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Conventional transformers for 50 or 60 Hz have more or less inrush current when switched on to the grid voltage. Worst-case inrush happens when the primary winding is connected at an instant around the zero crossing of the primary voltage.

But is there a secondary voltage overshoot when switched on?

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    \$\begingroup\$ Capacitors have inrush current. Inductors, not so much. \$\endgroup\$
    – jonk
    Commented Oct 4, 2022 at 8:15
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    \$\begingroup\$ @jonk core saturation if switched at zero volts AC supply, well known phenomenon which is why we have time delay mains fuses \$\endgroup\$
    – Neil_UK
    Commented Oct 4, 2022 at 8:19
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    \$\begingroup\$ @Neil_UK Yeah. There is that fact. It's just that core saturation doesn't instantly occur. It takes the accumulation of Webers to get there. \$\endgroup\$
    – jonk
    Commented Oct 4, 2022 at 8:20
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    \$\begingroup\$ The slow fuses are slow due to the inrush of the capacitor banks, which may flow through some inductance. But that inductance is not the cause of the inrush. \$\endgroup\$
    – tobalt
    Commented Oct 4, 2022 at 8:22
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    \$\begingroup\$ @tobalt inductance is not the cause of the inrush, it's the sudden decrease of inductance when the core saturates. \$\endgroup\$
    – Neil_UK
    Commented Oct 4, 2022 at 10:14

1 Answer 1

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is there a secondary voltage overshoot when switched on?

Non-saturating core scenario

The secondary voltage is proportional to the rate of change of primary current and, in the inrush situation, the initial rate of change of current is the same as when it settles down so no, there won't be an over-voltage situation on the secondary.

Saturating core scenario

Due to the DC offset in primary current when inrush occurs, the core can saturate. The effects of core saturation can mean large current spikes but, these are simultaneously met with significant reductions in the core permeability. In effect, the two "problems" cancel each other and, there is no secondary overshoot.

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  • \$\begingroup\$ I agree with Andy's answer. I think it's worth mentioning, that due to core saturation, some distortion of the secondary on waveform will occur. \$\endgroup\$
    – Luiz Oliveira
    Commented Oct 4, 2022 at 17:07

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