What is the Polarity Test of Transformer?
The term polarity means the flow of the current direction. In the DC circuit, one terminal is positive, and the second terminal is negative. And current always flows from the positive to the negative terminal.
In a DC circuit, current flows only in one direction. But in the case of the AC circuit, the current changes direction periodically among positive and negative.
If we want to connect two transformers in parallel, we must know the polarity of the primary and secondary winding of the transformer.
Parallel operation of a transformer is done only by connecting the same polarity terminals of the primary winding and secondary winding. If the polarity of both transformers is not matched, it may result in short-circuit or, worst condition, the transformer may damage.
Therefore, it is most important to know the polarity of the transformer.
There are two types of the polarity of a transformer;
- Additive Polarity
- Subtractive Polarity
In additive polarity, the voltage of the primary side (high voltage) and secondary side (low voltage) will be additive, meaning similar terminals of the transformer are connected. Then the polarity is said to be additive polarity.
Additive polarity can be used for small-scale distribution transformers. The circuit diagram of additive polarity is shown in the figure below.
In subtractive polarity, the primary winding and secondary winding voltage will differ between high and low voltage.
The circuit diagram of subtractive polarity is shown in the figure below.
For the step-up transformer,
For the step-down transformer,
Subtractive polarity is used for the large-scale transformer.
Steps to Follow for Polarity Test of Transformer
The connection diagram of the polarity test of the transformer is as shown in the figure below.
Step-1 Connect voltmeters as shown in the above connection diagram. A voltmeter connected to the primary winding measures the primary voltage VP. And the second voltmeter is connected in secondary winding that measures the secondary voltage VS.
Step-2 Connect a voltmeter between primary and secondary winding. The reading of this voltmeter decides the transformer connected in additive or subtractive polarity.
Step-3 Read the transformer ratings if available.
Step-4 Switch ON the single-phase supply using an auto-transformer. And give voltage to the primary winding.
Step-5 Take a reading of voltmeters.
If the reading of VV shows the value of the addition of VP and VS, the transformer is connected in additive polarity.
If the reading of VV shows the value of the difference of VP and VS, the transformer is connected in subtractive polarity.
Suppose you need the transformer in an additive connection, and the result is subtractive. It would be best to have the transformer subtractive, and effects are additive; you can get the required connection by changing terminals on one of the sides (primary or secondary).
While the polarity test of the transformer, the maximum measuring voltage of voltmeter VV was chosen wisely. Because, in additive condition, the voltage of primary and secondary both appear to this voltmeter.
So, take a voltmeter whose maximum measuring voltage range is more the primary and secondary voltage summation.
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