ac motors with brushes are universal motors. im not too framiliar with them except that i remember them being frequency independant. as though they could run on anything from 50hz to 400hz. i seem to recall a 400hz generator you could attach to youre truck motor with a v belt next to the alternator to power universal motors. i guess this is usefull for a recluse building a log cabon with no electricity. universal motor MIGHT generate power, but i dont think they have magnets so its doubtfull. i stay away from them since i have little expierence.

you can find a nice induction motor in smaller hosepowers in things like table saw, drill press, air conditioning fans, etc. but my main interest is in l;arger 3 phase motors. single phase motors top out at about 5hp. do a search on ebay for a single phase motor and youll be hard pressed to find anything bigger than 5hp. but 3 phase is everywhere in huge sizes as big as cars to the size of 1/4hp and less.

you are right that you can connect a 3 phase motor to single phase power, but as you said it wont start turning on its own. it will just sit there and vibrate. you have to spin the shaft and plug it in and it will catch and spin on its own. but one big part of the motor wont be used for power, but it can be used to generate power. thats how machinists use 3 phase cnc's in their single phase houses. they have a big 3 phase motor running off 220v and the 3rd leg generates power. a friend of mines got a big 3 phase motor as big as a 55 gallon barrel to convert single phase to 3 phase for his cnc, bridgeport and lathe in his garage.

like i said this stuff facinates the crap out of me.

That would work, but that's not the audience I'm trying to reach - unfortunately :/


I was going to use an ammeter - but that sounds a lot easier :D

Hey trebuchet03, I got a chance to test a cheap inefficient 6 pole, shaded pole box fan motor today on our motor test stand. I was curious to see what would happen if I tried what you proposed at the start of the thread. I directly coupled it to a variable speed dc motor and plugged the box fan motor in thru a directional watt meter (like a utility power meter). When I powered up the fan motor I noted the direction of rotation and speed, then I powered up the dc motor and checked it with a tach until i was going around 1256 rpms (not quite 5% over speed). The watt meter was still turning the same direction but much slower. I continued to speed up the dc motor and could never get the meter to spin backwards :( it did slow way down though. I will try this test with other motors when I can get the chance.

I tried a different motor today with good results. It is a 6 pole capacitor start/run induction motor off an industrial airmaster fan. I was over driving it with a 3/4 hp 1725 rpm motor thru a step down pulley set. With the 3/4 hp motor at full speed it would be trying to spin the fan motor at 1400 rpm. I used the wattmeter and an inline multimeter reading amps on the fan motor. Running the fan motor by itself it was drawing 1.5 amps and spinning the wattmeter ccw. As I brought the 3/4 hp motor up with a variac (variable output transformer) I noted the speed when the wattmeter stopped as 1233 rpm. At this point you'd think the ammeter would show zero amps but it still showed around 1.3 amps. This is because there is still current in the stator but it is 180 deg out of phase with the voltage (90 deg is the normal phase shift). That is why the wattmeter stopped spinning. Anything above this speed started spinning the wattmeter cw and causing a phase shift in the other direction. I was able to get the wattmeter to spin faster cw than it did ccw as a motor :thumbup: . The fan motor did start to get warm though so I wouldn't expect to be able to do that continuosly. Looks promising :thumbup: :thumbup: .

I would like to eventually check the input vs output power though. That will require the torque meter in between the motors so I can use actual shaft input power and not worry about the inefficiency of the drive motor (which could be any type of prime mover). I'd like to monitor the voltage and current phasing on the Tekscope too.

Nice :) I still need to get to the surplus store... perhaps this weekend or next... Curses I hate waiting so long :p

Interesting results with the shaded pole....

I've read that motor temperature as a generator should excess 170-180F or so for continuous use. 180 is about the limit for touching something - although briefly. 170, you should be able to hole on for a few seconds before it feels too hot.... Not sure how empirically accurate 170-180 is though :p

First I have to say the power loss converting power this way is a LOT, but the cost of a sine wave inverter is a LOT as well ;)

Before inverters were common they had motor-generators, basically a 12vdc motor tied to a 120vac generator. With this you could easily create 120vac from 12vdc. The problem is they were not very efficient, like 50-60% at load. The huge advantage of them was motor starting, since you physically had two motors spinning when the load hit the flywheel effect kept them moving when an inverter would trip out.

I had one of these as a kid and it did work both ways. If you plugged the 120vac side in it made nice 13.4vdc power and yes I even tried pushing the 12v side up to 18vdc and it did push power back out to the grid.

That’s what I know about it...

Here is a good on-line source for cheap motors and other stuff. https://www.surpluscenter.com/electri...tname=electric

They even have a Wincharger made rotary converter :) . https://www.surpluscenter.com/item.as...tname=electric

Brock, your right about low efficiency, but low dollar and ease of DIY is a plus.

A home built wind generator with a speed switch to enable grid power only when the turbine attains a speed high enough to back feed would be ideal.

I wonder how the total system efficiency would compare between a carefully selected induction motor as a direct drive grid tied wind generator (grid acting as "storage") vs. a system that charges batteries and uses an inverter? How efficient are lead acid batteries anyway?

Sweet! Even with the low price, it's still a bit high for a 1/2hp (as compared to, I found it for free in an abandoned appliance). I guess it won't hurt divulging the plan :)

So the task to complete is: converting energy from a bike trainer into mains power. I must go from the bike (perhaps through a trainer stand) to some sort of conversion to electrical and then plug into the grid.

Is it safe to assume an a/c motor with a capacitor is suitable for generation?
This would have been perfect if it wasn't for it's non continuous duty cycle :/

I think 1/2hp ideal, even a 1/4hp should work. cruising at moderate pace around 100 or so watts, 200 watts for more strenuous training... And then the occasional non sustainable burst...

From what I've seen so far it should be safe (especially if it is free :) ).

I have tested a couple more motors lately and noticed a couple things. All have been capacitor run motors. The one I just finished testing was off my Lakewood 20" high velocity fan. It seemed to generate best on the lowest speed setting (it has 3). With a fixed input on low I was seeing 136 watts going out, on medium I saw 130, and on high I only saw 32 watts. It is rated for just over 2 amps on high and is the size of most box fan motors but axially longer.
I tested a 1/2 hp garage door opener motor and had trouble getting any output power from it. I think it was needing more power than the 3/4 hp motor I was driving it with could provide. It was quite small for a 1/2 hp motor (about the same as the lakewood motor) but it was intermittant duty rated for around 5 amps.
So far the motor that seemed the most efficient was the 6 pole airmaster industrial fan motor. It did best on the high speed setting (it has 2). It's a low torque motor rated at 1100 rpms 2.5 amps input on high and air over continous duty. No hp rating is given but physically is the largest motor I've tested (6" diameter x 6" long).
I plan to try some appliance motors soon, I think I have a dryer motor and a washing machine motor plus some 2 pole pump motors.

So far the lower power density motors look to work the best. For the application you are thinking of trying I would try 1/4hp or less, 1/8 or 1/6 hp may be best to start with.

^^ wait, that high velocity fan... Is that a typical $10-$20 box fan - shaded pole? Or a more "industrial" fan (the type with metal blades and a metal cowl)?

I was thinking about that after I posted.... It would seem prudent to do a very low scale proof of concept - on a bike and everything before going to a full scale larger capacity system.