8.1.3. Current transformation

Load behaviour of the transformer

If the transformer is output-loaded, current I₂ flows into coil N₂. Current I₂ generates the magnetic flow F_2K. According to Lenz’s Law this magnetic flow is counter-positioned to the cause (F_1K).

Figure 126 - Loaded transformer

In this manner the magnet flow F_1K is weakened and induction voltage U₁₀ decreases. Given uniform rated voltage, the difference increases between the two voltages U₁₀ and U₁.

Consequently, a greater input current I₁ flows whereby the magnetic flow F_1K is increased. The magnetic flow F in the iron core thus remains virtually constant:

F = F_1K - F_2K = constant

This also applies to the output voltage of the transformer.

The input current I₁ increases as the load current I₂ becomes greater.

Transformation ratio

Without heeded the losses of the transformer, the following applies according to the energy conservation law:

s₁ = s₂

U₁ · I₁ = U₂ · I₂

If we arrange the equation so that the voltage and current values appears on respective sides, then

.

The following relationships may be cited for current ratio:

Conversely the currents are proportional to the voltages or numbers of turns. A transformer converts high currents into low ones or low currents into higher ones.

Example:

A welding transformer takes up 220 (current being 10A). The output voltage is 20V. How great is the welding current?

Solution:

I₂ » 110A