Output imbalance across the phases

[makenzie71]

So here's a weird one:

I got a turbine up that is making 50v across one phase, 25v across the second phase, and 12v across the third.

Anyone ever seen anything like that?  Earlier this week when it was calm I ohmed out the cable at the end of the run and I got a nice 15ohm across all three phases.  I thought maybe I had a poor connection up top so I just climbed the tower and checked everything out and my connections are solid with clean copper...no arcing or corrosion or soot...and my slip ring looks good on the outside.

Ideas?

I should add that this is a new problem that arose last week.  It's been working properly for the last (nearly) three years.

[ruddycrazy]

shorted turns in the gen head more than likely

[makenzie71]

Seems like shorted coils would give me uneven ohm readings

[DamonHD]

Maybe not if intermittent, eg only when the coils warm up or are vibrated?

A "Heisenbug" if it was software!

Rgds

Damon

[mab]

A shorted turn wouldn't necessarily make a big difference to the dc ohms readings, but if there's a shorted turn it ought to be possible to 'feel' it:-  if you spin the generator with the output wires open circuit it ought to spin freely (don't know if your generator normally has cogging or not, but even if so, once it's spinning it shouldn't have much resistance).

If there's a shorted turn it will cog, (or cog more than usual) and if your trying to spin it fast it should drag and slow.

[makenzie71]

Maybe not if intermittent, eg only when the coils warm up or are vibrated?

A "Heisenbug" if it was software!

Rgds

Damon

I thought about that but it seemed to be pretty constant...hard to test really.  I had it in a slow wind, just enough for it to spool up, and the readings seemed consistently unbalanced even at really low voltage and cold.

A shorted turn wouldn't necessarily make a big difference to the dc ohms readings, but if there's a shorted turn it ought to be possible to 'feel' it:-  if you spin the generator with the output wires open circuit it ought to spin freely (don't know if your generator normally has cogging or not, but even if so, once it's spinning it shouldn't have much resistance).

If there's a shorted turn it will cog, (or cog more than usual) and if your trying to spin it fast it should drag and slow.

I climbed the tower yesterday because my assumption was I had a bad connection between the generator and the cable run...it's happened before...but the connections were solid.  I tried turning the rotor by hand and it seemed really smooth, no cogging or pulling.  And I tried it with each phase shorted and then with all phases shorted and could definitely feel a difference in how it turned.

I'm choosing the route of optimism and hoping it's the slip ring.  When I first noticed this the wind was blowing from the south...I went out and tested it again a couple days later witht he wind coming out of the north and it seemed to be working properly.  Now it's jacked again and pointed south.

[Mary B]

Maybe not if intermittent, eg only when the coils warm up or are vibrated?

A "Heisenbug" if it was software!

Rgds

Damon

I thought about that but it seemed to be pretty constant...hard to test really.  I had it in a slow wind, just enough for it to spool up, and the readings seemed consistently unbalanced even at really low voltage and cold.

A shorted turn wouldn't necessarily make a big difference to the dc ohms readings, but if there's a shorted turn it ought to be possible to 'feel' it:-  if you spin the generator with the output wires open circuit it ought to spin freely (don't know if your generator normally has cogging or not, but even if so, once it's spinning it shouldn't have much resistance).

If there's a shorted turn it will cog, (or cog more than usual) and if your trying to spin it fast it should drag and slow.

I climbed the tower yesterday because my assumption was I had a bad connection between the generator and the cable run...it's happened before...but the connections were solid.  I tried turning the rotor by hand and it seemed really smooth, no cogging or pulling.  And I tried it with each phase shorted and then with all phases shorted and could definitely feel a difference in how it turned.

I'm choosing the route of optimism and hoping it's the slip ring.  When I first noticed this the wind was blowing from the south...I went out and tested it again a couple days later witht he wind coming out of the north and it seemed to be working properly.  Now it's jacked again and pointed south.

Slip rings can wear unevenly, especially if you have a prevailing wind direction. They sit in that one spot moving just a little bit back and forth.

Farmer had a downwind machine near me, constant slip ring issues. Turns out the slip rings weren't sealed and when fall winds hit field debris was getting in there and grinding to dust coating the surfaces... with NW winds most common that time of year the south side had a build up that was getting wet and forming a layer of hard crud that made the brushes ride up. So when it spun around that spot had no output. And SE happens to be the other prevailing wind direction here...

[Adriaan Kragten]

Most 3-phase PM-generators are connected in star and the star point is hidden somewhere inside the generator. This is the case if only three wires are coming out of the generator. So if you measure the AC voltage in between two phases, you measure the voltage which is generated by two coils. This voltage is a factor square root of three higher than the voltage in between the star point and one phase if all three coils are in good condition. Lets call the phases U, V and W. If phase U makes internal short-circuit, you should measure a high AC voltage in between phase V and W and a much lower but the same voltage in between U and V and in between U and W. But as you measure three totally different voltages, I think that the problem is not caused by the coils. Can't you measure the voltages directly on the generator terminal?

[makenzie71]

Can't you measure the voltages directly on the generator terminal?

Only while standing on a 2x6 bolted to the tower stub 35ft up in the air why the turbine is spinning

[Mary B]

Can't you measure the voltages directly on the generator terminal?

Only while standing on a 2x6 bolted to the tower stub 35ft up in the air why the turbine is spinning

That so reminded me of this picture(don't have to watch the whole vid, just the "walking the plank" pic... alto Paul was a very interesting guy!)

165' in the air!

https://youtu.be/W1oLD38de6s?t=345

[makenzie71]

I was going to grab a picture while I was up there but wind picked up while I was doing the film bit and I resolved to keeping both hands on the tower...definitely (probably) the slip ring, though.  Lots of corrosion underneath it, I'm sure during one of my "let put the brakes on in 60mph wind" experiments I cooked the thing lol.  You can kind of see the discoloration where it burned through the insulation.  New slip ring is on the way.

https://youtu.be/w4yUIaqYSs8

[Mary B]

I was going to grab a picture while I was up there but wind picked up while I was doing the film bit and I resolved to keeping both hands on the tower...definitely (probably) the slip ring, though.  Lots of corrosion underneath it, I'm sure during one of my "let put the brakes on in 60mph wind" experiments I cooked the thing lol.  You can kind of see the discoloration where it burned through the insulation.  New slip ring is on the way.

https://youtu.be/w4yUIaqYSs8

Not a good day to try tower work! Sounds like water intrusion into the connections... copper oxide is an insulator. Will also make things run hot.

[makenzie71]

nah it was fairly calm when I climbed up, we usually calm down pretty good in the evenings so I wasn't expecting it to pick back up.

[Adriaan Kragten]

All my larger VIRYA windmills have no slip rings but a 4-phase cable with a rubber casing and flexible copper wires which is going down through the centre of the tower. There is a box at the bottom of the tower in which there is a short-circuit switch and a 3-phase rectifier. The star point is also short-circuited to get the highest braking torque and therefore I use a 4-phase cable. The cable enters the box from below and so there is a loop in the cable. If the head would move several times in the same direction, the cable is twisted. Il the cable twist becomes too strong, you will see that the loop is twisted. Then you have to disconnect the cable and allow the cable to twist back. Or you have to climb the tower and turn the head back on a windless day. This construction has never given any problems and it takes some years untill twisting of the cable is too strong.

[makenzie71]

My wind can do 180 degree shifts twice a day.  I had one of my turbines slip ring fail in a fixed position, if it failed on day one it took it three months to twist the wire apart in the conduit.