Re: [CAD_CAM_EDM_DRO] Servo issue

Jason Cox wrote:

>Jon,
>
>I have stripped everything down and have my servo's running on a bench. I
>have swapped the axies over but have not been able to replicate the problem
>I had on the mill.
>
>One thing I did notice however, is that the motor which was over heated
>seems to be weak when I try to turn the shaft anti-clockwise. When the servo
>is holding position I am able by hand to turn the shaft about 1degree anti
>clockwise, this requires some strength, it then get a slow pulsation going.
>I cannot turn the shaft by hand clockwise. And this happens regardless of
>the g320 I use. What could be causing this weakness and could it somehow
>relate to my machine going crazy?
>
>

Ohh, this is VERY odd. I might suggest reversing the motor wires and
swapping the two signal pins on the encoder. this completely reverses
everything between the Gecko and the motor. If it now is weak in the
opposite direction, it indicates that both Geckos have an asymmetrical
current limit, which they shouldn't have. If it stays the same, then it
has to be the motor. But, I have taken classes on DC machines, and I
can't think of any reasonable mechanism where a DC motor would have
such an asymmetrical response.

One possibility is a bad armature winding, which is just on the end
of one of the field poles. You should try this test at, say, 90 degree
intervals of motor rotation (just disable, turn 90 degrees, reenable)
and try the manual torque test. If you only get this effect at certain
positions, you have a burned-up coil on the motor. To make a certain
determination, disconnect the motor from the Gecko and machine, and
turn by hand. One trick is to hold the motor by the pully, anf give the
motor housing a flip. If you feel a jerking, like something intermittently
grabbing inside, that is pretty much proof of a shorted winding.
If that test shows nothing odd, then connect a length of small wire, or
a low-value resistor (a few ohms) across the motor terminals, and turn
the motor by hand. If it is easier to turn during part of the rotation,
that indicates an open winding. Open windings can sometimes be fixed,
as the open is very often at the commutator blocks, and they can be
resoldered. Shorts are usually deep inside the windings, and you have
to rewind the entire armature. You might try these tests before the
one above with rewiring. I think you are likely to find a shorted
winding - that's the most common failure in an overheated motor.

Jon