DC Machines Lab Manual

8/21/2019 DC Machines Lab Manual

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NIE Institute of Technology, Mysore - 18 DC Machines & Synchronous Machines Lab (06EEL67)

Department of Electrical and Electronics Engineering 1

NIE Institute of TechnologyDepartment of Electrical and Electronics Engineering

List of experiments for DC machines and Synchronous machines laboratorySubject code:06EEL67 IA marks: 25

Exam Hours:03 Exam marks: 50

I cycle Experiments

1 Speed control of a DC motor by armature voltage control and flux control.

2 Load Characteristics of a DC Shunt and Compound Generator

3 Hopkinson's test..

4 Field test on a DC Series motor..

5 Swinburne's Test

6 Ward Leonard method of speed Control of a DC motor.

II cycle Experiments

7 Load test of a DC motor-determination of speed-torque and HP-efficiency characteristics.

8 Retardation testelectrical braking method.9 Slip test.

10 V' and inverted 'V' curves of a synchronous motor.

11 Voltage regulation of an alternator by EMF and MMF method.

12 Voltage regulation of an alternator by ZPF method.

13 Performance of synchronous generator connected to infinite bus, under constant power and

variable excitation & viceversa.

Lab Incharge

Teaching Staff: 1. Smt. Ushasurendra2. Sri. B.S. Srikanthan

3. Mr. Sandeep kumar .K.J

4. Mr. Mohan .N

Technical Staff: 1. Sri. Arun Kumar .L.S2. Mr. C. Suresha.3. Mr. Arun .M

HOD E&EE


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Contents

Sl

No Name of the experiment Page no

1 Speed control of a DC motor by armature voltage control and flux control. 3 - 6

2 Load Characteristics of a DC Shunt and Compound Generator. 9 - 16

3 Hopkinson's test. 19 - 23

4 Field test on a DC Series motor. 25 - 27

5 Swinburne's Test 29 - 32

6 Ward Leonard method of speed Control of a DC motor. 35 - 36

7 Load test of a DC motor-determination of speed-torque and HP-efficiency

characteristics.

39 - 41

8 Retardation testelectrical braking method. 43 - 45

9 Slip test. 47 - 50

10 V' and inverted 'V' curves of a synchronous motor. 53 - 55

11Voltage regulation of an alternator by EMF and MMF method. 57 - 61

12 Voltage regulation of an alternator by ZPF method. 63 - 66

13 Performance of synchronous generator connected to infinite bus, under

constant power and variable excitation & viceversa.67 - 69

Viva questions. 71 - 73


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Experiment No.01

Speed control of a DC motor by armature voltage control

and flux control

Aim:To control the speed of a DC motor by1) armature voltage control method

2) flux control method

CIRCUIT DIAGRAM

Name Plate details of the machines:

Motor:

Apparatus Required:

Circuit

Ref.Description Rating Quantity

A1 Moving coil ammeter 0-2.5A-5A 2

V1 Moving coil voltmeter 0-250V 1

Rfm Rheostat 200,1.7A 2Ram Rheostat 50,5A 1


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THEORY: The working principal of a Dc motor can be stated as when a current carryingconductor is placed in a magnetic field it experiences a force. In the practical DC motor, the

permanent magnet is obtained by a field winding which produces the required flux is called the

main flux and all the armature conductor mounted on the periphery of the armature drum, get

subjected to the mechanical force. Due to this overall armature experiences a twisting force

called torque and armature of the motor starts rotating. As the armature starts rotating it cuts the

main flux and hence an emf gets induced in the conductors the direction of which is against the

supply voltage and hence an emf gets induced in the conductors, the direction of which is

against the supply voltage and hence it is termed as back emf Eb. (Eb = )

Therefore, the supply voltage has to overcome this back emf to keep the conductor rotating.

The speed of the motor automatically adjusts its self to the load so that the electrical power

required to drive the current through the armature is equal to the mechanical power required to

drive the load. The back emf is always less than the supply voltage V.

Therefore Eb= V - IaRa ------------ voltage equation

N 1/ -------------------- Speed equation (voltage kept constant)

From the above equations it can be seen that the speed is inversely proportionally to the flux per

pole.

Thus, the speed of the DC motor can be increased or decreased by varying the flux.

Armature control method: (rheostatic control) in this method an adjustable resistance R isplaced in series with the armature resistance Ramaking the total resistance in the armature equal

to (R + Ra), then the back emf for any armature current Iais given by Eb = V- (R + Ra) Iait can

be seen that maximum drop and the voltage which actually gets impressed is minimum and

hence the speed is minimum. Similarly when the resistance is fully cut out, then speed is

maximum. The draw backs of this method are 1) for a given value of the resistance the speed is

not a constant but a function of the load current. The value of the resistance has to be changed for

a rapidly changed load if the speed is to be kept constant. 2) there is considerable wastage of

power and the power wasted is proportional to the reduction of speed (current drawn will be

more at lesser speed) 3) only speeds below the rated speed can be obtained.

Field control method: (flux control)it is seen that when the flux is varied speed will also vary.

The resistance of variable rheostat Rfis fully cut out, the speed is minimum (equal to the rated

speed ) the field is current maximum and maximum flux is produced. This is accomplished by

means of shunt regulator in the case of a DC shunt motor and a diverter in case of a series motor

as shown in figures.

This method of speed control is both convenient and economical, but obviously it gives speed

greater than the normal. By a combination of both rheostatic and field control methods speeds

below or above normal can be obtained.


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E Volts

R

RI E - RI

Rheostatic Control

R

SHUNT SERIES

FIELD CONTROL

METHODS OF SPEED CONTROL

Procedure

Flux control method:

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

2. Keep Ramin cut in position and Rfm in cut out position.

3. Close the supply switch starts the DC motor.

4. Voltage across the armature voltage kept in constant value say (160-180V).

5. The field current is decreased in steps by cutting in Rfm and at each step, the speed of the

motor and corresponding field current are noted and tabulated.

6. Do not exceed more than twice the rated speed of the motor.

7.

Repeat the procedure with another value of armature voltage.8. Plot a graph of speed v/s field current for each constant armature voltage.

Armature voltage control method:


2. Keep Ram in cut in position and Rfm in cut out position.

3. Close the supply switch starts the DC motor.

4. Adjust the field current to some constant value say (0.5A) by cutting field rheostat.

5. Now by cutting out Ram in steps note down corresponding speed and armature voltage

and tabulate the same.

6.

Experiment is repeated for different constant field current by adjusting the field resistance

and step 5 is repeated.

7. A graph of speed v/s armature voltage is plotted for each constant field current.


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Inference

Flux control

Armature voltage = __________ Armature voltage = __________

Sl

NoIf Amps N rpm

Voltage control

Field current = __________ Field current = __________

SlNo

V Volts N rpm

Typical graph:

N

If

N

V

In rpmIn rpm

AmpsVolts

Conclusion:

SlNo If Amps N rpm

Sl

NoV Volts N rpm


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Experiment No.02

Load Characteristics of a DC Shunt generator and

Compound generator

Aim:To draw the load characteristic of a dc shunt generator.

CIRCUIT DIAGRAM


Motor: Generator:

Apparatus Required:

Circuit


A1, A2 Moving coil ammeter 0-20A 2

V1, V2 Moving coil voltmeter 0-250V 2

Rfm Rheostat 200, 1.7A 1Rfg Rheostat 500, 1.2A 1Ram Rheostat 50, 5A 1


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Theory:

DC generator works on the principle of dynamically induce emf. The DC generator has the

following characteristics.

Magnetization Characteristics: This characteristic is obtained by plotting a graph of generated

no load voltage E against the field current If,when speed of the generator is maintained

constant. It is also called as no loadCharacteristicor open circuit Characteristic, since it plottedwithout load with output terminals kept open.

LoadCharacteristics: this is further divided into two Characteristics

1) External Characteristics 2) Internal Characteristics

External Characteristics: it is the graph of terminal voltage Vt against load current Il.

If a shunt generator is loaded, due to armature resistance the terminal voltage decreases

and its load current increases, IaRa drop and greater de magnetizing effect occurs . The net

effect is that the terminal voltage progressively diminishes with the increase in load current.Hence the graph of Vtv/s Il i.e. external Characteristic is a dropping curve.

Internal Characteristics: it is the graph of generated induced emf E against armature currentIa.

Compound generator : There are two ways of connecting the shunt field in a compound

generator called short and long shunt. The short shunt is the more usual arrangement as it gives

a somewhat higher voltage due to the fact that the shunt field has the full armature voltage

across it.In the long shunt arrangement, the voltage across the shunt is the armature voltage less

the ohmic drop in the series field.

Load Characteristics of a DC Shunt generator

External Characteristics:


2. The motor field resistance Rfmis kept in cut-out position armature resistance Ramis kept

in cut in position and generator field resistance Rfgin cut in position.3. Keep all the load switches in off position.

4. Close the supply switch and ensure that the motor is rotating in the proper direction.

5. Gradually cut out the Ram completely and slowly cut in the Rfmuntil the motor reaches

the rated speed.6. Slowly cut out the generator field rheostat Rfguntil the rated voltage is obtained.

7. Now load the generator in steps upto its rated current by closing the load switches one by

one.8. Corresponding load current Iland terminal voltage V are tabulated at each step by

maintaining the rated speed of the motor by adjusting Rfm.

9. Switch off all the loads and bring back the rheostat Rfgto its original position.10.The rheostats Rfm, Ram are brought back to their original positions and switch off supply

switch S1 to stop the motor.

11.Find out the values of Rshand Ra by VA method and tabulate corresponding V and I.


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12.Plot the external characteristics V v/s IL

Internal Characteristics:

1. Plot the external characteristic AE.2. Measure Rshfor different V and I and plot shunt field resistance line OF3. Measure Ra and plot armature resistance line OB.

4.

Chose any point Con the OAline.5. Draw a horizontal line from C. the lice CDrepresents field current corresponding to

terminal voltage OC.6. Mark Gon this horizontal line where it touches external characteristic. Mark GH=

CD.7. Drop a vertical line till it touches the Xaxis and mark that point as Jand on Raline

mark it as I.8. Measure IJand mark from Gand mark it as K. the point Krepresents one of the

point of internal characteristic because IJis equal to GKrepresents internal drop,which you are adding with terminal voltage.

9.

KLrepresents total EMF generated.10.Similarly no of such points are obtained and internal characteristics is obtained.

Procedure

Starting the DC shunt motor:


2. To start the DC shunt motor ensure that the rheostat in series with the motor armature is

completely cut-in and the rheostat in series with the motor field is completely cut-out.Further ensure that the rheostat in series with the generator field is completely cut-in.

3.

Close the supply switch.4. Cut-out the armature rheostat RamSlowly and completely.5. Cut-in the motor field rheostat Rfmslowly until the motor reaches the rated speed.

6. This procedure is followed in all experiments where a DC shunt motor is to be brought to

its rated speed.

Load Test

1. Ensure that the load is disconnected and bring the motor to rated speed as described

above.

2. Now slowly cut-out the generator field rheostat Rfgun-till the voltmeter reads the rated

voltage. This is the no load voltage of the shunt generator.3. Now close the load switch.

4. Load the generator. For each load measure the generator field current If from ammeter

A1, the load current from ammeter A2and the terminal voltage V from the voltmeter V1.

5. Continue loading until the rated current of the generator. Ensure the reading are taken at aconstant motor speed.

6. Continue loading and note down the point at which the load current starts decreasing.

This gives the critical load current.7. Now keeping the load constant at some value, vary the speed and note the effect of speed

on the terminal voltage.


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8. Now remove the load, reduce the generator voltage, cut out the motor field rheostat, cut

in the motor armature rheostat and disconnect the main supply switch.

Load test

N = _____________rpm

InferenceTabular Column

ILAmps IfAmps V volts Ia= If+ IL IaRa Eg=V+IaRa

Critical load current = ___________

Measurement of armature resistance

Connect a low voltage power supply with current limiting to the armature winding as shown in

fig. Apply different voltages and measure the current flowing through the armature winding for

each voltage. The ratio of V/I gives the resistance

Resistance of the armature:

V (Volts) I (Amps)Ra= V/IOhms

Armature resistance = _____Ohms


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Measurement of field resistance:

Connect a low voltage power supply with current limiting to the field winding as shown in fig.

Apply different voltages and measure the current flowing through the field winding for each

voltage. The ratio of V/I gives the resistance

Resistance of the field:

V (Volts) I (Amps) Ra= V/I Ohms

Typical Graph for Shunt generator characteristics:


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Circuit Diagram for a Compound Generator


Motor: Generator:

Apparatus Required:

Circuit Ref. Description Rating Quantity

A1 Moving coil ammeter 0-2.5A 1

A2 Moving coil ammeter 0-20A 1


Rfm Rheostat 200, 1.7A 1Rfg Rheostat 500, 1.2A 1Ram Rheostat 50, 5A 1

THEORY

The shunt generator gives a terminal voltage which falls of somewhat with increase of load. It isusual to include an adjustable resistance called shunt regulator in the field circuit, and cutout

some of this resistance when the load increases. The raise in the terminal voltage obtained by

providing shunt generator with additional series excitation, the design being such that, over the

series range, working characteristics does not drop. The characteristics for shunt and series turns

separately are shown in the fig below and that of the compound generator is obtained by adding

ordinates of two curve. If series excitation is such that the terminal voltage on full load is the

same as on no load, the generated is level compounded. If the terminal voltage raises with loadit is over compounded.


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Procedure: Make connections as shown in the circuit diagram. Ensure that the load is NOT

switched on. Cut in resistance Ramin series with the compound motor armature, cut out resistance

Rfmin series with compound motor shunt field and cut in Rfgin series with generator shunt field.

1. Close the supply switch. Cut out Ramcompletely. Cut in Rfm to bring the compound

motor to rated speed.2. Now slowly cut out the generator field resistance Rfgun-till the voltmeter reads the rated

voltage. This is the no load voltage of the shunt generator.

3. Now close the load switch.

4. Load the generator. For each load measure the generator field current Iffrom ammeter

A1, the load current from ammeter A2 and the terminal voltage V from the voltmeter V1.

5. Continue loading until the rated current of the generator. Ensure that the readings are

taken at a constant motor speed.

6. Now remove the load, reduce the generator voltage, cut out the motor field resistance, cut

in the motor armature resistance and remove the main supply switch.

7.

Repeat the steps for all different connections of the compound generator.

8. Measure the resistances of the armature winding and the series field winding.

Measurement of shunt Field, series Field and armature resistance are found using VI

method as mentioned in the shunt generator experiment.

Load test

N = __________rpm

Tabular Columns

Long shunt cumulative compound generator

ILAmps IfAmps v volts Ia= If+ ILamps Ia(Ra+Rse) volts Eg=V+Ia(Ra+Rse) volts


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Long shunt differential compound generator

ILAmps IfAmps v volts Ia= If+ ILamps Ia(Ra+Rse) volts E =V+Ia(Ra+Rse) volts

Short shunt cumulative compound generator

ILAmps IfAmps v volts Ia= If+ ILamps IaRa+ ILRsevolts Eg= V + IaRa+ ILRsevolts

Short shunt differential compound generator

ILAmps IfAmps v volts Ia= If+ ILamps IaRa+ ILRsevolts E = V + IaRa+ ILRsevolts

Typical Graph Shunt generator characteristics

Conclusion:


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Experiment No.03

Hopkinson's test

AIM: To conduct Hopkinson's test on a pair of identical machines and to calculate their

efficiency

CIRCUIT DIAGRAM

Name plate details:

Motor: Generator:

Apparatus Required:

CircuitRef.

Description Rating Quantity

A1,A3,A4 Moving coil ammeter 0-20A 3

A2,A

5 Moving coil ammeter 0-2.5A 2

V1,V2 Moving coil voltmeter 250V/500V 2

Rfm Rheostat 200, 1.7A 1Rfg Rheostat 500, 1.2A 1Ram Rheostat 50, 5A 1S1 SPST Switch 220V , 16A 1


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THEORY:This is also called as regenerative test, which is carried out preferably on a pair of

identical machine. These are mechanically coupled and are so adjusted electrically that one of

them act as a motor and other acts as a generator. The motor supplies the mechanical power

required to drive the generator, while the electrical power developed in the generator

is utilized in the motor, resulting in no wastage of their outputs.

Thus two machine of any size can be tested under full load condition and power taken from the

supply will be that required to overcome the losses only. The method is therefore invaluable

where tests of long duration under full load condition have to be made on very large machines.

Such test is called heat runs, because the aim of the test is to determine the final temperature

rise. Regenerative tests were first introduced by HOPKINSON and hence the name

HOPKINSONS TEST.

Merits of HOPKINSONS TEST:

a. The power required for this test is small as compared to full power of the two machine.

b. Since the machine are tested under full load conditions, results can be more accurate as

regards to temperature rise and commutation quantities

Procedure


2. Keep the motor field rheostat cut out, motor armature field rheostat cut in, generator field

rheostat cut in and the switch S open3. Close the supply switch.

4. Cut out the motor armature resistance and cut in the motor field rheostat until the rated

speed is reached.

5. Now cut out the field rheostat of the generator slowly and observe the reading of the

voltmeter across the switch. If it decreases, continue cutting out the resistance until thevoltmeter reads zero and close the SPST switch. If the voltmeter reading increases when

the resistance is cut out, then interchange the armature terminals, cut out the resistance

and bring the voltmeter to zero before closing the switch. Now the generator voltage is

equal to the supply voltage.

6. Note down the no load readings.

7. Now over excite the generator (increase the field current) and under excite the motor to

maintain the constant speed.(decrease its field current). Note down all the meter readings.

8. Continue until rated current flows through the motor armatures by maintaining rated

speed of the motor.

9.

Slowly bring back field rheostat Rfgto cut in position by maintain rated speed throughadjusting motor field rheostat Rfm until the generator armature current shows zero (A4)

then open SPST switch. Reduce the speed and open the main supply switch.

10. Measure the armature resistance of motor and generator by VA method.

11. The efficiency of the machines are calculated. The graph of efficiency verses output of

both the machines are plotted.


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Measurement of armature resistance

Connect a low voltage power supply with current limiting to the armature winding as shown infig.

Apply different voltages and measure the current flowing through the armature winding for each

voltage. The ratio of V/I gives the resistance.



Armature resistance (Rarm)= _____Ohms


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Inference:

Hopkinsons test

N = __________rpm

Sl.

No.

IL

Amps

Ifm

Amps

Iam

Watts

IfgWattsIag

Watts

V volts

Efficiency of generator

Output (Watts) Losses (watts)

Input = output +losses %

Efficiency of Motor

Input (

Watts)Losses (watts)

output = Input -

losses%

Calculations:

Power drawn by supply = V1 X IL=____________Watts

Motor armature copper losses = Iam2X Rarm=____________Watts

Motor field copper losses = V1X Ifm=____________Watts

Total motor loss = Iam2

X Rarm + V1 X Ifm=____________Watts

Generator armature copper losses = Ifg2X Rarm=____________Watts

Generator field copper losses = V2X Ig =____________Watts

Total generator loss = Ifg2X Rarm+ V2X Ig =____________Watts


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Total stray losses of the two machines (W) = V1 X IL[ (V1X Ifm+ Iam2X Rarm) +( V2X Ig + Ifg2X Rarm)] =____________Watts

Assumeing stray losses to be equally distributed in both the machines, stray losses in each

machines =2

W=____________Watts

Efficiency of the motor = (Motor inputlosses)Motor input

Efficiency of the generator = Generator output

(Generator output + losses)

Conclusion:


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Experiment No.04

Field test on a DC Series motor

Aim:To conduct Field test on DC Series motor.

CIRCUIT DIAGRAM

Name plate details:

Motor: Generator:

Apparatus Required:

CircuitRef.


A1,A2 Moving coil ammeter 0-20A 2

V1,V2,V3 Moving coil voltmeter 0-250V 3

R1 Rheostat 18, 12A 1

Theory: There are three ways of exciting the DC motor; these are series, shunt and compound,

and the characteristics of a motor are determined by the method of excitation. In case of DCseries motor, the flux varies with the motor current and the speed is inversely proportional to flux

and hence the series motor is essentially a variable speed motor. The speed being low on heavy

loads and dangerously high on light loads and for this reason, the DC motor should never be run

with-out load. No-load tests are impossible because of the dangerously high speed attained by

the series motor and hence, tests such as swinburns test, can-not be performed on large size DC

series machines. In view of this field test is quite suitable for DC series machines, because DC

series machines are used for traction purposes and are therefore usually available in pairs.


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In this test two similarly dc series machines are required. These two series machines are

mechanically coupled and their fields are connected in series, in order to make Iron losses of

both the machines equal. This necessitates equal excitation and this is achieved by connecting

the fields in series. One of the machine operates as a motor and drives the other machine

operating as a separately excited generator, the output of the latter being wasted in the adjustable

load R. The connection of the field windings of motor and generator in series, ensures that thestray losses in each machine will be the same. It should be noted that Fields test is not a

regenerative test, because output of the generator is not fed back to the motor, but is dissipated in

the resistor R.

Procedure


2. Keep rheostat R1cut in and apply at least 50% of load to the generatorbefore

Starting.

3. Switch on the DC supply and start the motor, ensure the direction of rotation.

4. If the motor rotates in the opposite direction stop the motor and interchange the field

connections and repeat the above steps.

5. Cut out resistance R1completely and bring the motor to the rated speed.

6. Vary the load till the motor current reading reaches its full load value.

7. Simultaneously note down the ammeter and voltmeter readings.

8. With load switches in ON position only the machine is switched off.

9. Cut in the rheostat R1.

10.Switch off the supply.

11.Measure the resistance of the series field winding and armature winding of both the

motor and generator separately.

ObservationsField test:

V1Volts

V2Volts

V3Volts

N rpmIam

ampsIag

amps% m % m

Resistance of the Motor armature: Resistance of the Generator armature

V (Volts) I (Amps)Rag= V/IOhms

Armature resistance Ram= _____Ohms Armature resistance Rag= _____Ohms

V (Volts) I (Amps)Ram= V/IOhms


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Resistance of the Motor Field: Resistance of the Generator Field

V (Volts) I (Amps)

Rfg= V/I

Ohms

Field resistance Rfm = _____Ohms Field resistance Rfg= _____Ohms

The stray losses and efficiency of the motor and generator are calculated as follows

Calculations:Input of motor = VmIam= ________Watts

Output of generator = VgIag =________Watts

Total Losses = W1= InputOutput = VmImVgIag =________Watts

Motor armature copper loss Wcam= Iam2

Ram=________WattsMotor or Generator field copper loss Wcagor Wcfm= Iam2Rmf =________Watts

Generator armature copper loss Wcag= Iag2Rag =________Watts

Total copper loss WC= Wcam+ 2 Wcfg + Wcag =________Watts

Stray Losses Ws= WtWc =________WattsTotal losses of the motor Wm= (Wcam+ Wcfm) = Ws =________WattsOutput of the Motor = InputTotal losses = VmIm - Wm =________WattsEfficiency of motor = (output of motor) / (Input of motor) =________Watts

Total losses of Generator Wg = Wcag + Wcfg + Ws =________Watts

Input of Generator = Output of Generator + Total losses, WgEfficiency of Generator = (Output of Generator)/ (Input of Generator) * 100 =______%

Conclusion:

V (Volts) I (Amps)

Rfm= V/I

Ohms


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Experiment No.05

Swinburne's Test

AIM: To determine the efficiency at any load by conducting Swinburnes test.

CIRCUIT DIAGRAM


Motor:

Apparatus Required:

Circuit





Rfm Rheostat 200, 1.7A 1Ram Rheostat 50, 5A 1

Theory: The methods of testing electrical machines can be divided in to 3 classes, direct,

indirect & regenerative. In the direct method, the motor or generator is put on full load and the

whole of the power developed is wasted & thisis especially, so in case of large machines. The

indirect method consist of determining the losses and predetermining the efficiency from this

data. The amount of power required thus is to supply the losses only, so that there is no difficulty


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in applying the method even to very large machines. The only disadvantage is that the machine is

running light during the test so that, all though the efficiency is calculated fairly accurate, it does

not reflect the performance to the machine during temperature raise or to the commutation

qualities of the machine. Swinburns test is simplest one such indirect method in which losses aremeasured separately and from this efficiency at any load is predetermined later on. The machine

is made to run under no load. This test applicable to shunt and compound machines, where the

flux remains constant.

The motor is run at its rated speed by applying the rated voltage under no load. The armature

resistance is found decreased slightly by increasing armature current because of the fact that

brush contact resistance is approximately propositional to the armature current.

Following are the advantages and disadvantages of this test.

The advantages of this test are.

1. It is convenient and economical because power required to test large machine is small i.e

only no load input power.

2.

The efficiency can be pre-determine at any load because constant losses are known.

The disadvantage of this test are.

1. No account is taken of the change in iron losses from no load to full load. At full load

flux gets distorted due to armature reaction resulting in more iron losses.

2. As test is on no load we cannot know whether commutation is satisfactory at full load and

weather temperature rise is within specified limit.

Procedure

1.

Connections are made as shown in the circuit diagram.

2. Keep Ramin cut in position and Rfm in cut out position.

3. Close the supply switch starts the DC motor bring the motor to the rated speed by Cut-out

Ram and cutting in Rfm.

4. Note down all the meter readings and are tabulated.

5. Armature resistance is found out by VA method and calculations are carried out to obtain

efficiency of DC machines when running as a motor and as a generator.

6. A graph of efficiency v/s different loading is plotted for motor and generator.

Measurement of armature resistanceConnect a low voltage power supply with current limiting to the armature winding as shown in

fig. Apply different voltages and measure the current flowing through the armature winding foreach voltage. The ratio of V/I gives the resistance


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Inference:

NOLoad readings

V = _______ ILO= ______ Ifo = ________

CALCULATION:

Specimen calculation to calculate efficiency of machine when running as MOTOR:

No Load armature copper loss = I2

a Ra = (Il+If)2

Ra= _______Watts.

No Load Input power = VI1 = ________Watts.

Constant Losses = WC = VIl (Il+If)2Ra= _____Watts.

The of the ;machine when running as a motor for different % of loading is given by,

[ ] [

]

[ ] [

]

[X is fraction of load][ Iris rated current of the machine]

= __________%

V (Volts) I (Amps)

Ra= V/I

Ohms

Fraction of load X %


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Specimen calculation to calculate efficiency of machine when running as GENERATOR:

No Load armature copper loss = I

2a Ra = (IlIf)2Ra= _______Watts.

No Load Input power = VI1 = ________Watts.

Constant Losses = WC = VIl (IlIf)2Ra= _____Watts.

Total losses = constant losses + armature copper loss=________________ Watts.

The of the ;machine when running as a generatorfor different % of loading is given by,

[X is fraction of load]

=_______% [ Iris rated current of the machine]

Conclusion:

Fraction of load X %


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Experiment No.06

Ward Leonard method of speed Control of a DC motor

AIM:To control the speed of DC motor by Ward Leonard speed control method.

CIRCUIT DIAGRAM


Induction Motor: DC generator: DC Motor:

Apparatus Required:

Circuit



Rfm Rheostat 200,1.7A 1Rfg Rheostat 500,1.2A 1R2 Rheostat 50,5A 1S1 DPDT or reversible switch 16A 1

Theory: This method is commonly used where a very delicate speed control over the wholerange from zero to full speed is required, such as in paper mills, elevators, colliery, winders etc.

The method consist simply in working the motor with a constant excitation and applying to its

armature sufficient voltage to get the required speed. i.e. the field of the main motor, whose

speed is to be controlled is connected permanently against the fixed supply terminals and the

supply voltage for this main motor is obtained from a motor generator set.

The variable voltage of the generator can be obtained by varying the resistance in its field

circuit. Thus the applied voltage to the main motor can be changed from zero to maximum value.The direction of rotation of main motor can also be reversed. Although the system requires a


35/72



large capital for providing a MG set, but it is still preferred as it gives very fine and unlimited

speed control in either direction of rotation.

Procedure


2. Keep Rfmcompletely cut out and the field rheostat of the DC generator completely cut in

position.3. Close the AC Supply switch. The induction motor runs at its rated speed.

4. Now close the DC supply so that the field of the DC motor is energized. Keep the field

rheostat Rfmat cutout position.

5. Now through the DPDT switch to position 1-1. Adjust the field rheostat of thegenerator, Rfgto obtain different terminal voltages.

6. For each value of the voltage, note down the corresponding speed of the DC motor

whose speed has to be controlled.

7. Now cut in the generator field rheostat so that the voltage is minimum and open the

DPDT switch to 0-0 position ensure that motor stops, now throw the DPDT switch to

position 2-2. So that DC motor will run in the reverse direction.8. Again vary the voltage by varying Rfgto obtain different voltages and note down the

corresponding speed for each voltage in the reverse direction.

9. Bring back Rfgto cut-in position Open the reversible switch to 0-0, Open the DC supply

switch and AC supply switch.

10.Graph of speed v/s voltage is plotted.

Inference:

Forward direction Reverse direction

Conclusion:

V Volts N rpmV Volts N rpm


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38/72



Experiment No.07

Load test of a DC motor-determination of speed-torque and

HP-efficiency characteristics.

AIM:To draw the load characteristics of DC shunt motor

CIRCUIT DIAGRAM

Name plate details:

Motor:

Apparatus Required:

Circuit





Rfm Rheostat 200, 1.7A 1Ram Rheostat 50, 5A 1


39/72



THEROY:

A motor is a device which converts electrical energy into mecha nical energy . in Dc shunt

motor, the field winding is connected in parallel with the armature winding and the combination

is connected across the supply, one of the characteristic of Dc shunt motor is the speed torquecharacteristic. Its the mechanical characteristic of the DC motor. The curve of speed torquecharacteristic remains constant though torque changes from no-load to full load is small. DCshunt motor is called constant speed motor because as long as supply voltage is constant, flux

produce is also constat hence the speed. Therefore it has wide applications and is used in

blowers, fans, centrifugal and reciprocating pumps, lathe machines, machine tools, milling

machines, drilling machines.

PROCEDURE:

1. Connction are made as shown in circuit diagram.

2. The rheostat Ramis kept in CUT-IN position and Rfmis CUT-OUT position.

3. Close the supply switch and start the DC motor.4. CUT-OUT the armature rheostat Ramslowly and completely.

5.

CUT-IN the motor field rheostat slowly until the motor reaches the rated speed.

6. Note down the initial readings of all the meters.7. Now load the motor (machanical loading) in steps nearly to its rated current.

8. For each loading all the meter reading and speed of the motor are tabulated.

9. Release all the loads.

10.Reduce the speed by keeping Rfm in CUT-OUT position and Ram in CUT- IN positionand open the supply switch.

11.Torque, BHP and efficiency of the motor are caculated using formula.

12.The graph - % versas BHP - N versas Torque is plotted

Tabular coulmn

Sl.no V in

volts

Load

current

in amps

W1

Kg

W2

Kgs

W = (W1-

W2)

in Kgs

Tsh =

9.81xWxR

N-m

N

rpm

Output=

60

2 NTsh

Watts

% =

Input

Output


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Calculation:

Motor input = VxI=______ Watts

Motor output =60

2 NTsh Watts

Motor torque = 9.81xWxR

Where R = Radius of the break drum

Efficiency =Input

Output

Conclusion:


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42/72



Experiment No.08

Retardation testelectrical braking method.

AIM: To determine the stray losses of the Dc machine.

CIRCUIT DIAGRAM

Name plate details of the motor:

Apparatus Required:

Circuit


A1 Moving coil ammeter 2.5A/5A 1

V1 Moving coil voltmeter 250V/500V 1

R1 Rheostat 200,1.7A 2R2 Rheostat 50,5A 1S1 DPDT Switch 16A 1

T Stop Watch 1

Theory: This is also one of the indirect method of testing DC shunt machines where is the

separation of losses can be conveniently found.

The machine is made to run up to a little way beyond the normal speed and the supply to the

motor is switched off, while keeping the field excited. As a result, the armature slows down and

its kinetic energy is used to supply the various stray losses ( iron, friction and windage losses)

produced by rotation. Hence this test is also called as RUNNING DOWN TEST.


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Procedure


2.

Keep shunt field resistance Rfmin cut-out position and resistance in series with the motor

armature Ram, in cut-in position.

3. Keep SPDT switch in position 1

4. By cutting out slowly armature rheostat (Ram) the speed is built up and cutting in Rfmof the

motor the speed is built up to slightly more than its rated speed.5. Now open the SPDT switch and hence speed of the motor and the voltage across the armature

false down.

6. The time taken to drop in voltage say about 50 V is noted down.

7. Experiment is repeated for different drop in voltage and corresponding time required (T1) is

tabulated. (Example: 220V-170V, 220V-120V, 220-70V, 220V-0V)

8. Graph is plottedvoltages time taken required drop.9.

Now experiment repeated as given as steps2 to 5 by throwing the SPDT switch on position 1-

2side i.e. cut of armature supply and include the load in the circuit.

10.Again time (T2) taken to drop in voltage and corresponding current is noted down

simultaneously

11.

Graph of voltage verses current is plotted and average current is obtained.12.The average drop and average current product will give the electrical losses.

Tabular columns

Without load With load

Sl

No

V1Voltage

drop in volts

Time T1in

seconds

1.

2.

3.

4.

Where,

T1is time required to drop in required voltage of V1(without load)

T2 is time required to drop in required voltage of V2(with load)

I1 is initial current when SPDT thrown from 1-2 position (towards the load)

I2 is the final current when voltmeter reading comes near the final voltage.(example:220V is theinitial voltage and 170V is the final voltage)

Sl

No

V2Voltage

drop in volts

Time T2in

seconds

I1in

amps

I2in

amps

1.

2.

3.

4.


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Calculations

W1= V * I (watts)

= average value of drop in voltage value of corresponding current

=____________V

Where, W1 is electrical losses.

T2 is time required to drop in required voltage of V2(with load)

T1is time required to drop in required voltage of V1(without load)

T2

WITH LOAD

WITH OUT LOAD

VOLTAGE

TIME

T1

T1>T2

V1

V2

V1

V2

I1I2

I

Vt

Conclusion:


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46/72



Experiment No.09

Slip test

Aim :Determination of Xd and Xqfor an alternator and thereby to determine the regulation of the

alternator.

CIRCUIT DIAGRAM


Motor: Salient pole alternator:

Apparatus Required:

CircuitRef.


A2 Moving iron Ammeter (AC) 0-5A 1

V1,V2 Moving iron Voltmeter (AC) 0-250-500V 2Rfm Rheostat 200,1.7A 1Ram Rheostat 50,5A 1AT1 3 phase auto transformer 0 - 470V, 10A 1


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Theory:

the effect of armature reaction, fluxes and induced emf are assumed to be constant because of

uniform air gap in a synchronous machine with a cylindrical rotor. However a salient pole

synchronous machine has non uniform air gap because of which its reactance varies with the

rotor position. Thus the salient pole machine possesses two axes of geometric symmetry.

1.

Field pole axis called direct axis or d axis.2.

Axis passing through the centers of inter polar space called the quadrature axis or qaxis.

In case of a cylinderical rotor machine there is only one axis of symmetry ( pole axis or direct

axis). Hence in case of salient pole machines, the reluctance of the magnetic path are different

along the direct axis and q axis. The reluctance of the magnetic circuit on the d axis is due toyoke, teeth of the stator, pole and air gap, and core of the rotor. In quadrature axis the reluctance

is mainly due to large air gap in the inter polar space. Because of the non uniformity of the

reluctance of the magnetic path the mmf of the component is divided into two components

namely a) a direct acting component b) a quadrature acting component. When the armature is in

phase with the excitation voltage, the entire mmf of the armature acts at right angles to the axisof the salient pole and therefore all the mmf is in quadrature. On the other hand if the armature

current is in quadrature with the excitation voltage, Eothen the entire mmf of the armature acts

directly upon the magnetic paths and thus all the armature mmf is direct acting.

In an alternator excitation is given to the field winding and voltage gets induced in the armature.

But in slip test a three phase supply is applied to the armature having the voltage much lesser

than the rated voltage while the field circuit is kept open. The alternator is run at a speed close to

the synchronous speed. The three phase current drawn by the armature from the three phase

supply produces a rotating magnetic field. This is similar to the rotating magnetic field existing

in a induction motor, since the armature is stationary. When the stator mmf is aligned with theaxis of the pole the effective reactance offered by the alternator is Xd. when the stator mmf is

aligned with the q axis of the poles then the effective reactance offered by the alternator is Xq.

Procedure

1. Connections are made as shown in the circuit diagram. The field of the alternator is open.

The motor armature rheostat is cut in and the field rheostat is cut out.

2. The AC main supply switch is closed. And vary the autotransformer till the voltmeter

across the field winding reads about 80V

3. The DC motor is started observe that volt meter across the field winding decreases so as

to ensure the correct phase sequence

4. Bring the motor to the rated speed.

5. The applied voltage is increased (say around 150V) until the ammeter of the armature of

alternator reads the rated current.

6. By varying the field rheostat of the DC motor, the speed is slightly reduced by 2-3% of

the rated speed.

7. Oscillations are observed in the ammeter and voltmeter. The speed is adjusted until the

oscillations are maximum.

8. The maximum and minimum values of the ammeter and voltmeter are noted down.


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9. The applied voltage is reduced. The motor speed is reduced and the main supply switch

opened.

10.The armature resistance per phase is measured.

Inference

Imax=

Imin=

Vmax=

Vmin=

Measurement of armature resistanceConnect a low voltage power supply with current limiting to the armature winding as shown in

fig. Apply different voltages and measure the current flowing through the armature winding foreach voltage. The ratio of V/I gives the resistance




Calculations

Direct axis synchronous impedance Zd

Zd =

Qudrature axis synchronous impedance

Zq =


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Direct axis synchronous reactance Xd

Xd= Qudrature axis synchronous reactance Xq

Xq= Determination of regulation:

Therefore, % Regulation = (

) x 100

Xd = = Xq =

=

Direct axis synchronous impedance Zd Zq

Conclusion:


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51/72




52/72



Experiment No.10

V and inverted V curves of synchronous motorAim:To obtained V and curves of synchronous motor.

Name plate details:

Synchronous Motor:

Apparatus:

Circuit

reference


A1 Moving coil Ammeter 0-2.5 A 1

A2 Moving iron Ammeter 0 -10 A 1

V1 Moving iron voltmeter 0 -500 V 1

W1,W2 Watt meters 10 A, 600 V 2

Rfm Rheostat 400, 1.7 A 1


53/72



THEORY:Synchronous motor is a machine that operates at synchronous speed and has wide

applications as regards to (i) power factor correction (ii)Voltage regulation (iii) Constant speedConstant load drives. The main drawback of the synchronous motor is that it is inherently not

self starting and needs a prime mover to starts. This drawback can be addressed by providing a

special winding on the rotor poles called the damper winding or squirrel cage winding. The

damper winding consists of short circuited copper bars embedded in the face of the field poles.

Therefore when Ac supply is fed to the stator in the beginning, the motor starts as a squirrel cage

induction motor because of the presence of damper winding. The exciter moves along the rotor.

As soon as the motor attains 95 % of the rated speed, the rotor is energized with DC. Now the

rotor is magnetically locked with the stator rotating magnetic field and thus motor runs at

synchronous speed. These motors can be constructed with wider air gap than induction motor,

which make them better mechanically.

Application:

1. In substation and power houses in parallel to the bus bars for PF improvement. For this

purpose they are run on without load and are over excited

2.

In factories having large numbers of induction motor for power factor improvement.

3. For voltage regulation at end of the transmission line.

4. Because of constant speed independent of the load, it can be used to drive another

alternator to generate a supply at different frequency.

In case of a synchronous motor driving a constant load, variation in the field

current affects not only PF but also current drawn by the motor.

The power drawn by synchronous motor is given by P = 3 VLILcos

Since input power P and supply voltage V are constant, any decrease in the power factor causesincrease in armature current and vice versa. The curves drawn between the armature current and

field current for different constant loads are known as V curves due to the shape of the English

letter V. The V curve of the synchronous motor gives the relation between armature current and

field current for different power inputs. Similarly, the variation of power factor with a variation

in field current (DC excitation) for a constant load gives inverted V curves. From V-curves it is

observed that with low value of field current the armature current is large and lagging. As the

field current increases the pf increases and armature current decreases and reaches its minimum

value. When armature current is minimum, the pf is unity and corresponding field current is

known as normal field current or excitation of the motor for that particular load. The region in

which the field current is less then its normal value is known as region of under excitation or

region of lag. The field current is more then normal value or armature current is known as region

of over excitation or region of lead.


54/72



Procedure:

1. Connections are made as shown circuit diagram.

2. Now close the AC supply switch and ensure proper direction of rotation of the motor.

3. Close the DC supply switch with the field rheostat completely cut-in, thus exciting thefield of the synchronous motor. Let the motor run on no-load.

4. Synchronous motor runs at synchronous speed.

5. Now vary the excitation of the synchronous motor in steps by varying the field rheostat.

6.

Note down the corresponding reading .7. Further variation of field excitation cause line current to reach to its minimum and

increases from minimum to higher values of load current.

8. Care has to be taken to see that excitation should not be increase further beyond the ratedvalue.

9. Reduce the excitation to a minimum and repeat the procedure step 5 to 8 for different

loads.

Observations:

Without load With load

If in

Amps

Ia in

Amps

W1in

watts

W2in

wattsIf in

Amps

Ia in

Amps

W1in

watts

W2in

watts


55/72



Ideal graph:

WITH LOAD

WITH OUT

LOAD

WITH OUT

LOAD

WITH LOAD

Ia

If

Cos

If

UNDER EXCITATION

REGIONOVER EXCITATION

REGION

UPF

Conclusion:


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Experiment No.11

Regulation of an alternator by EMF and MMF method

Aim: To determine the voltage regulation (by EMF and MMF method) by conducting open

circuit and short circuit test on a given alternator.

Name plate details:

Motor: Alternator:

Apparatus Required:

CircuitRef.



A2 Moving iron ammeter 0-10A 1

V1 Moving iron Voltmeter 0-500V 1

Rfm Rheostat 200,1.7A 1Ram Rheostat 50,5A 1R1 Rheostat 500,1.2A 1S1 TPST 32A/220V DC 1


57/72



Theory: voltage regulation of an alternator is defined as the increase in terminal voltage

expressed as percentage of the rated terminal voltage when the full load is thrown off with speed

and field current remaining constant.

x 100 Viz Eo= No load terminal voltageV = Full load terminal voltage

It should be noted that voltage raise in the terminal voltage when the full load is thrown off isnot the same as the full load terminal voltage when the full load is applied.

The change in the terminal voltage of an alternator with the change in load supplied by it, is due

to the following reasons

I. Voltage drop on account of armature effective resistance.II. Voltage drop on account of armature leakage reactance.

III. Voltage drop on account of armature reaction.

To determine the regulation of an alternator, open circuit and short circuit tests are to beperformed. Before these tests are conducted it is necessary to know about synchronous reactance,

synchronous impedance, effective resistance etc.

synchronous reactance- The emf setup due to armature reaction MMF is always in quadraturewith the Load current I and is proportional to it. Thus it is equivalent to an EMF induced in an

inductive coil and the effect of armature reaction can therefore considered as equivalent to

reactance drop IXa ,where Xa is the fictitious reactance. The armature winding possesses of acertain leakage reactance XL. The sum of leakage reactance XLfictitious reactance Xais called

as the

Xs=XL+ Xa.Effective Resistance of the armature winding is somewhat greater than conductor resistance,

called the dcresistanceas measured by direct current.This is due to additional losses, over the

purely I2R loss, inside some time outside the conductor, owing to alternating current. The main

source of this additional loss is. 1 ) Eddy currents in the surrounding material, 2 ) magnetic

hysteresis in the surrounding material. 3)Eddy currents OR unequal current distributions in theconductor itself. Hence it is sufficiently accurate to measure armature resistance by dc and

increase it to a fictitious value say effective resistance, Rewhich varies widely from 1.25 to 1.75times more than the DC resistance.

Synchronous impedance - when the Synchronous reactance Xs is combined with armature

effective resistance Re, quantity obtained is called synchronous impedance

Zs = Re+ j XsOpen circuit test: The machine is made to run to its rated speed, by keeping the armature

winding open. With field current raised in suitable steps, until voltage between any pair of

armature terminals little above the rated EMF and corresponding values of V (no-load voltage)

are noted. the Open Circuit voltage per phase, Eoare obtained by dividing the voltmeter readings

by

3 .A curve is drawn between Eo and field current If and is known as open circuitcharacteristics. The initial straight part of curve yields the air gap characteristics since at that lowExcitation, reactance offered by core is negligible.Short circuit test: in this test all the three phases are shorted and since the current in all the three

phases are equal, it is enough to measured current in any one phase. Rheostat of sufficiently high

Ohmic value is inserted in the DC field circuit, to keep the current very low. The machine is runat synchronous speed and the field excitation is so adjusted to circulate 150 to 200% of full load

current .the short circuit characteristics is drawn by plotting a curve between SC current Iscand

If.


58/72



Synchronous impedance method OR EMF method:-The regulation obtained in this method is always higher than actual values, because here Z s is

assumed to be constant, but it is not so. Hence this method is called pessimistic method. Thismethod is theoretically accurate for non-salient pole machines.

MMF method OR Ampere- turn method:-

In this method the data obtained from open circuit test are utilized. This method of determining

synchronous impedance is known as optimistic method since it gives values lower than actualvalues. The reason being that the excitation to overcome armature reaction is determined on

unsaturated part of the saturation curve.

Procedure for EMF and MMF method:

Open circuit test:


2. Armature rheostat Ram is kept in cut-inand Field rheostat of DC motor is kept in cut-out

position. The field rheostat of alternator is kept in cut-inposition.3. Supply is given to the DC motor by closing the switch S1and the DC motor is started.

4.

Bring the motor speed to that of the rated speed of synchronous machine.5. Close the DPST switch to excite the field of the alternator.

6. Rheostat of the alternator field is cut-out gradually in steps and the corresponding fieldcurrent (If) and open circuit voltage of the alternator are tabulated, till the rated voltage of

the synchronous machine is reached.

7. The field rheostat of the alternator is brought back to its original position.(Cut-inposition)

Short circuit test:

1. The TPST switch is closed, so that the stator terminals are short circuited.

2.

Field rheostat of alternator is cut-out gradually in steps and at each step armature current(Ia) and corresponding field current are noted till rated armature current is obtained.

3. Field rheostat of alternator is brought back to cut-in position and DPST switch is open.

4. Armature rheostat & Field rheostat of the motor is brought back to their original position.5. Supply is switched off.

6. The DC resistance(Rdc) per phase of the stator winding are measured by VA method.

Tabular column:

Open circuit test: Short circuit test:

Sl. No. Vphin volts Ifin amps Sl. No. Ifin amps Iscin amps


59/72



TYPICAL GRAPH

Calculation:

EMF method or synchronous impedance method:

Zs=Isc

Voc= ______

Ra=_______

Xs= =________Eo = = _______ Volts

[Note: + for lagging PF _ for leading PF]

Percentage regulation =ph

pho

V

VE X 100 = ________%

MMF or ampere turn method

1. If1 Field current corresponding to the voltage E1from the graph of OCC

E1=V + IaRacos

If1 = Amps

2. If2 Field current required to circulate the rated armature current during short circuit

If2 = Amps

3. Ift= + for lagging pf.

_ For leading pf.


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4. % Regulation=

For each voltage E1 , If1is noted from the graph

For each IfT , E is noted from the graph.

Tabulation for regulation:

% Reg (lag) % Reg (lead)

0.2

0.4

0.6

0.8

1.0

Conclusion:


61/72




62/72



Experiment No.12

Voltage regulation of an alternator by ZPF method

Aim:-To determine the regulation of an alternator by Z.P.F. method

CIRCUIT DIAGRAM

Name plate details:Motor: Alternator:

Apparatus Required:

Circuit


A1 Moving coil ammeter 0-2.5 1


V1 Moving iron Voltmeter 0-500V 1Rfm Rheostat 200,1.7A 1Ram Rheostat 50,5A 1R1 Rheostat 500,1.2A 1S1 TPST 32A/220V DC 1

L1 3 phase variable inductive Load 10A 1


63/72



Theory : this method is also called as Potier method of determining the voltage regulation of

an alternator. This method is more accurate than EMF and MMF methods of determination of

voltage regulation. This method is based on the separation of reactances due to leakage flux andthat due to armature reaction flux, therefore it is more accurate, whereas EMF and MMF

methods are based on the total synchronous reactance. In this method XL is called Potier

reactance and hence the name Potier reactance method. The data required for determination of

voltage regulation by ZPF method are

Field current to circulate full load current in the stator

Effective resistance of the armature winding

Open circuit characteristics

Zero power factor full load characteristics a curve between terminal voltage andexcitation, while the machine being run on synchronous speed and delivering full load

current at zero power factor.

Procedure:

1.

Connection is done as per circuit diagram.2.The open circuit test and short circuit test results conducted as per EMF and MMF

method are used.

3.The machine is run at synchronous speed by the prime mover.

4.Close the DPST switch to excite the field of the alternator, Rated voltage is built up by

varying the excitation.

5.Now close the TPST switch and connect the purely variable inductive load across the

armature terminal.

6.The value of the reactance is then adjusted and the excitation is varied in such a way that

the rated full load current by maintaining rated voltage and rated speed.

7.

Note down the corresponding meter readings.

8.Decrease the inductive load as well the excitation by maintaining rated speed. And open

the TPST Switch.

9. Field rheostat of alternator is brought back to cut-in position and DPST switch is open.

10.Armature rheostat & Field rheostat of the motor is brought back to their original

position.

11.Supply is switched off.

12.Plot a graph between the terminal voltage and excitation current as shown in fig. (a) will

give ZPF characteristics.

Observation: ZPF test

Rated armature current on zero PF load = _________Field current = _________

Terminal voltage of the alternator = ________


64/72



Procedure to draw ZPF characteristics:ZPF full load voltage excitation characteristics can be

drawn by knowing two points A and B. point A is obtained from the short circuit test with full

load armature current. Hence OA represents excitation (Field Current) required to overcome the

de-magnetization effect of armature reaction and to balance leakage reactance drop at full load.

Point B is obtained when full load current flows through the armature and wattmeter reads zero.

From B line BC is drawn equal and parallel to AO. Then a line is drawn through C parallel to

initial straight part of OCC (parallel to OG), intersecting the OCC at D. BD is joined and a

perpendicular DF is dropped on BC. The triangle BFD is imposed on various points of the OCC

to obtain corresponding points on the zero factor curve. In triangle BDF, the length BF

represents armature reaction excitation and length DF represents leakage reactance drop IXL.

This is known as Potier Reactance voltage. The Potier reactance is given by

XP =

Fig (a)

Potier regulation diagram:this diagram is drawn as fallows


65/72



OV is drawn horizontally to represent terminal voltage, V on full load and OI is drawn to

represent full load current at a given power factor. VE is drawn perpendicular to the phasor OI

and equal to reactance drop (IXL), neglecting resistance drop. Now phasor OE represents

generated EMF E from OCC field excitation I1corresponding to generated EMF is determined,OI1 is drawn perpendicular to Phasor OE to represent excitation required to induce EMF OE on

open circuit. I1I2is drawn parallel to load current Phasor OI to represent excitation equivalent to

full load armature reaction. OI2 gives total excitation required. If the load is thrown off, than

terminal voltage will be equal to generated EMF corresponding to field excitation OI 2. Hence

EMF E0may be determined from OCC corresponding to field excitation OI2.Phasor OE0will

lag behind Phasor OI2 by 900. EE0 represents voltage drop due to armature reaction. Now

regulation can be obtained from the relation

% regulation =

x 100

E

E0

I1

I2

V

I

90 IXL

90

Conclusion:


66/72



Experiment No.13Performance of synchronous generator connected to infinite

bus, under constant power and variable excitation

& viceversa.AIM:To determine the characteristic of Armature current Vs field current and power factor Vs

field current for fixed power and the characteristic of power factor Vs power and armaturecurrent Vs power, for fixed excitation in an alternator.

CIRCUIT DIAGRAM

Name plate details:Motor: Alternator:

Apparatus Required:

Circuit




V1 Moving coil Voltmeter 0-500V 1

W1,W2 Watt meters 10A-600V 2

Rfm Rheostat 200,1.7A 2Ram Rheostat 50,5A 1R1 Rheostat 400,1.7A 1SP Synchronizing panel 1


67/72



Theory: In most of power stations (whether DC or AC), it will be found that the power is supplied from

several smaller units (generators) operating in parallel rather than form a single larger unit capable of

taking care of the maximum peak loads. There are a number of good reasons for this practice. The

demand for electrical energy continues to grow at steady space, electrical utilities and other find it

necessary to increase generating capacity at regular intervals. Since it is not economical to discard

serviceable alternator in favour of larger ones, it becomes necessary to operate alternators in parallel. This

provides greater reliability improves efficiency, facilitates repairs and maintenance and makes the supply

of load possible when demands exceeds the capacity of largest single unit available.

Conditions for parallel operation:

1. Terminal voltage of the incoming machine is equal to that of the others (Infinite bus)

2. The frequency of the incoming machine is mach with the bus bar frequency.

3. Phase sequence of the incoming machine voltage must be as same as that of the bus bar voltage.

The behavior of the alternator when running in parallel is quite different from its performance when

operating in the stand alone mode. The underlying principle when operating on infinite bus is that the

excitation controls the reactive power output and power factor where as the power input from the prime

mover controls the active power output and the power angle of the alternator.

At constant power:If the excitation of the alternator is reduced, to make up for the required air gap flux,

the armature current is delivered at a leading power factor as leading power factor armature current is

known to have a magnetizing armature reaction effect. Hence, as the excitation is gradually increased

from a small value, the power factor becomes unity and for further increase if excitation becomes lagging,

i.e. an overexcited alternator delivers lagging armature current. For a fixed power, the product of armature

current and its power factor should be constant. Hence, the change of armature current magnitude is

inverse to the change in the magnitude of power factor, as the excitation is increased. Obviously, the

minimum armature current occurs at the maximum power factor i.e, at unity power facor.

With constant excitation: When the power input is increased there will be commensurate increase of

power output from the alternator i.e. the active component of armature current increases, whereas the

reactive component remains fairly constant since the excitation remains constant. Consequently, the

power factor increases with an increases of power input at constant excitation.

Procedure

1. Keep Rfmcut out, Ramcut in and R1cut in.

2. Keeping the AC supply switch open, close the DC supply switch.

3. Cut out Ramcompletely and cut in Rfmtill the DC motor reaches the rated speed.

4.

Close the AC supply switch and note down the supply voltage from synchronizing panel .5. Adjust the excitation of the alternator by varying R1until the alternator terminal voltage is equal

to AC supply as indicated by the synchronizing panel voltmeters.

6. Observe the lamps on the synchronizing panel. If they glow on and off simultaneously the phase

sequence of the alternator and the infinite bus bar are matched. If the lamps glow on and off in a

sequence, one after the other, open the AC supply switch and inter change any two phase

terminals.

7.

After ensuring correct phase sequence adjust the speed of the DC motor by varying Rfmuntil the

lamps flicker very slowly.


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8. Close the switch on the synchronizing panel. The alternator is now synchronized to the infinite

bus.

9. Vary the excitation of the alternator by varying R1until alternator reaches its rated current. Note

down the readings of the meters. Do not vary the prime mover field.

10.Decrease the alternator current by varying alternator field excitation by ensuring 0 armature

current open the synchronizing switch, TPST2.

11.Field rheostat of alternator is brought back to cut-in position.

12.

Open AC switch TPST1, bring back all rheostats to original position and open the DC supplyswitch.

13.Armature rheostat & Field rheostat of the motor is brought back to their original position.14.Supply is switched off.

Observations:

Input current to DC motor = I (read from A1)

Input voltage to DC motor = Vs(read from V1)

Field current of alternator = If (read from A2)

Armature current of alternator = Ia(read from A3)

Line to line terminal voltage of alternator = V (read from V2)

Three phase power delivered by alternator = W1 + W2

tan= Three phase reactive power Q = VIaSin

Constant power

If (amps) Ia (amps)W1

(watts)

W2

(watts)

P = W1 +

W2

(watts)

Cos IaCos Q (vars)

Conclusion:


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VIVA QUESTIONS ON DC M/CS

What is the working principle of a DC Generator ?

Can a DC machine be worked as a motor/generator?

Why are starters used for DC motors?

Mention the methods to control the speed of DC shunt motor

What is meant by build up of a generator?

What are the indications and causes of an over loaded generator?

How do we conclude that connections between field coils and armature of a generator are

corrected?

The series field winding has low resistance while the shunt field winding has high

resistance why?

Define critical field resistance in DC shunt generator.

Define the term critical speed in dc shunt generator.

Define the term critical load resistance referred to DC shunt generator.

How can one differentiate between cumulative compound and differential compound

generator?

How will you change the direction of rotation of a DC motor?

DC series motor should never be started on no load why?

Name applications of DC series motor.

Why is field control considered superior to armature resistance control for DC shunt

motor?

Name the different types of losses that occur in DC machines.

What are the special features of Hopkinsons test?

Explain the purpose of conducting Swinburnes test.

Hopkinsons test is also calledas regenerative test why?

What are the advantages of Hopkinsons test over Swinburnes test? Mention the limitation of Hopkinsons test.

Why Swinburnes test cannot be performed on DC Series machines.

What is retardation test and on what type of machines is it performed?

How are large size DC series machines tested?

What is the best way of minimizing eddy currents in an armature?

Retardation test is also called as run down test why?


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Why are starts used for DC motors?

Name any two applications of 1) DC Series motors 2) DC shunt motor 3) DC series

generator

What is armature reaction? And what are the methods used to reduce armature reaction?

What are compensating windings?

What are inter poles and how do they neutralize cross magnetizing effect?

If the load is removed from a DC series motor in operation, what will happen?

What are stray load losses?

Explain back EMF of an DC motor and what is its significance?

Why do we call a shunt motor as a constant flux motor?

What are the conditions to be satisfied to run shunt generators in parallel?

Why name plate details have to be taken before conducting experiments?

What is the difference between mechanical load and electrical load?


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VIVA QUESTIONS ON SYNCHRONOUS M/CS

Based on the construction, name the two types of constructions employed in synchronous

machines?

Which type of synchronous generators are used in hydro electric plants and why?

What are the causes of changes in voltage in alternators when loaded?

Define the term voltage regulation in alternator.

Name the various methods for pre-determ

Documents

DC Machines Lab Manual