Re: Using SSR to switch transformer

Paul E. Schoen wrote:
"Arlet Ottens" <usenet+5@xxxxxxxxxx> wrote in message news:47e7e311$0$14354$e4fe514c@xxxxxxxxxxxxxxxxx
Jan Panteltje wrote:
On a sunny day (Mon, 24 Mar 2008 17:36:05 +0100) it happened Arlet Ottens
<usenet+5@xxxxxxxxxx> wrote in <47e7d876$0$14342$e4fe514c@xxxxxxxxxxxxxx>:

After reading the various bits of advice, I decided to order a big SSR (50A continuous) to build a prototype.
Some solid state relais already have a zero crossing chip.
Does yours?
No, I picked one with random turn on (Crouzet 84 134 320), otherwise there's no way to control the inrush currents.

The ones with zero crossing logic seem to be more prevalent, but there are still plenty of choices for random turn on types.

I picked this big one for my prototype because it had a better chance of surviving any timing errors. For production, I'll look for something smaller and cheaper.

Your solution seems to be ideal. For my application, I cannot use a soft start, because I must apply a full current waveform to a circuit breaker to simulate a fault, and measure its response time accurately. The best I can do is time the initial application of the test voltage to coincide approximately to the expected zero crossing of current, based on the impedance of the source and the load, which are both mostly inductive.

Also, in my case, the load itself opens while the test set has voltage applied, and it may open at any point on the waveform. Thus arcing and inductive spikes are inevitable, and the transformer may have some remanant magnetism after the breaker trips, which will then cause a high inrush on the next test.

But I have considered adding a demagnetizing sequence after the trip is detected. I think a series of phase modulated pulses with gradually reduced duty cycle might reduce the magnetization of the transformer to a minimal level for the next test.

I tried some experiments using phase control on the SSR to slowly turn off the transformer over 50 cycles (1 second), followed by a test using a few whole cycles starting at a zero crossing.

I noticed there was still some increased current surge (2 times normal on the first half cycle), depending on the polarity of the first half cycle of power. Apparently, the demagnetizing wasn't complete. However, it is a lot better than starting with the core fully magnetized in a unfavorable direction. Results may vary with transformer type.

You may also be able to put the core in a consistent magnetized state after a test, and always start at opposite phase.
.

Relevant Pages

  • Re: Using SSR to switch transformer
    ... Some solid state relais already have a zero crossing chip. ... and the transformer may have some remanant ... magnetism after the breaker trips, which will then cause a high inrush on ... duty cycle might reduce the magnetization of the transformer to a minimal ...
    (sci.electronics.design)
  • Re: AC power control.
    ... >> half cycles) into a transformer. ... > That's why I said "whole cycle". ... If you switch a transformer at zero crossing you get maximum inrush, ... saturate the core, note that this occurs even if you use whole cycles ...
    (sci.electronics.design)
  • Re: AC power control.
    ... Initially these will be used on-off, with zero crossing switching. ... possibly capacitive loads is rather silly. ... > half cycles) into a transformer. ... That's why I said "whole cycle". ...
    (sci.electronics.design)
  • Re: AC power control.
    ... >> Zero crossing causes massive inrush surges with all transformer loads as ... > It will be used as a zero crossing switch of course fuckwit. ... > but you have to use either a whole cycle, ...
    (sci.electronics.design)
  • Re: PLL confusion
    ... >>zero crossing of a single cycle or something like that), ... >>When taking a reference for phase it's good practice to take something ... >>signals DO have, ...
    (sci.electronics.basics)