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Power-User

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Measuring of Non-Sinusoidal Output of UPS

05/02/2012 3:32 AM

Dear Friends,

I have been working for testing of efficiency and wave forms of different available UPSs. Most of them are "modified square wave" type. For output side, the energy meters (digital and analog) were not measuring the energy correctly because of non-sinusoidal output of UPS. So I am arranging an old fashioned "Disc Type Energy Meter". For measuring DC current of input side of UPS, I use a digital clamp on meter (very good quality HIOKI 3285) and an analog meter and found a little bit difference in their reading i.e. digital was measuring 10.5A (252W) and analog 9.1A (218W) at 24VDC. So please help that which one should I follow?

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Anonymous Poster #1
#1

Re: Measuring of non-sinusoidal output of UPS

05/02/2012 3:57 AM

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Power-User

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#2
In reply to #1

Re: Measuring of non-sinusoidal output of UPS

05/02/2012 7:37 AM

I do understand why you are saying this.

First of all i am sorry for my poor English as some time i dont have words to write what i want to ask and may be this is another case.

In this thread, i written that i measured the DC input current to UPS by a clamp on meter and i think this is the point which enabled you to write the sentence. Actually the current drawn from battery is non-linear type and thus the clamp on meter collects the samples and then calculates the RMS value. That is why that during my checking, the clamp on meter was measuring 9.1A and analog (series) meter was measuring 10.5A.

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Anonymous Poster #2
#3
In reply to #1

Re: Measuring of non-sinusoidal output of UPS

05/02/2012 7:49 AM

Good question. Response totally uncalled for.

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#4

Re: Measuring of Non-Sinusoidal Output of UPS

05/02/2012 9:35 AM

Hopefully someone will be able to help you rather than post nothing of value.

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Guru

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#5

Re: Measuring of Non-Sinusoidal Output of UPS

05/02/2012 10:28 AM

I would use the one I have checked the calibration on...Generally speaking, it has been my experience that analog meters tend to be more accurate than digital, out of the box...but it's a crap shoot, and can go either way...I always check the calibration on all my test instruments, and if I can't correct the problem, I am at least aware of the amount that the instrument is off, and make allowances...There can also be circumstances where one works better for the application than the other one....

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#6

Re: Measuring of Non-Sinusoidal Output of UPS

05/02/2012 12:38 PM

Only meters that are designated as "True RMS" meters will give you the correct readings. They tend to be electronic and they sample the waveform and then calculate the "area under the curve" to produce the correct number. They tend to be more expensive than most meters.

That square wave is a pretty rough looking approximation of a sine wave. I can see interaction between the sudden change in voltage (vertical axis) and the impedance of your oscilloscope probes. The actual transitions are probably a little flatter.

It is very very important to know your test equipment and how it performs. If not, you will have a lot of trouble getting the same results when you check later. I like to "test the test" sometimes to be sure I know that the equipment will work for what I am trying to measure.

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#9
In reply to #6

Re: Measuring of Non-Sinusoidal Output of UPS

05/02/2012 8:16 PM

The same holds true for any type of meter...The digital types can be as much as 5% +/- as factory calibration standard...Of course the more money you spend the greater the accuracy generally speaking....

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#7

Re: Measuring of Non-Sinusoidal Output of UPS

05/02/2012 4:44 PM

I agree with Joe. You need a true RMS reading meter. I don't believe the disk meter will give you the correct answer. Check you HIOKI 3285 and see if it is a True RMS meter. If so I would go with that reading.

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#8

Re: Measuring of Non-Sinusoidal Output of UPS

05/02/2012 5:23 PM

I know this is off topic, but, assuming that the UPS is for an IT or electronic equipment load.

I would exclude any of UPSs that do not produce a pure sine wave.
Yes, It's a personal preference, but most if not all equipment runs better on a "pure" power source.

Regards,
Sapper

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#10

Re: Measuring of Non-Sinusoidal Output of UPS

05/03/2012 8:51 AM

Most if not all industrial power meters will correctly measure the power and give you the harmonics content.

They cost between $2000 and $10 000 (CDN/US) or can be rented from specialized local business.

Many modern digital oscilloscopes in the medium price range will do the same for a single phase device.

I would let you use my power meter if you drive by my office in Montreal.

Regards,

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#11

Re: Measuring of Non-Sinusoidal Output of UPS

05/03/2012 12:32 PM

If the current clamp impedence was matched a little better to the oscilloscope, the waveform would probably look more square. It doesn't look like pulse with modulation was attempted, just a rough approximation of a/c power.

Anyway, for a square wave such as this, the RMS power can be calculated from the RMS voltage and RMS current. Those values are calculated by taking the peak values divided by the percentage of time it maintains that value with respect to the overall cycle time. Therefore, if the frequency is 60 Hz. then the total time the signal is above or below zero divided by 1/60 or 16.667mSec, is the percentage of peak power to use in the calculation.

By the way, when I say peak, I'm not talking about the little spike that follows the transition because that is a function of unbalanced probes. I'm talking about the average value of the horizontal line while the output transistors are on. Fancy o'scopes will do that for you.

One last note: This square wave is loaded with harmonics. Since most devices that are going to use this power have to step it down to some value close to the DC level that it needs, a large amount of the harmonic content will be stripped off as heat in that transformer. That has two implications. First, you won't get all of the calculated or measured RMS power through the step-down transformer. Second, that transformer is going to suffer due to heat buildup. It will cause accelerated aging of every power supply that expecting a nice sinusoidal pure frequency. Just thought you should be aware of that.

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Power-User

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#12
In reply to #11

Re: Measuring of Non-Sinusoidal Output of UPS

05/05/2012 2:52 AM

The clamp on meter Hioki 3285 is a true RMS meter. If we talk about the output side of UPS then we can measure the rms current and voltage only but can not measure the power fector to calculate the real power consumption of load. So when i measured the power by using a digital and analoge (needle type) energy meter, i found a difference in their measurment. I discussed with an expert and he told that as the output is not sino-suidal, so a disc type energy meter can work well here.

but here i am talking about DC input to UPS from battery. For a 200W load at UPS, i measured the input DC current by using above mentioned meter and an analoge meter and found a difference i.e. digital was measuring 10.5A (252W) and analog 9.1A (218W) at 24VDC. So this is the point of my confusion that which one should i follow, digital or analoge?

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#13
In reply to #12

Re: Measuring of Non-Sinusoidal Output of UPS

05/09/2012 1:14 PM

You should go with the digital (Hioki 3285) meter since it is true RMS. Any other meter calibrated for a sinusoidal wave shape at either 50 or 60 Hz will only give accurate values for those kind of signals.

Other circuits can cause you problems as well. For instance, if you were to take measurements of fluorescent lamps operating on an electronic ballast, you will need other equipment because the frequency will be too high.

So, in conclusion, it is important to know something about the signal you are trying to measure and you have to make sure your equipment is rated for it, if you are trying to get accurate information. Sometimes you only need to know that it is working, instead.

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Anonymous Poster (2); Baxter (1); marcot (1); NotUrOrdinaryJoe (3); Sapper (1); Signode (2); SolarEagle (2); wareagle (1)

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