EE-241 Electrical Machines I _2012

8/13/2019 EE-241 Electrical Machines I _2012

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PRACTICAL WORK BOOKFor Academic Session 2012

Electrical Machines-I (EE-241)For

S.E (EE)

Name:

Roll Number:

Class:

Batch: Semester/Term:

Department :

Department of Electrical EngineeringNED University of Engineering & Technology


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Electr ical Machines-I Contents

NED University of Engineering and Technology Department of Electrical Engineering

Revised 2012

CONTENTS

Lab.No.

Da te d List of Experiments Pa ge

No . Re ma rk s

01Energization of Benches installed in

electrical machines lab, through mainpanel.

01

02 Reading and explanation of the name plate

data of DC & AC rotating machines 03

03To draw the magnetization curve of self

exited DC shunt generator (open circuitcharacteristics curve O.C.C).

0 6

04 To draw the load characteristic curve of self

excited D.C shunt generator.09

05To draw the external and internal

characteristics of separately excited DCgenerator.

12

06 Speed control of a DC shunt motor by flux

variation method. 14

07 Speed control of a D.C. Shunt Motor by

armature or rheostatic control method.16

08To study rotors of electric machines

18

09 To study parallel operation of two dc

generators and shift of load on one another. 21

10 To find out the Cu losses of a single phasetransformer by short circuit test.

23


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Electr ical Machines-I Contents


Revised 2012

11 To find out the Core losses of a single

phase

transformer by

open circuit

test.

25

12To observe the effect of increasing load on

DC shunt motors speed, armature current,and field current.

27

13 To observe the starting of synchronous

motor29


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Electr ical Machines-I Lab Session 01


1

LAB SESSION 01

OBJECT

Energization of Benches installed in electrical machines lab, through main panel.

APPARATUS

1. Bench2. Main Panel

THEORY

Every motor-generator set bench located here in electrical machines laboratory is not

directly connected to KESC supply. Instead of connecting bench with KESC supply, we have

main panel which is connected with KESC supply and benches are connected with main panel.

Every bench has DC supply, 3-supply, 1-supply and supply given to bench for services.

Services supply is used for energization of any equipment through that specific bench for

instance if we want to connect tachometer to abench, it is energized through service supply etc.

Main panel has three major portions, namely

1. Main supply-services-fix DC lines

2. Main supply-single phase AC fix lines-three phase AC fix lines

3. Main supply-interconnections

1. Main supply-services-fix DC lines

This is first portion and it includes a knife switch, three ammeters connected through CTs

measuring phase current and a voltmeter connected through PT measuring Line voltage. Upon

energization of this portion fans installed inside every bench start running and services switches

of every bench are also energized. Except this one line diagram of fix DC supply is also on this

portion.


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2

2. Main supply-single phase AC fix lines-three phase AC fix lines

As the name suggest, one line diagram of single and three phase supply is located on thisportion, mean you can connect three phase and single phase supply to any bench through this

portion.

3. Main supply-interconnections

The function of this portion is to interconnect different benches. If we are generating three

phase AC supply or DC supply with the help of any motor-generator set and intend to give our

generated supply to any DC or three phase AC motor located on any other bench, in this situation

we can connect both benches through this portion.

PROCEDURE

1. Select any bench, to which you intend to energize2. With the help of single line diagram drawn on main panel, connect dc, single phase ac

and three phase ac supply

RESULT


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El ectrical M achin es-I

NED University of Engineering and

OBJECT

Reading and expl

APPARATUS

1. DC Motor2. DC Generator

3. 3- Induction Motor4. 3-Synchronous Mot

5. 3-Synchronous Gen

CIRCUIT DIAGRAM

THEORY

Name plate, is a shee

Rated parameters are the para

of immense importance to kn

operation. In addition to this

designing any controlling cir

ambient temperature, number

field current and voltage (in c

Technology Departme

3

LAB SESSION 02

anation of the name plate data of DC & AC r

or

erator

t fixed on every electrical machine, shows

eters on which machine perform at best effi

w about the rated parameters of ny machi

these parameters are also necessary for th

uitry for that machine. Name plate data inc

of poles, operating frequency, enclosure ty

se of doubly excited machines/generator) etc

L ab Session 02

t of Electrical Engineering

tating machines.

the rated parameters.

ciency. Therefore it is

es before putting it in

further analysis like

udes voltage, current,

e, cooling employed,

.


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4

PROCEDURE

Check out name plate data of machines given below, installed at different benches.

1. NAME PLATE DATA OF DC MOTOR:

2. NAME PLATE DATA OF DC GENERATOR:

3. NAME PLATE DATA OF 3-INDUCTION MOTOR:


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5

4. NAME PLATE DATA OF 3-SYNCHRONOUS MOTOR:

5. NAME PLATE DATA OF 3-SYNCHRONOUS GENERATOR:


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Electr ical Machines-I LAB SESSION 03


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LAB SESSION 03

OBJECTTo draw the magnetization curve of self exited DC shunt generator (open circuit

characteristics curve O.C.C).

APPARATUS

1. Bench 10-ES/EV or Bench 14-ES/EV

2. DC multi-range ammeter

3. DC multi-range voltmeter

CIRCUIT DIAGRAM

THEORY

The magnetization characteristics also known as No load or Open circuitcharacteristics is the relation between emf generated and field current at a given speed.

Due to residual magnetism in the poles, some emf is generated even when filed current is zero.Hence the curve starts a little way up. It is seen that the first part of the curve is practicallystraight. This is due the fact that at low flux densities reluctance of iron path is being negligible,

total reluctance is given by air gap reluctance which is constant. Hence the flux and consequentlythe generated emf is directly proportional to exciting current. However at high flux densities iron


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path reluctance is being appreciable and straight relation between emf and field current no longer

holds good. In other words saturation of poles starts.

PROCEDURE

1. Connect the shunt field to armature terminal through the ammeter, switch and rheostat.

2. Connect the multi-range voltmeter across the terminals of armature.3. Press yellow switch on and increase AC voltage of induction motor (prime mover) by

the help of 3-phase autotransformer until it reaches at normal speed.4. Note the reading of voltmeter which indicates the voltage due to residual magnetism.5. Close field switch and excite the field at low current.

6. Increase the field current in steps and note the voltage each time.7. Take at least 11-12 readings.

8. Tabulate the reading and draw the curve between armature induced e.m.f and excitingcurrent

OBSERVATIONS

S.No. FIELD CURRENTIF (A)

TERMINAL VOLTAGEVT (volts)

1

2

3

4

5

6

7

8

9

10

11

12


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RESULT

1. The curve starts somewhat above the origin. The voltage at zero excitation is due to

residual magnetism of the field, which is necessary for building up the voltage of self-excitation generator.

2. The voltage increases rapidly at first and then changes a little in value at higher excitationsindicating the effect of the poles saturation.


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LAB SESSION 04

OBJECT

To draw the load characteristic curve of self excited D.C shunt generator.

APPARATUS


2. DC multi-range ammeter3. DC multi-range voltmeter

CIRCUIT DIAGRAM


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THEORY

Load characteristic curve is the graphical representation which shows change in terminal

voltage with respect to change in load. After building up of voltage, if a shunt generator is loadedthen terminal voltage drops with increase in load current. There are three main reasons for the

drop of terminal voltage for a shunt generator under load.

i) Armature Reaction

Armature reaction is the effect of magnetic field set up by the armature current on the

distribution of flux under main poles of a generator. Due to demagnetizing effect of armaturereaction , pole flux is weakened and so induced e.m.f in the armature is decresed.

ii)Armature Resistance

As the load current increases, more voltage is consumed in ohmic resistance of armature

circuit. Hence the terminal voltage (Vt=EIaRa) is decreased where E is the e.m.f induced inarmature under load condition.

iii)Drop In Terminal Voltage

The drop in terminal voltage (Vt) due to armature resistance and armature reaction resultsin decreased field current, which further reduces e.m.f induced.

For a shunt generator

Ia = IL+ IfE = Vt+ IaRa

PROCEDURE

1. Make the connections as shown in circuit diagram.

2. Press yellow switch on and increase AC voltage of induction motor (prime mover) bythe help of 3-phase autotransformer until it reaches at normal speed.

3. When motor reaches rated speed, close the shunt field switch.

4. Increase field current by changing the field resistance until the terminal voltage reaches to220 volt.

5. Close the switch of load and vary the load current by means of load rheostat.6. Note down the meter readings from all meters carefully.


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OBSERVATIONS

S.No If(A) IL(A) VT(V) Ia=If+ILVd=IaRa

Ra=0. 5 ohm

1

2

3

4

5

6

7

8

RESULT

The terminal voltage of a D.C. generator is maximum at no load, which decreases withincreasing load.


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LAB SESSION 05

OBJECT

To draw the external and internal characteristics of separately excited DC generator.

APPARATUS


2. DC multi-range ammeter3. DC multi-range voltmeter

CIRCUIT DIAGRAM

THEORY

The load or external characteristic of a generator is the relation between the terminalvoltage and load current. The characteristic expressed the manner in which the voltage across theload varies with I, the value of load current. The internal or total characteristic of a generator is the

relation between the e.m.f actually induced in the generator Eaand the armature current Ia.Theinternal characteristic of the generator, which is separately excited, can be obtained as below:

Let:

Vt = Terminal voltage, Ia= Armature current, Ra= Armature resistanceThen,

Ea= Vt+ IaRa


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Ia = ILTherefore if we add drop of armature (IaRa) to terminal voltage Vtwe get actually induced e.m.f

(Ea).

PROCEDURE

1. Make the circuit as shown in circuit diagram.2. Press yellow switch on and increase AC voltage of induction motor (prime mover)

by the help of 3-phase autotransformer until it reaches at normal speed.3. When motor reaches rated speed, close the shunt field switch.4. Increase field current by changing the field resistance until the terminal voltage reaches

to 220 volt.5. Close the switch of load and vary the load current by means of load rheostat.

6. Note down the meter readings from all meters carefully.

OBSERVATIONS

S.No IL(A) If(A) VT(V) Ea= Vt+

I R (V)

1

2

3

4

5

6

7

8

RESULT

From the graph it is observed that the terminal voltage across generator decreases as theload increases.

Armature Resistance = 0.5 Ohms


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LAB SESSION 06

OBJECT

Speed control of a DC shunt motor by flux variation method.

APPARATUS


2. DC multi-range ammeter3. DC multi range voltmeters

4. Digital tachometer

CIRCUIT DIAGRAM

THEORY

This method is used to increase speed of DC motor above base speed.To understandwhat happens when the field resistance of dc motor is changed, assume that the field resistance is

increased then the following sequence of cause and effect will take place

1. Increasing Rfcauses Ifto decrease

2. Decreasing IfDecreases

3. Decreasing lowers Ea

4.Decreasing Ea Increases Ia5. Increasing Ia increases T ind6. Increasing Tindmakes Tind> Tload, hence speed increases.


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7. Increasing speed increases Ea

8. Increasing Ea decreases Ia9. Decreasing Ia decrease Tind until Tind = Tloadat higher speed.

Naturally decreasing Rfwould reverse the whole process and speed of motor will decrease.

It is important to bear in mind, changing field resistance does not effect torque induced ,at the endits magnitude remains same but at higher or lower speed depending upon change in resistance.

PROCEDURE

1. Make connections as shown in the circuit.2. Keep the motor starting rheostat at its maximum position and field rheostat at its minimum

position while starting motor.

3. Start the motor by pressing yellow switch "ON" without load.4. Adjust the motor start rheostat to its minimum value.5. Decrease field current by the help of field rheostat step by step and take readings of field

current and speed from digital tachometer at every step. Adjust the field rheostat to givemaximum speed at which it is safe to operate the motor.

OBSERVATIONS

S. NoField Current Speed

If(A) N (RPM)

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

RESULT

Speed increases as the field excitation decreases.


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LAB SESSION 07

OBJECT

Speed control of a D.C. Shunt Motor by armature or rheostatic control

method.

APPARATUS

1. Bench 13-ES/EV or Bench 15-ES/EV2. DC multi-range ammeter

3. Voltmeters

4. Digital tachometer

CIRCUIT DIAGRAM

THEORY

This method is used to decrease speed of DC motor below base speed. To understand whathappens when the armature resistance of DC motor is changed, assume that the armature

resistance is increased then the following sequence of cause and effect will take place1. Increasing Racauses Ia to decrease


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2. Decreasing Ia deccreases Tind

3. Decreasing Tindmakes Tind< Tload, hence speed decreases.4. Decreasing speed decreases Ea

5. Decreasing Ea increases Ia again.6. Increasing Ia increases T induntil Tind= Tloadat lower speed.

Naturally decreasing Ra would reverse the whole process and speed of motor will increase.It is important to bear in mind, changing armature resistance does not effect torque induced ,at the

end its magnitude remains same but at higher or lower speed depending upon change in resistance.

PROCEDURE

1. Make connections as shown in the circuit.

2. Keep the motor starting rheostat at its maximum position and field rheostat at its minimumposition while starting motor.

3. Start the motor by pressing yellow switch "ON" without load.

4. Adjust the motor start rheostat to its minimum value.5. Increase the value of starting resistance by the help of motor start rheostat step by step and

take readings of voltage across armature and speed from digital tachometer at every step.

OBSERVATIONS

S. No

Armature Voltage Speed

Va(V) N (RPM)

1.

2.

3.

4.

5.

6.

7.

8.

RESULT

Speed is very nearly proportional to the applied voltage in the case of armature controlmethod.


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LAB SESSION 08

OBJECT

To study rotors of electric machines

APPARATUS

1. Rotor of DC Machine

2. Rotors of Asynchronous AC Machine

CIRCUI T DIAGRAM


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THEORY

One of the classifications of parts of electric motors is according to the state of

component i.e. stationary or rotating. If it is rotating, it is called rotor otherwise it is known asstator. Construction of rotor in DC motor is different from rotor used in 3-phase induction motor.

Therefore here we will discuss them individually.

ROTOR OF DC MOTOR

It is also known as armature, it consists of armaturecore, armature winding and

commutator.

(i) Armature core:It is cylindrical in shape and made up of silicon steel sheets. It carriesarmature winding, causes them to rotate and hence cut the magnetic flux produced by

field winding- located in stator core. It is important to bear in mind, function ofarmature core is to provide very low reluctance path to the flux through the armaturefrom N-pole to S-pole.

(ii) Armature winding:There are two types of armature winding(i) Lap winding (ii) Wave winding

Lap winding is used in high current and low voltage machines whereas use of wave winding is for low current and high voltage machines.

(iii) Commutator:Function of commutator in DC motor is to produce unidirectional

torque. In DC motors we are providing DC supply through commutator. It is ofcylindrical structure and built up of segments of hard drawn copper. These segmentsare insulated from each other by layer of mica. The number of segments is equal to the

number of armature coil.

Rotors of 3- Phase Induction Motors:

Process of energy conversion in induction motor took place by induction principalhence rotor is not electrically connected either with stator or with external supply. There are two

types of rotors used in induction motor, discussed below:

Squirrel Cage Rotor:

Almost 90 percent of induction motors are squirrel-cage type, because this type of rotorhas the simplest and most rugged construction imaginable and is almost indestructible. Therotor consists of a cylindrical laminated core with parallel slots for carrying the rotor

conductors which, it should be noted clearly, are not wires but consists of heavy bars ofcopper, aluminum or alloys. One bar is placed in each slot; rather the bars are inserted from the

end when semi-closed slots are used. The rotor bars are brazed or electrically welded or boltedto two heavy and stout short-circuiting end-rings, thus giving us, what is so picturesquelycalled, a squirrel-case construction.


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It should be noted that the rotor bars are permanently short-circuited by themselves; hence it is

not possible to add any external resistance in series with the rotor circuit for starting purposes.The rotor slots are usually not quite parallel to the shaft but are purposely given a slight skew.

This is useful in two ways:(1) It helps to make the motor run quietly by reducing the magnetic hum and

(2) It helps in reducing the locking tendency of the rotor i.e. the tendency of the rotor teeth to

remain under the stator teeth due to direct magnetic attraction between the two.

Phase-wound Rotor

This type of rotor is provided with 3-phase,double-layer, distributed windingconsisting of coils as used in alternator. The rotor is wound for as many poles as the number o

stator poles and is always wound 3-phase even when the stator is wound two-phase `.The three phases are starred internally; the other winding terminals are brought out and

connected to three insulated slip-rings mounted on the shaft with brushes resting on them.These three brushes are further externally connected to a three-phase star-connected rheostat,

this makes possible the introduction of additional resistance in the rotor circuit during thestarting period for increasing the starting torque of the motor and for changing its speed-torque/current characteristics. When running under normal conditions, the slip-rings are

automatically short-circuited by means of a metal collar, which is pushed along the shaft andconnects all the rings together. Next, the brushes are automatically lifted from the slip-rings to

reduce the frictional losses and the wear and tear. Hence, it is seen that under normal runningcondition, the wound rotor is short-circuited on itself just like the squirrel-case rotor.


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Electrical Machines-I LAB SESSION 09


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LAB SESSION 09

OBJECTTo study parallel operation of two dc generators and shift of load on one another.

APPARATUS

1. Two Voltmeters (0 600V)

2. Two Ammeters (0 15A)

THEORY:

This arrangement of operation in parallel can be made to meet the load demand easily andwork them near to their maximum efficiency. It also helps to prevent the complete shut down in

case any generator fails.

1. If two or more generators run at full load then it is more economical and also improvesefficiency.

2. Periodical over halving and general repairs can be carried out without shut down to totalsupply; only one generator can be shut down.

3. If load on the power station increases, additional generators can be added to the already

working generators.

For proper synchronizing of generators the following condition must be achieved.

1. The terminal voltage of incoming generator must be the same as that of the runninggenerator.

2. Polarity of the incoming generator should be the same as line polarity.

PROCEDURE

1.Connect the generators and meters as shown in the diagram and check connections.

2. Run first generator at its normal speed and connect to bus bar.3. Adjust the voltage at 220V with the help of field excitation.4. Now put he load on first generator and increase the load slowly.5. Run second generator at its normal speed and regulate the voltage till it equals the bus bar

voltage.6. Check the polarity of second generator and connect it to the bus bar.

7. Note readings of ammeters of both generators and that of load ammeter.8. Shift the load of first on second by weakening the field strength of first generator but at the

same time increasing field strength of second generator.9. When all the load of first generator has shifted to second generator, disconnect first from

bus bar .Note readings of all ammeters as before.


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OBSERVATIONS For first generator

S.No IL(A) If(A) VT(V)

1

2

3

4

5

6

7

8

For second generator

S.No IL(A) If(A) VT(V)

1

2

3

4

5

6

7

8


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LAB SESSION 10

OBJECT

To find out the iron core losses of singlephase transformer (open circuit test).

APPRATUS

1. Voltmeter (0 300V )2. Ammeter ( 0 2A )

3. Wattmeter ( 0 120 W )

CIRCUIT DIAGRAM

THEORY

The purpose of this test is to determine no load loss or core loss and no load current Io

which is helpful in finding Xo and Ro.One winding of the transformer which ever is convenient but usually high voltage winding

is left open and the other is connected to its supply of normal volt and frequency. A wattmeter,voltmeter and ammeter are connected in low voltage winding i.e. Primary winding in the presentcase.Normal voltage is applied toprimary normal flux willbe set up in the core hence normal iron

loss will occur which are recordedby the wattmeter. As theprimary no load Io is small usually


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Electri cal Machines-I LAB SESSION 10


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2-10% of rated load current Cu losses is negligible small in primary I will in secondary b/c it is

open. Therefore the wattmeter reading will show practically the core loss under no load condition.

OBSERVATIONS

S.No W (watts) V (Volts) I (Ampere)

RESULT

The iron losses of single phase transformer are ___________ watt.


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LAB SESSION 11

OBJECT

To find out the Cu losses of a single phase transformer by short circuit test.

APPARATUS

1. Voltmeter (0-15V)

2. Wattmeter (0-750)3. Ammeter (0-15A)

THEORY

In this test one winding (usually low voltage winding) is short circuited by a thick conductor orby means of ammeter (Which may serve an additional purpose of indicating rated load). A low

voltage (5-10% of the normal voltage) at normal frequency is applied to the primary andgradually increased, till full load current is flowing in both primary and secondary.

Since in this test the applied voltage is a small percentage of the normal voltage themutual flux produced is also a small percentage of its normal value. Hence core losses are verysmall with the result that the wattmeter reading represents the full load copper loss.

PROCEDURE

1. Make connections according to the given circuit.

2. Connect primary of transformer with variable ac voltage supply.3. Note down transformer rated current from name plate data and keep on increasing

voltage until you get rated current read by Ammeter connected.


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4. Once you get rated current at any specific voltage level, note down reading ofinstruments connected and calculate different parameters.

OBSERVATION

S.No W (watts) Vsc (Volts) I (Ampere)

CALCULATIONS

RESULT The copper losses of single phase transformer are__________ Watts


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LAB SESSION 12

OBJECT

To observe the effect of increasing load on DC shunt motors speed, armature current, and field

current.

APPARATUS

1. Bench 13-ES/EV2. DC multi-range ammeter

3. DC multi-range voltmeter

4. Tachometer

CIRCUIT DIAGRAM

THEORY

Here AC generator is used as load on dc shunt motor. As we know generator has counter torque

which opposes input power given by dc shunt motor and counter torque is dependant loadcurrent. Hence on increasing load on generator, it will develop more counter torque, thus more

load will be reflected on dc shunt motor.

N = (Speed Equation of DC Motors)

Ia = (Armature current equation)


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Electri cal Machines-I LAB SESSION 12


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If= (Field current equation)

In shunt dc motor field is connected in parallel with armature. From its speed equation it is vivid,on increasing load speed will drop due to increased armature drop.

Decrease in speed will decrease back e.m.f. and consequently armature current will increase. Asfor as field current is concern, it will remain constant until and unless terminal voltage remains

constant.

PROCEDURE

11. Make the circuit as shown in figure.

12. Keep the motor starting rheostat at its maximum position and field rheostat at its minimumposition while starting motor.

13. Start the motor by pressing yellow switch "ON" without load.14. Adjust the motor start rheostat to its minimum value

15. Note down the readings of instrument connected.16. Now connect electrical load on generator and start increasing load in steps.

17. After every step, note down readings of instrument connected.18. Draw curves between armature current and load current and between speed and load

current

OBSERVATIONS

RESULT

1. Speed decreases and armature current increase with increase in load but field

current remains constant.

S.No ILoad IArmature IField N rpm I1 I2 I3


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-29-

LAB SESSION 13

OBJECTTo observe the starting of synchronous motor

APPARATUS

1. 3- Synchronous motor

2. Variable 3- AC supply

3. DC Supply

4. Techometer

CIRCUIT DIAGRAM

THEORY

To understand how the synchronous motor works, assume that the application of three-

phase ac power to the stator causes a rotating magnetic field to be set up around the rotor. The

rotor is energized with dc (it acts like a bar magnet). The strong rotating magnetic field attracts

the strong rotor field activated by the dc. This results in a strong turning force on the rotor shaft.

The rotor is therefore able to turn a load as it rotates in step with the rotating magnetic field. It

works this way once its started.

However, one of the disadvantages of a synchronous motor is that it cannot be started from

a standstill by applying three-phase ac power to the stator and dc to its rotor. When ac is applied

to the stator, a high-speed rotating magnetic field appears immediately. This rotating field rushes

past the rotor poles so quickly that the rotor does not have a chance to get started. In effect, the

rotor is repelled first in one direction and then the other. A synchronous motor in its purest form

has no starting torque. It has torque only when it is running at synchronous speed. A squirrel-

cage type of winding is added to the rotor of a synchronous motor to cause it to start. The


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squirrel cage is shown as the outer part of the rotor in figure. It is so named because it is shaped

and looks something like a turn able squirrel cage. Simply, the windings are heavy copper bars

shorted.Hence, three phase synchronous motor is not self started. At the starting time, it behaves

as induction motor and gets accelerated. Once it approaches speed near to synchronous speed, its

rotor winding is excited then synchronous motor start rotating at synchronous speed. If we have

given rotor supply at start, motor will just produce humming sound.

PROCEDURE

19. Make the circuit and switch on both ac and dc supply and observe the performance.

20. Disconnect dc supply, switch on ac supply and observe the performance.21. When motor run near to synchronous speed, which already calculated, switch on dc

supply also and observe the behavior.

OBSERVATIONS:

Starting of Synchronous Motor

Applied Voltage toInduction

Motor StatorCurrent

R.P.M

Synchronous Motor

AppliedVoltage

StatorCurrent

FieldCurrent

D.CVolts

R.P.M

EE-241 Electrical Machines I _2012

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Transcript of EE-241 Electrical Machines I _2012