PRESENTED BY :
JESTY JOSE – 130800109027
JOBIN ABRAHAM – 130800109028
SAGAR JAISWAL – 130800109029
DANISH KHATRI - 130800109030
ACTIVE LEARNING ASSIGNMENT
 SUBJECT
– DC MACHINE & TRANSFORMER
 SUBJECT
CODE – 2130904
 BRANCH
– ELECTRICAL
GUIDED BY :
Prof. MAYURI TAHILRAMANI
TESTING OF DC MACHINES
CONTENTS :
Objectives Of Testing
 Methods For Testing
 Brake Test
 Swinburne’s Test
 Hopkinson’s Test
 Field Test For DC Series Motor
 Retardation Test

OBJECTIVES OF TESTING :
A
DC machine has to be tested for
proper fabrication and trouble free
operation.
 From the tests one can determine the
external characteristics needed for
application of these machines.
 Also, one can find the efficiency, rating
and temperature rise of the machine.
METHODS FOR TESTING :

1.
2.
3.
There are three methods for testing of DC
Machines. They are :
Direct Method
Indirect Method
Regenerative Method
(1) DIRECT METHOD :
 In this method, the DC machine is loaded
directly by means of a brake applied to a
water cooled pulley coupled to the shaft of
the machine.
 It is not practically possible to arrange
loads for machines of large capacity. So this
method is used only for testing of small dc
machines .
 Brake Test is an example of direct test.
(2) INDIRECT METHOD :
• In this method of 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.
• Swinburne’s Test is an example of Indirect
Test.
(3) 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.
• The electrical output of the generator is feedback
into the supply. Thus the power drawn from the
supply is small only to overcome the losses of
two machines.
• Hence the large machines can be tested at rated
load without consuming much power from the
supply.
• Hopkinson’s Test is an example of Regenerative
Method .
•
BRAKE TEST OR LOAD TEST :
To assess the rating of a machine a load test has to be
conducted.
 When the machine is loaded, certain fraction of the
input is lost inside the machine and appears as heat,
increasing the temperature of the machine.
 If the temperature rise is excessive then it affects the
insulations, ultimately leading to the breakdown of the
insulation and the machine.
 The load test gives the information about the efficiency
of a given machine at any load condition. Also, it gives
the temperature rise of the machine.
 The load test alone can give us the proper information
of the rating and also can help in the direct
measurement of the efficiency.

LIMITATIONS :
1.
2.
Large amount of power is required to test a large
machine and the entire output power is wasted at the
mechanical brake .
Non- availability of large capacity load for testing large
motors in the laboratories.
SWINBURNE’S TEST




Swinburne's test is the most commonly used and simplest method of
testing of shunt and compound wound dc machines which have
constant flux.
In this test the efficiency of the machine at any load is pre-determined.
We can run the machine as a motor or as a generator.
In this method of testing no load losses are measured separately and
eventually we can determine the efficiency.
The following shows the circuit diagram of Swinburne’s Test :
Calculation of Efficiency When the Machine is
Motoring on Load
•
Power input = VI

Armature copper loss, PCU = I2 Ra = (I - Ish)2Ra

Constant losses, WC = VI0 -(I0 - Ish)2 Ra

Total losses = PCU + WC
∴ Efficiency of the motor:
Calculation of Efficiency When the Machine is
Generating on Load

Power input = VI

Armature copper loss, PCU = I2 Ra = (I + Ish)2 Ra

Constant losses, WC = VI0 - (I0 - Ish)2 Ra

Total losses = PCU + WC
∴ Efficiency of the generator:
ADVANTAGES :


This test is very convenient and economical as it is
required very less power from supply to perform the test.
Since constant losses are known, efficiency of Swinburne's
test can be pre-determined at any load.
DISADVANTAGES :




In dc series motor, the Swinburne’s test cannot be done to
find its efficiency as it is a no load test.
Iron loss is neglected though there is change in iron loss
from no load to full load due to armature reaction.
We can’t measure the temperature rise when the machine
is loaded. Power losses can vary with the temperature.
We cannot be sure about the satisfactory commutation on
loaded condition because the test is done on no-load.
HOPKINSON’S TEST




It is a full load test and it requires two identical machines
which are coupled to each other.
One of these two machines is operated as a generator to
supply the mechanical power to the motor and the other
is operated as a motor to drive the generator.
For this process of back to back driving the motor and the
generator, Hopkinson's test is also called back-to-back
test or regenerative test.
If there are no losses in the machine, then no external
power supply would have needed. But due to the drop in
the generator output voltage we need an extra voltage
source to supply the proper input voltage to the motor.
Hence, the power drawn from the external supply is
therefore used to overcome the internal losses of
the motor-generator set.

Efficiency of Generator


Total losses in the generator, WG = (I1 + I3)2 Ra + VI3 +
W/2
Generator output = VI1
Then, efficiency of the generator :
•
Efficiency of Motor

Total losses in the motor, WM = (I1 + I2 - I4)2 Ra + VI4 +
W/2
Motor input = V(I1 + I2)
Then, efficiency of the motor:



Advantages of Hopkinson's Test :
1. This test requires very small power compared to full-load
power of the motor-generator coupled system. That is why it
is economical.
2. Temperature rise and commutation can be observed and
maintained in the limit because this test is done under full
load condition.
3. Change in iron loss due to flux distortion can be taken into
account due to the advantage of its full load condition.
Disadvantages of Hopkinson's Test :
1. It is difficult to find two identical machines needed for
Hopkinson's test.
2. Both machines cannot be loaded equally all the time.
3. It is not possible to get separate iron losses for the two
machines though they are different because of their
excitations.
4. It is difficult to operate the machines at rated speed
because field currents vary widely.
FIELD TEST ON DC SERIES MOTOR

The field test is applied to two similar series motors
which are coupled mechanically. The connection diagram
for the test is shown in the fig.



One machine is made to run as a motor while the other as a
generator which is separately excited. The field of the two machines
are connected in series so that both the machines are equally excited.
This will make iron losses same for the two machines. The two
machines are running at the same speed. The generator output is given
to the variable resistance R.
The resistance R is changed until the current taken by motor reaches
full load value. This will be indicated by ammeter A1.
For Motor :
For Generator:
RETARDATION TEST :
This method is generally employed to shunt generators
and shunt motors. From this method we can get stray
losses.
 Thus if armature and shunt copper losses at any given
load current are known then efficiency of a machine can
be easily estimated.
 The following shows the circuit diagram of Retardation
Test :

 The
machine whose test is to be
taken is run at a speed which is slightly
above its normal speed.
 The supply to the motor is cut off
while the field is kept excited.
 The armature consequently slows
down and its kinetic energy is used in
supplying the rotational or stray losses
which includes iron, friction and
winding loss.
REFERENCES :
Theory & Performance Of Electrical
Machines By – J.B. Gupta
 www.electrical4u.com
 www.nptel.ac.in
