Turn-On of AC Voltage - John Dunn, Consultant, Ambertec, P.E., P.C.

You might think intuitively that by switching AC power into any load at the zero crossing of the line voltage's sine wave, that you would minimize input current surges, but that conception is not always true.

If your load involves a power transformer with which you will have a magnetization inductance, you will be better off to close the on-switch at the peak of the AC voltage waveform at its 90° (or 270°) time.

This point is illustrated in the following sketch which is derived using the recursive differential equations method.   

 

Eventually, the current through the inductance will settle to a waveshape that is in quadrature with the waveshape of the line voltage. If you switch on at the peak of the AC line voltage, that current waveshape will immediately have its zero crossing in the proper places.

However, if you switch on an the zero crossing of the input voltage waveform, the current waveform will not be in its proper relationship to the input voltage, but will take some time to get there with a time constant of essentially L/Rs which if the line impedance is low, can be a very long time indeed.

Did you ever hear a power transformer make the noise: "HHMMMMMMmmmmmm.........." ?

Comments

[George Storm]

Agreed - but the message needs to be "understand and/or check the full system before committing.
The overdrive can of course be a lot worse than this if the transformer saturates because it is not sufficiently derated to handle this condition - and this used to be relatively common with US (designed-for-60Hz) equipment that was imported into Europe.
On the other hand, if the equipment also includes a capacitor to correct power factor, the peak will be limited only by the drive impedance and the series resistance of the capacitor; it will however be of relatively short duration.
Things can become even more interesting if the transformer is driving a capacitor-smoothed rectifier...

[Hans Hammarquist]

This explains why, sometimes, when you switch on a toroid transformer (like a VARIAC) it "blows" the fuse. The DC component will saturate the core with excess current as result.

[John Dunn]

Hello, Hans.

Your point is very well taken. I had forgotten that toroidal transformers are especially prone to this problem.

Thank you for the reminder!