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Old 10-27-2007, 12:29 AM   #1
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Grid Tie and Induction Motors

Yes, I know it's nearly 3am on the east coast... but and idea struck me. So the idea is to feed small amounts of mechanical power into the power grid. Not necessarily run the meter backwards, but supplement power consumption.

I've researched grid tie inverters - which are very expensive. For those wondering, a grid tie inverter is feeds mains power back into the grid by syncing phase angle and phase (no dead shorts ) and applying slightly higher voltage. They are very efficient and really not within a college student experiment budget

So I was thinking... Rather than go from mechanical to DC to AC to grid - go from mechanical to AC to grid VIA an induction motor. As a proof of concept, use a DC motor + battery to turn an induction motor. Plugged into the grid, in theory, should apply current if spun fast enough with enough torque.

Oh, but the phase! you say? How do you prevent a dead short?"
I've thought of this -- before applying mechanical power - have the grid bring the induction motor up to speed. Then try to turn faster (apply a torque) with the DC motor, for example. In theory, the amount of extra power put into the grid will be related to the slip angle of the motor - which will also control the speed of the input (so you can't go over speed by too much).

Keep in mind that this whole battery business is just a proof of concept sort of thing - I'm not talking perpetual motion or any hohaa craziness. In the end, the final mechanical input will be around 200 watts. I expect this to be very low efficiency (likely 50%ish), 100W isn't an answer to the energy issues - but it's an experiment. It's also not going to come even close to driving the meter backwards, but it should run (as supplement) my laptop + two to three 13w CFL's

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I think the theory is feasible -- the inspiration comes from flywheel driven UPS systems. An induction motor is driven while mains power is on to keep a flywheel in motion. When the power goes out, the FW drives the motor and feeds to local grid.

I'm thinking of using a "low" rpm induction motor.... If I recall, ceiling fans are 16 pole? So that's 60Hz2*2/16=450rpm... Add ceiling fan motor to the list of things to hunt for Looking at the one above my head, it looks like it even has a nice bolt pattern for some sort of pulley shenanigans

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Can someone either throw some ice water on me and slap me for being an idiot -- or let me know if I've found a boat to Valhalla.

Oh, and my apologies for dancing around the "mechanical input" details.... There's a reason for this, I promise In any case, insight and information is appreciated
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Old 10-27-2007, 01:28 AM   #2
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Why not use a "dead" box fan motor and gear the DC motor appropriately, too much slip already w/ a fractional hp motor?
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Old 10-27-2007, 01:49 AM   #3
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Quote:
Originally Posted by omgwtfbyobbq View Post
Why not use a "dead" box fan motor and gear the DC motor appropriately, too much slip already w/ a fractional hp motor?
Totally possible -- I'm just looking for a proof of concept at the moment All I need to see is the current flow change directions, even if it's small. I'm just looking for a "yeah, that works" or "No, you need to do {insert action} first" before I dive in
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Old 10-27-2007, 06:30 AM   #4
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By all means, experiment, that's what circuit breakers are for . I don't quite understand what is going to keep it in synch. I suspect the slip angle will be more like different rotational speeds, but I definately would like to see a simple mechanical tie-in happen. Maybe I'm missing something here.

You start out in synch, yes, then you manually keep it in synch, at a resolution of 60 hz?
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Old 10-27-2007, 11:21 AM   #5
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Quote:
Originally Posted by skewbe View Post

You start out in sync, yes, then you manually keep it in sync, at a resolution of 60 hz?
Think of what happens when you put an induction motor under load -- you start at the rated speed, and slow down a little... Say from 3600 to 3550 - this is related to the motor slip and the motor is always in a state of attempting to catch up to the field. If you apply too great a load that the motor slows down too much, the motor will just completely stop. This is why (generally speaking) you can hold a motor in place with a heavy load, apply power, and nothing happens - induction motors require some sort of excitation to get moving (today's design and motor controls allow for such starting though).


Now, to my understanding, it works in the opposite direction. If you apply a load in the same direction as the motor - the motor slips in the same direction of rotation - say from 3600 to 3650 rpm. But, because you are coupled to the grid, you're governed by the grid itself -- the grid itself keeps you from going over speed AND under speed. Go too fast, and you'll feel more resistance - go to slow and the motor will start drawing power from the grid.

Of course, this is what I think is going on - based on a physics II course I took 3 years ago and haven't touched since
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Old 10-27-2007, 01:03 PM   #6
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Quote:
Originally Posted by trebuchet03 View Post
Totally possible -- I'm just looking for a proof of concept at the moment All I need to see is the current flow change directions, even if it's small. I'm just looking for a "yeah, that works" or "No, you need to do {insert action} first" before I dive in
Oh yeah, portable fan motors are easy peasy to find/fix. The manufacturers, instead of spending an extra quarter and using a thermal switch, use a thermal fuse, so if/when the fan binds because of the crappy lube getting contaminated, it'll blow and the fan won't work no more. So, free fan you can fix for a couple bucks or so, or even freerer if you just bypass the blown thermal fuse.
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Old 10-27-2007, 02:12 PM   #7
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Quote:
Originally Posted by omgwtfbyobbq View Post
Oh yeah, portable fan motors are easy peasy to find/fix. The manufacturers, instead of spending an extra quarter and using a thermal switch, use a thermal fuse....
Cool, I didn't know that - but makes a lot of sense
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Old 10-29-2007, 06:21 AM   #8
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This was done with some commercially available home sized wind generators back in the 1970s. I think that Enertech was one brand name.

Here's a bit of info:
http://www.awea.org/smallwind/succes...ories_026.html

And just a little down the page here:
http://www.kansaswindpower.net/used_wind_generators.htm
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Old 10-29-2007, 01:30 PM   #9
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Interesting, because a co-worker and I have been talking about grid-tie for a while and we came to a similar conclusion that this would be the cheap way to do it. The theory is sound and should work. It would be great to have a set-up with an over running clutch so you can start the motor and then bring your primemover (you) up to speed and attempt to overspeed the induction motor. You could use the rear end of a bicycle with the freewheel. It could run off the perimeter of the tire, or grab a bent up rear wheel and remove the spokes and rim. Connect the motor so it spins the hub thru a sproket and chain or something then start pedaling and shifting gears until you can overrun it(all while mounted on a stationary exercise thing of course ).
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Old 10-30-2007, 06:08 AM   #10
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If that bike test set-up works out good, then you could find the right DC motor to run off your homemade windgen and/or PV array to overrun the right induction motor thru a direct drive set-up. Some simple relay logic could make things more efficient so the induction motor doesn't pull from the grid when no alternative energy is available to overrun it. It could be fairly efficient if the motors were carefully chosen and some motor control were used on the DC motor to effectively load the system.

I love alternative energy but am put off by the thought of massive battery banks and grid tie seems like a good alternative. Of course you still lose power when the grid goes down though .
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