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#49666 - 03/12/05 01:22 PM Cheap way to test harmonics?
tsolanto Offline
Member

Registered: 09/03/02
Posts: 131
Loc: Long Island NY
Does anyone know how to test for harmonic distortion without purchasing and expensive testing instrument. I priced one for around $4000 ouch... need an alternate method. one of the schools that we work in is blowing power supplies in their computers and their engineer thinks it might be caused by harmonics. I would like to test over time, like a chart recorder would be used.

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#49667 - 03/12/05 02:38 PM Re: Cheap way to test harmonics?
capt al Offline
Member

Registered: 06/20/04
Posts: 203
Loc: Norton, Ma USA
Take at look at the TPI 440 Scopemeter. Around $300.00 on sale.

Al

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#49668 - 03/12/05 02:54 PM Re: Cheap way to test harmonics?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Here is the meter Al is talking about.




You can check it out here.

http://www.kilowattclassroom.com/Equip440.htm

That said I have never heard of harmonics damaging a power supply does anyone have any info on that?

Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#49669 - 03/12/05 03:03 PM Re: Cheap way to test harmonics?
Redsy Offline
Member

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
I believe that if you are only looking for "qualitative" testing (i.e. whether or not harmonics exist) and not "quantitative" testing (an actual value of distortion), and you already own an average sensing and a True RMS amp meter, you can determine if harmonics are present. http://support.fluke.com/find-sales/download/asset/1260362_k_w.pdf

Other than that I have rented sophisticated logging analyzers several times and they are not that expensive overall.
http://www.us-instrument.com/

Look at the "Dranetz" products




[This message has been edited by Redsy (edited 03-12-2005).]

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#49670 - 03/12/05 03:23 PM Re: Cheap way to test harmonics?
e57 Offline
Member

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
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Mark Heller
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#49671 - 03/13/05 05:09 PM Re: Cheap way to test harmonics?
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
PC supplies are pretty tolerant of power line problems. The first thing they do is convert to 160v DC and chop that to 25kz square waves. They make harmonics. It doesn't see harmonics.

BTW the quick and dirty is to use a clamp on the neutral. Harmonics (power factor etc) are the difference between the actual current and the expected current if you had a clean load.
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Greg Fretwell

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#49672 - 03/13/05 05:37 PM Re: Cheap way to test harmonics?
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Measure current with standard amp-clamp. Measure again with "true RMS" amp=clamp. More than 15% difference is enough to start worrying about.
And- remember- there are no harmonics on single phase systems.

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#49673 - 03/13/05 06:47 PM Re: Cheap way to test harmonics?
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
Why don't you think there are harmonics in single phase systems? The triplen harmonic isn't as big a problem but you can certainly have harmonics.
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Greg Fretwell

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#49674 - 03/14/05 02:00 AM Re: Cheap way to test harmonics?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Greg the damaging harmonics cancel each other out on a single phase supply system as in a 120/240 system.

Only from a 3 phase supply like 208Y/120 do you need to worry about overloading the neutral with harmonic currents.

Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#49675 - 03/14/05 07:23 AM Re: Cheap way to test harmonics?
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
He wasn't talking about overloading the neutral, they said the harmonics were damaging the PC supplies. You can have nasty harmonics on a single phase system but I still don't think it will hurt a switcher supply. I won't get into whether a situation where current lags voltage by 8ms could exist.
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Greg Fretwell

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