Electrical Machines Lab
for
B.Tech / B.E /Diploma
&
Electrical And Electronics Engineering
Open Circuit Test :
OC & SC TEST ON 1–PHASE
TRANSFORMER.
The difficulties in testing
of large transformers by direct loading
(I) large amount of energy has to be
wasted in such test
(ii) it is a stupendous (impossible for large transformers)
task to arrange a load large enough for direct loading.
Thus performance
characteristics of a transformer must be computed from a knowledge of its
equivalent circuit parameter which, in turns, are determined by conducting
simple tests involving very little power consumption, called no loading test.
In these tests the power consumption is very small. The two-non loading tests
are (I) open circuit test (ii) short circuit test. These two tests help to
determine (1) the parameters of equivalent circuit (2) predetermine voltage
regulation (3) pretermine efficiency.
Open Circuit Test :
As the name suggests, the
secondary is kept open circuited and nominal value of the input voltage is
applied to the primary winding and the input current and power aremeasured. In Fig. V,A,W are the
voltmeter, ammeter and wattmeter respectively.
Let these meters read V1, I0 and W0 respectively.Fig.
shows the equivalent circuit of the transformer under this
test. The no load current at rated voltage is less than 1 percent of nominal current and hence
the loss and drop that take place in primary impedance r1 +jxl1 due to the no load current I0 is negligible. The active
component Ic of the no load current I0 represents the core losses
and reactive current Im is the current needed for the
magnetization.
Thus the watt meter reading The parameters measured
already are in terms of the primary. Sometimes the pri- mary voltage required may
be in kilo-Volts and it may not be feasible to apply nominal
voltage to primary from
the point of safety to personnel and equipment. If the secondary
voltage is low, one can
perform the test with LV side energized keeping the HV side open
circuited. In this case
the parameters that are obtained are in terms of LV . These
have to
be referred to HV side if we need the
equivalent circuit referred to HV side.
Sometimes the nominal
value of high voltage itself may not be known, or in doubt,especially in a rewound
transformer. In such cases an open circuit characteristics is firstobtained, which is a graph
showing the applied voltage as a function of the no load current.This is a non linear curve
as shows This graph is obtained by noting the currentdrawn by transformer at
different applied voltage, keeping the secondary open circuited. Theusual operating point
selected for operation lies at some standard voltage around the kneepoint of the
characteristic. After this value is chosen as the nominal value the parametersare calculated as mentioned above.
Short Circuit Test
The purpose of this test
is to determine the series branch parameters of the equiv-
alent circuit of Fig.. As the name suggests, in this test primary applied voltage, the current
and power input are measured keeping the secondary terminals short circuited.
Let these values be Vsc, Isc and Wsc respectively.
The supply voltage required to circulate rated current through the transformer
is usually very small and is of the order of a few percent of the nominal
voltage. The excitation current which is only 1 percent or less even at rated voltage
becomes negligibly small during this test and hence is neglected. The shunt
branch is thus assumed to be absent. Also I1 = I2 as I0 ≃ 0.
Therefore Wsc is the
sum of thecopper losses in primary and secondary put together. The reactive
power consumed is thatabsorbed by the leakage reactance of the two windings.
If the approximate
equivalent circuit is required then there is no need to separate r1
and r′2 or xl1 and xl2. However
if the exact equivalent circuit is needed then either r1 or r′2 is
determined from the resistance measurement and the other separated from the
total.
As for the separation of xl1 and xl2 is
concerned, they are assumed to be equal. This is a fairly valid assumption for
many types of transformer windings as the leakage flux paths are hrough
air and are similar.
Source : Nptel
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