LAB MANUAL ELECTRICAL machines - I :

Transcription

DC Machines Lab
CHRISTU JYOTI INSTITUTE OF TECHNOLOGY & SCIENCE
(Affiliated to JNTU Hyderabad), Colombonagar, Janagoan : 506167
DEPARTMENT OF ELECTRICAL & ELECTRONICS
ENGINEERING
ELECTRICAL machines - I
LAB MANUAL
Website: http://www.cjits.org/
Ph no: 08716 202101
08716
E mail ID: cjits68@yahoo.co.in
Year/Semester
Branch
HOD EEE
: II/II
: EEE
Principal
__________________________________________________________________
Electrical & Electronics Department – CJITs
-1-
DC Machines Lab
PREFACE
The significance of the Electrical machines Lab-1 is renowned in the
various fields of engineering applications. For an Electrical Engineer,
it is obligatory to have the practical ideas about the Electrical DC
machines. By this perspective we have introduced a Laboratory manual
cum Observation for Electrical machines lab-I.
The manual uses the plan, cogent and simple language to explain the
fundamental aspects of Electrical DC machines in practical. The manual
prepared very carefully with our level best. It gives all the steps in
executing an experiment.
__________________________________________________________________
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DC Machines Lab
GUIDELINES TO WRITE YOUR OBSERVATION BOOK
1. Experiment Title, Aim, Apparatus, Procedure should be on right
side.
2. Circuit diagrams, Model graphs, Observations table, Calculations table should be left
side.
3. Theoretical and model calculations can be any side as per your convenience.
4. Result should always be in the ending.
5. You all are advised to leave sufficient no of pages between experiments for
theoretical or model calculations purpose.
DO’S AND DON’TS IN THE LAB
DO’S:1. Proper dress has to be maintained while entering in the Lab. (Boys Tuck in and
shoes, girls with apron)
2. All students should come to the Lab with necessary tools. (Cutting Pliers 6”,
Insulation remover and phase tester)
3. Students should carry observation notes and record completed in all aspects.
4. Correct specifications of the equipment have to be mentioned in the circuit
diagram.
5. Student should be aware of operating equipment.
6. Students should be at their concerned experiment table, unnecessary moment is
restricted.
7. Student should follow the indent procedure to receive and deposit the equipment
from the Lab Store Room.
8. After completing the connections Students should verify the circuits by the Lab
Instructor.
9. The reading must be shown to the Lecturer In-Charge for verification.
10. Students must ensure that all switches are in the OFF position, all the
connections are removed.
11. All patch cords and stools should be placed at their original positions.
DON’Ts:1. Don’t come late to the Lab.
2. Don’t enter into the Lab with Golden rings, bracelets and bangles.
3. Don’t make or remove the connections with power ON.
4. Don’t switch ON the supply without verifying by the Staff Member.
5. Don’t switch OFF the machine with load.
6. Don’t leave the lab without the permission of the Lecturer In-Charge.
__________________________________________________________________
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DC Machines Lab
JAWAHARLAL NEHRU TECHNOLOGICAL
UNIVERSITY HYDERABAD
II Year B.Tech. EEE-II Semester
L T/P/D C
0
-/3/- 2
(54602)ELECTRICAL MACHINES LAB-1
The following experiments are required to be conducted
Compulsory experiments.
1. Magnetization characteristics of DC shunt generator. Determination of
critical field resistance and critical speed.
2. Load test on DC Shunt Generator. Determination of characteristics.
3. Load test on Dc series Generator .Determination of characteristics
4. Load test on Dc Compound Generator .Determination of characteristics
5. Hopkinson’s test on Dc Shunt generator .predermination of Efficiency
6. Field’s test on Dc series machines. predermination of Efficiency
7. Swinburne’s test and Speed Control of DC shunt motor . Pre-determination
of Efficiency
8. Brake test on DC Compound motor. Determination of performance curves.
In addition to the above experiments, atleast any two of the experiments from
the following list are required to be conducted.
9. Brake test on DC Shunt motor. Determination of performance curves
10. Separation of losses in DC shunt motor.
11. Retardation test on DC shunt motor. Determination of losses at rated speed.
12. Parallel operation of two DC shunt machines
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DC Machines Lab
S.NO NAME OF THE EXPERIMENT
1
2
3
4
5
6
7
8
PAGE
NO
Magnetization characteristics of
DC Shunt Generator.
Brake test on DC Shunt motor
Brake test on DC compound motor.
Load test on DC Shunt generator.
Load test on DC Series generator.
Speed control DC Shunt motor, and
Swinburne’s test on DC shunt machine
Load test on DC Compound generator.
Field’s test on two identical Series machines.
9
Hopkinson’s test.
10
Separation of losses in a DC shunt machine
Additional Experiments:11
Retardation test on DC shunt motor.
Determination of losses at rated speed
12
Parallel operation of two DC shunt machines
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DC Machines Lab
1. Magnetization Characteristics of DC Shunt Generator
Aim:
To conduct an experiment on a D.C shunt generator and draw the
magnetization characteristics (OCC) and to determine the critical field
resistance and critical speed.
Name plate details: (To be noted Down from the Machine)
S. No
Parameters
1
2
3
4
5
Rated voltage
Rated current
Rated speed
Rated power
Rated field current
Motor
Generator
Apparatus:
S. No
Apparatus
1
2
3
4
Voltmeter
Ammeter
Rheostats
Tachometer
Type
Range
Qty
Theory:
Open circuit characteristics or magnetization curve is the graph between
the generated emf and field current of a dc shunt generator. For field current is
equal to zero there will be residual voltage of 10 to 12V because of the residual
magnetism present in the machine .If this is absent there the machine can not build
up voltage to obtain residual magnetism the machine is separately excited by a dc
source from OCC we can get critical field resistance and critical speed.
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DC Machines Lab
Circuit diagram
DPST Switch
3 point starter
A
L F
+
Fr1
+
230
V DC
230V
Supply
DC
Supply
A
A
Fr2
F
M
F
−
FF
FF
V
−A
G
M
AA
AA
Fuse 20A
DPST Switch
+
+
A
230 v
DC
supply
−
P.D
−
Fuse 5A
Shunt Field Resistance (Rsh):DPST
FUSE 5A
+
+
A
230 V
DC
Supply
−
F
+
V
FF
−
−
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DC Machines Lab
Procedure:
1. Connect the circuit as per circuit diagram shown in above figure.
2. Set the potential divider to zero and output keeping the motor field rheostat
in minimum position.
3. Switch on the supply and start the motor with 3-point starter. Check the
speed of the motor, if below the rated speed then adjust the speed by
controlling the field rheostat. Bring the speed to the rated speed.
4. The speed of the motor should be maintained constant over the experiment.
5. Note down the voltmeter reading s at zero field current If.
6. Increase the Ish of the generator in steps by changing the potential divider
position up to little higher value than the rated voltage of the generator.
7. Take the readings of the If and Eg at each step by increasing and decreasing
values of the potential divider.
8. Make all the equipment to initial positions and open the DPST.
Graph:
Eg (V)
A
C
O
B
Z
Y
If (A)
Y
If v/s Eg
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DC Machines Lab
Tabular column:
S.NO
Field current
If
Generated Voltage (Eg)
Decreasing
Increasing (If)
(If)
Average Eg
1.
2.
3.
4.
5.
6.
7.
Calculations:
Critical field resistance (Rc)
= OA/OY
Field resistance (Rf)
=
The maximum voltage the Generator can induce
With this field resistance.
=
Critical Speed
= YZ/BZ * N
Result:
a. The OCC of a DC shunt generator has been observed
b. Critical field resistance=400Ω
c. Critical speed=1035rpm
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DC Machines Lab
2. BRAKE TEST ON DC SHUNT MOTOR.
DETERMINATION OF ITS PERFORMANCE CURVES
Aim:
To conduct brake test on DC Shunt motor. And to determine its
performance characteristic curves.
Name plate details: (To be noted Down from the Machine):
S. No
1
2
3
4
5
Parameters
Rated voltage
Rated current
Rated speed
Rated power
Rated field current
Motor
Apparatus:
S. No
Equipment
1
2
3
4
5
6
Voltmeter
Ammeter
Ammeter
Rheostat
Tachometer
Connecting wires
Range
Type
Qty
Theory:
When if is required to determine directly efficiency if comparatively small
motors, the motor is loaded directly by means of Mechanical Break. Hence in the
case of shunt motor there is no drastic change in speed. The Torque
T = (S1 ~ S2) g. r – Nm. where S1 S2 is the spring balance reading, r = Break drum
Radius and g=9.81.
P = Power developed. Efficiency of DC motor = Po/ Pi x 100
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DC Machines Lab
Circuit diagram:
DPST Switch
+
+
A
−
3 point starter
L F A
Fr1
S1
+
230 V
DC
Supply
V
−A
+
A
Fr2
A
−
M
F
−
FF
-AA _
Fuse 20A
Procedure:1.
2.
3.
4.
Connect the circuit as per the circuit diagram shown in the figure.
Keep the field regulator Rsh at minimum position.
Check the belt is very loose so that there is no load on the motor.
Start the motor with the 3-point starter; check the speed of the motor with
the help of a tachometer. If the speed is below the rated speed then adjust
the field regulator. So that the motor runs at rated speed.
5. Apply the load on the motor by slowly by tightening the belt.
6. Take the readings of the voltmeter ammeter and the spring balance weights
readings at each of increasing the load up to the rated current of the motor
7. Cool the break drum by pouring the water in it though out the loading
period.
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- 11 -
S2
DC Machines Lab
Tabular column:
S.No
Voltage
(V)
Current
(I)
Speed
(N)
Spring
Balance
Readings
S1
S2
Torque=
9.8 1(S1 -S2) .r
Nm
Pout =
2 NT/60
Watts
Pin =
VI Watts
Eff =
Pout / Pin
x100.
1
2
3
4
5
6
Graphs:
1. Plot the graph between torque(T) versus current(I)
2. Plot the graph between speed(N) versus current(I)
3. Plot the graph between speed(N) versus torque(T)
4. Plot the graph between efficiency() versus output(Pout )
EXPECTED GRAPHS:
Y
T
a
T
X
0
I
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DC Machines Lab
X
N
0
N v/s I
I
Y
X
N
T
N v/s T
Y
X
η
Y
O/P
 Vs O/P
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DC Machines Lab
Calculations:
1. Total torque T=9.81 (S1-S2) r n-m
2. Power output Pout = 2NT/60
3. Power input Pin = VI
Result:
The performance of DC shunt motor is obtained by brake test
Efficiency =
__________________________________________________________________
- 14 -
DC Machines Lab
3.BRAKE TEST ON DC COMPOUND MOTOR
DETERMINATION OF PERFORMANCE CURVES
Aim:
To conduct brake test on dc compound motor and to determine the
performance curves.
Name plate details (To be noted Down from the Machine)
S. No
1
2
3
4
5
Parameters
Rated voltage
Rated current
Rated speed
Rated power
Rated field current
Motor
Apparatus:
S. No
Equipment
1
2
3
4
5
Voltmeter
Ammeter
Rheostat
Tachometer digital
Connecting wires
Range
Type
Quantity
Theory:
A Compound motor has a shunt field winding as well as series field
winding. If the series field mmf and shunt field mmf help each other it is a
cumulative compound motor. If the series and shunt fields appose each other it is a
differentially compound motor. The operation of differential compound motor is
unstable In a cumulative compounded motor the fluxes are add each other at light
loads the shunt field is stronger than series field so motor behaves shunt motor. At
high loads series field is stronger than shunt field so the characteristics like nearly
to series motor
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DC Machines Lab
Circuit diagram:
3 point starter
DPST Switch
+
+
A
L
−
F
A
Y
Fr1
S1
+
230 V
DC
Supply
V
−A
YY
+
A
Fr2
A
−
M
F
−
FF
AA
Fuse 20A
Procedure:
1.
2.
3.
4.
5.
6.
7.
8.
Connect the circuit as shown in the circuit figure.
Keep the D.C compound motor field rheostat at minimum.
Check that pulley should be very loose so that there is no load.
Start the motor with the help of a 3-point starter check the speed with the
help of the tachometer, of the speed is below the rated speed, adjust the
field regulator, so that motor is rotating at rated speed.
Increase the load by tightening the belt slowly up the the ratrd current of the
motor.
Note down the readings of the voltmeter, ammeter and spring balance.
In the breaking arrangement entire power is eliminated in the form of heat
at the brake drum, so pore the water in it.
Make all the equipment to initial values and open the DPST.
__________________________________________________________________
- 16 -
S2
DC Machines Lab
Tabular columns:
S. No
Voltage
(V)
Current
(I)
Speed
(N)
Spring balance
readings
S1
S2
Torque=
9.81x(S1-S).r
Nm
Output
power
2πNT/60
Watts
Input
power
VI
Watts
Efficiency
Pout/Pin x
100.
1.
2.
3.
4.
5.
6.
Graph:
1.
2.
3.
4.
5.
6.
7.
Draw graphs O/P Vs Speed,
Draw graphs O/P Vs Current,
Draw graphs O/P Vs Torque,
Draw graphs O/P Vs Efficiency.
Draw graphs speed(N) Vs current(I)
Draw graphs torque(T) Vs current(I)
Draw graphs speed(N) Vs torque(T)
Y
Cum
Diff
N
X
0
T
N v/s I
a
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- 17 -
DC Machines Lab
X
Diff
T
Cum
Y
0
I
T v/s I
X
N
Cum
Diff
Y
T
N v/s T
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- 18 -
DC Machines Lab
X
Η
I
T
N
I
Cum
Diff
T
N
O/P
Y
 v/s O/P
Result:
The performance of DC compound motor is obtained by brake test
Efficiency =
__________________________________________________________________
- 19 -
DC Machines Lab
4. LOAD TEST ON DC SHUNT GENERATOR.
Aim:
To conduct a load test on the given DC Shunt generator and to obtain the
following performance characteristics.
1. Internal characteristics (Eg v/s Ia)
2. External characteristics (V v/s IL)
Name plate details: (To be noted Down from the Machine):
S. No
1
2
3
4
5
Parameters
Rated voltage
Rated current
Rated speed
Rated power
Rated field current
Motor
Generator
Apparatus required:
S. No
Equipment
1
Ammeter
2
Voltmeter
3
4
5
6
5
Rheostat
Rheostat
Load
Tachometer
Connecting wires
Range & Type
Qty
Theory:
By conducting load test on DC shunt generator we can get load
characteristics i.e, Internal & External characteristics. By exciting the m/c, the field
current increases and voltage build up. After the machine has attained 220V the
rated load is switched on. With increase in load, the voltage will be dropped.
__________________________________________________________________
- 20 -
DC Machines Lab
Circuit diagram:
Armature
3 point starter
L
DPST Switch
F
DPST Switch
A
+
A
+
Fr1
−
230 V
DC
Suppl
y
A
Fr2
A
M
FF
F
AA
A
Fr1
+
Fr2
V
G
AA
F
−
FF
Resistive Load
−
Fuse 20A
Fuse 20A
Resistance (Ra):DPST
FUSE 5A
+
+
A
−
A
+
230 V
DC
Supply
M
V
−
AA
−
Procedure:
1.
2.
3.
4.
Connect the circuit the circuit as shown in the figure
Start the motor with the help of 3-point starter
Adjust the field regulator of the motor so that the motor runs at rated speed.
And adjust the potential divider so that rated voltage is obtained at its
terminals.
5. Gradually apply the load on the generator in steps and note down readings
of the load as given on the name plate details of the generator keeping the
speed constant by adjusting the field regulator of the motor.
6. Plot the external and internal characteristic graph.
__________________________________________________________________
- 21 -
DC Machines Lab
7. Determine the armature resistance experimentally using the circuit diagram.
8. Take at least six readings of the voltmeter and ammeter by varying the load
resistance and determine the average value of armature resistance
Observations:
S. No
Load
current(IL)
Field
current
(If )
Ia= IL+If
Terminal
voltage
(Vt)
Eg=Vt+IaRa
1
2
3
4
5
6
Graph:
Y
Armature
reaction drop
V Eg
Armature
resistance
drop
0
IL, Ia
X
Result:
Internal and external characteristics are ploted by conducting load test on
DC shunt generator.
__________________________________________________________________
- 22 -
DC Machines Lab
5.LOAD TEST ON DC SERIES GENERATOR
Aim:
To conduct load test on the given DC series generator and to obtain its
performance characteristics.
Nameplate Details: (To be noted Down from the Machine)
S. No
1
2
3
4
5
Parameters
Rated voltage
Rated current
Rated speed
Rated power
Rated field current
Motor
Generator
Apparatus required:
S.NO
1
2
3
4
5
Equipment
Range
Type
Qty
Ammeter.
Voltmeter.
Rheostat
Load
Tachometer
Theory:
The load characteristics curve of DC series generator shows the relation
b/w its terminal voltage and load current. The characteristics are rising in nature
and excitation increases with load. At large values of load current, the terminal
voltage must be start decreasing owing to the saturation of the machine iron &
rapidly increasing voltage drop of armature and armature resistance.
__________________________________________________________________
- 23 -
DC Machines Lab
Circuit diagram:-
+
DPST Switch
3 point starter
L F A
DPST Switch
A
YY
Fr1
Y
230 V
DC
Supply
Fr2
A
A
G
M
F
+
V
−
FF
AA
AA
Resistive Load
−
Fuse 20A
Fuse 20A
Armature Resistance (Ra):DPST
FUSE 5A
+
+
A
−
+
A
230 V
DC
Suppl
y
M
V
−
AA
−
Series Field Resistance (Rse):DPST
FUSE 5A
+
+
A
230 V
DC
Suppl
y
Y
−
+
V
YY
−
−
__________________________________________________________________
- 24 -
DC Machines Lab
Procedure:
1.
2.
3.
4.
5.
6.
7.
8.
Connect the circuit the circuit as shown in the figure
Start the motor with the help of 3-point starter
Adjust the field regulator of the motor so that the motor runs at rated speed.
And adjust the potential divider so that rated voltage is obtained at its
terminals.
Gradually apply the load on the generator in steps and note down readings
of the load as given on the name plate details of the generator keeping the
speed constant by adjusting the field regulator of the motor.
Plot the external and internal characteristic graph.
Determine the armature resistance experimentally using the circuit diagram.
Take at least six readings of the voltmeter and ammeter by varying the load
resistance and determine the average value of armature resistance
Graph:
Plot the graph b/w terminal voltage and load current by taking ‘V’ on Yaxis and ‘IL’ on X-axis, and Eg on Y axis and Ia on X axis.
X
OCC
Eg & V
Internal
External
Ia = Ise
Y
Eg&V v/s Ia=Ise
__________________________________________________________________
- 25 -
DC Machines Lab
Observations:
S.no
Terminal voltage (V)
Load current
IL=Ia=Ise
Eg=V+IaRa+IaRa
1.
2.
3.
4.
5.
6.
7.
8.
Result:
Internal and external characteristics are ploted by conducting load test on
DC series generator.
__________________________________________________________________
- 26 -
DC Machines Lab
6.SPEED CONTROL OF DC SHUNT MOTOR
Aim:
To conduct speed controls on DC shunt motor.
The methods are
1. Armature voltage control method
2. Flux control method
Nameplate Details (To be noted Down from the Machine)
S.No
1
2
3
4
5
Parameters
Rated voltage
Rated current
Rated speed
Rated power
Rated field current
Motor
Apparatus:
S.No
1
Equipment
Range
Type
Qty
Ammeter
2
3
Voltmeter
Rheostats
4
5
Tachometer
Connecting Wires
Theory:
i) Armature voltage control method:
For a load of constant Torque, the speed is proportional to the applied to the
armature. Therefore speed voltage characteristic is linear and is a straight line. As
the voltage is decrease across the armature the speed falls. This method gives
speeds less than rated speeds.
Eb α ΦN
Eb α N
V-Ia(Ra+R) α N
As the voltage is decreased speed decreases.
__________________________________________________________________
- 27 -
DC Machines Lab
Flux Control Method:
ii)
With rated voltage applied to the motor, the field resistance is increased i.e field
current is decreased. I t is observed that speed increases.
Eb/Φ α N
N α Eb/If
The characteristics If Vs N is inverse (or) if it is hyperbola.
Circuit diagram:
DPST Switch
F
L
A
Ar1
Ar2
+
-
A
Fr1
230 V
DC
Supply
A
+
Fr2
M
F
−
FF
V
−
AA
Fuse 5A
Armature Resistance (Ra):-
DPST
FUSE 5A
+
+
A
230 V
DC
Suppl
y
−−_-
A
+
M
AA
V
−
−
__________________________________________________________________
- 28 -
DC Machines Lab
Procedure:
i) Armature Voltage Control Method
1) Make connections as per the circuit diagram.
2) Keeping both rheostats at minimum, Start the motor with the help of starter and
by adjusting field rheostat bring the motor to rated speed.
3) By increasing armature circuit rheostat in steps note down voltage, Ia and speed
at every step.
4) The corresponding graph is draw between armature Voltage Vs speed.
ii) Flux Control method:
1) Make connections as per the circuit diagram.
2) Keeping both rheostats at minimum, Start the motor with the help of
starter and by adjusting field rheostat bring the motor to rated speed.
3) By increasing field circuit rheostat in steps note down current, If and
speed at every step.
4) The corresponding graph is draw between field current Vs speed
Tabular Column:
Armature Voltage Control Method:
S. No
Armature Voltage in volts
Speed in RPM
1
2
3
4
5
Flux Control Method:
S. No
1
2
3
4
5
Field Current in amps
Speed in RPM
Expected graphs:__________________________________________________________________
- 29 -
DC Machines Lab
Y
N
O
X
If(I)
N Vs If
Y
N
O
X
Va(V)
N Vs Va
Result:
The variation of speed with armature voltage and field current in
case of DC shunt motor is studied.
__________________________________________________________________
- 30 -
DC Machines Lab
6. SWINBURNE’ S TEST ON DC SHUNT MACHINE
PREDETERMINATION OF EFFICIENCIES
Aim:
To perform no load test on dc motor and to predetermine the efficiencies of
the machine acting as a motor and generator.
S. No
1
2
3
4
5
Parameters
Rated voltage
Rated current
Rated speed
Rated power
Rated field current
Motor
Equipment:
S.No
1
2
Apparatus
Voltmeter
Voltmeter
3
Ammeter
4
Ammeter
5
Rheostats
Type
Range
qty
Theory:
It is simple indirect method in which losses are measured separately
and the efficiency at any desired load can be predetermined. This test applicable to
those machines in which flux is practically constant i.e. shunt and compound
wound machines. The no load power input to armature consist iron losses in core,
friction loss, windage loss and armature copper loss. It is convenient and
economical because power required to test a large machine is small i.e. only no
load power. But no account is taken the change in iron losses from no load to full
load due to armature reaction flux is distorted which increases the iron losses in
some cases by as 50%.
__________________________________________________________________
- 31 -
DC Machines Lab
Circuit diagram:
DPST Switch
+
+
A
L
−
F
A
Ar1
Ar2
+
-
A
Fr1
+
230 V
DC
Supply
A
V
−
+
Fr2
M
F
−
FF
AA
V
−
Fuse 5A
Circuit diagram to find out Ra:
+
DPST Switch
+
A
−
230 V
DC
Supply
A
+
M
AA
−
V
A
−
Fuse 5A
Procedure:
1. Make connections as per the circuit diagram.
2. Show the connections to the lab instructor.
3. Keeping both rheostats at minimum, Start the motor with the help of starter and
by adjusting field rheostat bring the motor to rated speed.
4. Note down all the meter readings at no load..
5. Do necessary calculations and find out the efficiency of the Machine as a motor
and as a generator.
6. Draw the graphs between output Vs efficiency of the Machine as a generator and
as a motor.
__________________________________________________________________
- 32 -
DC Machines Lab
Observations:
vt
IL
If
VtIf
Iam =ILIf
Iag =
IL+If
Iam2Ram
Iag2Rag
Wom
Wc=
Wom+
VtIf
TLM
Wc+
Iam2Ra
TLG
WC+
Iag2Ra
O/Pg=I/Pm=
VtIL
I/Pg=
O/Pg+
TLG
O/Pm=
VtILTLM
Ƞm
Expected graphs:-
η
(
%
)
Generator
Motor
Efficiency v/s Output
O
Output
Calculations:
No load armature copper losses =Ia 2 Ra =(Il –If)2 Ra
Constant losses Wc=V l–(Il-If)2 Ra
Efficiency as a motor:
I= Assumed load current
Motor i/p=VI
Ia=IL-If
Motor armature losses=I2a .Ra
Total losses=I2a Ra+ Wc
Efficiency of motor= VI- I2a Ra+ Wc / VI x (100)
Efficiency as generator:
__________________________________________________________________
- 33 -
Ƞg
DC Machines Lab
I=assumed load current
Generator O/P =VI
Generator armature cu. Losses= I2a .Ra
Total losses= I2a Ra+ Wc
Efficiency of generator=VI / VI+ I2a Ra+ Wc x (100)
Results:
The Swinburne’s test has been performed on DC shunt machine and
efficiency of both motor and generator is obtained.
7. LOAD TEST ON COMPOUND GENERATOR
__________________________________________________________________
- 34 -
DC Machines Lab
Aim:
To conduct load test on DC compound generator and to determine its
characteristics.
S. No
Parameters
Motor
1
2
3
4
5
Rated voltage
Rated current
Rated speed
Rated power
Rated field current
Generator
Apparatus:
S. No
Equipment
1.
2.
3.
4.
5
Voltmeter
Ammeter
Rheostats
Tachometer
Connecting wires
Range
Type
Qty
Theory:
D.C. Compound generator consists of both series and shunt field
windings. The shunt and series fields can be connected in two ways.
1. Short shunt.
2. Long shunt.
When the MMF of series field opposes the MMF of shunt field, the
generator is differentially compound. The terminal voltage decreases sharply with
increasing load current. Evidently this connection is not used.
In cumulative compound the connections of the two fields are such that
their MMF’s added and help each other. If the series field is very strong, the
terminal voltage may increase as the load current increases and it is called over
compounding. When terminal voltage on full load and no load are equal, it is
known as flat compounded generator. If the series field is not strong, the terminal
voltage will decreases with increase in load current (under compound)
Circuit diagram for cumulative compound generator:
__________________________________________________________________
- 35 -
DC Machines Lab
+
DPST Switch
3 point starter
A
F
L
−
+
DPST
DPSTSwitch
Switch
A
Y
+
230 V
DC
Supply
YY
A
A
M
F
A
+
V
G
FF
AA
AA
F
−
FF
Resistive Load
−
Fuse 20A
Fuse 20A
FUSE 5A
+
+
A
−
A
230 V
DC
Suppl
y
+
M
V
−
AA
−
Series Field Resistance (Rse):DPST
FUSE 5A
+
+
A
230 V
DC
Suppl
y
−
Y
+
V
YY
−
−
Procedure:
__________________________________________________________________
- 36 -
DC Machines Lab
1. Make the connections as shown in the figure
2. Keep the D.C motor field in minimum position and generator rheostat in
minimum position initially load should not be connected to generator out put
terminals and are open circuited.
3. Motor started with the help of a 3-point starter and its field regulator is adjusted
so that rated speed of motor obtained
4. The gen field rheostat is also adjust such that the rated voltage is obtained at
the gen terminals.
5. By keeping the gen field excitation constant load is switched on varied in steps
and meters readings are tabulated.
6. In each step of load variation readings are taken the motor is running at rated
speed is achieved by adjusting motor field rheostat variation of load in speed is
continued till the generator supplies rated current.
7. When the current exceeds gen rated current decrease the load to zero and open
the DPST
Tabular Column:S. No
IL, Amps
VT Volts
If , Amps
IA= IL+ If
Eg = VT +IA (RA+Rsc)
1
2
3
4
5
6
7
Model graphs:__________________________________________________________________
- 37 -
DC Machines Lab
Internal characteristics
Y
Eg
Cumulative
Differential
O
X
IA
Eg v/s IA
External characteristics
Y
V
Cumulativ
e
Differential
O
IA
X
V v/s IA
Result:By conducting load test the performance characteristics of DC compound
generator are obtained.
8. FIELDS TEST ON TWO IDENTICAL DC SERIES
__________________________________________________________________
- 38 -
DC Machines Lab
MACHINES
Aim:
To determination the efficiency of two mechanically coupled series
machines by conducting field’s test.
Name Plate Details(To be noted Down from the Machine)
S.No
1
2
3
4
Parameters
Rated voltage
Rated current
Rated speed
Rated power
Motor
Generator
Apparatus:
S.No
1.
2
3.
4.
Equipment
Voltmeter
Ammeter
Resistive load
Connecting wires
Range
Type
Qty
Theory:
This test is applicable for two series machines which are coupled
mechanically. Series machines cannot be tested on no load conditions due to
dangerous high speeds. One machine normally run as motor and drives generator
whose output is wasted in a variable load R. The fields of two machines are
connected in series in order to make iron losses of both the machines equal.
Circuit diagram:
__________________________________________________________________
- 39 -
DC Machines Lab
DPST Switch
+
+
A
−
2 point starter
L A
DPST Switch
−
+
A
Y
230 V
DC
Supply
+
YY
V
+ A
+
V
−
M
M
V
_ AA
−
A
G
−
AA
Y
YY
Resistive Load
Fuse 20A
FUSE 5A
+
+
A
−
A
230 V
DC
Suppl
y
+
M
V
−
AA
−
Series Field Resistance (Rse):-
+
DPST
FUSE 5A
+
A
230 V
DC
Suppl
y
−
Y
+
V
YY
−
−
Procedure:
__________________________________________________________________
- 40 -
DC Machines Lab
1. Connect the circuit as per the circuit diagram.
2. There is always more than half load on the gen to avoid the dangerous
speed.
3. Care must be taken while performing the experiments, because there are
series machines, thus it has dangerous starting torque as well as speed to
avoid the condition more than half load should be maintained on motor.
4. Start the D.C motor by using the 2-point starter while load on the gen
should be adjusted to rated value by changing the load switch.
5. Tabulate the readings of the motor and gen using V & I should be rated
values.
6. Open the DPST for stopping the machine.
7. Take readings of armature and series field resistance of machine by using
DMM.
Observations:
V1 = 196V
I1 = 12.3A
RAM = 1Ω
RseM = 1.1Ω
V2 =230V
I2 = 7.5A
RAG = 2 Ω
RseG = 1.2Ω
V = 204V
Calculations:
V= voltage supplied
V1 = motor input voltage
V2 = generator output terminal voltage
I1 = motor input current
I2 = generator output current
1)
2)
3)
4)
5)
Power input of M-G set = VI1 watts
Power output of M-G set = V2I2 watts.
Total ohmic losses WC = I12 (RAM + RseG + RseM )+ I22 RAG Watts
No load rotational losses in M-G set WO = W- WC Watts
Rotational losses of each machine WS = WO/2 Watts
Motor efficiency:
Motor input = V1I1 Watts
Motor losses = (RAM + RseM) I12+ WS Watts
Efficiency = Motor input - Motor losses / (Motor input)
Generator efficiency:
__________________________________________________________________
- 41 -
DC Machines Lab
Generator output =
Field loss =
Armature copper losses =
Total losses WG =
Efficiency =
V2I2
Watts
2
I1 RseG Watts
I22 RAG Watts
I22 RseG+ I22 RAG+ WS Watts
(output )/ output + losses
Model graphs:η
(%)
Generat
or
Motor
O
Output
Result:
By conducting the field’s test we found the efficiency of series machines
(motor and generator).
__________________________________________________________________
- 42 -

LAB MANUAL ELECTRICAL machines - I :

Transcription

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