below 2%THD on power supply?

[AdeelR]

Hi
Can somebody help me with selecting a powersupply that will provide less than 2%THD upto the 13th harmonic, and provide a constant voltage at 115V 60Hz. Any help will be greatly appreciated.

Thanks

 

[Overvoltage]

Is this a homework problem....?

 

[AdeelR]

No, This is one of the measurement condition for testing Battery chargers in a standard. I have everything to perform the test except for this constant powersupply. The wall cannot give less than 2% THD, so i need to get a power supply, or generator that will give that.

 

[jraef]

THD is usually thought of as what the power supply will reflect BACK to the source. Are you saying that your test fixture must have a source with less than 2% THD on the incoming terminals? That's fairly strict. The best I have ever seen is Sola ferroresonant power conditioners with output THD at 3%. Most others, even "medical grade" conditioners are 5%, if they have a rating at all. You may need to contact someone like them to see if they can customize one for you. You could theoretically make your own harmonic trap filter on the output to see if you can improve on that, but it would be tricky given the nature of ferroresonant transformers. I would not attempt it, but then, it isn't my field.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[AdeelR]

Thanks alot jraef..Thats what i thought aswell less than 2% is too strict.

 

[VEBill]

If the THD spec is a 'measurement condition', then try the test in question using normal commercial power and see if it passes under those more strenuous conditions.

It depends on the nature of the test and the test spec.

 

[AdeelR]

Well you see, The UUT is a battery charger. I have to supply 115V at 60Hz, and maitain this test voltage, because i want to check the efficiency of the charger. So the application requires me to follow this condition. I am not allowed to measure efficiency at another test voltage with another harmonic distortion content.

 

[IRstuff]

Why not? Seems like somehow the designer is trying to fudge the results. Is the power supply ever going to see such a pristine signal in actual usage? I think not.

So, the only plausible purpose is someone writing BS test requirements, or the designer knows that without a "perfect" sinewave input, the efficiency will be REALLY crappy.

TTFN

FAQ731-376

 

[BobM3]

Maybe the spec is tight because the designer is trying to minimize test variations that would cause the test to be non-repeatable from unit to unit or vendor to vendor.

 

[AdeelR]

I know what you mean, the most accurate test would be if the test voltage was coming from the mains. But, The IEC standard has this requirement, and i cant deviate from it.

 

[VEBill]

My gut tells me that increased THD would reduce efficiency. And thus if you can achieve the required minimum efficiency with available ~5% power, then it should be even better with 2% THD. I suspect that the test condition limit of 2% THD is there to help you pass the test.

I wonder if a nice, good quality gas-powered generator might provide a clean sin wave (?). But I know that some are horrid.

 

[itsmoked]

Good idea VE1BLL.

Perhaps a sinewave inverter generator.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[AdeelR]

what do you guys think about putting a line filter to reduce THD?

 

[Comcokid]

You might check out some of the Elgar AC sources.

http://www.elgar.com/products/Products_AC.htm

Some of these are designed for product testing and testing to various CE standards. The TW series indicates 0.3% THD - but I didn't read all of the fine print.

 

[VEBill]

The problem with the filter idea is that you're trying to filter a very low frequency and a high current. A passive filter that dealt with low-order harmonics would need a forklift to move.

How about a function generator feeding a high power audio amplifier. Assuming your UUT is under a few hundred watts, you might be able to produce audiophile quality AC power. There are "70 volt" PA transformers available to help get the voltage into the right range if required.

But Comcokid has a good pointer.

 

[IRstuff]

OK, something akin to this, apparently:

http://www.energystar.gov/ia/partne...nloads/Battery_Charger_Draft2_Test_Method.pdf

Seems like an odd approach, but it is what it is.

So, Comcokid's link leads to

http://www.elgar.com/products/CW/CW_Specifications.htm

which would seem to meet the requirements.

TTFN

FAQ731-376

 

[AdeelR]

Thanks for the help. The Elgar products do look good. The CW series looks like a good bet. I will get a quote.

Thanks

 

[ScottyUK]

Pacific have a nice line of prgrammable power sources with very high performance. Probably better to hire one than buy unless this for a production run.

http://www.pacificpower.com/

----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[jimkirk]

Agilent also has AC power sources.

http://www.home.agilent.com/agilent/product.jspx?nid=-536902250.0.00&lc=eng&cc=US