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Old 7th August 2012, 07:33 PM   #1
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Default Preventing the inrush current saturation in a toroidal/EI transformer

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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Old 7th August 2012, 09:01 PM   #2
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i think ur problem might be that the toroid may not like partial sinewaves, nor the high slope transitions... seems like a long way to go when a 555 timer or a transistor or two with a cap and a relay to control a series current limiting resistor for a few seconds...

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Old 7th August 2012, 09:08 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by tauro0221 View Post
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)

Quote:
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.

Quote:
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.


Quote:
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.
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Old 7th August 2012, 09:32 PM   #4
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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.
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Old 7th August 2012, 09:35 PM   #5
DF96 is offline DF96  England
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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.
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Old 7th August 2012, 10:01 PM   #6
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Right now I am waiting for the triacs to run the test. What I do is to switch ON the transformer with a switch and read the current. Then do the same thing using the micro and read the current and compare both.
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Old 7th August 2012, 10:02 PM   #7
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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.
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Old 7th August 2012, 10:10 PM   #8
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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.
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Old 8th August 2012, 07:11 AM   #9
RJM1 is offline RJM1  United States
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I posted this link once before in another thread, it is a good explanation of why you should switch on a transformer at maximum voltage (minimum dV/dT).


http://relays.te.com/appnotes/app_pdfs/13c3206.pdf
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Old 8th August 2012, 08:15 AM   #10
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by tauro0221 View Post
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.
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