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Preventing the inrush current saturation in a toroidal/EI transformer
Preventing the inrush current saturation in a toroidal/EI transformer
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Old 8th August 2012, 11:10 AM   #11
AndrewT is offline AndrewT  Scotland
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Simple.
Just stick a big resistor in the primary circuit and short that resistor out after a short time delay.
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Old 8th August 2012, 12:41 PM   #12
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Perhaps the trouble in this case is the trafo itself, it may have partially shorted turns.

TRIAC driving a toroid is definitively a bad idea. It's asymetric current flow will give a DC current flowing the primary and really saturating it. A NTC, or a resistor shunted by a relay are better ideas.

BTW, I have seen thousands of toroidal transformers without the problems you mention here. In fact, I saw a 800V 800W toro charging 4 * 470µF 400V in series, for triggering flash lamps that must be fired several times at a minute, in an bottle inspector (Heuft) , and it hasn't any start up current limiter.
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Old 8th August 2012, 01:38 PM   #13
tauro0221 is offline tauro0221  United States
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Hi,
What I explained at the beginning is what the transformer companies are saying.
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Old 8th August 2012, 02:46 PM   #14
AndrewT is offline AndrewT  Scotland
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Tauro, forget what you wrote in post1.
Just do the resistor and measure what it allows.
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Old 8th August 2012, 03:12 PM   #15
tauro0221 is offline tauro0221  United States
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Elvee

I just got the parts and did some test and it ran OKAY. I have small glitch in the switch ON that I am overshooting the cycle wave at the beginning. It is in the software, If I terminated the cycle half way everything ran good.
I did a test switching the EI transformer with the switch and the current reading jump from 3.8 to 4.7 amps. Using the micro the current stay within the limits of 1.7 amp. No jump and it is cleaned.
Thank you for you advised.
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Old 8th August 2012, 06:54 PM   #16
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by tauro0221 View Post
Elvee

I just got the parts and did some test and it ran OKAY. I have small glitch in the switch ON that I am overshooting the cycle wave at the beginning. It is in the software, If I terminated the cycle half way everything ran good.
I did a test switching the EI transformer with the switch and the current reading jump from 3.8 to 4.7 amps. Using the micro the current stay within the limits of 1.7 amp. No jump and it is cleaned.
Thank you for you advised.
Which option did you choose? The peak switching or the soft-start?

Note that with a mechanical switch, there is a wide range of random variation possible: if the switching happens to be synchronized with the peak voltage, the disturbance will be minimal, and the opposite for the zero-crossings.
Therefore, a single test is not very significant.

Also, with an actual transformer and its winding resistance + the rectifier and filter cap, the optimum switching point will be displaced, normally advanced by a number of degrees.
If you want to fine tune the exact moment, simulation is your best option
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Old 8th August 2012, 07:16 PM   #17
tauro0221 is offline tauro0221  United States
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Hi,
I used the soft start it is running good. Yes with the switch I have some good reading. Maybe like you said I was synchronize with the zero crossing. One thing I noticed is that once you reached the half of the cycle wave the reading of the voltage it is at the max. I still need to do some adjustment in the software. I have a tube tuner and I am going to do a test to see how the filaments light up.
Again thank you very much for the advices.
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Old 13th August 2012, 11:35 PM   #18
tauro0221 is offline tauro0221  United States
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Hi,

This is reporting the status of the project. Finally I finished with the software after five different drivers. When testing the final one for the first time immediately a problem was encountered. “Call Houston I have a problem”. This problem was not expecting or prepared for it. When firing the triac from the end of the wave cycle to the beginning from about 30% or 30 degree of the wave cycle the transformer started humming badly and the current when up almost to 18 amps. It is sound like a bell ringing. Tried different thing but was unsuccessfully to fix the problem. During the testing I ran out of fuses so temporary fixed the problem by using a 20 amps fuse. The light from the blown fuse looks like a super nova explosion. The problem happened in both directions. So during the testing I noticed in the scope that this was happening after the wave cycle was at the peak going down. By doing some changes to the triggering timing in the cycle wave angle found that the problem when away at the peak. At this point no current spikes was noticed. Tried switching ON/OFF 3 to 5 times in sequences and every time was successfully completed without current spikes. Making the necessary adjustment to the software so the triggering of the triac start at the peak going down and from going up to stop at the peak everything started to run flawless. Try a soft start by ramping the wave angle cycle up to the peak and down from the peak and it didn’t show any current spikes.

Some of the members already pointed out some of the problems that I may possible encounter but the humming was a big surprise. One suggested to trigger the triac at the peak in both directions would worked out but what I was looking also was to be able to ramp slowly the voltage up and down so it can be use it also in the tube amplifiers. This will extend the life expectancy of the tubes by preventing the filament temporary brightly light up until the heat increase the resistance of the filament to the normal operational temperature condition. This will eliminate the stress in the filament.

Right now I am testing the circuit using a Dynaco 120 transformer with an output of 85 volts AC 3 amps with a load of 2.0 amps. Modified the software by adding a loop to repeat the ramping up/down test 100 times and stop. No problem encountered. Also need to do the same testing using the toroidal transformer. Everything up to now it is looking good and the project look like it is very feasible. I can not count victory yet until the test it is done using the toroidal transformer. That it is the main goals of the project. Need to go to Radio Shack to buy some fuses. Also working in the printed circuit board.

I would like to thank every body that participated with comments and suggestions. Every one was welcome and taken them in consideration for the project.

Please keep in mind that I am not a technical writer. It is hard to explain in words was going so any body can understand it.

Attached it is a sketch showing the final timing.
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File Type: jpg triac gate timing triggering.jpg (183.0 KB, 282 views)
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Old 16th August 2012, 05:17 PM   #19
nigel pearson is offline nigel pearson  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Simple.
Just stick a big resistor in the primary circuit and short that resistor out after a short time delay.
Andrew is 100 % right to say this . It is simple and allows ideal transformer design . Naim Audio in the past designed transformers at the absolute limit of our 13 Amp fuses . The Naim NAP 250 which was only a 2 x 75 Watt design would often blow these . Not a great problem as Naim owners are encouraged to never switch off the amps . Naim also have problems with DC on the mains as a result of the designs ( buzz ) . If I were Naim I would have gone even further and used this idea . I never knew entirely what Naim did . I suspect DC resistance of the primary is very low . If so it needs to build up a field to reduce current flow .

A 3.3 KVA auto transformer I use has very small surge current . It is an E and I type .
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Old 16th August 2012, 06:14 PM   #20
krokkenoster is offline krokkenoster  South Africa
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Default inrush current on transformers?

Quote:
Originally Posted by nigel pearson View Post
Andrew is 100 % right to say this . It is simple and allows ideal transformer design . Naim Audio in the past designed transformers at the absolute limit of our 13 Amp fuses . The Naim NAP 250 which was only a 2 x 75 Watt design would often blow these . Not a great problem as Naim owners are encouraged to never switch off the amps . Naim also have problems with DC on the mains as a result of the designs ( buzz ) . If I were Naim I would have gone even further and used this idea . I never knew entirely what Naim did . I suspect DC resistance of the primary is very low . If so it needs to build up a field to reduce current flow .

A 3.3 KVA auto transformer I use has very small surge current . It is an E and I type .
Hi Guys!
This is something that I never heard before! Is the problem not due to very large electrolytics in the secondary circuit.? I worked on large systems in battery chargers and so on and I never heard of this not even on three phase trannies. Everytime that I had this problem was there a fault in the secondary circuit or the load. IF ANYBODY SAY THEY KNOW ALL ABOUT ELECTRONICS THEY KNOW IN REALITY ZILCH! When I was still working for a government institution some fourty years ago, I had to repair a frequency counter and this thing had a 110 volt setting. The tranny had HALF of the primary burned . I then had the tranny rewound without the tap and the electrical engineer WITH VARSITY DEGREES, was mad at me for he said that he needed the 110volt tap.He wanted to use the 110 volt supply in the power station Now in the power stations for the trains, they had a 110volt batteries and he was connecting this counter on the batteries!! He was amazed when I told him that you can never do it. I demonstrated then to him a 380volt primary tranny on a 24 volt battery and soon SMOKE! I gave him then my old text book to read containing a valved car radio power supply with a vibrator to chop the 12 volt D.C. and boost it up to 250 volts.I am only a technician so if I sound silly then it is something that I never met in practise
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