Preventing the inrush current saturation in a toroidal/EI transformer

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Preventing the inrush current saturation in a toroidal/EI transformer with a micro controller.

I have been reading a lot of the different ways to prevent the high inrush current saturation when switching ON a toroidal /EI transformer. One of the most complained of the toroidal transformer is the blown of the fuses when switched ON. This is due to the high inrush current saturation resulting of the residual magnetic polarity for the last voltage half cycle before switching it OFF. If the switching ON of the voltage half cycle wave has the same polarity as the last residual magnetization then an inrush current saturation peak will occurred.. There are some methods to dampening it but it is not the real solution to the problem. I think the use of micro can prevent it and solve the problem permanently.

I was convinced that by switching ON the transformer at the zero crossing it would solve the problem but for the above explained will not solve the problem.

Here are some of the remedies used today to prevent the high inrush current when the transformer is switch ON.
1- Use of NTC combination in the primary/secondary
2- Slow blow fuse in the primary
3- In-line power resistor with bypass relay
4- NTC and a bypass relay
5- Different combinations of NTC, resistors and relays.
6- Air gaps

None of the above solution will solve the real problem. The real problem is the core magnetization polarity that remained in the iron core when the transformer is switched ON/OFF. This magnetization will remain in the iron core for a long period of time and it is hard to know /remember at what part of the wave cycle was switched OFF. The remained magnetization polarity can be negative or positive.

I like to point out that I am not by any mean an expert in transformers. What it is explained here it is what I think it is happening by the information found searching the Net. Maybe I am completely wrong but my design will used the above explained to come up with a solution.

My theory to solve the problem it is the use the microprocessor so at the zero crossing to start bringing the voltage slowly up by controlling the triggering of the triac from the end of the wave cycle to the beginning. This will slowly bring the voltage from zero to full voltage. To switching OFF I will used the same procedure by at the zero crossing slowly bringing the voltage down starting by controlling the angle of the wave cycle from the beginning to the end of the wave cycle. By doing it in this way it will leave the transformer in the dull state with no magnetization or the core will not have magnetic polarity? Hummmmmmm????


There are two question from the above explained that I need to corroborate and couldn’t find the answer searching the Net. If the toroidal transformer is switched ON/OFF always at the zero crossing will it have zero residual magnetic polarity in the core? Will it only have a barely small left over flux due to the hysteresis of the iron core?

I would like to ask if there is a member that has some transformer expertise that can corroborated/answer it. All members are welcome to advice/recommend in the design/development of the project.

I ordered a isolated chassis mount triac and a non-zero crossing SSR to built a full running prototype. Right now the status it is waiting for parts.

Attached is a timing sketch showing my theory and a picture showing the preliminary triac gate triggering circuit/software development. You can see the triac triggering pulse moving from the end of the wave cycle to the beginning. Both +/- wave cycle most be triggered to balance the magnetization of the core.
 

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This is due to the high inrush current saturation resulting of the residual magnetic polarity for the last voltage half cycle before switching it OFF.
No, it is only a very small part of the problem with currently used alloys.
The real problem lies in the design of the transformers: the well-known transformer's formula (E= 4.44*n*F*A*B) is based on steady state.
Ideally, the 4.44 factor should be 2.22, but it would double the quantity of copper, and also double the copper losses (it would reduce the iron losses though)

Here are some of the remedies used today to prevent the high inrush current when the transformer is switch ON.
1- Use of NTC combination in the primary/secondary
2- Slow blow fuse in the primary
3- In-line power resistor with bypass relay
4- NTC and a bypass relay
5- Different combinations of NTC, resistors and relays.
6- Air gaps

None of the above solution will solve the real problem. The real problem is the core magnetization polarity that remained in the iron core when the transformer is switched ON/OFF. This magnetization will remain in the iron core for a long period of time and it is hard to know /remember at what part of the wave cycle was switched OFF. The remained magnetization polarity can be negative or positive.
The real problem is the insufficient core area and/or number of turns.

I like to point out that I am not by any mean an expert in transformers. What it is explained here it is what I think it is happening by the information found searching the Net. Maybe I am completely wrong but my design will used the above explained to come up with a solution.

My theory to solve the problem it is the use the microprocessor so at the zero crossing to start bringing the voltage slowly up by controlling the triggering of the triac from the end of the wave cycle to the beginning. This will slowly bring the voltage from zero to full voltage. To switching OFF I will used the same procedure by at the zero crossing slowly bringing the voltage down starting by controlling the angle of the wave cycle from the beginning to the end of the wave cycle. By doing it in this way it will leave the transformer in the dull state with no magnetization or the core will not have magnetic polarity? Hummmmmmm????
Anyway, you arrive at a correct solution for the wrong reasons: starting with a small initial flux of either direction, and increasing it progressively to reach the full excursion after a number of cycles is a possible solution: it is a soft-start, in short.


There are two question from the above explained that I need to corroborate and couldn’t find the answer searching the Net. If the toroidal transformer is switched ON/OFF always at the zero crossing will it have zero residual magnetic polarity in the core? Will it only have a barely small left over flux due to the hysteresis of the iron core?
It will retain the maximum possible magnetization, because the voltage and current (the magnetizing one, which is important here) are in quadrature.

If you want to simplify the circuit and avoid the full soft-start option, you can switch on the transformer at the maximum of the voltage.
That's what SSRs for inductive loads do. They generally have a special suffix.
They won't be able to eliminate the power-on surge completely, but they will reduce it to a reasonable amount.
Of course, a good dimensioning of the transformer will also be of great help.
 
Elvee
I was thinking to do that to trigger the triac at the peak of the wave cycle but then how many trigger you do. Just once or trigger it at the peak few times. My idea it is to completely unsaturated the core because the voltage would be almost at zero in both wave cycle. I know I can't completely zero it because of the hystericsis of the core. Since in the switch ON I am trigger the gate of the triac at zero crossing the high inrush current would be minimizes. Another thing it is that I can use it to power tubes amplifier. I can bring slowly the voltage to allow the tubes filament to warn up making the tubes last longer.
 
I am no transformer expert either, but with low resistance transformers such as toroidal you can get a problem even when there is no residual core magnetisation. This is a DC current equal to the peak AC current, which can double the total peak current for a while. The DC current decays according to the L/R time constant. Zero crossing is the worst place to switch.
 
It is a very nice idea and will surely work fine if implemented correctly.

One useful feature of a transformer is the hysteretic losses will tend to reset the magnetization whenever you softly start it with chopped half-sines. You can do it with triacs or SCR thyristors, the soft-switch-off may even not be necessary.
 
Problem is not residual magnetism; problem is where on the current wave you energize, which ideally is at zero crossing. By assumption for inrush, this is 90 degrees lagging from the voltage wave, hence the rule-of-thumb to energize at the peak of the voltage wave.

Residual magnetism will make the problem either worse or better, depending on positive remanence or negative in relation to the applied current, but put your focus on energizing at the zero current crossing, letting the chips fall where they may with residual. This is solved best with either variac type soft start or basic current limiting per the recommendations above. Triac switching probably won't help much.
 
Elvee
I was thinking to do that to trigger the triac at the peak of the wave cycle but then how many trigger you do. Just once or trigger it at the peak few times.
You have to choose one of the methods: either you use your soft-start (it will be based on phase angle control, the peak switching is thus irrelevant), and the more cycles it takes to reach full conduction the better, or you switch it on once for all, but at the peak of the waveform.
 
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.
 
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.
 
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
 
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.
 
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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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 .
 
inrush current on transformers?

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 :scratch:practise
 
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