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MOT Draws 24AMPS, Not blowing 13A fuse/Tripping breaker?

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LAN_403

Muttyfutty!

Sun Aug 15 2010, 11:19AM

Registered Member #2915 Joined: Fri Jun 11 2010, 10:41AM
Location: Malaysia!
Posts: 101

Got one of those Awesome clamp meters and tested it on the input of my MOT,
I set it to the 20A range, thinking that it couldn't possibly be over 20Amps.
When I (Well my Dad) Flipped the switch, the readings went to 'Overload'. We quickly killed the input then reset the range to 200A.
Turned it on, According to the meter it draws 24 amps.
Have I killed my (New) Clamp ammeter?
or are the readings correct, and I need to look/check the quality of the breakers/fuses?
Thanks for your time,
MIke

radiotech

Sun Aug 15 2010, 01:36PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

What does your clamp ammeter have for functions? Was it set to peak or average? And where were the jaws of the meter with respect of the MOT's magnetic core. Those meters measure magnetic fields and read in amps.

If you listen to an old MO start, you hear a 'grunt' because it takes about 10 cycles of 60 Hz before the core has been booted up to smooth operation. Transformers have inrush issues.

To test your meter, measure the current to a 100 Watt incand lightbulb. Then try 2 passes (loops) through the jaws. The reading should double. If it is a true RMS meter with peak hold, you may get about 8 Amps inrush and 0.8 A rms.

This clamp ammeter can accomodate a 3 inch copper bussbar
and has absolutely no current parts, no coils, no wires. And it can measure both AC and DC amps by magnetic force only.

Muttyfutty!

Sun Aug 15 2010, 02:50PM

Registered Member #2915 Joined: Fri Jun 11 2010, 10:41AM
Location: Malaysia!
Posts: 101

Well...
I checked, but this is the bog standard model, the cheapest I could get. no real functions, so I can only assume it is set to average.
The distance from the core is about 12inches, sound enough?
I will do those tests you mentioned later, as it's 11:00 Pm in Malaysia, I'll post the results tomorrow.
The 'standing current' of the MOT (When I was not initiating an arc) was about 4 Amps, does this sound right?
Thanks you very much, Your help is appreciated!
Regards,
Mike

*Edit, corrected grammar*

Adam Munich

Mon Aug 16 2010, 12:37AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Have you considered the possibility your circuit breaker is faulty, and the MOT actually is drawing 24A?

Muttyfutty!

Mon Aug 16 2010, 01:51AM

Registered Member #2915 Joined: Fri Jun 11 2010, 10:41AM
Location: Malaysia!
Posts: 101

I have, But to me It seems EXTREMELY unlikely that the 20A circuit breaker would fail as well as the 13Amp fuse.
How many amps does an unballasted (fairly large?) MOT draw? Its 50Hz, 230V over here...

Bjørn

Mon Aug 16 2010, 02:35AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Also consider that a fuse does not break at the rated current just like that, the rated current is the minimum it can stand. Fuses are rated by speed and a very slow 13 A fuse may stand 26 A for a couple of minutes and 20 A for years if it has good cooling.

So it is quite possible that you really got 24 A.

Arcstarter

Mon Aug 16 2010, 03:35AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

The 'standing current' of the MOT (When I was not initiating an arc) was about 4 Amps, does this sound right?

I measured many a MOT, and it was always about 4 amps, but i live in 120v land... I still wonder if the clamp meter is picking up the EMF from the MOT...

And also, a question for radiotech, what is the bandwidth of that meter?

radiotech

Mon Aug 16 2010, 06:38AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The meter in the thread has Scale modules that fit into the tongs
that would be accurate from DC to 50Hz. Called a Tong Tester. Here's a 400 amp module.

Muttyfutty!

Mon Aug 16 2010, 09:33AM

Registered Member #2915 Joined: Fri Jun 11 2010, 10:41AM
Location: Malaysia!
Posts: 101

Thank you for your responses!

radiotech-I Did those tests, and I can confirm that the readings doubled with two loops. The 100W incandescent draws 0.4Amps on the 240V line.The meter inst 'rosak' then! VERY nice 'Tong Tester' By the way! :)

I Will do another test, this time with the clamp meter far, far away from the MOT, to prevent any EMI. How 'far' would you recommend? 1ft?
I thought it strange, that it draw so much current for a single Xformer...
Thanks again,
Mike

Adam Munich

Mon Aug 16 2010, 02:14PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

When an x-former saturates, the current it draws goes up. Way up.

Zoom

[Edit: Picture size]

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