# How Do Power Inverters Work?

Discussion in 'Physics & Math' started by cb767, Aug 21, 2005.

1. ### quadraphonicsBloodthirsty BarbarianValued Senior Member

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9,391
In a desperate attempt to bring this thread back on topic:

More interesting than the standard fixed-frequency, fixed-voltage inverter that has already been summarized are designs that produce variable frequencies and voltages. These are useful in applications where a motor must be powered to run at variable speeds.

http://en.wikipedia.org/wiki/Variable-frequency_drive

The design of systems like this gets very hairy for high-power applications, where the input power is usually multi-phase, and the cost and performance of the switches become problematic. Plus, you can't rely on a steep postfilter to reduce harmonics.

3. ### leopoldValued Senior Member

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17,455
more balls than most.
will definitely earn you some respect.

5. ### PronatalistRegistered Senior Member

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750
Power outage protection, on a cheap budget

post title: Power outage protection, on a cheap budget

No, I don't think that was my question. I am talking about using the ground in place of the common conductor, well probably not from the house to the telephone pole, but rather probably from the high voltage transformers somewhere to the electric power station. Obviously most homes need 2 or 3-conductor wiring, as houses aren't commonly made of conducting metal, but they like to use grounding to eliminate expensive current return conductors over long distances, that are apparently quite unnecessary to complete the circuit.

I think I understand the idea that grounding a metal case to some electrical device, helps insure that any ground leakage produces a short circuit rather than a shock hazard, which then hopefully trips the circuit breaker before anybody can get shocked. Ground fault breakers are a huge improvement upon this idea, which can trip at very low current leakage currents, detected by unequal current loads in the hot and common wires. But what applications should avoid ground fault breakers?

Hmmmm. I wouldn't know which way was better on a 2-conductor non-polarized plug. Old transformer plug power adapters don't "wear out" do they? What if they sit around rarely being used? Can they be much of a fire hazard should they stay plugged in for years? Should I go around every now and then, and feel them to make sure they aren't overheating?

Yeah, I am a packrat too. There's a rule of thumb that claims if you haven't used an item in over a year, you probably don't need it and should sell it, give it away, or throw it away. But then "Murphy's Law" probably would claim, the day after you throw it away, will be the day you finally find a use for it. I figure I can keep small stuff, but throw out the large stuff, as the apparent cost of storage (cost per square foot/(cubic foot if heavily invested into efficient shelving systems) of lost house storage space), likely exceeds the expected value of the item once a use for it is eventually found.

I think I have detected some mild "tingle" from touching a simple electric toaster. What's up with that? Does that mean I should have thrown it away? I tossed one out, since I received a replacement for Christmas or something.

But who watches TV for just a few minutes? Why not an hour? Are you suggesting that leaving one's headlights on, can ruin a car battery, and not merely result in having to get a jump start? I figure that heavy loads on the car battery, like watching a regular TV, would drain it within what? an hour? But even cranking up the engine to keep the charge, and buying replacement gasoline, is generally cheaper than buying a generator, provided it doesn't occur very often. A car already contains a handy "generator," but for that use, it isn't very efficient, but relies on equipment that most people already have anyway. The invertor is so small I can stick it in a drawer, out of the way. For seldom use, investing in a boat "deep cycle" battery wouldn't be particularly cost-effective, so should I?

Surely just watching TV wouldn't make my car battery get hot, but just mildly warm, if that? Isn't most of the heating in the invertor itself, which does have a fan within it. So are you suggesting that I power only the TV, and not add to the load, plugging up a lamp? Now most likely, during some power outage/storm, I would just go to sleep, or power up a battery radio, and not bother hooking up my invertor. It's more of one of those "just in case" scenarios. Hypothetically, I have guests or family members who say, "We need something to do, can't you get out your invertor and power up some video games or something?" What can I really do with 500 watts that is worthwhile?

I know, power some power tools in a place remote from electrical outlets. Like when the posts broke off my old car battery in the parking lot of the auto parts store. I couldn't put in the new battery, due to the posts being still in the car battery post clamps. My Dad talked some nonsense about leaving my car there and getting it towed. Huh? My Dad used to fix stuff like that. "Can't we just drill out the posts or something?," I asked. So I suggested, let's go to my house and get my invertor, since we had no cordless drills. I thought that better than stringing long extension cords across the parking lot. He tried to power my invertor with his extra "camping" battery from his van, which of course didn't work, because he previously (apparently) flipped off his charge toggle switch too much, and so my invertor "protested" with an audible low voltage alarm. So I suggested, well just start the engine and flip on the charge circuit. Which worked just fine.

And I do have gas logs, for power outage heating. (another excuse not to invest into a generator) Actually, I run my gas logs far more than the furnace anyway, as of course I selected a ventfree model, because my Dad bought me the accessory that almost nobody thinks to get. I have a gas logs wall thermostat, so why pay for natural gas to put heat up the chimney or to heat the basement? (And of course, seeing a small warm flame&glow, rather than hiding it down in the basement, just looks so cozy anyway.) Gas logs zone better, as any room I don't want to heat, I can just close the door. But they don't distribute the heat so well, good thing I don't currently need to heat every room. The wall thermostat works with gas log sets that have the "milivolt system," that provides an electrical connector for an optional regulation switch. The electricity comes from the thermocouple above the pilot light, so there's no batteries, nor any thermostatic remote to sit on. And it never "knows" that there even is a power outage, since its electricity is independent of the power company. It has to be a special thermostat, as ordinary thermostats use too much power to be powered by a pilot light thermocouple. Probably due to that resistor that helps shink the gap between on and off, with an adjustment lever hidden within the thermostat (speaking of the old furnace thermostat). My gas log thermostat is way back on the hallway wall, so that it can't be influenced overly by the heat of the gas logs, as with electric plug-in heaters, that sense their own heat and cut off leaving one cold, then make too much heat on a warm day, due to having to rarely cycle and cycling too much, not feeling their heat due to not having cycled for a while. I have heard that older gas log sets with thermostats on the unit itself, produced similar poor results, unable to maintain temperature from colder to warmer days. Actually, my gas logs thermostat is installed farther away than recommended. Supposedly, the wire should not be over 20 feet? long, lest it might lose too much voltage and fail to activate my gas logs. But come on, it's a sufficient gauge apparently, as wirenutting another extension wire to it, never caused any problems.

My gas logs are rated at 22000 to 32000 BTUs. No wonder they are able to keep me warm by themselves at down to 40 degrees F outside, on low, even in an old poorly insulated house that isn't all that big. That's more heat than running 4 electric heaters. 120V 15A circuits, just aren't all that much energy, thus most every electric hot water heater or air conditioner bigger than a cheap window unit, are all 240V. I usually run them on low, because cranking it up on high, as I might have to do during a power outage, would normally heat too much, cause the thermostat to cut them off, and then I have to see that cold dark hole of a fireplace. I rather they stay on continuously, and let the old furnace come on and push some heat to the extremities of my house, should they need some "help" to keep the house warm on the coldest days of winter. So I set the thermostats to give my gas logs priority in heating, since I think they cost less to operate than my old furnace. Newer high-efficiency gas furnaces capture more of the heat, and don't send as much up the chimney, but the water condensate from newer systems, if not installed exactly correctly, causes yet another reason for the furnace to shut down and require a service visit.

Huh? Are batteries so fragile? And yet they are pushing highly experimental (not ready for the consumer market) hybrid cars as a supposed "green" option? The biggest problem with the old rejected electric cars and the newer trendy hybrid car options, is that the battery technology is nowhere near where it needs to be to power naturally power-hungry cars.

Well I looked at the electrical data plate on my old ancient Philco refrigerator, which I think is pretty much on its last legs, as it's gotten more noisy when it starts up, and I have to turn the defrost timer every day to keep it from having defrosting problems. At least I can reach the defrost timer knob hidden behind the plastic front grill. The electrical data plate, suggests its power needs probably slightly exceed my invertors capabilities, so I haven't been brave enough to do a test or anything. If the invertor can't provide enough power, could that damage my refrigerator? I imagine that power-hungry refrigerant compressors, don't much care for low voltage situations. Low voltage might burn such motors up or something by stalling them. The surge voltage rating on my invertor is only 800 watts, which seems a little low. Could it be higher actually, due to manufacturers figuring conservatively what they are sure it can handle for safety or reliability or whatever, or could it be like Consumer Reports report on generators, in which useful load seems to be lacking compared to manufacturers' claimed load capacities? And do compressor motors even "care" that invertors don't produce perfect sine waves? Or is the average RMS all that matters?

Yeah, I know not to dare open the refrigerator door once, any time the power is out? (So how I am supposed to eat perishable food once the power fails? Gee, I guess I should have known the future, and got it out before the power failed?) It's scary how much heat a briefly open door would let in, without the compressor cycling on within a few minutes. But when my power was out for about a day, or many hours, some time ago, fortunately, I didn't have much food in there anyway. Must have been about time for another trip to the supermarket. I had some ice cream, which I took over to my Dad's house, since he still had power. After around 24 hours, my refrigerator was about 60 degree F. So much for "leaving the door closed." Of course during an ice storm, just put one's food in a plastic tote with a lid, and stick it outside. I have figured my car to be a "refrigerator" sometimes in the past. In winter, sometimes I can buy groceries on the way to Church, since it is so cold outside anyway. Cars don't hold heat for long, once the engine is shut off, one reason to, if not actually wearing a coat, at least bring one along, in case of car failure.

If I had a little camper refrigerator, like my Dad used to use when camping, well I then probably could get away with running on invertor power. But then those run on 12V or 120V anyway.

And who would remember to do that? Isn't it easier to blame the phone company for their lousy service? As much as the local phone monopoly charges for their 50-year-out-of-date slow dial-up service, when we should have had fiber optics run all the way to our homes by now, I think I would rather have the option to go with cheaper party lines. How long must we wait, to get cheap "modern" technology options? I want high-speed internet, but don't want to pay a cent extra for it, since I don't make that much money after taxes, and I mostly get on textbased forums anyway, which do okay even on dial-up.

Well that's a great benefit to some of these modern 4-handset-in-one cordless phone systems. Since the power comes from the AC power adapter, which has far more power than a low-voltage phone line strung for miles from the phone company, and from multiple handset batteries, I figure they don't weaken the talk volume, when somebody picks up a 2nd handset to join in the conversation. Which seems to sound better than the speakerphone option. But beware, not all cordless phones allow all handsets to be used, simutaneously. The system I bought for my sister, allows only up to 2 out of 4 handsets, to be used at the same time.