Transformer Inrush Current: What Causes It and How Can It Be Prevented?

What really happens when you plug in a transformer

At the precise moment of powering up, transformateur At the precise moment of powering up, the transformer can absorb a brief but very high current. This peak, which is completely normal, lasts only a few milliseconds, but it can be enough to trigger poorly calibrated electrical protection, both in a domestic installation and in an industrial environment. This phenomenon is often compared to starting an electric motor. The result is similar (a strong current draw), but the cause is totally different. An engine must overcome mechanical inertia. A transformer, on the other hand, does not move. Everything is played out inside, in its magnetic circuit.

The magnetic core: when it reaches saturation

The core of a transformer is made up of a magnetic core, usually made of iron or ferrite. Its role is to channel the magnetic flux created by the current that passes through the primary winding.

To imagine the phenomenon, we can compare this nucleus to a reservoir. As long as it is not full, it absorbs and guides the flow without difficulty. But like any tank, it has a limit. When the magnetic flux becomes too great, the material becomes saturated. From that moment on, the nucleus no longer plays its role as a regulator. The current is then limited only by the resistance of the copper wire... which is very low. As a result, the intensity rises sharply.

There are two situations

• Normal operation: The flow flows correctly in the core, the current remains moderate.
• Magnetic saturation: The core is "full", the current is no longer slowed down and skyrockets: this is the inrush current.

But this saturation does not happen randomly. It is highly dependent on when the transformer is energized.

The importance of the exact timing of the connection

The mains current is not constant: it follows a sine wave at 50 Hz (in Europe). Depending on the precise moment the switch is pressed, the behavior of the transformer can be radically different.

Two extreme cases allow us to understand it well:

• Connection at the top of the sinusoid (maximum voltage): the magnetic flux gradually increases. The core does not saturate. The start is smooth, with no noticeable current draw.
• Zero-crossing connection (zero voltage); This is the worst-case scenario. The magnetic flux is forced to reach a much higher value than normal, which causes an immediate saturation of the nucleus. The current can then reach 10 to 50 times its nominal value.

The effects observed:

• aximum Voltage Connection: Smooth start-up, no current draw
• Zero-crossing connection: Deep saturation, very high current

And that's not all: the transformer can react differently depending on... His past: the residual magnetization.

Residual magnetization: the memory of metal

When a transformer is de-energized, the core does not always return to a perfectly neutral state. Part of the magnetism can remain frozen in the material: this is called residual magnetization. This phenomenon is particularly pronounced on toroidal transformers, where the magnetic circuit is closed and continuous.

On reboot, this "memory" strongly influences the inrush current:

• If the new polarity reinforces the residual magnetization, saturation is reached very quickly (about 2 ms).
• If it compensates for it, the core takes much longer to saturate (up to 20 ms), which greatly reduces the current peak.

This is why two successive power-ups can produce completely different behaviors, without any change in load.

Ces précautions sont particulièrement importantes lorsqu’un transformateur est utilisé avec un gradateur.

Concrete effects and protection solutions

The consequences of the inrush current are very real. A peak of 16 A can cause a voltage drop of about 20 V on a local installation. This is enough to cause the lights to flash or disturb sensitive electronic equipment. A "fast" circuit breaker or fuse is not suitable: it would circuit or melt instantly, even if there is no transformer fault. The solution is to use timed protection, such as a "D-curve" circuit breaker or a timed fuse. They are designed to withstand a brief peak while remaining effective in the event of prolonged overload.

Two points of vigilance for the user:

• The resistance of the primary is decisive: The inrush current is limited only by the resistance of the winding. A simple multimeter in ohmmeter mode allows you to estimate it.
• Choose the right electrical protection: You should always provide protection that allows the peak of the inrush current to pass upstream of the transformers, a "D-curve" circuit breaker or a delayed fuse.