ia = IL

For self excitation Ia = IL + If

So, induced emf on load, Eg = V + IaRa

GRAPH: Open circuit characteristic is drawn by taking field current 'If' along x axis and generated

voltage Eo along y axis.

To find critical field resistance: From the origin a tangent is drawn to OCC at the linear

portion. The slope of the tangent will give the critical field resistance.

Calculate E0 at different speeds and draw O.C.C. for diff speeds in a separate graph.

External characteristic is drawn taking load current 'IL' along x axis and terminal voltage

'V' along yaxis.

Internal characteristic is drawn by adding laRa drop to external characteristic curve, 'la'

along x axis and 'E' along y axis.

24

25

Result:

QUESTIONS:

1. What is the principle of operation of DC generator?

2. Where the field winding is placed in DC generator? Why?

3. List out the factors involved in voltage build up of a DC shunt generator.

4. Explain briefly the function of a commutator in a DC machine. Can commutator action

be performed by a solid state device.

5. Define and explain critical field circuit resistance and critical speed of DC shunt

generator.

6. What is armature reaction?

7. What is internal & external characteristics of DC shunt generator?

8. What are the applications of DC shunt generator?

REFERENCES: 1. I.J.Nagrath & D.P.Kothari “ Electrical Machines.”


Machines.”


26

Circuit Diagram:

Name Plate Details:

Fuse Rating Calculation: D.C. Shunt Motor: D.C Compound Generator:


Measurement of Ra and Rf

D.C. Shunt Motor D.C. Compound Generator

Capacity: Capacity:

Voltage: Voltage:

Current: Current:

Speed: Speed:


YY Y (Dif.)

Y YY(Cum.) +

−

220 V

D.C.

D

P

S

T

S

L Z A

A

AA z

zz

M G

A

AA

V

+

−

D

P

S

T

S

AL

V

R

L

+

−

zz AF

S

P

S

T

z

+

27

Ex.No:

Date:

3.Load Test on D.C. Compound Generator (Cumulative & Differential Charactristics)

(P.336 & 337 of text book)

AIM:

To determine the external characteristics of a dc compound generator by conducting

load test.

APPARATUS REQUIRED:


1 Ammeter 0-10A MC 1

2 Voltmeter 0-300V MC 1

3 Rheostat(R1, R2) 1000Ώ/ 1A 2

4 Loading Rheostat 3 KW/ 230V

THEORY: The compound generators have both shunt and series field windings. If the series field

ampere –turns are such as to produce the same voltage at rated load as at no-load, then the

generator is FLAT COMPOUNDED.

If the series field ampere turns are such that the fated load voltage is greater than the no-

load voltage, then the generator is OVER COMPOUNDED.

If the rated load voltage is less than the no-load voltage, then the generator is UNDER

COMPOUNDED. But such generators are seldom used.

Also it can be stated that if the series field aids the shunt field, it is CUMULATIVELY

COMPOUNDED. And if the, series field opposes the shunt field, it is DIFFERENTIALLY

COMPOUNDED. It can be achieved by reversing the connections of the series field.

Further if the shunt field is connected across both armature and series field, it is LONG

SHUNT and if the shunt field is connected across armature only, then it is called SHORT

SHUNT.

28

29

Range fixing:


current.

The rated field current of motor is _____ A





Generator Load circuit rheostat rating is _____ Ω ; ____ A

The rated field current of generator is _____ A






Procedure:

Load Test:

PROCEDURE: 1. The connections are made as shown in the circuit diagram, which represents LONG

SHUNT compound generators. It may be differentially or cumulatively compounded. It

can be known only by seeing the voltmeter reading.

2. Keeping the load switch open, R1, in minimum resistance position & R2 in minimum

resistance, the supply switch is closed.

3. The motor is started using the dc 3 point starter. The motor is brought to its rated speed

by varying the field rheostat R1of the motor. The generator is brought to its rated

voltage by adjusting the field rheostat R2 of the generator.

4. Initial reading of ammeter and voltmeter are noted.

5. Now the load side DPST switch is closed. Different loads are applied and each step the

ammeter & voltmeter readings are noted and tabulated.

GRAPH:

Load current Vs Terminal voltage. Graph is drawn by taking load current 'IL' along x axis and

the terminal voltage 'V' along y axis.

30

For Load Test:

Cumulative

Sl.No.

Terminal

Voltage V

(Volt)

Load

Current IL

(Amp.)

Shunt field

Current If

(Amp.)

Armature

Current Ia

(Amp.)

Generated

Voltage Eg

(Volt.)

Differential

Sl.No.

Terminal

Voltage V

(Volt)

Load

Current IL

(Amp.)

Shunt field

Current If

(Amp.)

Armature

Current Ia

(Amp.)

Generated

Voltage Eg

(Volt.)

Measurement of Ra & Rse :

Sl.No. V (Volt) I (Amp.) Ra + Rse = V/I (ohms)

31

RESULT:

QUESTIONS:

1. Draw the diagrams of long & short shunt compound generators.

2. How will you obtain cumulatively & differentially compounded configurations?.

3. What is flat, over and under compound generators?

4. What are the applications of compound generators?

REFERENCES:

4. I.J.Nagrath & D.P.Kothari “ Electrical Machines.”


Machines.”


* * * * *

32

Circuit Diagram:

Name Plate Details:

.

Fuse rating calculation:

Model Graphs:

D.C. Shunt Motor

Capacity: Speed:

Voltage:

Current: Field Current:

+

D

P

S

T

S

A

V M

A

AA

220 V

D.C.

L F A +

F

FF

3 Point Starter

−

+

−

#

F1 F2

Brake drum

−

P0

N, T,

η, I

T

I P0

N

Ia

T

T

Ia

N

T

N

33

Ex.No:

Date:

4. Load test on D.C. Shunt Motor

(P.344 of text book)

AIM: To conduct the load test on a given dc shunt motor and draw the performance curves.

APPARATUS REQUIRED:


1

2

3

4

5

THEORY:

Principle of operation of a D.C. motor: To operate DC machine as motor, field windings are excited with DC source and North

and South Pole magnetic fields established in the air gap by the stator poles, a DC current has

to be passed through the armature terminals.

Current carrying armature conductors cause additional magnetic field to be set up.

Interaction of this magnetic field with the main magnetic field set up by the stator poles cause a

force to be developed around the conductors.

The force developed in all the conductors is unidirectional (anti-clockwise) due to

commutator action.

In case, clockwise direction is required either the direction of the main field or the

direction of the current through the armature conductor is to be reversed.

As the armature rotates, the system of conductors in the armature come across alternate

North and South pole magnetic fields. Therefore an emf is induced in the conductors. This is

called as Back emf.

The plot of speed, torque, efficiency and input current (vs) output power are called the

performance characteristics of a motor.

The speed characteristics is almost a horizontal line and the speed regulation is good.

Hence a d.c shunt motor is generally referred to as a constant speed motor and are used in

applications such as lathes, centrifugal pump, machine tools, blower fans and etc.

The torque varies directly with the output load and the armature current because the

field is approximately constant over the load range .As compared to other motors a shunt motor

has a low means that a motor draws more current from the main supply than a series motor or a

compound motor at the time of starting. The small input current on no load goes to meet the

various losses occurring within the machine.

The efficiency curve with is usually the same for all motors It is advantageous to have

an efficiency curve which is fairly flat over a wide range of loads and having maximum

efficiency at full loads.

34


Field circuit rheostat rating is _____ Ω ; ____ A (the current rating should be


Rated voltage of motor V = _______ Volts


FORMULAE:

Torque, (T) = (S1-S2)Rx9.81 Nm

Where S1, S2 -spring balance readings (kg) R- Radius of the brake drum (m)

Input power, Pi = VI watts

Output power, Po = 2IINT/60watts

Percentage efficiency = (Output power / Input power) x 100

PROCEDURE:

1. The circuit connections are given as shown in the circuit diagram. Ensure that there is

no load on the brake drum.

2. The supply is given by closing the DPST switch, keeping R1 in minimum resistance

position.

3. The motor is started using the dc 3 point starter, and it is allowed to run at rated speed

by adjusting the rheostat R1 which is connected to the field circuit. After setting the

speed rheostat position should not be altered.

4. At no load condition, the input voltage, current and speed are noted using voltmeter,

ammeter and tachometer. For load spring balance reading S1 & S2 are noted.

5. Now the load on the brake drum is increased, gradually and the corresponding

voltmeter, ammeter readings and speed, spring balance readings are noted down and

tabulated.

6. Then the load is gradually decreased and field rheostat is brought to the minimum

position and the supply is switched off.

7. Then torque, input power, output power and percentage efficiency are calculated by

using the formulae and tabulated.

Model Calculation:

Circumference of the brake drum = cms

Radius, r = mt.

Torque applied on the shaft of the motor, T = (F1 ~ F2) r 9.81 Nm

Output power, Po = WattsNT

60

2π

Input Power, Pi = V X I Watts

% Efficiency, η = 100×i

o

P

P

35

Observation:

Radius of brake drum, r = __________ mts.

Spring Balance Sl.

No. V I

F1 Kg F2 Kg Speed N Torque T

Output Power

P0

Input Power

Pi Efficiency η

37

GRAPH:

The performance characteristic curves are drawn as

i. Output power Vs speed

ii- Output power Vs current

iii- Output power Vs Torque and

iv, Output power Vs Efficiency.

RESULT:

QUESTIONS:

1. Explain the principle of operation of a DC motor

2. How does the back emf in a d.c. motor make the motor self regulating?

3. How can we reverse the direction of a d.c. shunt motor?

4. Justify the statement - “D.C. shunt motor will run almost at constant speed”

5. What is the need for starters for starting a d.c. machine?

6. What are the various D.C. starters?

7. What are the different types of d.c. motors?

8. What is the expression for torque developed ?

9. Write the condition for maximum power developed by d.c. motor.

10. Write the voltage equation of a d.c. shunt motor.

11. What is the relationship between back emf, speed of rotation and flux per pole?

REFERENCES:



Machines.”


38

Circuit diagram:

Name plate details:

.

Fuse rating calculation:

Model graphs:

Armature control Field control

D.C. Shunt Motor

Capacity: Speed:

Voltage:


Va

N

If1(rated)

If2<If1

If3<If2

Va1(rated)

Va2<Va1

Va3<Va2

N

If

220V

D.C

+

−

D

P

S

T

S

Af

V M

A

AA Z

ZZ

+

+

−

−

39

Ex.No:

Date :

5.Speed Control of D.C.Shunt Motor (P.359 to 376 of text book)

AIM: To determine the speed control characteristics of a dc shunt motor by

i. Armature control method and

ii. Field Control method.

APPARATUS REQUIRED:


1

2

3

4

5

THEORY: (page no 359 to 376 of the text book)

We know that back emf is given by the following equation.

Eb = V – Ia Ra = ΦZNP / A

N = ( V – Ia Ra / ΦZ ) * A/P rps.

N = K ( V – Ia Ra / Φ ) rps

Simply,

The speed of D.C. shunt motor = N α V/ Φ

Where V = applied voltage (or armature voltage)

Φ = flux. ( field current)

So the speed is directly proportional to armature voltage and inversely proportional to field

current.

it is clear that the variation of speed is Possible in three different ways:

(i) VARIATION OF FIELD (FLUX) CONTROL METHOD

A more economical method of speed control by rheostat adjustment of the field

current. If the field flux is reduced, the speed increases. Using this method, speed can be

increased above rated speed. In non- inter poles, it is in the ratio 2:1 and

in inter poles maximum to minimum speed of 6:1 is fairly common. But it is not possible to

reduce the speed below the rated speed.

The disadvantage of this method are i) Output torque gets reduced &

ii) Higher speed results in poor commutation.

(ii) .ARMA TURE CONTROL METHOD:

In this method, armature current is controlled by means of a rheostat in the armature

circuit. For a given armature current the larger the resistance of the controller, the smaller will

be the voltage across the armature and hence the speed will lower. This method is used when

Speed below the no-load speed is required.

The main disadvantages of this type of speed control are

1. The losses are more and hence it is relatively more costly.

2. The efficiency is low.

3. The speed cannot be increased.

40

Observation:

Armature control:

If1 = ------- A If2 = -------- A If3 = --------- A Sl.No.

Va Speed Va Speed Va Speed

Field control:

Va1 = ------- A Va2 = -------- A Va3 = --------- A Sl.No.

If Speed If Speed If Speed

41

PROCEDURE:

ARMATURE CONTROL METHOD:

1. Connections are given as shown in the circuit diagram and initially the field rheostat R1

is kept in minimum position and armature rheostat R2 is kept in maximum position.

2. Supply is given and the motor is started. The armature resistance is cutdown and speed

is measured.

3. If the motor runs below the rated speed, adjust the field rheostat and motor is brought

to its rated speed, The field current, 'If' is kept constant.

4. Now the armature voltage (ie potential drop across the armature) is varied by varying

the armature rheostat and corresponding voltmeter, ammeter (la) readings and speed are

noted.

5. The same procedure is repeated for another value of constant field current 'If'

FIELD CONTROL METHOD:

1. The motor is run at the rated speed as usual. Armature voltage and hence armature

current is kept constant.

2. The field current is varied and the value of the field current 'If' and speed are noted in

the tabular column.

3. The same procedure is repeated for another constant value of armature current.

GRAPH: ARMATURE CONTROL METHOD:

Graph is drawn between speed and armature voltage taking armature voltage along x axis

and speed along y axis.

FIELD CONTROL METHOD:

Graph is drawn between speed and field current, taking speed along y axis and field current

along x axis.

42

RESULT:

Questions:

1. Which speed control will give the speed greater than the rated speed and which one will

give less than the rated speed? State also the reason.

2. What are the factors that decide the speed of a dc machine?

3. What are the various methods of speed control in dc series motor?

4. Write few merits and demerits of the rheostatic control.

5. What will happen if the field winding of a running dc motor is opened?

REFERENCES:



Machines.”


******

43

Ex.No:

Date:

6. Load Test on D.C Series Motor

(P.347 to 350 of text book) AIM: To conduct the load test on a dc series motor and to draw the performance

characteristic curves.

APPARATUS REQUIRED:


1 Ammeter 0-20A MC 1

2 Voltmeter 0-300V MC 1

THEORY: In a DC motor Speed is inversely proportionally to the field current. But in a DC series

motor field current is equal to load current.

Normally no-load current of any motor is very low compared to full load current. So

when a DC series motor, operated at no-load will have enormous speed.

The dc series motor should always be started with some load. Otherwise the speed will be

enormous and fuse will blowout.

The speed of DC series motor is very large at low loads and decrease at heavy loads.So

DC series motor is not suitable for the applications where constant speed is required. Where

the motor has to be started with load, DC series motor is most suitable. So it is used in electric

traction.

FORMULAE:

Torque (T) = (S1-S2) x R x 9.81 (Nm), .

S1, S2 -Spring balance readings (kg). R -Radius of the brake drum (m)

Input power, Pi = VI watts

Output power, Po = (2ΠNT)/60 watts.

Percentage efficiency =Output power / Input power x 100

Range fixing:

The current drawn by the motor is 120% of full load current.

Current drawn by the motor IL = _______ A


The rated supply voltage is ______ V

∴ The range of voltmeter V is (0- ) V

Fuse Rating:

120% of rated current

∴ The fuse rating is ____ A

44

Circuit Diagram:

Name Plate Details:

Fuse rating Calculations:

Model Graph:

D.C. Series Motor

Capacity: Speed:

Voltage:


+

220 V

D.C.

2 Point Starter

D

P

S

T

S

−

V

+

−

A L A + −

M

A

AA

G

F1 F2

Brake drum

X XX

%η, T, IL , N

Po

N

T.

η

IL T, N

IL

T

45

PROCEDURE: 1. The circuit connections are given as shown in the circuit diagram. Ensure that some

load is applied to the brake drum. The dc series motor should always be started with

some load. Otherwise the speed will be enormous and fuse will blowout.

2. Supply is given and the motor is started, with the help of a two point starter. At that

initial load condition, voltmeter, ammeter ad spring balance readings are noted. The

speed is also measured using tachometer.

3. Now the load is increased and for various load conditions, the ammeter, voltmeter,

spring balance readings and speed are noted.

4. The torque, input power, output power and percentage efficiency are calculated using

the formulae and entered in the tabulation.

Modal calculation:

Circumference of the brake drum = _____ cms

Radius of the brake drum, r = _____ m

Torque applied on the shaft of the rotor, T = (F1 ~ F2) r × 9.81 Nm [

Output power, Po = WattsNT

60

2π

Input power Pi = V × IL

% Efficiency, η = 100×i

o

P

P

46

Observation:

Radius of brake drum, r = __________ mts.

Spring Balance Sl.

No. V I

F1 Kg F2 Kg Speed N Torque T

Output Power

P0

Input Power

Pi Efficiency η

47

GRAPH: The performance characteristic curves are drawn as

i. Output power Vs speed

ii- Output power Vs current

iii- Output power Vs Torque and

iv, Output power Vs Efficiency.

RESULT:

QUESTIONS:

1. Whether D.C series is motor is started on no-load? Justify

2. Why torque of a D.C series motor is high?

3. Why D.C series motor is called a variable speed drive?

4. What are the applications of D.C series motor?

5. Mention the methods to control the speed of the D.C series motor.

6. What type of starter is used for series motor? And Why?

REFERENCES:



Machines.”


48

INDEX

Name :………………………………Reg. No. …………………………….

Semester:………………………….. Sec :………………….

Ex.

No

Date of

Expt

Title of Experiment

Page

No Marks Signature

49

Ex.

No

Date of

Expt

Title of Experiment

Page

No Marks Signature

Documents

Ee2259 - Electrical Machines