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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:
1. Connections are made as shown in the circuit diagram.
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
Name Plate details of the machines:
Motor: Generator:
Apparatus Required:
Circuit
Ref.Description Rating Quantity
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:
1. Connections are made as shown in the circuit diagram.
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:
1. Connections are made as shown in the circuit diagram.
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
Name Plate details of the machines:
Motor: Generator:
Apparatus Required:
Circuit Ref. Description Rating Quantity
A1 Moving coil ammeter 0-2.5A 1
A2 Moving coil ammeter 0-20A 1
V1 Moving coil voltmeter 0-250V 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
1. Connections are made as shown in the circuit diagram.
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.
Resistance of the armature:
V (Volts) I (Amps)Ra= V/IOhms
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.
Description Rating Quantity
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
1. Connections are made as shown in the circuit diagram.
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
Name Plate details of the machines:
Motor:
Apparatus Required:
Circuit
Ref.Description Rating Quantity
A1 Moving coil ammeter 0-10A 1
A2 Moving coil ammeter 0-2.5A 1
V1 Moving coil voltmeter 0-250V 1
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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Resistance of the armature:
Armature resistance = _____Ohms
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
Name Plate details of the machines:
Induction Motor: DC generator: DC Motor:
Apparatus Required:
Circuit
Ref.Description Rating Quantity
V1 Moving coil voltmeter 0-250V 1
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
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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
1. Connections are made as shown in the circuit diagram.
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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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
Ref.Description Rating Quantity
A1 Moving coil ammeter 0-20A 1
A2 Moving coil ammeter 0-2.5A 1
V1 Moving coil voltmeter 0-250V 1
Rfm Rheostat 200, 1.7A 1Ram Rheostat 50, 5A 1
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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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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
Ref.Description Rating Quantity
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
1. Connections are made as shown in the circuit diagram.
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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Experiment No.09
Slip test
Aim :Determination of Xd and Xqfor an alternator and thereby to determine the regulation of the
alternator.
CIRCUIT DIAGRAM
Name Plate details of the machines:
Motor: Salient pole alternator:
Apparatus Required:
CircuitRef.
Description Rating Quantity
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
Resistance of the armature:
V (Volts) I (Amps)Ra= V/IOhms
Armature resistance = _____Ohms
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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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
Description Rating Quantity
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
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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.
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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
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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.
Description Rating Quantity
A1 Moving coil ammeter 0-2.5A 1
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
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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.
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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:
1. Connections are made as shown in the circuit diagram.
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
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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:
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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
Ref.Description Rating Quantity
A1 Moving coil ammeter 0-2.5 1
A2 Moving iron ammeter 0-10A 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
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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 = ________
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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
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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:
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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
Ref.Description Rating Quantity
A1 Moving coil ammeter 0-2.5A 1
A2 Moving iron ammeter 0-10A 1
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
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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
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