Re: Variac turn-on surge solution

• To: tesla-at-pupman-dot-com
• Subject: Re: Variac turn-on surge solution
• From: "Harri Suomalainen" <harri.suomalainen-at-pp.inet.fi> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Tue, 02 Nov 1999 07:56:43 -0700
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

> I note that the breaker probably only trips when the variac is energized
> near the peaks of the AC waveform.  You could install a big 25 amp solid
> state relay that is designed to switch at the zero voltage crossings.  Thus

u-hum. Variac (or any transformer) is an inductive load. It has 90 degree phase
lag (ideally, in practical device some resistive component is there). If
switching
would occur at zero-crossing point it would draw maxinum transient peak.

However, normally (pure) inductor current is zero at ac peak. So, that's when
the power should be applied. Zero-crossing switch would be much worse.

Simpliest thing is to use some resistor to feed the system and a time relay to
switch on full power somewhat later.

Harri

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