Transcript Advantages of Hopkinson`s test

Mahatma Gandhi Institute of Technical Education &
Research Center, Navsari
PRESENTATION ON
TESTING OF DC MACHINES
DEPARTMENT OF ELECTRICAL ENGINEERING
Year 2014-15
Prepared By
Name
Enroll No.
Vachhani Ankur P.
130330109120
Ukani Jignesh M.
130330109119
Vala Urvik B.
130330109121
Vegad Tushar N.
130330109122
Vyas Divyesh V.
130330109123
Yeshi Gopal N.
130330109124
Guided By
Mr. Animesh Patel
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The methods for testing of dc machines can be broadly classified in to three
methods:
A. Direct method
B. Indirect method
C. Regenerative method
(A) Direct method
In this method of testing, the dc machine is loaded directly by means of
the mechanical brake coupled to the shaft of the machine.
The efficiency of the machine is calculated directly by measuring the
power input and output.
(B) Indirect method
In this method of the testing , the losses are determined without actual
loading of the machine.
If the total losses in the machine are known , the efficiency can be
calculated .
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(C) Regenerative method
This method requires two identical dc machines.
One of the machines is operated as a motor and drives the other
machine as a generator.
Both the machines are mechanically coupled and electrically connected
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This test is the simplest Indirect method for finding out the efficiency of
dc machine.
In this method of testing , constant losses are determined experimentally by
operating the dc machine as motor running at no load .
This test is applicable to dc machines in which flux is practically constant i.e. shunt
and compound machines .
fig (A) circuit diagram of Swinburne's test on dc shunt motor
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Fig(B) Circuit diagram for the measurement of armature resistance
In this test the machine is run as a motor on No-load at its rated voltage.
The speed of the motor is adjusted to its rated value with the help of shunt
field regulator. The following observations are taken
Observation Table
Input voltage
V volts
No-load current
I0 A
Shunt field
current Ish A
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The power input to the armature at No-load supplies the following:
(1) Iron losses
(2) Mechanical losses
(3) No- load Armature Copper loss
Power input to the dc motor at No-load , W0 =VI0
Armature Cu loss at No-load = I²a0 ra
Constant losses of the machine Wc = VI0 - I²a0 ra
Calculation of efficiency when running as a motor
Power Input to the motor = VIL
Armature copper losses at full-load = I²ara
Total losses = Wc + I²ara
Efficiency of the motor
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Calculation of efficiency when running as a generator
Output of the generator = VIL+
Total losses = Wc + I²ara
 Advantages of Swinburne’s test: This test is very simple and convenient.
 The efficiency of the machine can be predetermined at any load condition ,
since constant losses are known.
 Disadvantages of Swinburne’s test: This test is applicable to dc machines in which flux is practically constant.
Series machines can not be tested by this method.
 The temperature rise and the commutation conditions can not be
checked under rated load conditions as the test is performed on No-load.
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This is a regenerative test which requires two identical dc shunt machines coupled
mechanically and connected electrical as shown in fig (C) .
One of the machines is operated as a motor and other as a generator.
The electrical output of the generator is fed to the motor. Hence the power input
from the supply is very small only to overcome the losses of both the machines.
In this test , the mechanical output of the generator is fed as input to the motor .
Hence this test is also called regenerative test or back-to-back test.
Fig(C) circuit diagram for performing Hopkinson's test on two dc shunt machines
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Observations Table
Input
Current
voltage v drawn
volts
from the
supply
I1A
Gen.
Arm.
Current
I2
A
Motor
Arm.
Current
(I1+I2)
A
Motor
field
Current
I3 A
Gen.
Field
Current
I4A
Calculation of efficiency hen running as a motor
Shunt Field copper loss of the motor = VI3
Total losses of the motor , Ptm = Wc + (I1 + I2)²rm + VI3
Total power input to the motor ,
Pi = V(I1 + I2 + I3)
Efficiency of the motor
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Calculation of efficiency when running as a generator
Shunt field copper losses of generator = VI4
Total losses of the generator, Ptg = Wc + I²2 + VI4
output of the generator , P0 = VI2
Efficiency of the generator
Advantages of Hopkinson’s test
 The temperature rise can be estimated during the test.
The efficiency of the machine can be accurately determined at various loads.
The commutation conditions can be checked under rated load conditions.
Disadvantages of Hopkinson’s test
 Availability of two identical dc machines.
 It is impossible to separate out iron losses of the two machines which are different
because of different excitations.
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Fig.(D) Circuit diagram for performing Field’s test on dc series machines
By above fig. small series machines can be tested by direct load test , but the large series
machines cannot be tested by Swinburne’s test because series motor can not run at
No- load due to dangerously high speed .
Field’s test is applicable to two similar series machines. These two machines are
mechanically coupled and electrical connected. One of machines are run as a motor
and drives the other machine as a generator.
A variable load is connected across the generator terminal. The output power of the
generator is wasted in a variable load resistance . This load resistance is varied till the
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motor carries its full load rated current.
Observation Table
Supply
Voltage
V
Motor
Armature
Current
I1
Generator Generator Motor
Armature Output
Voltage
Current
Voltage
V1
I2
V2
Power input to the whole set = VI1
Power output of the set = V2I2
Total losses in the set , Pt = VI1 - V2I2
Armature and field Cu losses of motor = I²1 (ra + rf )
Armature and field Cu losses of generator = I²2ra + I²1rf
Total copper losses of the set
Pc = I²1ra + I²1rf + I²2ra + I²1rf
= I²1 (ra + 2rf) + I²2ra
Stray losses (Iron + Friction) per machine , Ps = 0.5 (Pt - Pc)
The stray losses are equally divided between the machines because of their equal
excitation and speed .
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Motor Efficiency
Generator Efficiency
In this test , the two machines are mechanically coupled and electrically
connected but it is not a regenerative method of testing because the output
power of the generator is wasted in variable load instead of fed back into the
motor or supply as in the case of Hopkinson’s test
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