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Old 25th March 2011, 06:32 PM   #1
Einric is offline Einric  United States
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Default Inrush current limiting

I scrounged a 900VA+ transformer from an old battery backup and I can't find any information on it. I have plugged it in and it trips a 120V 20A breaker every time. I am wondering about using NTC Thermistors to limit my inrush current. Would it just be easier to build a soft start circuit?
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Old 25th March 2011, 06:35 PM   #2
ywara is offline ywara  United States
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I'd say that ICLs are probably easiest, though the on resistance could possibly be a problem if you don't have any headroom.
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Old 25th March 2011, 07:13 PM   #3
AndrewT is offline AndrewT  Scotland
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The recommended fuse rating for a motor and for a transformer is
3times the power divided by the supply voltage.

Your 900VA running on 115Vac would require approximately >=23.5A fuse to start up reliably.
You may find that you have to use a B or C rated 25A breaker to get it to start up.

Alternatively fit a soft start using either suitably rated Power Thermistors or Power Resistors.
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Old 25th March 2011, 07:16 PM   #4
Einric is offline Einric  United States
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I figured on using power thermistors. My breaker is a 20A Class C. It would make sense that I would trip the breaker if I was trying to pull almost 24A across it.
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Old 25th March 2011, 07:35 PM   #5
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Quote:
Originally Posted by Einric View Post
Would it just be easier to build a soft start circuit?
Probably. Have you noticed my group buy?
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Old 25th March 2011, 07:36 PM   #6
Einric is offline Einric  United States
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No I have not.
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Old 25th March 2011, 09:47 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
the transformer will be pulling a lot higher than 24Apk at start up.
Have a look at the time vs current of a ClassC breaker.
The transient that can pass for short periods is many times the rated breaker value.

At some point in the starting sequence your transformer is taking the breaker outside the ClassC curve. It might be at the very first instant of 1us after closing the switch , or it might be after 1ms after closing the switch.
More likely it is an accumulation of [i^2t] that opens the breaker.

The same happens with a fuse.
What you try to do is keep the current during the first few millisecond under some form of control so that the transient current limits for the chosen breaker an/or chosen fuse are not exceeded.
Soft start is designed to specifically control the starting currents.
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Old 26th March 2011, 01:49 PM   #8
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Quote:
Originally Posted by Einric View Post
I scrounged a 900VA+ transformer from an old battery backup and I can't find any information on it. I have plugged it in and it trips a 120V 20A breaker every time. I am wondering about using NTC Thermistors to limit my inrush current. Would it just be easier to build a soft start circuit?
It is strange that it trips every time, check there is not some other issue, Random trips are more characteristic of inrush current. 900VA is not a large transformer, 2 Kva transformers do not cause inrush current trips on the 240V domestic supply over here. (C curve breakers)
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Old 26th March 2011, 03:01 PM   #9
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light bulb tester time. Use a 40-60W light bulb in series with the transformer. If the bulb stays on after the initial surge there is an issue with the transformer.
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Old 26th March 2011, 04:52 PM   #10
Einric is offline Einric  United States
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I'll have to look into building a light bulb tester.
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