Auto-transformer and isolation transformer in-rush tripping 15A breaker ? - Page 1
Forum section: electronics / beginners

author: MathWizard
author_rank: Super Contributor
timestamp: June 12, 2024, 10:26:16 am
content: For the 2nd time I remember, I've tripped a 15A breaker, when turning on mains power on my 1-2kVA auto-transformer, that also has a 500-1000VA isiolation transformer hooked up to it. There was no other load with them.The only other thing's running off that breaker, as far as I know, are just a couple of low power, SMPS type light bulbs, a phone and modem, and this time for sure, 1 of my Siglent scopes.Last time, I had my soldering iron on, but not this time.So what kind of in-rush current is there to just transformers ? I would have though the inductance would limit it a lot. But yeah they would have very little DC resistance, and then there's any capacitance's, and then transformers and inductors, don't act quite the same.And don't breakers actually trip way over there rated amperage ? Something like 100A-200A, if you just short the wires together ? This wasn't a sustained 15A, it just went the moment I flicked the power switch on the auto-transformer.

author: Gyro
author_rank: Super Contributor
timestamp: June 12, 2024, 10:57:18 am
content: Quote from: MathWizard on June 12, 2024, 10:26:16 amIf you happen to close the circuit somewhere close to the voltage zero-crossing,a lot. Transformers, particularly lower end ones that run close to saturation, tend to rely on remnant reverse magnetisation  from the previous half cycle to prevent saturation on the next one. If you close the circuit at a zero crossing, the core will saturate as it reaches mains voltage peak. At that point, the only current limit is the winding DC resistance [Edit:, at mains peak voltage]. with big transformers, the resistance is low, and the peak current high. Transformer cores also run closer to saturation when the secondary is unloaded.This is the reason that you should never switch transformers with zero-crossing SSRs.

author: MathWizard
author_rank: Super Contributor
timestamp: June 12, 2024, 12:38:38 pm
content: Ok so when the voltage is changing at it's fastest, when crossing zero, is when it happens ? I had it happen again already today. I sort of think it's the combo of the 2, I've never had it happen using just the iso. trans, but I don't use the AT that much so IDK if it just does it. I can try with the AT voltage turned down 1st too.Sometimes I'll use a series 100W lightbulb limiter too, but I have it after those, with a proper AC light switch and plug after it, for the DUT.If it is in series 1st from the wall plug, then I'd assume it will flash with any in-rush current, and limit it (I hope). But I'd have to change my switch around, I want it just on the DUT.I just so happen to have some old 3-way switches, so maybe I could keep the bulb 1st, then switch in/out of circuit that way. Right now, I have to swap the bulb with a socket adapter, if I still what to use the nice switch (it's on the same 2x4 as the light socket).So is this a common issue tho? What does everyone else do ?

author: Circlotron
author_rank: Super Contributor
timestamp: June 12, 2024, 01:01:13 pm
content: When a transformer is up and running in steady state, one full half cycle of voltage changes the magnetism in the core from "negative" maximum to "positive" maximum. At the voltage zero crossing the magnetism is lagging 90 degrees behind at "negative" maximum so it has plenty of room to move.If the transformer is switched on at the voltage zero crossing the magnetism is at zero, so it only has half the latitude to move before it hits max and saturates, unlike the steady state running condition where it has twice as much room to swing at the instant the voltage pass through zero crossing.

author: soldar
author_rank: Super Contributor
timestamp: June 12, 2024, 04:32:38 pm
content: I used to have that problem with a transformer and I cut out and put in series the anti-inrush protection of a computer power supply (with NTC). Never again did it happen.I suppose you could insert a resistor in series, say 10 ohm and automatically short it with a relay after a fraction of a second. But the NTC solution seems to be the most widely adopted.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 12, 2024, 05:54:30 pm
content: The inrush current spike is not just a problem with cheap transformers. It also is an issue with larger ( > some 100 VA) torroidial transformers, as they have less effective air gap (and thus more remanence) and with grain oriented material can use higher magentization in normal operation. With high remanence even turning on at the voltage peak may not be enough to avoid the inrush peak, though it can limit it.With the core saturated the iductance really goes down. So for the peak it is mainly the DC resistance to still limit the inrush current, especially with larger transformers that have a really low resistance.The inrush pulse may well exceed 100 A for some 5 ms or so, maybe with a few smaller follow up peaks. So a 16 A fuse / breaker is supposed to trip on this.

author: MathWizard
author_rank: Super Contributor
timestamp: June 12, 2024, 09:04:18 pm
content: I have some good books on this stuff, but I barely got into them yet. Ok so I won't try and measure any currents like these with my DMM's.I do have some NTC4D-11, they are rated 4A max, I guess A_rms, but nothing I do uses much current. They don't give pulse current rating, but cold they are around 5 ohms (if I kill one thats ok). So I'll check and see how hard it would be to add one inside the auto-transformer.

author: Benta
author_rank: Super Contributor
timestamp: June 12, 2024, 09:39:14 pm
content: As a rule-of-thumb, you'll need soft start for any transformer over 300 VA to avoid tripping your breakers. I had the same problem with a 1 kVA isolaation transformer and devised this circuit. Works great:

author: johansen
author_rank: Super Contributor
timestamp: June 13, 2024, 12:07:05 am
content: It takes around 2 minutes for a 1GW sized transformer to get back to mostly sinusoidal magnetizing no load current when you first turn them on.And the only reason you can turn a transformer that large on, without taking the power grid down with it, is because the leakage inductance scales with the capacity of the transformer and it is to the tune of 20% at that size. So the highest peak currents are only 5 to 10 times larger than nominal, and the actual total power consumption o of those peak currents is not high at all.My 2kva variac takes about 3 seconds, you can hear it.Skip the complicated circuitry and use an NTC resistor.Now, why it trips the breaker: because its not a thermal breaker. Its partially magnetic. You send 100 amps through a 15 amp breaker and it trips on the first cycle. You can do this with a 200watt toroidal transformer (which has a 1 watt no load power consumption and is 90% efficient at full load.)This is why i love my old zinsco breakers, they didnt have that problem. 100 amps would take around 1 second to trip, and if you think that's bad, the 14 ga wire will barely warm up in 1 second at 100 amps

author: Benta
author_rank: Super Contributor
timestamp: June 13, 2024, 12:20:53 am
content: You're absolutely right, the modern resettable breakers cause the problem discussed here.But we have to live with it and solve the issue. And the solution is not reinstalling fuse wire as breakers.

author: bdunham7
author_rank: Super Contributor
timestamp: June 13, 2024, 12:39:59 am
content: Quote from: johansen on June 13, 2024, 12:07:05 amIn many cases they won't trip at all and will burn your house down instead.

author: themadhippy
author_rank: Super Contributor
timestamp: June 13, 2024, 01:15:08 am
content: Quotemaybe with a type B breaker,type C will need about 9 seconds and a type D 10 seconds.

author: Circlotron
author_rank: Super Contributor
timestamp: June 13, 2024, 02:42:16 am
content: For inrush limiting on SMPSs I have often seen either a resistor and shorting relay, or an NTC thermistor only. I have never seen a thermistor and relay combination. In my mind that would be the ideal combination. After the relay clicks on the thermistor would cool down and be ready for another soft start if the mains blinked off for a moment. And in particular, just before the relay came on at power up the hot thermistor would ensure a lower voltage across the contact, possibly making life easier for the relay.

author: johansen
author_rank: Super Contributor
timestamp: June 13, 2024, 03:01:09 am
content: Quote from: themadhippy on June 13, 2024, 01:15:08 amYes thats is what i expect from new breakers..The magnetic trip curve meets the thermal curve around 1kva toroidal transformer inrush, hence all the posts about it.Thermally i would be impressed if you can even measure a 0.1C temperature rise of the 12awg wires in your wall, when you plugin a 1kva toroid.

author: m k
author_rank: Super Contributor
timestamp: June 13, 2024, 08:09:40 pm
content: Quote from: Circlotron on June 13, 2024, 02:42:16 amI have it.First the relay was mechanic, but it didn't work.Now it's SSR and fine.Maybe mechanic one was too easy or NTC too slow.Or SSR is limiting.E,if memory serves it went so that first mechanical relay didn't work, maybe it was just bad luck.Next there were only NTC and then SSR was added.It's still not bullet proof, but pretty stable.The thing is probably(hiding under the table) 6A and wall is 10A.