EE201P- Electromechanics Lab
Sumpner’s back to back test on single phase transformer
Aim: To perform a back-to-back test on two identical transformers to evaluate
their efficiency
Apparatus:
Name
Single
phase
transformers
Ammeter
Ammeter
Voltmeter
Voltmeter
Wattmeter
Wattmeter
Rating
auto 230 V, 50 Hz
0-2A
0 - 10 A
0 - 100 V
0 - 300 V
75 V – 5 A
150 V – 5 A
Quantity
2
1
1
1
1
1
1
Theory:
This test is conducted simultaneously on two similar transformers to measure the
transformers’ temperature rise, efficiency and voltage regulation. This test is
efficient since the power required to conduct this test is equal to the total losses
of both the transformers. It might be noted that, to carry out this test, the two
transformers need to be identical.
The connection diagram of back to back test on two similar transformers named
T1 and T2 is shown in fig.A. The two low voltage primaries are connected in
parallel across a rated single phase supply of 150 volts. The high voltage
secondary windings are connected in phase opposition so that at rated voltage
input the net output voltage is zero. In this condition both the transformers draw
only the no-load current from the supply and net power in put is equal to the core
losses in the both the transformers. After this, voltage is injected from the
secondary side variac till the rated current flows in the secondary. This rated
current results in the full load ohmic losses in the winding of the transformer.
Secondary side wattmeter reading shows the net ohmic losses in both the
transformers.
Circuit diagram:
Figure A Circuit diagram of Sumpner’s back to back test
Procedure:1. Make the connection as shown in the figure. A.
2. With the help of autotransformer, the rated voltage of 150 V should be
applied to the primary winding.
3. With no secondary supply and correct polarity connections, the reading of
secondary voltmeter should be zero. If it is not zero, check the polarity of
transformers.
4. Insert a voltage in series with the help of secondary autotransformer such
that the rated current flows in the secondary winding.
5. The readings of ammeter, wattmeter, and voltmeter are taken on the
primary and secondary side.
Observations:Sr. No.
Primary Side
Secondary side
Voltage
(V)
Current
(A)
Power
(W)
Voltage
(V)
Current
(A)
Power
(W)
Calculations:-
Precautions:1. Before applying the 1-phase supply, make sure of the polarity of the
transformer windings.
2. In the secondary windings, the current should not go beyond the rated
value.