Transformer Inrush Analogy

Imagine a big water slide at a water park.

When the water slide is off, no water is flowing down it. When you first turn the water on, there is a sudden rush of water to fill up the slide before it settles down to a steady stream.

This is the analogy I think of for the inrush current in a power transformer. Now, the intensity of this water surge (or the inrush current) can depend on a few factors.

If the water slide already had some water on it (akin to residual flux in the transformer) when you turned the water on, the initial surge could be bigger or smaller depending on whether that water was flowing in the same direction or against it (analogy breaks down a bit 😂). If the transformer was previously energized, it is likely there is some positive or negative flux left.

If the water system is at minimum pressure (similar to maximum positive voltage), there might not be a transient surge at all (meaning just normal exciting inrush). A scenario of maximum transient inrush is when the energization occurs at the voltage zero-point. This can be as high as 30x full-load amperage. In the analogy, this could be thought of as a point of maximum water pressure (pretending that water pressure fluctuates like a sine wave 🙈).

Also, the size of the water slide (similar to the size of the transformer), the water supply source, the material of the slide (like the type of iron in the transformer), and the previous history of water flow (or the L/R ratio of the transformer and system) can affect the intensity of the initial surge. In a three-phase circuit, it's like having three water slides side by side. You might get a surge in one, two, or all three slides when you turn them on. Even if they're turned on at slightly different times (like the voltages being 120° apart in the three-phase circuit), you might still get a surge in at least one of the slides.

The magnetizing inrush can occur under three conditions - when you first turn on the transformer (initial), when the transformer has to recover from an interruption (recover), or when another transformer (or water slide) nearby is turned on (sympathetic).

Lastly, the technology and materials used in making transformers have been getting better over the years, which has helped reduce these inrush currents. Just like how the design and materials of the water slide could be improved to control the initial water surge.