Inrush current limiting

Einric · 25th March 2011, 05:32 PM

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?

ywara · 25th March 2011, 05:35 PM

I'd say that ICLs are probably easiest, though the on resistance could possibly be a problem if you don't have any headroom.

AndrewT · 25th March 2011, 06:13 PM

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.

Einric · 25th March 2011, 06:16 PM

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.

peranders · 25th March 2011, 06:35 PM

Quote:

Originally Posted by Einric

Would it just be easier to build a soft start circuit?

Probably. Have you noticed my group buy?

Einric · 25th March 2011, 06:36 PM

No I have not.

AndrewT · 25th March 2011, 08:47 PM

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.

metalsculptor · 26th March 2011, 12:49 PM

Quote:

Originally Posted by Einric

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)

BobEllis · 26th March 2011, 02:01 PM

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.

Einric · 26th March 2011, 03:52 PM

I'll have to look into building a light bulb tester.

25th March 2011, 05:32 PM	#1
Einric diyAudio Member Join Date: Feb 2011 Location: Bozeman, MT	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?

25th March 2011, 06:13 PM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. __________________ regards Andrew T.

25th March 2011, 08:47 PM	#7
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. __________________ regards Andrew T.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Inrush limiting circuit	pona	Parts	1	16th October 2010 09:32 AM
Inrush current limiting heater filaments	boywonder	Tubes / Valves	14	2nd February 2009 07:33 AM
Inrush current limiting necessary for EI transformers	Bama Slamma	Power Supplies	10	6th November 2006 05:08 PM
Limiting Inrush??	Deltat	Power Supplies	11	31st March 2006 03:38 PM
Inrush Current Limiting Circuit	haldor	Parts	22	8th October 2004 03:06 PM


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

25th March 2011, 05:35 PM	#2
ywara diyAudio Member Join Date: Mar 2011	I'd say that ICLs are probably easiest, though the on resistance could possibly be a problem if you don't have any headroom.

25th March 2011, 06:16 PM	#4
Einric diyAudio Member Join Date: Feb 2011 Location: Bozeman, MT	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.

25th March 2011, 06:36 PM	#6
Einric diyAudio Member Join Date: Feb 2011 Location: Bozeman, MT	No I have not.

26th March 2011, 02:01 PM	#9
BobEllis diyAudio Member Join Date: Sep 2003 Location: Upstate NY	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.

26th March 2011, 03:52 PM	#10
Einric diyAudio Member Join Date: Feb 2011 Location: Bozeman, MT	I'll have to look into building a light bulb tester.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read