Turns Ratio in Instrument Transformers: CTs and PTs

This article explains the concept of the turns ratio in instrument transformers, specifically focusing on current transformers (CTs) and potential transformers (PTs).

Turns Ratio Definition

The turns ratio of an instrument transformer is simply the ratio of the number of turns in its primary winding to the number of turns in its secondary winding. However, the way this ratio is expressed differs slightly between current and potential transformers, as detailed below.

Turns Ratio for Current Transformers (CTs)

For current transformers, the turns ratio is defined as the ratio of the number of turns in the secondary winding to the number of turns in the primary winding.

Mathematically, this is represented as:

n = Ns / Np

Where:

• n represents the turns ratio of the CT.
• Ns is the number of turns in the secondary winding.
• Np is the number of turns in the primary winding.

Essentially, this means the CT steps down the current, typically with the secondary having many more turns than the primary.

Turns Ratio for Potential Transformers (PTs)

In contrast to current transformers, the turns ratio for potential transformers is defined as the ratio of the number of turns in the primary winding to the number of turns in the secondary winding.

The equation is as follows:

n = Np / Ns

Where:

• n represents the turns ratio of the PT.
• Np is the number of turns in the primary winding.
• Ns is the number of turns in the secondary winding.

So, a PT steps down the voltage, meaning the primary usually has more turns than the secondary.