It’s All about the Instantaneous Polarity!
Polarity in an AC circuit is constantly changing direction and magnitude. When we talk about the polarity of a transformer, we mean an instantaneous polarity.
The high-voltage windings are always referred to as H1 and H2. The low-voltage windings are always referred to as X1 and X2. If we take an instantaneous polarity, H1 and X1 will always have the same polarity.
Transformers are referred to as having either additive polarity or subtractive polarity. What this refers to is the relative position of high-voltage terminals with respect to the low-voltage terminals as they are brought out of the transformer case. Observing the transformer from the side where the low voltage terminals are brought out, H1 is always located on the left-hand side of the transformer, as shown in Figure 4. Then:
- If the X1 terminal is directly across from the H1 terminal, the transformer has subtractive polarity.
- If the X1 terminal sits diagonally across from the H1 terminal, the transformer has additive polarity.
Sometimes we must determine the polarity of a transformer (the markings have worn off, for example).
- Identify which leads are H leads and which are X leads. This can be done from the gauge and insulation of the windings. High voltage (H), thinner wire, thicker insulation. Low voltage (X), thicker wire, thinner insulation.
- Pick one terminal of the H windings and mark it as H1. The other terminal will be H2 (Thanks to Captain Obvious!).
- Place a jumper between H1 and the adjacent X terminal.
- Apply a voltage to the high side of the transformer. To be safe, keep the value relatively low.
- Place a voltmeter between H2 and the other X terminal. If the voltmeter has a subtractive polarity, the voltmeter will read the difference between the high-side voltage and low-side voltage. If the transformer has an additive polarity, the voltmeter will read the sum of the high-side voltage and low-side voltage.
- Using the turns ratio, we can put a lower voltage than rated on the primary.
- Place a jumper from one H lead to an X lead.
- Read the voltage across the two leads not jumped.
- If the voltage is the sum of the primary and secondary, then the transformer is additive.
- If the voltage is the difference of the primary and secondary, then the transformer is subtractive.