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Matching Battery Backup "Waveshape" to the Right Equipment?
Controlio asks: "I see all sorts of battery backups out there, from as low as $100 to the multi-$1000 range. One difference I've noted in battery backups is the 'waveshape' it outputs (stepped sine wave, trapezoid, etc). I've heard different types of outputs are good for different types of equipment, and certain types of equipment can be destroyed by the wrong UPS (motors, etc). So what's the best type of output for different kinds of electronic equipment? I'm looking at what exactly I can backup in my room - from computers to TiVos to a rather large TV, large speaker amp, etc etc etc. Are there any deadly combinations that I need to avoid when making my purchase?"
Most inverters spit out a sinusoid-like CURRENT waveform because they have a large inductor or transformer to smooth out the current (inductors oppose fast current changes). The industry hasn't made linear-mode UPSes for at least a decade now, as the losses are too high.
Motors don't harm UPSes, UPSes harm motors. (more on this later.) A UPS is essentially a cheap VFD (Variable Frequency Drive). Unlike DC motors, you can't get full torque out of an AC motor by just reducing the voltage. The frequency and voltage must be kept in a specific ratio in order to keep the torque characteristics of the motor.
Unfortunately for UPSes, the voltage waveform they spit out is more or less a nasty-ass chopped up PWM waveform. Bad for anything, really. With motors (and most inductors) you will end up breaking down the insulation on the first few windings of the stator (in the case of a motor). That's why motor manufacturers (I'm talking 3-phase here) use "inverter-grade" wire; the insulation is better and can withstand this nasty voltage spiking.
And no, the more expensive UPSes still spit out a nasty-ass voltage wave form. You are paying for either a) a higher switching frequency, b) better PWM control (meaning a more balanced waveform), c) more stages (instead of just on-off, on, 1/2way and full) and/or c) heavy inductors or output transformers to smooth out the current waveform. I haven't seen a commercial step-like UPS in ages (too many components for not much advantage). Stick a scope probe across your high-end UPS; it's still got a nasty voltage waveform but chances are it's giving you a truer (closer to sine) current output than your $100 special.
Most PCs come with cheap-ass switchmode power supplies. There are MOVs in almost all equipment. Unfortunately most suppliers use 130V MOVs. The peak-to-peak voltage on standard North American outlets is about 170VAC (120VRMS is 170V p-p) -- those MOVs eventually fail because they're turning on at the peaks of each cycle for the briefest of moments. Sketch out a sinewave with 84V peaks; the amount of time you're over 60 degrees is only a few tens of degrees. Depending on the construction of the power supply you'll either blow the shit out of the MOV or the internal fuse eventually. I've fixed countless computer power supplies by replacing the MOV and/or fuse.
What's that got to do with inverters and UPSes? Simple, really: the PWM voltage output has lots of nice sharp edges and causes all kinds of ringing in any length of cable. It can help lead to premature failure of power supplies but when it does fail it looks no different than a normal equipment failure. Yeah there's a $25k replacement value given but that's only from lightning strike or power surge on the incoming side of the UPS. Read the fine print; it does not cover failures as I've described.
The only time that an AC motor will cause grief to a UPS is if you're powering it and your load is actually driving the motor; i.e. you're using a motor to power an uphill conveyor and the load on the conveyor gets to great as to start moving the belt downhill and drive the motor. This causes the motor to generate instead of.. uh.. "motorate" and raises the bus voltage on your poor UPS which was NOT designed to do so. This usually overvoltages the caps and as I've seen many times in our power lab, things come apart.
First, let's define some terms. A UPS (in its most basic form) consists of a battery, a battery charger, and an inverter. For this discussion, we only care about the inverter, for that is what takes the low voltage DC provided by the battery and converts it into standard AC (at whatever voltage is typical where you live).
:^)
Switching power supplies (such as that in your PC) are, these days, remarkably unaffected by being powered with an odd waveform. At best, they work justfine. At worse the odd waveform confuses them to such an extent that they shut down harmlessly, or they just run a little warm. In theory, anyway.
That said, what the electronics really want is a pure 60 or 50Hz sine wave. It is, after all, precisely what they were designed for.
I forget what sort of waveform my (old) APC Back-UPS 450 uses (sine, square, stepped square, sawtooth, whatever). I did at one point attempt to drive a relatively serious audio system, an aquarium, lava lamp, and a bunch of computers from it.
The computers worked fine. The stereo produced a low 60-cycle hum, but functioned otherwise. The flourescent lights in the aquarium got pissed and flickered. The pumps made noises of certain distress. The lava lamp produced a figure of startling likeness to Roseanne Bar, and was promptly disconnected.
I've done similar experiments with a Best Ferrups 800 that I happened across, which offers a real-live sinewave output. Nothing budged; no lights flickered, the aquarium filter continued to run silently, Roseanne Bar did not appear, and there was much rejoicing. I did note an increased buzzing sound from the Best box, but it was quickly overcome by the sounds of Iron Butterfly's Inna Gadda Da Vida (my 17-minute, 6-second treat for my otherwise dark, silent, and powerless neighbors during an inexplicable outage).
Given this experience, there's a -lot- of things I don't trust to a non-sine inverter like my Back-UPS. For this reason it sits disconnected in the corner of the room (also, because it once rebooted a machine as the power went out - hint, hint). On the other hand, there's absolutely nothing I would hesitate to plug into the Ferrups.
Most other good UPSs (mid-grade APC, Best, Tripplite seems OK) should fair about as well at powering whatever you feel like plugging them into, the key being that it output a sinewave.
[ObRant: it's presently more expensive to produce a high-current, high-voltage sinewave than any other waveform. In other words, all of these units are bloody expensive. And, it's the harmonics that make the rest of them so bothersome to use, but I'll leave those anecdotes to some other less-tired poster. Also, UPS makers have a bad habit of making a big deal out of the VA rating of a UPS, possibly the single most useless figure in a home computing enviroment, while they should be pushing battery size (in Amp-Hours) and inverter efficiency. Oh, and that too-good-to-be-true $99 super-whizbang 1.6kVa UPS you saw at some online discount whore really is too good to be true. If you can carry it around with one hand, it's not worth having, and is akin to replacing the battery in a car with a 12 volts worth of D-cell NiCads and expecting it to fire. Unless, of course, you've got some special application in mind that doesn't need much current, like keeping an ISDN line up during short blackouts or somesuch.]
Kid-proof tablet..
Let me get this right: When you have a power outage, you want to make sure that you can watch your really big TV, and use your "large speaker amp"? On my UPS, (yes that is a boat battery which it uses) I only have my 2 important systems and one 13 inch monitor. That way, it can last longer. I don't need my big MP3 machine or my 19 in. monitor running when the power goes out, I only need to be able to safely shut the important machines down if I have to. You put the bare minimum of stuff on a UPS.
For your purposes, I recommend an old mainstay of power backups: a nuclear reactor. That way, you only have to refuel it every 20 years or so, AND you don't need a connection to the lousy power grid!
I personally have had great luck with the APC brand, and as a general rule, I won't buy anything else. Your mileage may vary.
Where you run into problems is where you start trying to run traditional "appliances". Fridges, Air Conditioners, Heaters, Microwaves, etc. etc.
Most low-end UPS's (such as the APC Back-UPS) put out a pseudo sine wave, where as the more expensive ones (APC Smart-UPS for instance) put out a "real" sine wave. In theory the "real" sine wave models should work for even the fridge if there is enough wattage available, however, I wouldn't try it without taking to the UPS manufacturer first.
Essentially the difference between the real sine wave and the pseudo one is that the real sine wave is the standard "smooth" sine wave you'd expect to see by plotting the sine function on a graph. The pseudo version is basically a modified square wave. The power is off (at zero) for a while, then at full positive power, then off, then at full negative power and then back to off, repeat. The ratio between on and off is set up so that the average voltage is the same as a real sign wave, and that the peak voltage is also identical.
This works for almost 100% of the electronics out there. Quite frankly, a switching power supply or even an old fashioned transformer supply doesn't really care about the waveform as long as the voltage is in spec.
The reason why some other loads do is that the waveform itself is important to the operation of the device. For instance, an AC motor actually operates by taking advantage of the reversing voltage to cause rotation. In addition, most inductive loads such as motors, etc. can cause spikes back into the UPS which may either blow the UPS up or cause it's regulation circuit grief.
In short, if you're just wanting to run the electronics, just use any old UPS. If you're wanting to run applicances, well, good luck!