Transformer Polarity TestPublished on 24/2/2012 & updated on Thursday 3rd of May 2018 at 04:33:52 PM
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We use dot convention to identify the voltage polarity of the mutual inductance of two windings. The two used conventions are:
- If a current enters the dotted terminal of one winding, then the voltage induced on the other winding will be positive at the dotted terminal of the second winding.
- If a current leaves the dotted terminal of one winding, then the polarity of the voltage induced in the other winding will be negative at the dotted terminal of the second winding.
When we look into the operations of the distribution transformers, we find that they need to work all the time and also need to supply at high demand at peak times. So, to cope up with these situations, we connect the transformers in parallel.
Paralleling is done by connecting same polarity terminals of the primary winding together. A similar procedure is done for the secondary winding. Paralleling will increase the power supplying capacity and also the reliability of the system. We do polarity test on parallel transformers to ensure that we connect the same polarity windings and not the opposite ones. If we accidentally connect the opposite polarities of the windings, it will result in a short-circuit and eventually damage the machine. We can categorise the polarity of the transformer to two types,
- Additive Polarity
- Subtractive Polarity
Additive PolarityIn additive polarity, the voltage (Vc) between the primary side (Va) and the secondary side (Vb) will be the sum of both high voltage and the low voltage, i.e. we will get Vc = Va + Vb
Subtractive PolarityIn subtractive polarity, the voltage (Vc) between the primary side (Va) and the secondary side (Vb) will be the difference of both high voltage and the low voltage, i.e. we will get Vc = Va – Vb
We use additive polarity for small-scale distribution transformers and subtractive polarity for large-scale transformers.
Procedure of Polarity Test of Transformer
- Connect the circuit as shown above with a voltmeter (Va) across primary winding and another voltmeter (Vb) across the secondary winding.
- If available, take down the ratings of the transformer and the turn ratio.
- We connect a voltmeter (Vc) between primary and secondary windings.
- We apply some voltage to the primary side.
- By checking the value in the voltmeter (Vc), we can find whether it is additive or subtractive polarity.
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