Generator made out of a 'cheap' AC motor (wound field induction motor)

[Mads]

Hi All

This is my first post and I cant wait to get started making wind turbines!

I found this video: http://www.youtube.com/watch?v=DkbZizJvDCI
(You can put subs on it)

In the video he got a generator that he made from a AC motor where he feeds DC current into the rotor which makes a magnet field (Instead of using perm. magnets ). I really like that idea!

I have searched the web without figuring out how to convert a AC motor into a wound field induction motor. Anyone knows how to do it? or anyone tried it?

Thank you so much
Mads

[electrondady1]

welcome mads.
my understanding is an induction motor can be used as a generator by simply over spinning it.
i think capacitors are involved as well.

there is a method used on this site that uses a ordinary brushed ac motor and replaces the armature with a body of magnets .
the field coils are then used to produce a current
the method was perfected by much loved member, "Zubbly"
he has passed on now but his postings are still accessible.
just use the search function

[Flux]

Wound field induction motors used to be used for starting fairly large motors on line. They probably haven't been produced for years except possibly in odd places.

If you fed dc to the rotor they worked just as wound field synchronous alternators, which  are far more readilly available and will do the job just as well.

Such things are ok for large turbines but are less common for smaller projects as the field requires most of the low wind output of a small machine. If you have plenty of wind and don't mind getting nothing in winds below 10 mph then the wound field alternator is fine.

I wouldn't suggest converting a cage induction machine but of course it can be done and it is certainly no more difficult than making a permanent magnet rotor.

You should be able to find a wound field alternator that is a better starting point than converting a cage motor. If you are lucky to find an ancient wound field induction motor then fine it will behave just as a wound field alternator. Just depends on what you can find.

You could even convert a capacitor excited brushless alternator by adding slip rings to the rotor and you could take the output from the main winding and get some help from the quadrature winding. The main thing against these is that they tend to be 2 pole machines and for low speed operation you tend to get better results with more poles but it will still work.

Flux

[Mads]

Thank for the reply!
It sounds like it is not as good an idea as I had hoped.

[Janne]

Hi,

Like Flux mentioned, those motors can be useful in larger machines, when you're not interested about the lowest-wind performance. Like direct heating, or feeding to the grid. But at least on here they're getting hard to find, and have a bit of a maintenance issue with the sliprings and the brushes. So using one, I say it depends on the application and if you can find one affordably.
You can forget about direct drive with them though. For a small machine speed up gearbox dont make much sense to me(others may argue..), so it only starts being useful in big machines.

If you're curious about the  windings, I made a post about it some years back; http://www.fieldlines.com/index.php?topic=140188.0 Hope it makes sense

[joestue]

I wouldn't suggest converting a cage induction machine but of course it can be done and it is certainly no more difficult than making a permanent magnet rotor.

You should be able to find a wound field alternator that is a better starting point than converting a cage motor. If you are lucky to find an ancient wound field induction motor then fine it will behave just as a wound field alternator. Just depends on what you can find.

http://johansense.com/bulk/repulsioninduction_1.JPGhttp://johansense.com/bulk/repulsion-induction_2.JPG
i happen to have one of these things.
that's a half hp motor, 1725 RPM, about the same size as a 5 HP 3 Phase 3450 RPM motor.
the commutator is arranged with a steel spring inside the cup, which shorts out all the bars until it gets about say 800 rpm, after which the current is passes through carbon brushes. i modified the brush arrangement so the angle can be changed, its fun to play with but that's about it. i suppose i could hook a 60 hz MSW inverter to the rotor, and the line to the stator and see what happens when i try and spin it with VFD..

there is a thread on this forum where i did a bunch of experiments with making a wound rotor synchronous generator from an induction motor.
the biggest problem i ran into was: small induction motors are designed to run with EVERYTHING completely saturated. the shell of the motor helps pass the flux around the core lol, and about half of the flux doesn't make it through the rotor core, it runs through the teeth back to the core. (count the turns and use the standard transformer formulas to find what the flux would be...)
I was able to hit about 40 watts no load rotor loss on the equivalent of a 3 HP 3400 RPM motor core, running it with 4 poles instead, and the core maxed out at about 1.3T or so, but that's with no load.
once you throw a load on it, you have to add that many more amp turns to the rotor, which means in practicality you'd be looking at for a 3-5 HP motor, probably about half of your copper losses are in the rotor.

Unfortunately there is no good way to augment the rotor with magnets, unless you have a milling machine (which i do) and you have a solid block of steel to start over with for the rotor core. (which i don't, and don't know where to get around here)
oh, and if you want to run it at 1700 RPM, the rotor probably has to be laminated.