UPS Backup


Old 12-31-06, 12:15 AM
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UPS Backup

I hope I posted this in the right area. I have a 750 watt UPS backup with a 26AH battery. When running off of line power it outputs 120 volts. When running off of the battery it outputs 160 volts. Wierd? I plugged a 5 amp jigsaw in it and the voltage dropped to 130 volts. What could cause this? Maybe a trimmer off somewhere.

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Old 12-31-06, 07:04 AM
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The load caused the voltage to drop.
Old 12-31-06, 07:53 AM
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Why the variation on output,depending on the sorce?

I was thinking of getting one myself. In this rural area we get momentary outages, lasting a min or two,(except for storms).
My experience is with the larger industrial types, which also regulate/condition the power. Data rms. etc...
Old 12-31-06, 08:33 AM
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What is the make and model of the UPS? I have never seen one be that far off under load; the measurement without load attached is fairly meaningless. They're usually centered on 120V or 115V with a 5% variance.
Old 12-31-06, 09:45 AM
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It is quite likely that the output of this UPS is not a clean sine wave, but rather some sort of modified square wave. Unless you have a good 'TRUE RMS' volt meter, all bets are off on the voltage measurement.

Old 12-31-06, 10:01 AM
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It is an older Tripplite BC-750. It is a I think they said rectangular wave? I am reading it with a cheapo volt meter. So it could be that it is outputing 120 just the meter won't read a modified sine wave right? I also have a 2000 watt power inverter in my car the is a modified sine wave, and my meter will read it as 127 no load and 120 with load. I don't know if this makes a difference.
Old 12-31-06, 10:33 AM
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The type of meter you are using is probably the issue.

When we talk about voltages we use the term "nominal" this is because not all sources are created equal, this means that other than exact readings are ok, as long as they are within tolerance. We are looking at a 120v nominal system, but for the actual voltage to be a bit high or a bit low is not uncommon or a problem.

So, why are you getting the 130v reading under load?

A typical electrical signal from a utility is a sine wave. Most of the time it is a curve, starting at zero curving up to max, turning back down to zero, then going down to minimum in the other direction. (ac goes both ways)

At a nominal voltage of 120, max is about 190 (those in the know feel free to correct me, i dont remeber the exact voltage). Minimum is about -190.

120 is a measurement called "root mean square" this is one method of calculating an average on a sine wave or curve. it is the method used for electrical meters. Most cheep meters do not read true "RMS" This could be the cause of your problem, since, as you stated, the sine wave from the UPS is rectangular not curved. This likely means that the max and min values are also different than they would be comming from the utility, but the mfg of the UPS has taken all that into consideration when making thier product.

Basicly, since the max and min values of the wave are different, and the duration of time that the voltage stays at that level is also different, you meter is giving you a false average value. (because it is not using the true root mean square method to determine it's displayed value)
Old 12-31-06, 10:34 AM
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I hope that some of that last post helps, I am not very good at explaining things.
Old 12-31-06, 10:47 AM
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Great that is a wonderful explanation. I understand now. So the UPS is ok, the meter just won't read the correct voltage. Thank you. I just put a new battery in it, and wanted to use it but was afraid to plug my computer into it. Thank you very much guys.

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