testing for shorted stator?

[defed]

i was putting my anemometer up on my test tower at near the same height as the turbine.  while up there, i spun the blades and they seemed to not turn as easily as i remembered.  doesn't seem as stiff as full short, but not as easy as no short.  is there a way to check for a short to narrow down if the stiffness is caused by this or something else?  i unhooked it from the rectifier, and same thing, so if it is a short, it's in the drop line or the stator itself.

i ohm'd each phase against another and all are open, but i think that is correct since it is wired 3 phase star.

[defed]

found an old thread where Dan B says to run 12 vdc up each pair of phases and see if they draw the same current.  tried it, and all 3 pairs are equal...so i guess it's not the stator causing the stiffness.  i'll have to lower it down and check the rest of it out.

it's always been a slower starter, needs some pretty good winds to get going, but has always kept going in low winds once started, even tho it turned easily enough for the slightly heavier blade to settle to the bottom.

[Flux]

Not much to go on. I can't remember what type of machine it is but if it has a shorted stator it is unlikely to start and if it does it will soon burn out.

You won't find anything measuring resistance with normal meters but if star connected all leads should show a circuit and it will be as  equal as you can measure.

If it is an axial and it is stiff then you may have issues with bearings or seals, there is nothing else to drag. If it is a motor conversion then you will have core loss at start up and it may need considerable wind to get it turning but once the prop is out of stall it will run down to light winds.

It is very difficult indeed to judge anything from rotating the blades, the torque due to losses in these machines is small and even a shorted stator will not offer much resistance at low speed at the tips of the blades. The most common effect of a short is to have blades rotating slowly but they refuse to pick up speed. Cable faults will be so bad as to act as a permanent brake. A small coil fault may not prevent start up in a reasonable wind but it will  soon produce local  heating and burn and get worse.

Flux

[DanB]

diagnosing a shorted stator can be tricky...  especially if its on the tower.
Obviously checking that all three phases seem equal by checking resistance - and the best way I think to do that is run a voltage up and check current because your ohm meter is probably not good enough.  that often doesnt tell you much anyhow though.

If it's on the ground, then you could run current through it and check polarity on the coils with a compass (or a magnet or something) - if you put a 12V light bulb in series with that sort of 'experiment' it might keep the current low enough to give you time to play before the battery goes dead (or the coils over heat).

A real simple test somtimes is just take the machine down, remove one magnet rotor and remove the nuts the hold the stator down tightly to the stator bracket.  The shorted part will want to 'levitate' away from the back magnet rotor as you spin it - that sometimes helps to located shorted areas in the stator.

best of luck there... I've fixed some shorted stators and wasted days on other ones with no luck at all.

[tecker]

I look for shorts all the time . The only tool for that job is an infrared scanner . You can also use thermal printer paper .  I use thermal paper on pwr supplies.

[defed]

sorry Flux, i thought i mentioned that it was a 4' Piggott, single rotor, but i guess i didn't.

i think it was a bearing problem.  the spindle and hub i'm using are from the rear end of a front wheel drive car.  the bearings aren't the typical tapered type i'm used to.  it seems that the nut tightened a bit...it's just a nylock nut and the nylon is probably worn out, i need a new one.  i thought i tested it and it didn't matter how tight i made it, the bearings still were free...but when i lowered it to check, it seems it was causing the resistance from being too tight.

one problem that hasn't helped is i had 1 windy day since i put it up, and it worked well...ever since, not a LICK of wind.  even the anemometer has barely turned since.  hard to check/monitor stuff when NOTHING can happen anyway.

[ChrisOlson]

diagnosing a shorted stator can be tricky...  especially if its on the tower.
Obviously checking that all three phases seem equal by checking resistance - and the best way I think to do that is run a voltage up and check current because your ohm meter is probably not good enough.  that often doesnt tell you much anyhow though.

If you don't have the star point hard wired and encased inside the stator I've diagnosed them by checking individual phase voltage too.  If one phase is low compared to the other two I know I got wires where the insulation rubbed thru someplace.

I've never tried fixing one.  It costs less than 50 bucks to wind a new one, so that's what I do.
--
Chris

[Flux]

sorry Flux, i thought i mentioned that it was a 4' Piggott, single rotor, but i guess i didn't.

With  a small machine bearing friction is critical to start up, especially on an axial capable of producing power in low winds. Many of the smaller commercial machines don't start in low winds and if they do they don't produce any power below 10 mph so  the effect is less noticeable.

Any significant short beyond perhaps a single turn would prevent start up on a machine of that size so I suspect your stator is fine.

Flux

[gizmo]

I have a shorted turns tester from my TV repair days, its a handy bit of gear that gives a go / no go indication of shorts within a coil. I use it to check F&P stators for my windmill projects.

A coil with a internal short is very hard to detect with a ohm meter, or even the Dans 12v test, if the short is between adjacent windings.  But a shorted turn will generate a lot of heat and could eventually lead to a complete coil failure. A shorted turns tester works by pulsing the coil and listening for the ringing oscillations. If there is a short, the ringing is quickly dampened and easily detected.

I found this PDF of a shorted turns detector kit that was/is available from Dick Smith Electronics (Australia ). The PDF includes the circuit, so you could make one for yourself if you have the tools. Most TV technicians would have one, they are also called LOPT testers.

Glenn  

Oops, forgot to add the PDF link....

http://members.ozemail.com.au/~bobpar/k7205.pdf