Alternator Testing

[Larlaeb]

I've completed the alternator for the 7ft (scaled down from the 10ft plans as indicated in the Homebrew book).  Since this is my first machine and I don't have any reference point, I have a few questions to check my work.  This is the single rotor machine and it's wound for 12 volts. The gap between rotor and stator is currently at 1/8 inch.  My questions are:

1.  How hard should this be to turn?  It turns smoothly but offers a good deal of resistance even with no load on it.  No problem turning it by hand at 60+ rpm or so though.
   
2.  If I short any two of leads the resistence increases and it has a noticeable cog as it is turned and if I short all three of the leads it turns smoothly with even more resistence.  I think that is as it should be.
   
3.  Putting a meter on the leads and turning the disk at roughly 60 rpm I get the following voltages:
   
   

     Center to outer right  - 2 volts

     Center tor outer left  - 2 volts

     Outer right to outer left - 1.5 volts

The difference in the last voltage reading concerns me.  I expected some variation but that seems a little much.  

Any help in know what and how to check this before I go further would be appreciated.

Thanks,

[Flux]

 "How hard should this be to turn?  It turns smoothly but offers a good deal of resistance even with no load on it.  No problem turning it by hand at 60+ rpm or so though. "

The only resistance should be that of the bearings of the hub. If you have seals and heavy grease it may be not negligible. If you remove the stator and the thing is much easier to turn then you may have a problem. I would expect you to hand crank it to 200 rpm with no trouble.

 "If I short any two of leads the resistence increases and it has a noticeable cog as it is turned and if I short all three of the leads it turns smoothly with even more resistence.  I think that is as it should be."

Yes that is as it should be.

I assume you are measuring your volts from the star point to each output lead when you say centre.

The difference in voltage does seem a lot, I would expect things to be very close. This and the fact that you can only crank it to 60 rpm does make me think you may have a short in one coil.

You are on the low end of voltage and frequency for accurate measurements with some cheap ac meters but even so the comparison between like things should be correct even if the absolute voltage is in error.

Remove the stator and see if you can crank it easily, if there is a difference with stator removed then I do suspect there is a problem.

Flux

[Larlaeb]

Ok, there is a large difference in resistence with the stator removed.  It will spin freely and make several revolutions free wheeling after I spin it.  With the stator on it will not spin at all unless I'm pushing on it.  

As far as the voltage checking is concerned, I'm just using the three leads from the stator.  Reading between the inner lead and either of the outer ones gives me 2volts but if I put the meter on the outer two I get about 1/2 a volt less at that speed.  Doesn't sound good.  

Looking through the stator with a light the connections look 'smuggy' almost as if the shrink wrap was melted by resin.  I wonder if that could be the problem.

Suggestions?  Or do I need to start over.

Thanks for the response.

[DanB]

It sounds like there's an internal short unfortunately...

Sometimes you can trace this.  If you hook a battery to a phase then 6 coils should become 'magnetic' - look for a 'dead' coil (or three maybe) and perhaps you can find a short.  Also while doing that...  check current if you can, that's a good way to check resistance in these stators.  

[Flux]

OK so you are measuring the 3 line voltages not to neutral. They should all be equal and I also think your figures are low if measured at 60 rpm. the dc volts will be 2 x 1.4 = 2.8v . To reach 12v you need 60 x 12/2.8 = 257 rpm. This may be ok but seems high for a 7ft prop.

In view of the drag issue I am sure you have some sort of fault in that winding.

I assume it is a 9 coil winding and you have star connected it. If you have delta connection the drag might possibly be ok but if star connected it is certainly faulty and the drag can only be caused by shorted turns. wrong connections would cause unbalanced volts but would not cause drag.

If it is completely potted I think you will struggle to sort it out but if you are fairly experienced you could try passing a controlled current between each pair of leads in turn ( say limit to 10A from a car battery with limit resistance). You should feel the same pull on all the coils in two phases to a magnet held over them( half will be repulsion and you will need to reverse the battery to compare them as a pull. If you have one coil very much weaker than the others you may be able to get into its connections to check for a short. If you are not very experienced this will probably be too confusing for you to follow.

If the fault is within a coil and not the interconnections then it is a dead duck anyway unless you can excavate that coil, fit a new one mad make good the potting.

Bad luck, sorry I can't be more hopeful.

Flux

[Flux]

Dan got there first but I think we both agree you have a problem. Between the 2 descriptions you may be able to follow the method of testing we described.

Flux

[Flux]

Pleased to hear you managed to fix it. As you say the back side is not important, just cover it with plenty of resin to keep the wet out and it should be fine.

Some of the heat shrink sleeving is not as tough mechanically as I would like and I am not really sure how it behaves when potted with solvents and lots of shrinkage forces from the resin and possibly from the temperature of cure shrinking the sleeve onto soldered joints.

Sometimes we have to use what can be got rather than what is ideal.

Flux

[kurt]

Larlaeb posted with a very oversize pic this comment.

Gentlemen,

Thanks so much for your help on this.  Luckily since I'm only using one mag disk, one side of the stator is not that critical in terms of smoothness.  I got out the dremel and pulled out a couple of connections.  It appears that the shrink wrap either melted or rubbed through.  I pulled them out (see pic) and stuck it back on the hub to check it out.  No resistence and over 3 volts on all leads.  I'll continue checking but it appears this was the problem.  I'll need to re-insulate and stick it back together.  It might be a little ugly but at least this part will work.  Serves me right for congratulating myself on a good casting. =)

Anyway, you guys are tops and the input was much appreciated.  I'll let you know how the project comes out.

Thanks again,

the photo had to go therefore the comment had to go but i wanted to preserve his text so i reposted it here.

Larlaeb if you are going to post photos here they must be sized to less than 150 kb and under 640 x 480 pixels.

[Larlaeb]

Sorry about the oversize photo.  I see the rules now. Thanks.