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Old 3rd December 2008, 02:12 AM   #1
kl is offline kl  Europe
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Default Toroid inrush current in case of short blackout

Hello,

What happens with inrush current in case of SHORT blackout for one cycle (20 or 16.6ms - 50/60Hz), few cycles, one second or more?
How big is the current compared to power-on inrush current?
For example: 600VA/230V toroidal transformer.
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Old 6th December 2008, 06:38 AM   #2
kl is offline kl  Europe
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I found some info at noinrush.com. It seems that conventional soft start circuits and NTC are useless in case of voltage dips . I will try to redesign my softstart.
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Old 6th December 2008, 11:15 AM   #3
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I tried to address this problem with a circuit that has a timing capacitor that can discharge much faster than it charges. But the chart shows how dire the situation appears to be.
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Old 6th December 2008, 11:55 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
a delayed on and instant off for the bypass relay is OK for a few seconds of no power.
For a few hundred mS of loss you would need a fast acting instant off and a fast acting relay to get the resistor and/or NTC back into circuit in time.
For a one cycle loss, one would need a missing pulse detector and a very fast bypass, probably not a relay, maybe a FET bypass.

But, I had not realised the significance of a missing part cycle.
Would some kind of phase comparator be able to release in time for the re-establishing of power?
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Old 6th December 2008, 12:51 PM   #5
Mooly is offline Mooly  United Kingdom
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kl, Is this a hypothetical question or do you have a problem with poor mains. I used a 600va toroid for years, but found I needed at least a 6.3 AT fuse. Even then it "parted" once on switch on.
It is possible as Andrew say's to have a missing pulse detector, but is it worth it all ? Providing the secondaries are suitably protected any risks are minimal with a large primary fuse, indeed the toroid manufacturer possibly has info on recommended input protection. Relays are OK but the contacts need to be suitably rated to prevent arcing.
What seems a simple problem can either be " a non problem " or if you are determined to do something about it can become a fairly major design exercise.
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Old 7th December 2008, 05:50 AM   #6
kl is offline kl  Europe
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Thanks for replies!

I have good mains, but ... just trying to design best soft start. I will continue to build my current triac-based soft start project.
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Old 7th December 2008, 06:50 AM   #7
Mooly is offline Mooly  United Kingdom
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A PTC thermistor and relay to short it out gives the ideal result. Burst control using triacs can generate RF noise and "irregularities" at the zero crossing. It's a no win situation really. The thermistor and relay offer no delay in the event of a mains drop out, but does that matter, the caps in the amp wil be charged so the toroid isn't seeing a low impedance.
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Old 7th December 2008, 10:44 AM   #8
AndrewT is offline AndrewT  Scotland
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it's not the smoothing cap load that is the problem.
That is solved differently.
It's the loss of flux in the transformer on power failure that we are trying to solve. A relay cannot drop out instantly. The best are of the order of 2mS and big relays that can handle input currents are very possibly >10mS to drop out.
This drop out time gets much worse the more effectively we snub (with a diode) the back EMF of the coil.

If the problem is as bad as that posted graph shows, then a relay can never solve that quarter cycle current surge. A whole cycle loss of power may even be too short to detect and for the relay to open in the 20mS available to detect and drop out.
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Old 7th December 2008, 10:56 AM   #9
Mooly is offline Mooly  United Kingdom
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No easy answer is there ? To be honest I think it's a non problem really. I mentioned the reservoir caps because from fully discharged it must increase the primary current over many cycles I would imagine. If the secondary circuit wiring is low impedance it may well " overload " the transformer briefly, although I have never done any measurements.
Relays will never work for drop outs of a cycle or two.
Anyone any data from the toroid manufacturers on what they recommend ?
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Old 7th December 2008, 11:49 AM   #10
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But is that short time loss really a problem?

If it were to happen then transformers and motors *everywhere* will saturate flattening the mains waveform and your fuses should survive.
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