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