What is the need of polarity test on transformer ?
Consider two single phase transformers A and B with
their primaries connected in parallel.
If the voltages induced in primary is such that A1
is positive when B1 is negative, then, the two induced voltages (A1A2
and B1B2) will add up and a large circulating current
would flow in the transformers and the transformers’ windings get melted.
Therefore, these relative polarities of transformer windings must be known if
transformers are to be operated in parallel or in a polyphase circuit, without
damage.
Transformer Polarity Test
Consider a single phase two winding transformer. Connect the primary winding to the supply and
secondary winding to the load. This will induce voltages in primary and
secondary winding of the transformer.
Now, connect a voltmeter between one terminal of
primary and one terminal of secondary winding as shown below.
Short the other terminals of the two windings.
If the voltages induced in the two windings are of
same polarity, as shown in above diagram, then, the voltmeter will give reading
equal to the substraction of the two voltages i.e E1-E2
(assuming step down transformer). This is called substractive polarity, shown
below.
On the other hand, if the induced voltages in the two
windings have opposite polarity, as shown below, the voltmeter will read value
equal to the sum of the induced voltages in the two windings i.e E1+E2
.
This way we can find the correct polarities before
connecting transformers in parallel.
NOTE :- Since the
maximum voltage appearing across the voltmeter during the test is equal to the
sum of peak value of E1 and peak value of E2 i.e (E1)max+(E2)max
. Therefore, voltmeter upper range must be at least equal to (E1)max+(E2)max
.
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