Electrical Science for Technicians by Waygood Adrian

Author:Waygood, Adrian
Language: eng
Format: epub
ISBN: 9781317534907
Publisher: Taylor and Francis
Published: 2015-06-18T16:00:00+00:00

Figure 11.11

If, at any given instant, the polarities of the adjacent high- and low-voltage terminals are identical – e.g. both ‘positive’, as in Figure 11.11 (left) – then we describe the transformer as having ‘subtractive polarity’. If, on the other hand, the polarities of the diagonally opposite high- and low-voltage terminals are identical, as in Figure 11.11 (right), then we describe the transformer as having ‘additive polarity’. The significance of the terms, ‘subtractive’ and ‘additive’ polarity, will become apparent, shortly.

If we want to connect two transformers in parallel, exactly the same rule applies as for batteries: i.e. the ‘correct’ connection is when terminals having like (instantaneous) polarities are connected together.

So how do we determine whether a particular single-phase transformer has ‘subtractive’ or ‘additive’ polarity, and what exactly do these two terms actually mean?

If a transformer’s terminals are already labelled, then whether that transformer is ‘subtractive’ or ‘additive’ isn’t really an issue (although you may wish to confirm those labels are correct)! It only becomes an issue if the terminals are unlabelled, in which case we need to be able to label them ourselves.

In the UK, the practice is to label a single-phase transformer’s terminals with a letter, followed by a numerical subscript, where:

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the high-voltage terminals are labelled with the upper-case letter ‘A’, and the low-voltage terminals with the corresponding lower-case letter ‘a’. When three singlephase transformers are used to form a three-phase transformer bank, the other transformers’ high-voltage terminals are labelled ‘B’ and ‘C’ respectively, and their corresponding low-voltage terminals are labelled ‘b’ and ‘c’.

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one end of a winding (arbitrarily termed the ‘start’ end) is labelled with an odd-numbered subscript (e.g. ‘A1’), while the other end (termed the ‘finish’ end) is identified with an even-numbered subscript (e.g. ‘A2’).

So, for a straightforward, single-phase, two-winding transformer, the high-voltage terminals will be labelled ‘A1–A2’, and the low-voltage terminals will be labelled ‘a1–a2’. If the transformer has any additional windings, then their terminals can be labelled ‘A3–A4’, ‘a3–a4’, etc., as necessary.

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