Transformer Current Ratio

The current in the windings of a transformer is inversely proportional to the voltage in the windings.

This relationship is expressed in below Equation.

Transformer Current Ratio

where

IP = primary coil current

IS = secondary coil current

Since the voltage ratio is equal to the turns ratio, we can express the current ratio in terms of the turns ratio, as in below Equation.

Transformer Current Ratio Equation

Example 1:

When operated at 120 V in the primary of an iron core transformer, the current in the primary is 4 amps. Find the current in the secondary if the voltage is stepped up to 500 V.

Solution:

Transformer Current Ratio

Next, we solve for Is

Is = (Vp/Vs) Ip

Is = (120/500) 4

Is = 0.96 amps

Example 2:

A transformer with 480 turns on the primary and 60 turns on the secondary draws Solution: 0.6 amps from a 120 V line. Find Is.

Solution :

Transformer Current Ratio Equation

Next, we solve for Is

Is = (Np/Ns) Ip

Is = (480/60) 0.6

Is = 4.8 amps

We should note from the previous examples that a transformer that “steps-up” voltage, “steps-down” the current proportionally.

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