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Joined: Dec 2004
Posts: 354
K
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Yup, Dapo AC21 is a fluorescent load rating. G.E. has very comprehensive tables of loading and switching specs for their contactors in their contactor catalogue. In my opinion you should just get contactors that are well over-rated for the task ( provided they fit in the board). If your contactors are for a boning room then there could be a considerable "down-time" cost to your customer. An extra couple of bucks for over-size contactors may be economically viable.

Joined: Jul 2002
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Gray,
Just a small Health and Safety question if I may?.
Are the light fittings spread over 3 phases?.
Reason I ask that is because before I started my time as an Electrical Apprentice, I worked in a Meatworks as a Slaughterman and if all of the lights went out at once, the last thing you would want to be doing is using a razor-sharp knife. [Linked Image]

Joined: Mar 2003
Posts: 52
D
Dapo Offline OP
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Yes Mike, they are across three phases, and they are in mixed rows to eliminate the strobing effect I am told.

Joined: Sep 2003
Posts: 650
W
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Did the contractor actually _measure_ the load on the various phases, or was this calculated?

What is the THD of the ballasts on these fixtures?

Given your description of the neutral bar heating, I'd urge checking for harmonic neutral currents. From your description, these ballasts are fitted with power factor correction capacitors, which won't do much for harmonic content. Any triplen harmonic content will show up on the neutral; and it is possible that the neutral current exceeds the phase current.

If this is the problem, then upsizing the contactors may simply mean that the next problem seen will be neutral conductor overheating.

A meter used to measure the neutral current should be known to read correctly at 180 and 360 Hz. (Or 150/300 Hz, as appropriate)

-Jon

Joined: Mar 2003
Posts: 52
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Dapo Offline OP
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Thanks for the reply Winnie.

You have raised some good questions. Which will lead me to ask more questions, since I know very little about harmonics on neutrals (other than I have read about it occurring).

1. I don't know whether the contractor measured the neutral load, can a normal clamp tester measure the harmonic current? For example a Fluke true RMS eg a model 33 or 35.

2. THD ? I assume is total harmonic distortion, do fittings have this marked on them?

3. I always thought that non linear loads such as switch mode power supplies on computers caused harmonics, can normal ballasts and capacitors cause harmonics as well?

Triplen Harmonics is definately not a term I had heard before, though a quick search on Google showed it is a common term. Are there sites which explain harmonics, triplen etc.




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

Joined: Sep 2003
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If the meter is a Fluke 33 as described here: http://www.metrictest.com/product_info.jsp?mfgmdl=FLU%2033 then it should be _perfect_ for determining if harmonics are the problem. Apparently the device can measure true RMS current up to 10000 Hz, and can measure crest factor, which is a measure of harmonic distortion. I've never used one of these myself, but take a look at the datasheet on the page mentioned above.

Get measurements of the crest factor on each phase, and the neutral current, and that should tell you the story.

THD is total harmonic distortion. I doubt that fittings have this marked. Some devices might have power factor and crest factor marked.

_Any_ 'non-linear' load will have a certain amount of harmonic current flow. Computer switching power supplies are one example, because they have input rectifiers that only conduct at the peak of the AC waveform. But transformers are also non-linear, especially when close to saturation. Arc lamps are also non-linear, because the voltage has to get high enough each cycle before they start conducting.

Triplen harmonics are harmonics which are third multiples of the fundamental. In three phase systems, triplen harmonics flow through the neutral, since they are not balanced phase to phase.

I don't really have a list of web sites that would help you; I had to read up on this stuff about a year back, but all I could do now is more google searching. If you search on the term 'IEEE 519' and look for 'tutorial', you'll probably find some good stuff.

-Jon

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