Turns ratio
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
- A control transformer has a 480 V primary and a 120 V secondary. (Practice numbers.)
- Turns ratio: 480 ÷ 120 = 4 : 1.
- The control circuit draws 2.5 A at 120 V: 120 × 2.5 = 300 VA.
- 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 people ask
Is the turns ratio the same as the voltage ratio?
For an ideal transformer, yes. A real one sags a little under load, so the secondary voltage at full load is a touch under the ratio; the nameplate voltage is the maker’s figure at rated load.
Does the turns ratio change with load?
No — it is fixed by the windings. What changes with load is the current in both windings and the small voltage drop inside the transformer.
What about three-phase transformers?
The winding turns ratio still applies winding to winding, but wye and delta connections add a factor of √3 between line voltages on the two sides.
Related
- Transformer basics
- Single-phase transformers — the lesson
- Transformer calculator
- Wye and delta
- Glossary: turns ratio
- Back to the glossary
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