The turns ratio is the transformer's identity card: a single number that tells you how much the voltage is stepped up or down. Understanding it precisely is the foundation for everything that follows: reading the nameplate, the TTR test, and tap-changer adjustment.
The Basic Formula
- The ratio of the two voltages equals the ratio of the number of turns (directly proportional).
- The ratio of the two currents is inverse: the side with the higher voltage carries a lower current.
A 13800/400 volt distribution transformer. Turns ratio = 13800 ÷ 400 = 34.5. If the secondary has 40 turns, then the primary has approximately 40 × 34.5 = 1380 turns.
Important Practical Notes
- The ratio written on the nameplate corresponds to a specific tap-changer position; changing the tap changes the actual ratio.
- The ratio is checked in the field using a TTR device, and a deviation exceeding the tolerance (around 0.5% in many specifications) calls for investigation.
- In three-phase transformers, note that the ratio between line voltages is affected by the connection type (star/delta), not just by the number of turns.
Sample answer: The secondary voltage = 2200 ÷ 10 = 220 volts, and the secondary current = 5 × 10 = 50 amps, neglecting losses. The apparent power is equal on both sides: 2200 × 5 = 220 × 50 = 11 kVA.
Forgetting that the current relationship is inverse, so some multiply the current by the ratio instead of dividing (or vice versa). Always remember: the higher voltage corresponds to the lower current.
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