Rapport de transformation

The turns ratio is the number of turns on a transformer’s primary winding divided by the number on its secondary. Voltage follows the ratio one way and current follows it the other, because the power in is the power out (less a little heat).

Voltage one way, current the other

A 4 : 1 transformer puts out a quarter of its input voltage. Since power in equals power out, the secondary can deliver four times the primary current. Step the voltage down and the current steps up; step the voltage up and the current comes down. The ratio of the voltages is the ratio of the turns; the ratio of the currents is the same number upside down.

Where you meet it

Every control transformer, every step-down transformer feeding a panel, every current transformer on a meter. A CT is the same idea used backwards: many turns on the secondary around one primary conductor, so a big current becomes a small one a meter can read. Never open a CT secondary while the primary is carrying current — the voltage across the open terminals can be dangerous.

One worked, so the idea sticks

A control transformer

1. A control transformer has a 480 V primary and a 120 V secondary. (Practice numbers.)
2. Turns ratio: 480 ÷ 120 = 4 : 1.
3. The control circuit draws 2.5 A at 120 V: 120 × 2.5 = 300 VA.
4. Primary current: 300 ÷ 480 = 0.625 A — the secondary current divided by 4.

Real transformers lose a little in heat, so the primary draws slightly more than the ideal figure.

Questions fréquentes

Voir aussi

• Les bases des transformateurs
• Single-phase transformers — the lesson
• Transformer calculator
• Étoile et triangle
• Glossary: turns ratio
• Back to the glossary