Generator made from wound rotor induction motor

[Janne]

Hi guys!

I've got a large wound rotor generator, from a rather large windmill. Unfortunately it's busted, beyond possible repair. But to convert another motor for it I really needed to study the blown one further, to learn how the rotor is connected.

The generator I'm studying is an old 33kW, 950rpm(6pole) motor. It's been converted to work like a car alternator, supply DC to the field coil, and it makes 3 phase AC current. The beauty of this design is, that the windmill output is very easily controlled by varying the current supplied to the rotor, (or field coil). The stator is just like in a normal induction motor.

This is the generator I'm talking about. Most likely due to too high RPM's the field windings had came loose, and caused a catastrophic failure. It's got 3 sliprings in the rear part of it, originally used for soft starting purposes.

Finally after almost a days work the rotor is out of the motor. It has 72 slots on it, each slot has 4 turns of wire in it. Each coil in the rotor has 16 turns, so it occupies 4+4 slots. (4 going forward, and 4 coming back) I'll call these sets of 4 slots "bundles", so there is a total of 18 bundles in it. These coils of 16 turns are obviously wound in the factory, and they have not been tampered with. Instead, all that has been done is that the coils start and end leads have been reconnected.

Part of an old note, made by the guy who had done the conversion. The numbers there state the bundles, for example 1 means the start of coil 1 and 4 means the end of coil 1. It's rather confusing, but I hope you can follow. The dotted lines state about the required changes in the original connections. It would be easier to

understand if each of the coils where current runs clockwise would be first wired up in series, then the ones where current runs clockwise after that. But this picture shows how to do the process with minimal changes to the original winding. Also in the drawing is shown the 6 magnetic poles formed, 4 bundles make up a single pole.

I drew a picture that hopefully makes it more clear. Basically this tells how the coils sit in the slots on the rotor. Remember, each slot has 4 turns on it, though there is only 4 turns showing it is 4x4=16 turns in that coil.

Another picture that clarifies more the bundles and how the magnetic poles are formed. 4 bundles form a single pole. The connections between the bundles is not shown, they are all pictured in the drawing (in the note)

Anyways, a long post, but I hope if someone has one of these old motors can now find some good use for it

[Flux]

Yes those wound rotor induction motors were simple to use as alternators. All you need to do is to fed dc to a pair of rings on the rotor, or more usually to short 2 rings together and use those as one connection and use the 3 rd ring for the other dc connection. Leaving the 3 rd phase shorted let it act as a damper winding, but either way it works ok.

I haven't the patience to plough through your connections but I suspect the person who converted it reversed one phase to make it add to the turns on the rotor as a zig zag winding. Something that seems logical but was probably counter productive in the long run.

Sadly such things went extinct just after Noah's ark and few will be lucky enough to find one. Wound field excitation had some advantages but also has some considerable disadvantages compared with the present neo magnets. You can track the prop characteristic easily but you need to supply the excitation current and that has to knock the low wind performance. The things also showed significant reactance limiting and you had to hammer the field to get an output remotely close to that you can get with neos. Normally there was no chance of direct connection and you have all the snags of a speed increase.

Flux

[dinges]

Guess I'm one of the lucky ones then, Flux

http://www.anotherpower.com/gallery/album93

We've discussed about this motor a few times in IRC. Zubbly once told me to guard it with my life, otherwise I'd probably have taken it apart for its sliprings by now. Sad truth is, I don't really know how to put it to use. Discussing a bit with Janne, and studying his notes, opens some perspectives.

The problem is that there is very little information on the web (or this forum, for that matter) on using such motors in windapplications.

Thanks for the posting Janne. Hopefully more information about using these motors will come up.

Peter.

[Flux]

There is basically nothing on the web about wind power pre permanent magnet. Only a few odd people played with home built wind power, there was no information and it was mostly learning from experience.

The commercial big machines used this type of motor as induction generators synchronised to the grid, the wound field let you add resistance to increase slip and keep the prop speed up. The very clever ones used slip power recovery schemes ( electronic Kramer drives) to feed the slip energy back rather than dissipating it as heat.

The problem with all wound field alternators charging batteries via rectifiers were very similar to those of the original dc generators. The dc machines lent themselves to compounding. Alternators made little impact before the coming of the silicon rectifier.

They were not as easy to compound as the dc machine but the coming of the transistor changed all that.

The big snag is that they were large and heavy  for the output and to manage reasonable power to weight it was necessary to use a speed increasing drive.

I got results comparable to using permanent magnets but with slightly worse low wind performance. Control to avoid stall was easier and in general the overall performance was good.

Most people nowadays aim for the very best possible output in the region above cut in and that makes a lot of sense on most wind sites, but it comes at a heavy price in output above 20 mph. Fortunately lack of battery capacity makes the higher outputs less attractive as if you have enough power in low wind you spend most of the time dumping in high wind. If it were not for this the present stalled machines with low efficiency in high winds wouldn't compare with what we managed in the old days.

Flux

[Ungrounded Lightning Rod]

Most people nowadays aim for the very best possible output in the region above cut in and that makes a lot of sense on most wind sites, but it comes at a heavy price in output above 20 mph. Fortunately lack of battery capacity makes the higher outputs less attractive as if you have enough power in low wind you spend most of the time dumping in high wind.

And that's the big difference between off-grid site power machines and commercial wind farms:

 - The wind farms have a grid to feed and can make good use of that high-wind energy.  In fact it's their big money maker, thanks to the cube-law relationship of power to wind speed.

 - Off-grid systems may be able to use some of the high-wind power on optional loads.  But mainly they need to be optimized for low-to-typical winds (to keep the storage from running dry when wind is light) and protect themselves from destruction in high winds (when the excess energy would be mostly wasted anyhow).

[hvirtane]

A really nice generator and description of it.

I've seen some of those engines, too. One of them not so far from me. Don't own myself any, however.

I'm wondering if something similar, but axial flux design could be made at home...

- Hannu

[Electron Skipper]

I agree, it is nice item.

If you currently do not have some of the Lindsay publications on motor rewinding, and the Lejay manual reprint, you might want to consider adding these to your library.  

The Lejay manual could give you some ideas for rewinding as could some of the older motor winding books like Audell's, and a couple books Lindsay also reprints from time to time.

Worst case- Rewind it and attach an internal combustion engine to it; use it as a welder.