how to judge bearing friction loss in continuous alternators

[werner]

Hi,

I obtained a nearly new alternatior, 5kVA, continous use. I placed it with a pelton wheel and found that actual output seems a bit below calculated output.

On the alternator standing alone, if I give it my best to give it a turn, the axle may turn about a 1/2 turn. i.e. I grab the axle end and give my best to make it rotate, with the above result. As the axle is supported by 2 bearings and supposedly does not touch anywhere else, and the stator has a mass of possible 20-30 kg, I am not sure ii should rotate easier.

I measured roughly with a scale and would say that friction is overcome at about 0.75 N/m when the axle starts to move.

I hope someone may be able to interpret this for me. If in essence there is a bit too much friction, then possible the bearings are no installed correctly, or something is touching on the inside, possible a lubrication issue.

best

Werner

[picmacmillan]

hi there........as a mechanical engineer my self......i don't think we have to put nuetons into the bearing problem...i found it too tight also,...all i did was to clean out the grease on the bearings with varsol and just put a little grease  back in the bearing.....if you think there is some equation that may help just ask and maybe we can take the dust off the frictional force books muew=FN.muew is the greek letter and "f"is the force and "N" is the normal...in short try the removal of grease approach i am sure you will be happy.....one more thing....i found it alot easier to turn the spindal after the blades were on the genny...good luck...pickster

[hiker]

whats your water pressure-the more jets hitting the wheel = higher tork and speed..

[Flux]

You don't say what type of alternator it is. If it is permanent magnet( unlikely ) it is probably normal.

If it has brushes they add a fair bit of drag and it still could be normal.

If it is brushless and not permanent magnet then it does seem a bit stiff.

As stated above too much grease can cause this, if it has grease nipples, someone may have pumped the bearings full, this causes more failures than lack of grease./

Another possibility is that the bearings have not seated properly in the housing and are pre-loaded.  If it has been taken apart and re-assembled this may be the case.

Thump the end of the shaft with a soft faced hammer or a block of wood and this will usually set the bearings in their right place and free it.

If this doesn't work you may have a manufacturing problem with the fit of the bearings on the shaft, an over tight fit on the shaft or in the housing will reduce the clearance in the bearing and cause this.

If you wrap a piece of string round the shaft and pull, it ought to turn fairly easily at least at low speed.

If something is touching, you ought to feel or hear it.

If there is serious bearing loss and you run it for a while on the turbine you should be able to feel heat from the bearing that is tight.  If there is no heat after half an hour there is probably nothing wrong.

Hope this helps, difficult to judge without seeing it.

Flux