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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.