Transformer Inrush current

[metal]

No problem, I am reluctant to changing the topic, is this oscilloscope any good MP720011? if posts become more than 4 for MP720011 I will open a new thread. This one is available in stock, I don't have to wait for a long time to get it. I will use it for SMPS designs and this project, of course.

[PM3295]

also you did not pulse the transformer for some cycles,

There are about 21 pre-mag pulses before switching, that should be enough I think.

Posted on: October 14, 2020, 07:40:37 19:40 - Automerged
You can't complain about the price for sure. I wonder if it is some re-branding of another popular name?

[metal]

Yes, the price is good, I m not sure about rebranding thing, may be other members can shed light on this..

[PM3295]

This is about the best I can set the balance between pre-mag level and inrush-current. It doesn't take much of a variation to flip it either way (too much or too little). I think smaller transformers may be more sensitive. When I get a chance, I will test it on the 1 kVA isolation transformer I have in the lab.

[metal]

How much current at relay engage moment?

[PM3295]

It appears that with no-load, a bit more pre-mag is required than when it is under some load.

[metal]

can you try to see if more pulses at no load can make any difference? also do you know the actual inrush current for the tranfo you are working with?

[PM3295]

If I catch it on the zero crossing it peaks at about 5A, so roughly twice the peak TSR gives me. With the smaller transformers, the copper resistance of the primary winding will help to limit the peak inrush. I expect the difference to be way more with a large transformer due to the low primary resistance.

I will experiment with more pulses to see what difference it makes.

[metal]

PM3295, look at this, it is interesting!
I am wondering what your response will be concerning post #98 which is the actual scheme used. I still don't understand what he means by 500 times!

[sadman]

PM3295, look at this, it is interesting!
I am wondering what your response will be concerning post #98 which is the actual scheme used. I still don't understand what he means by 500 times!
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PM3295, look at this, it is interesting!
I am wondering what your response will be concerning post #98 which is the actual scheme used. I still don't understand what he means by 500 times!

He run or on/off the load 500 times like repeatbility test and circuit work flawlessly and as expected

[metal]

the scheme is kinda strange, the delay used on each half-wave is like this: 9ms, 8ms, 7ms, 6ms, then fully ON. May be repeating each delay for 125 times? I noted he doesn't use 5ms as he mentioned it caused inrush current to occur.

[PM3295]

It appears that he implemented fundamentally a digital controlled soft-start scheme. Before full switch-on, he slowly increases the flux swing in the core and relies on the losses in the core to balance out any residual offset. This is basically the same method used in industry as I pointed out in one of my previous posts (#15), where they use a low-voltage  variable AC source to reset the residual component in the core.

He the applies the same technique (when he wants to switch off the transformer) by slowly ramping the voltage down, before switching off near zero. This will eliminate most of the residual  magnetism offset that can cause problems with the next initial turn-on cycle of the transformer.

He then tested this up/down ramping action 500 times. At least, this is how I understand this.