How to run a 120 VAC fan off 12 VDC.

Hi, EE's I could use some ideas on a domestic project that has hit a glitch - one that, as an EE, I should have seen coming!
We have installed a wood burning stove insert in our family room to provide a third fuel source in the event of an electrical power failure (we have electricity, a gas furnace (needs electric power, of course), and now wood.) The wood stove insert has a 50 watt turbo-fan to push air around the firebox - this is needed to get the up to 35,000 BTU/hour heat out (it is not a free-standing stove, it is in the old fireplace.)
To run this fan when the power is out , I acquired a 12 VDC deep cycle marine battery and a 300 watt ,12 VDC to 120 VAC converter. Well, it runs after a fashion but with a nasty buzzing noise. Reason: the motor (a shaded pole AC induction type) does not like the massive harmonics in the convertor's AC output.
Waveform and harmonics: I put a 'scope on the AC waveform. It is virtually a 60 Hz square wave... but not quite. While a true square wave goes from + to - instantly, this waveform stops at zero and holds for about 4 mS, it also holds + and - for about 4 mS (all aproximate times.) The promotion for this unit calls it a "modified sine wave" - I'd call it a "brutalised sine wave"! It's OK for resistance heating, tungsten lamps and 120 VAC power supplies with a FW or HW rectifier, but not for AC motors. Of course, this waveform allows the converter output devices to be either fully on or fully off for negligable internal power loss using only two high current DC voltage rails (+ and -). My understanding is that a better, stepwise (but lossless) synthesis of a sinewave needs multiple switched high current DC lines - much more expensive than this little converter.
I presume the harmonics are all odd since a true square wave is sine (wt) + (1/3)sine(3wt) + (1/5)sine(5wt) ... etc. I've not done a Fourrier analysis on this "brutalised sine wave" but it may be all odd harminics, too (anyone done this?) Anyway, I tried using a 780 mH series choke to reduce the 180 Hz, 300 Hz, ... etc. No go. With a resistive load (75 watt lamp, not the motor) this choke dropped the voltage to about 40 VAC rms with a nearly triangular waveform across the lamp. Tried a smaller, 21 mH choke and it took out the higher harmonics but left a very significant 180 Hz component such that you could hardly recognise the sinewave (it was in there somewhere!) So, series chokes don't work.
Next I tried a 5 MFD AC capacitor across the lamp with the series 21 mH choke in. Result: 120 VAC RMS across the load but huge increase in AC current, presumably mostly leading VARS - but not measured. I did not measure the DC current to check this. I could do more measurements but I think the problem is clear.
Musings... 1. Is there any work around that still uses the 12 VDC/120VAC converter? 2. A large part of the AC energy is in the higher harmonics. Attentuating them on the AC side, even if possible, would seem to cut efficiency. 3. I can't replace the fan motor with a 12 VDC type - it's mechanically infeasible (but it's the best solution as a 12 VDC power supply for regular 120 VAC use would be very easy.) 4. Any scheme that delivers a true sine wave at 120 VAC is very lossy (low efficiency), like a class A amplifier. 5. Running a simple rotary fan (from a microwave oven) directly from the crude AC is actually much quieter, no buzz. Perhaps the stove fan has more vibrating metal parts. 6. The stove fan does not need to run at full speed to be effective. Indeed, it's OK down to 70 VAC from a variac, or with the rheostat speed control quite a way down to, say, 1/4 speed. 7. I've run out of ideas... does anyone have any more?
Here's the fan voltage/current curve on pure AC: Fan speed control rheostat on full. Supply by variac (pure sine wave, not converter)
VAC        Current (AC amps) 60        0.2 (this is minimum for self start) 70        0.25 80        0.3 90        0.34 100        0.38 110        0.43 120        0.46 125        0.48 No PF info - assume ~0.9) 70 VAC seems to be the practical minimum for lowest speed operation.
Feel free to email me directly at "analogdino at(taboy) rogers dot(ty as they) com" (decoded) Thanks for all replies. Cheers, Roger
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On Fri, 30 Jan 2009 11:22:31 -0800 (PST), Engineer

Can you get a blower from a car heater? That sounds cheaper than trying to get pure AC out of an inverter you can buy at a reasonable price.
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On Jan 30, 3:52 pm, snipped-for-privacy@aol.com wrote:

(snip)
Good idea, thanks. But very dificult to fit into the stove insert (the original is a horizontal turbofan), or even outside - would need some sort of manifold and seal. Cheers, Roger
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With a fixed source and fixed load it should be possible to make a high efficiency tuned LC filter to do the job.
Don Young
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I had thought about this but was defeated by the design issues. A series L+C, tuned to 60 Hz, in series with the load might work. The shaded pole motor is inductive (adding to the series choke value ) but taking mechanical power from it makes it lossy so the overall Q factor might be rather poor. Also, the load presented to the unit at harmonic frequencies would be mainly inductive - not sure what that would do to it. But the AC source impedance is very low (it delivers up to 2.5 A rms, some of it very peaky), so that may help. I may give it a try unless there is experience out there that says it can't work!. I get the sense that many people must have run up against the motor problem with these cheap invertors - I'd like to hear from them. Cheers, Roger
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Go to Walmart and buy a 400 watt Maxx power inverter for $57. I have one that has alligator clips so I can connect it to the truck battery in case of a power outage, I then run an extension cord into the house and plug the Toyoto stove in and am good to go. As an alternative, a person could connect it to one side of the panel after turning off the main and all the breakers except one for lighting and one for the heater. I live where it gets down to 60 below zero F and if the power goes off for more than 30 minutes we are in trouble.
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Gerald Newton wrote:

Umm...no....you really shouldn't connect local sources to your panel. Use a set of plugs, or better yet spend the money on a proper transfer switch. Linemen restoring electrical services after an outage make mistakes too, and you don't want to be the one who supplied the volts that zapped a lineman. Relying on your main breaker as the isolating means is reckless optimism.
Bill
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You are right, Bill. You should spend the big bucks and do it right. Unfortunately, the common practice is to connect a generator using a male to male 120 volt plug. In tough economic times people use shortcuts. However, if they turn off the service disconnect before they do these illegal acts, the linemen should be saved. I still keep my little 400 watt unverter handy for emergencies. I have a generator but it will not start at 50 below so I will bring it in the house and use the inverter long enough to run the heater to warm up the generator. Linemen are required to test circuits before working on them. I attended the trial of one that didn't. He lost an arm and a leg in the accident. The NESC has work rules that are very explicit about disconnecting circuits and locking them out,
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wrote:

These "kill a lineman" threads are really getting silly. How long do you think a 400w inverter will be powering the grid? The inrush of a totally unloaded transformer would trip out a 400w inverter. I can't even get one of those Walmart 400w inverters to handle the inrush of a 27" CRT TV.
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snipped-for-privacy@aol.com wrote:

It's shorthand. "Kill a lineman" stands in for "you shouldn't do a dumb thing like this because the people who may want to fix your power supply may just this once forget to do the ground cables that they should do *every single time* because they've been doing it all night in a storm." Doing even slightly unusual things with residential power distribution should not be recommended. The inverter doesn't have to power anything past your local pole pig to be at best a nuisance. I wouldn't touch the HV terminal of a pole pig that I suspected had any kind of AC source still kicking on its secondary side. And it costs *nothing* to do it the *right* way - why would *anyone* want to jury rig a connection to a panel when it makes more sense to plug in the fan directly to the inverter?
Imagine the light show when the power comes back up and Joe Homeowner's teenager figures he'll be helpful by flipping back on the main breaker *without* first disconnecting Pop's jury-rigged internet special.
Bill
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wrote:

Bill there are plenty of good reasons to use proper switching equipment and I would never tell someone it is OK to do otherwise. It is just the "kill a lineman" thing that bothers me. They have procedures they follow that prevent this. A lineman who doesn't follow procedures is a dead man walking anyway. Just the capacitive coupling to a floating line is enough to kill you. That is why they ground anything they touch and assume every wire is live until they prove otherwise
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When it is 50F below zero we do things that are necessary for survival. I have seen one person use stove oil for ant-freeze because they could not afford ant freeze. Using an inverter to back feed the panel is an emergency act that just might save a house from freeze up. Each to his own. For me this is a viable solution and I intend to keep that inverter handy in case the power goes off.
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In article <94ab2489-cf61-4cae-a288-8e6641b1ccf4

Given the choice, I'd rather save the lineman. ...but that's just me.
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If the linemen are dumb enough to work in 50 below maybe they are too much of a liability.
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In article <90f5e0dd-f7ee-4087-b662-c441ad271b85

Nice attitude. Perhaps you believe murder is justified for trying to save your ass. <what a dolt>
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After reading your post, I read the inverter nameplate that said for purely inductive loads also add a resistive load such as a 100 watt light bulb.
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(snip)
Hi, All. I think I've found a workable solution. The series LC tuned to 60 Hz did not work at all... However, a low-pass series L = 0.5 Henries (nominal) and shunt C = 6 MFD across the fan seems to work. It gives about 86 VAC RMS across the fan and the 'scope waveform is "sort of smooth" but still not a good sine wave. Fan runs at about 1/4 speed, which is what I want, and it does not hum much. The DC current from the 12 V battery is about 1.7 amps (say, 21 watts.) This will be fan mechanical work plus converter losses plus some harmonic heating the fan. Larger caps across the load give higher speed but higher voltage acros the fan - over 200 VAC, even, with DC current up to 2.6 amps... not pursued, Cheers, Roger
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