Transformer Inrush current

[metal]

Don't tell me you have not looked at the crappy code I wrote :] I will modify that, for sure. But, Relay switching times, frankly I don't know when the relay should switch ON, do you mean during the +ve half-wave, -ve half-wave, or should I wait few cyclyes after the SCR half-waves then switch the relay ON, guide me and I will translate that to the code using a POT and reup the simulation.

[dikris]

Yes PM3295 raised an important point because the relay switching time (from the moment of applying voltage to the coil till the relay contacts close) can be anywhere from 3ms to 20ms depending on the relay's type and contact's current. You could reduce the switching time by a factor of 2 or 3 by applying a higher (than the nominal) voltage to the coil for a short time. This requires unfortunately an additional voltage source. For example for a 12V relay  you should have a say 10V source to keep the relay's contacts closed without too much power dissipation in the coil and another 24V or 48V source that will be applied to the coil trough a series RC network at the moment of turning on. By playing with the RC values that control the duration (and the current) of the higher voltage pulse applied to the coil one can determine (experimentally) the shortest switching time achievable with the additional voltage source.

[metal]

Honesntly, my first approach was using a TRIAC that switched at 17~18ms of each +ve half-wave for few cycles and then switched ON at 90 degrees of the +ve half-wave permanently for few seconds, then arelay switched ON. I am starting to see complications in what you have suggested.

[PM3295]

No, I have not looked at the code itself.

As dikris said;
When you send the signal to switch on the relay to bypass the SCR, the relay will have a delay before the contacts actually close. This can vary from 5 ms to as much as 50 ms, depending on the size of the relay. So, if you send the signal to close the relay and the delay is too long, you may actually switch on the wrong polarity!

[metal]

I see, then I better go with a TRIAC instead of a relay for the thyristor which can manager switching ON in a smoother manner than a relay. The relay might just short the TRIAC if necessary.

[PM3295]

One way is to use dual SCR's for better control. Then you don't have to worry about relay switching times.

I modified my circuit to do this. So, you can still have the uni-polar pulses on the one SCR and at the switch point fire both while waiting for the relay to close.

In a practical circuit, one should use an MOC 3021/2 device for triggering.

Attached is the Proteus file for the modified dual SCR control. I added an RC time constant in the relay switch circuit as the current model doesn't have such a parameter.

[metal]

I also modified mine to use a TRAIC - again indeed - and I no longer care about when the relay has to switch ON. Proteus and code attached. It is worth noting that I also modified the ZCD, and replaced the SCR.

Posted on: September 08, 2020, 12:30:40 00:30 - Automerged
BTW, if I wanted to add a "POT/DIP switch" this time, then I would do it to define how many pulses to do before fully switching ON :-D

[PM3295]

I looked at your new sim, but I don't see switching on the negative half after the positive pre-mag pulses. After these pulses, the flux will be pushed up towards positive saturation, and you need to switch at the right moment, forcing it in the opposite direction. It appears from the waveform that you switch on the positive side, and that will push it up into hard saturation. The Proteus simple transformer model unfortunately can't display this saturation effect, so the peak current will be way more than shown.

I placed a cursor where the relay contacts should ideally be closed.

Your previous sim switched in the right sense.

Last attachment is from my modified Proteus cct. The relay switch time is set very long to be able to visually see it happening in the sim.

  

[metal]

In my last sim, I am dependent on the TRIAC, not the relay, so I no longer care when the relay switches ON. The relay is switched ON after 1s from the time TRIAC is fully switched ON on both directions, when the relay stabilizes, I just turn the TRIAC off, I no longer need it, and this way also prolongs the relay life. Please look at the sequence in the code as well. Also if you look at the first attachment in this post and the sim in the current post here, I am imitating the exact thing.

Looking at your sim, seems I have to keep the rhythm of pre-mag pulses as is i.e. on the +ve half-wave, and on the last pulse I have to keep the TRIAC ON so that I don't have to care about the relay switch ON timing. This also ensures that the transformer has actually continued on the -ve half-wave as per your sim.

I don't trust Proteus sim that much, so I don't trust that the relay in the previous sim has actually closed at the right moment, don't believe this lie urself too.

Look at the attachment, is that what you are looking for?

[PM3295]

That looks perfect.

Are you going to include a way to vary (shift pulse timing) the amount of pre-mag current to cater for various transformer sizes? Larger transformers will need a higher current.

See first document, Sideshow Bob reply #12

[metal]

sure, I can do that, may be with a 3-bit DIP switch 16ms to 19ms in 0.5ms steps, sth like 1,2 ,3 ,4, and 5..

[PM3295]

Can you post the latest file to test?

[metal]

Here you go, 2-bit DIP switch, TSR3.rar, 6.PNG

and 3-dip switch, TSR4.rar, 7.png

[PM3295]

Looking good!

[metal]

Now I will re-write the code to become a real code, this code is a big lie! There will be an INT and a delay timer...

[metal]

What about starting small and then go bigger in per-mag then switch ON, what's wrong with that?

[PM3295]

The pre-mag level needs to be set to a fixed level (after experimentation) according to the type and size of the transformer you switching.
Naturally smaller transformers will have lower currents to get to +Bmax, which will be too low for a large transformer.

[PM3295]

metal,

Have you made a working proto of this design yet?

[metal]

I built a prototype, but I was not able to test it because my oscilloscope is broken.. I can't buy a new one because the business is completely destroyed these days..

[PM3295]

I made a quick proto of my analog design to at least see if it works as per simulation. Below is a screen capture switching a 200W toroidal transformer. It seems to be working in principle. After the initial negative peak current, it settles down quickly to normal operating current.

Red CH    :  Is the AC half-cycle reference
Yellow CH : The gate firing pulses
Blue CH    : The transformer primary current
Green CH  : The bypass relay-ON signal, NOT the actual contact-closed point

[dikris]

I made a quick proto of my analog design to at least see if it works as per simulation. Below is a screen capture switching a 200W toroidal transformer. It seems to be working in principle. After the initial negative peak current, it settles down quickly to normal operating current.

Red CH    :  Is the AC half-cycle reference
Yellow CH : The gate firing pulses
Blue CH    : The transformer primary current
Green CH  : The bypass relay-ON signal
[/quote

You didn't switch at the peak of the mains. The surge amplitude would have probably been smaller if you did.

[metal]

Why are you switching at the zero-crossing? You are supposed to do pulses starting at nearly 80~90 degrees and start from there to see how things go, also you did not pulse the transformer for some cycles, look at my sims
you might want to proto mine, I will write the good the code for you

http://www.sonsivri.to/forum/index.php?topic=68555.msg197978#msg197978
http://www.sonsivri.to/forum/index.php?topic=68555.msg197983#msg197983

[PM3295]

You didn't switch at the peak of the mains. The surge amplitude would have probably been smaller if you did.

It worked in principle, but I still have to fine-tune the timing to switch exactly on the negative peak.

The previous waveform of the relay switch-on may be a bit misleading; The trace is where the relay gets the on-signal, but it actually switches sometime later, as expected. If you look at the current though the relay contacts alone below, it is switching very near the peak. The level of pre-magnetizing is also important, and you need to be able to monitor the current for the correct setting.

[metal]

you are right, regardless, +ve or -ve pulses, still I am willing to see you trying my approach as it has lower components count and is much simpler to be honest with you.

[PM3295]

I always make a point to at least test any design I came up with, regardless if it will be put to use or not.