Turns Ratio Definition - What is turns ratio meaning

The turns ratio (often denoted as a or N) is a fundamental parameter of a transformer that defines the relationship between the number of turns of wire in its primary coil to the number of turns in its secondary coil.

It is expressed as:
Turns Ratio (a) = Nₚ / Nₛ
where:

Nₚ = Number of turns in the primary winding (connected to the input/source).

Nₛ = Number of turns in the secondary winding (connected to the output/load).

Key Relationships Derived from the Turns Ratio

The turns ratio directly determines how the transformer changes voltage and current between its primary and secondary sides, assuming an ideal transformer (100% efficient).

1. Voltage Relationship

The voltage across the windings is directly proportional to the number of turns.
Vₚ / Vₛ = Nₚ / Nₛ = a

If a > 1 (Nₚ > Nₛ), it is a step-down transformer. The secondary voltage is lower than the primary voltage (Vₛ < Vₚ).

If a < 1 (Nₚ < Nₛ), it is a step-up transformer. The secondary voltage is higher than the primary voltage (Vₛ > Vₚ).

If a = 1, it is an isolation transformer. The voltages are equal, but the circuits are electrically isolated.

2. Current Relationship

The current in the windings is inversely proportional to the number of turns (to conserve power).
Iₚ / Iₛ = Nₛ / Nₚ = 1/a

In a step-down transformer (high voltage to low voltage), the secondary current is higher.

In a step-up transformer (low voltage to high voltage), the secondary current is lower.

3. Impedance Relationship

The transformer also transforms the apparent impedance of the load. This is crucial for impedance matching.
Zₚ / Zₛ = (Nₚ / Nₛ)² = a²
Where Zₚ is the impedance seen by the source looking into the primary, and Zₛ is the impedance of the actual load connected to the secondary.

Types of Turns Ratio Notation

It's important to note the convention, as the ratio can be written in two ways:

Primary to Secondary (Nₚ/Nₛ): This is the most common definition used in electrical engineering. A ratio of "2:1" means the primary has twice as many turns as the secondary (a step-down transformer).

Secondary to Primary (Nₛ/Nₚ): Sometimes used, especially when focusing on voltage step-up. The same transformer would have a "1:2" ratio in this convention.

Always check the context to see which definition is being used.