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Author Topic: Transformer Inrush current  (Read 4303 times)
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metal
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« on: August 31, 2020, 10:32:48 10:32 »

Hello,

I am trying to understand how I can do a replica for this https://www.tauscher-transformatoren.de/assets/pdf/R_current_inrush_avoider.pdf
I can't understand how this work, how I should switch the traic, when, for how long? which quadrant, etc..
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Sideshow Bob
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« Reply #1 on: August 31, 2020, 02:35:31 14:35 »

From what I understand TSR is using unipolar voltage impules to premagnetise the transformer. In the PDF you linked to the look to be in applied in the posetive half on the falling edge. Probably because this will most easy usning a solidstate switching element. Is also looks like the transfomer are switched fully on some time during the second pulse. The width of the pulse needed will depend on the transformer so TSR devices are have an adjusting pot.
Ps I have also seen seen pictures using the negative period. I do not think it matter as long as the premagnetise pulses are unipolar.
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vern
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« Reply #2 on: August 31, 2020, 03:19:48 15:19 »

to minimize inrush current you should switch on the transformer near the positive or negative voltage maximum of the AC sinus.
Gate pulse length does not matter, once the triac is triggered it will stay ON until the load current drops to almost zero.
Just check the datasheet of your triac for minimum trigger puls length.
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metal
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« Reply #3 on: August 31, 2020, 03:58:55 15:58 »

finally http://ww1.microchip.com/downloads/jp/AppNotes/jp587377.pdf
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PM3295
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« Reply #4 on: August 31, 2020, 07:23:31 19:23 »

to minimize inrush current you should switch on the transformer near the positive or negative voltage maximum of the AC sinus.

This is correct. The high inrush current is normally a result of core saturation during turn on, pushing the flux density into the saturation regions of the particular B-H curve. If you apply voltage at one of the peaks, you only accumulate 1/2 of the volt-seconds before the B-H trajectory moves through zero and in the reverse direction. Switching on at zero volts you accumulate a full half cycle of volt-seconds and  can push the core into saturation.
« Last Edit: September 02, 2020, 07:01:28 07:01 by PM3295 » Logged
metal
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« Reply #5 on: August 31, 2020, 07:30:47 19:30 »

Do you mean that I have to switch either -ve or +ve - but not both - at 90 degrees for like 20 cycles before applying full power?
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« Reply #6 on: August 31, 2020, 09:12:56 21:12 »

You just have to catch it on one of the two peaks and keep it on after this "trigger" point. Since you will be moving in the safe areas of the B-H curve after this, you should not experience a massive over-current.
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sadman
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« Reply #7 on: September 01, 2020, 05:37:24 05:37 »

I personally used this soft start with inrush current limiter
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metal
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« Reply #8 on: September 01, 2020, 06:58:33 06:58 »

You just have to catch it on one of the two peaks and keep it on after this "trigger" point. Since you will be moving in the safe areas of the B-H curve after this, you should not experience a massive over-current.

I see, then it is just a matter of switching at 90 degrees instead of zero crossing point and keep on, and that's it? I don't need to repeat switching at 90 degrees for many cycles? this is much easier than I thought, but the implementation of microchip and some papers I read is different, still in the papers there are missing information, unfortunately. Honestly I still don't get it.
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« Reply #9 on: September 01, 2020, 07:04:43 07:04 »

Hello,

I am trying to understand how I can do a replica for this https://www.tauscher-transformatoren.de/assets/pdf/R_current_inrush_avoider.pdf
I can't understand how this work, how I should switch the traic, when, for how long? which quadrant, etc..

more finding

https://www.youtube.com/watch?v=SVLGHB2IxxU

https://www.instructables.com/id/How-to-Make-a-Softstarter/
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Sideshow Bob
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« Reply #10 on: September 01, 2020, 09:21:56 09:21 »

to minimize inrush current you should switch on the transformer near the positive or negative voltage maximum of the AC sinus.
Gate pulse length does not matter, once the triac is triggered it will stay ON until the load current drops to almost zero.
Just check the datasheet of your triac for minimum trigger puls length.
No you are wrong on this matter. Switching on toroidal transformer on peek voltage is NOT a good idea.
Also @Metal and @Vern take a look st this document https://www.noinrush.de/fileadmin/_migrated/content_uploads/01-speech-from-berlin-at-cwieme-2004_01.pdf
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vern
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« Reply #11 on: September 01, 2020, 10:06:45 10:06 »

Sideshow Bob,

very good document indeed.
But it covers some very high power assemblies, where the extra effort for inrush protection does not matter.

However, for lower power and simpler designs it is still advisable to switch near the voltage peaks, because switching at zero crossing is far worse.

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« Reply #12 on: September 01, 2020, 10:52:44 10:52 »

Sideshow Bob,

very good document indeed.
But it covers some very high power assemblies, where the extra effort for inrush protection does not matter.

However, for lower power and simpler designs it is still advisable to switch near the voltage peaks, because switching at zero crossing is far worse.


Yeh I can agree on it is kind of a lesser evil to switch near the peak. And to be pedantic the "correct" thing is to use a train of  premagnetise pulses with the correct with. The number of pulses and width may vary dependent of the transformer. But well as you said this will be needed only for transformers of some size. Kind of not common in the hobbyist world. I also found some more docs about TSR, they may only appeal to those who are interested in the TSR topic though. Both links are public.
https://www.emeko.de/fileadmin/_migrated/content_uploads/01-Doc-101-r4-en_TSR_Initial_Adjustment.pdf
https://www.noinrush.de/fileadmin/_migrated/content_uploads/01-Press-TSR-Speech-2009-en.pdf
https://www.noinrush.de/index.php?id=98&L=1
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metal
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« Reply #13 on: September 01, 2020, 02:24:18 14:24 »

I think I am getting some where, I am premagnetizing at the +ve half, while TSR at the -ve half, doesn't really matter. But actual number of pulses that is needed for premagnetization is an unknown to me. Am I getting some where, or it is just an illusion Cheesy
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« Reply #14 on: September 01, 2020, 03:04:15 15:04 »

From what I have understod As long as the pulses are unipolar it does not matter. In the 01-Press-TSR-Speech-2009-en.pdf document I think I read that 40 pulses was needed. But that was not a torroid. They are somewhat iffy about how many pulses that are needed
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« Reply #15 on: September 01, 2020, 06:43:50 18:43 »

The technique of switching at one of the peaks will only be very effective if you assume there is no residual magnetism left in the core and the B-H curve is close to zero crossing. If there is any residual magnetism left in the core, you should try and reset it back to the zero point on the curve.

This can be done by applying a lower amplitude AC voltage which decays towards zero over a period of time, effectively demagnetizing the core and moving the B-H point to near zero. After this period, you could switch safely on one of the peaks. This could be accomplished by using some PTC based current limiting circuit, but you should still monitor the current to help determine how long the decay must be to reset the core.

Using the pre-magnetization pulse method, you are also a bit in the dark on how many pulses will be required for a certain transformer without monitoring the current as well. There will be a steep rise in the current when one of the extremes of the B-H curve is reached.

I only see the inrush to be a real problem with large transformers above 1 kVA. With smaller transformers there will be enough copper losses to limit the inrush current.
  
« Last Edit: September 01, 2020, 06:53:55 18:53 by PM3295 » Logged
metal
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« Reply #16 on: September 05, 2020, 02:54:47 14:54 »

here is the schematic of TSR
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« Reply #17 on: September 05, 2020, 03:44:31 15:44 »

here is the schematic of TSR
only schematic  Cheesy Cheesy
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« Reply #18 on: September 06, 2020, 12:54:03 00:54 »

Here is a simple design concept for a TSR to play with. It works in simulation, but I have not tested the actual hardware.
« Last Edit: September 07, 2020, 04:00:53 04:00 by PM3295 » Logged
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« Reply #19 on: September 06, 2020, 03:29:10 03:29 »

I see, then it is just a matter of switching at 90 degrees instead of zero crossing point and keep on, and that's it? I don't need to repeat switching at 90 degrees for many cycles? this is much easier than I thought, but the implementation of microchip and some papers I read is different, still in the papers there are missing information, unfortunately. Honestly I still don't get it.
No you don't have to repeat the switching, just make sure it turns on initially at the peak of the mains and keep the switch on. The inrush current will be the highest during the initial cycle and then will  gradually cease. That is the maximum you could do to limit the inrush current without adding hardware. I don't know what Microchip says but what I am saying is backed up by my experience. The only improvement you could do, if you are using digital control, is to store information at what point of the mains sinusoid the transformer switched off. That will determine at what point (-Br or +Br) the transformer core will remain. So if it is at -Br, you turn the switch on at +Vmax of the mains. Similarly, if the core is at +Br you turn the switch on at -Vmax. This will reduce the inrush current even further.
« Last Edit: September 06, 2020, 03:36:23 03:36 by dikris » Logged
metal
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« Reply #20 on: September 06, 2020, 09:39:02 09:39 »

Here is a simple design concept for a TSR to play with. It works in simulation, but I have not tested the actual hardware.

old school guru Smiley

were you able to simulate the inrush occurring? I shall share my Proteus simulation and PIC code, it is darn simple.

Posted on: September 06, 2020, 11:37:22 11:37 - Automerged

here is my proteus and PIC code. Unfortunately, I have no access to an oscilloscope to measure actual values concerning inrush current with/out this solution. If someone can check if this is an effective design, it will be really awesome. This is a test code which is very simple, sure I will change it to use an interrupt rather than just simple delays as in the simulation.
« Last Edit: September 06, 2020, 09:52:11 09:52 by metal » Logged
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« Reply #21 on: September 07, 2020, 04:06:45 04:06 »


were you able to simulate the inrush occurring?

Yes, I added the simulation of the inrush current for hard switching and soft switching to my previous post. You can also see the comparison of the flux swing for both operations.

Attached is a Proteus file for the circuit I posted before., if someone want to play with it.
« Last Edit: September 07, 2020, 07:59:07 07:59 by PM3295 » Logged
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« Reply #22 on: September 07, 2020, 07:04:24 07:04 »

I personally used this soft start with inrush current limiter
smart and simple.
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metal
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« Reply #23 on: September 07, 2020, 08:58:19 08:58 »

@PM3295

If you tried my sim and been OK with how things work, let me know, so I will modify the code to use an interrupt and I will design a PCB for it as well, in case you are satisfied. The PIC solution is a lot simpler and less components to build.
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« Reply #24 on: September 07, 2020, 02:58:17 14:58 »

metal, I have played around with your sim and it looks good to me. Do you have a way to vary the  switch pulse timing to compensate for the relay switching times? It will be nice if you could have an external preset pot that you can adjust to get it spot on.
« Last Edit: September 07, 2020, 03:00:25 15:00 by PM3295 » Logged
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