Is the internal voltage measuring TRUE RMS?

[Anonymous user]

In my country, UPSes and Voltage Regulators are required by code since the begining of 2008 to all be true-rms which means they must measure the input voltage correctly even if the sinewave is very distorted, in order to correct the output voltage precisely.

As some of you may already know, if you use a regular multimeter or voltmeter to measure distorted sinewave shapes, the result will not be accurate. For the result to come out correctly you need to have a true-rms multimeter...

So, are all APC UPSes TRUE RMS? In my country it's been a while that they put the word "TRUE-RMS" on the box of voltage regulators and UPSes.

I'm asking this because my APC UPS tends to activate its voltage regulator when it shouldn't do it, well I think so. The input voltage numbers the PCPE shows me is 4 volts lower then the numbers my two multimeters show me when I measure directly from the wall outlet.

Thanks for any information!

Message was edited by: rau

[Techie_apc]

Good Question...As far as I know, APC UPSs use different techniques to protect against voltage disturbances, depending on the type of voltage disturbance. The "Sensitivity" feature is used to fine tune the UPS's response to different power problems.Voltage disturbances on AC power lines can be divided into three general classes: sub-cycle transients, short duration voltage variations and long duration under or over voltages. APC UPS is designed to protect a user against any voltage disturbances but uses different techniques for each class.

APC UPS uses multiple techniques to monitor the AC input line and is designed to give the customer the maximum flexibility in responding to power problems.

[Anonymous user]

I think it is not a question of protecting against all voltage disturbances because there are at least 5 diferent kinds of voltage disturbances that some has nothing to do with voltage measuring techniques. For exemple, passive components that doesn't require prior measuments in order to correct the problem is used to filter out noise and spikes and the voltage regulation side of the UPS has nothing to do with it nor the voltage measuring system.

In this case I'm talking about voltage regulation only and volage regulation requires basically two process:

1 - Measuing the input voltage

2 - Commanding relays to selecting one or more transformer taps corresponding to the amount of voltage is required to correct the input voltage, it can either trim ou boost the input voltage.

In order to do this correctly the measuring system must do correct readings, right? This is a logical thing. And it must do it no matter what. It doesn't matter if the sinesoidal wave shape is rectangular, trapezoidal or no matter what shape it is at the moment. And to do that the voltmeter of the unit must be TRUE RMS.

The "low", "medium" and "high" sensitivity setting is only there to tell the UPS when it should transfer to battery regarding the noise on the line that the input filter was not able to correct, it doen't change the UP and LOW transfer points nor has anything to do with the voltage regulation side of the UPS, AFAIK. If the line is noisy, it doesn't necessarily means the sine wave shape is distorted... It can be distorted and noisy at the same time or not. Interestingly when the UPS transfers to battery it puts out a more distorted kind of wave shape than the one usually available in a normal wall outlet...

The question here is about the voltage regulator only, if it is measuing the input voltage correctly so as to select the correct transformer compensation tap at the output. If it doesn't do this when the wave is some what distorted or very distorted do to harmonics or other sources of wave distortion, it means the voltage regulation side of the UPS won't work as it should, it will start selecting the compensations taps when it shouldn't do it because the internal logics of the UPS now thinks the input voltage is at "x" voltage when it is actually at "y".

Thanks for the answer anyway!

[Techie_apc]

Hi Rau,

Hmm...I agree with you..You really raised a valid question. I leave this specific question to other APC folks...

[Anonymous user · Accepted solution]

To be realistic I think know one from APC will tell me their UPSes are not true RMS even if it's just the basic back-ups line. This could make people think their UPSes are not good because of that.

Thanks for spending your time trying to answer my question anyway!