TESTING

1. Testing of transformer
THERE ARE FIVE TYPES OF
TESTING
 OPEN CIRCUIT TEST ON TRANSFORMER
 Short Circuit Test on Transformer
 LOAD TEST ON SINGLE PHASE TRANSFORMER
 SUMPNER'S TEST
 POLARITY TEST

2. OPEN CIRCUIT TEST ON TRANSFORMER
One of the winding is connected to the supply at rated
voltage,while the other winding is kept open circuited(HV).
A voltmeter, wattmeter, and an ammeter are connected in(LV)
winding.
 The voltage at rated frequency is applied to that LV side with the
help of a variac of variable ratio auto transformer .

4.  The HV side of the transformer is kept open. Now with the
help of variac, applied voltage gets slowly increased until the
voltmeter gives reading equal to the rated voltage of the LV
side.
 After reaching at rated LV side voltage, all three instruments
reading (Voltmeter, Ammeter and Wattmeter readings) are
recorded.
 The ammeter reading gives the no load current Ie. As no load
current Ie is quite small compared to rated current of the
transformer, the voltage drops due to this current that can be
taken as negligible.
 Since, voltmeter reading V1 can be considered equal to
secondary induced voltage of the transformer, the input power
during test is indicated by watt-meter reading.
 As the transformer is open circuited, there is no output, hence
the input power here consists of core losses in transformer and
copper loss in transformer during no load condition.

5. Short Circuit Test on Transformer
 The connection diagram for short circuit test on transformer is
shown in the figure. A voltmeter, wattmeter, and an ammeter
are connected in HV side of the transformer as shown.
 The voltage at rated frequency is applied to that HV side with
the help of a variac of variable ratio auto transformer.
 The LV side of the transformer is short circuited. Now with the
help of variac applied voltage is slowly increased until the
ammeter gives reading equal to the rated current of the HV
side.
 After reaching at rated current of HV side, all three
instruments reading (Voltmeter, Ammeter and Watt-meter
readings) are recorded.

7.  The ammeter reading gives the primary equivalent of full load
current IL.
 As the voltage applied for full load current in short circuit test
on transformer is quite small compared to the rated primary
voltage of the transformer, the core losses in transformer can
be taken as negligible here.
 The input power during test is indicated by watt-meter reading.
As the transformer is short circuited, there is no output; hence
the input power here consists of copper losses in transformer.
 Therefore it is seen that the short circuit test on transformer is
used to determine copper loss in transformer at full load and
parameters of approximate equivalent circuit of transformer.

8. LOAD TEST ON SINGLE PHASE
TRANSFORMER
Load test on single phase Transformer aim to
draw the efficiency and regulation characteristics
of single phase transformer.
 Each of the terminals of primary as well as secondary
winding of a transformer is alternately positive and
negative with respect to each other.
 It is essential to know the relative polarities at any
instant of the primary and secondary terminals for
making correct connections under the following type
of operation of the transformer.

10. CHARACTERISTICS CURVES OF SINGLE
PHASE TRANSFORMER

11. SUMPNER'S TEST
 Sumpner's test or back to back test can be employed
only when two identical transformers are available.
 Both transformers are connected to supply such that
one transformer is loaded on another.
 Primaries of the two identical transformers are
connected in parallel across a supply.
 Secondary are connected in series such that emf's of
them are opposite to each other.
 Another low voltage supply is connected in series with
secondary to get the readings, as shown in the circuit
diagram shown below.

13. T1 and T2 are identical transformers.
Secondary of them are connected in voltage
opposition.
as transformers are identical. In this case, as
per superposition theorem, no current flows
through secondary.
Thus input power measured by wattmeter W1
is equal to iron losses of both transformers.
i.e. iron loss per transformer Pi = W1/2.

14. A small voltage v2 is injected into secondary
with the help of a low voltage transformer.
the voltage v2 is adjusted so that, the rated
current i2 flows through the secondary. in this
case, both primaries and secondary carry rated
current.
thus short circuit test is simulated and
wattmeter w2 shows total full load copper
losses of both transformers.
i.e. copper loss per transformer pcu = w2/2.

15. From above test results,the full load efficiency
of each transformer can be given as-

16. POLARITY TEST
Polarity in electrical terms refers to the Positive or
Negative conductors within a d.c. circuit, or to the
Line and Neutral conductor within an a.c. circuit.
the resultant voltage appearing across a voltmeter
will be the sum of the high and low voltage
windings.
This is useful when connecting single phase
transformers in parallel for three phase operations.

17.  Polarity is a term used only with single phase
transformers.

18. Let A1 and A2 denotes the ends of the primary
winding.
Let a1 and a2 denotes the ends of the
secondary winding.
Ends A1and a1 are joined and a voltmeter is
connected across A2and a2.
Single phase supply is given to A1 and A2.
If the direction of the induced emf in primary
(Ep) and that of the secondary (Es) is the same.

19. The voltmeter will read the difference of
emf(Ep ~ Es).
If the windings are wound in the opposite
direction on the magnetic core,Ep and Es will
be in the opposite direction.
The voltmeter will read Ep+Es.in this case, the
polarity is know as additive.
Generally,the polarity used is subtractive.
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