Current transformers differ from voltage transformers in that the primary winding is generally part of the power line. The primary winding of a current transformer must be connected in series with the load, Figure 21–1. Current transformers are used to change the full scale range of AC ammeters. Most in-line ammeters (ammeters that must be connected directly into the line) that have multiple range values used a current transformer to provide the different ranges, Figure 21–2. The full scale value of the ammeter is changed by changing the turns ratio. Assume that the ammeter illustrated in Figure 21–2 is to provide range values of 5 amperes, 2.5 amperes, 1 ampere, and 0.5 ampere. Also assume that the meter movement requires a current flow of 100 mA (0.100) to deflect the meter full scale and that the primary of the current transformer contains 5 turns of wire. Transformer formulas can
A current transformer of this type is basically a toroid transformer. A toroid transformer is constructed with a hollow core similar to a donut in that it has a hole in the middle. When current transformers are used, the main power line is inserted through the opening in the transformer, Figure 21–3. The power line acts as the primary of the transformer and is considered to be one turn.
The turns ratio of the transformer can be changed by looping the power wire through the opening in the transformer to produce a primary winding of more than one turn. For example, assume a current transformer has a ratio of 600:5. If the primary power wire is inserted through the opening, it will require a current of 600 amps to deflect the meter full scale. If the primary power conductor is looped around and inserted through the window a second time, the primary now contains two turns of wire instead of one, Figure 21–4. It now requires 300 amps of current flow in the primary to deflect the meter full scale. If the primary conductor is looped through the opening a third time, it would require only 200 amps of current flow to deflect the meter full scale.