Re: Here we go again - more trouble

Thanks so much Jon, I have something to work with now.

Keith

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
> Keith Clark wrote:
>
> >Something bad has happened and I can't figure out what. When the
> >Gecko blew I found a loose wire. I replaced the wires and checked
> >all connections. Checked motor to see no connection between
> >windings or internal shorts. New fuse then blows when powered
up.
> >Now with the new Gecko in place, and all the wires checked, there
is
> >a full 75 volts across the fuse holder (fuse removed)
> >
> Of course. The fuse is not there, so you will see the DC supply
voltage
> across the open fuse holder. This tells you nothing except that
the power
> supply is on, and the fuse is missing.
>
> > leading from
> >the power supply to the drive while no voltage across the good
> >fuses. The only thing I can figure is that the motor has been
> >damaged and there is a short. There is continuity between the
phase
> >a and b and between c and d (as there should be) but no
connection
> >between a and c or a and d etc that would suggest a short.
> >
> >I am not sure what to check at this point. When I put in the
fuse
> >it blows. Not sure if the new Gecko is dead or not. Can't
imagine
> >that the motor is bad though.
> >
> >
> Ah, this is different. Disconnect ALL motor wires from the new
Gecko and
> see if it still blows fuses. Check all the screw terminals very
> carefully, it
> is REAL easy to have one strand of the wire hanging out and
shorting to
> another terminal. You might even take that Gecko out and tap it
gently on
> a bench to make sure there are no wire strands or other debris in
it, and
> get a good look at the screw terminals to make sure there are no
strands
> or wire anyplace they shouldn't be.
>
> The loose wire could be a red herring that was caused when you
started
> looking
> into why the fuse blew the first time. It CAN be a bad motor,
although that
> is not real common. And, a motor CAN be bad, with internal shorts
in one
> winding, without grounding the winding or shorting out to the
other winding.
> Again, not real common, but it CAN happen. And, the Gecko drive
could
> have killed the motor when whatever happened, after a transistor
blew.
> Of course, you may not want to hook the possibly bad motor to a
known-good
> drive and see what happens. You might be able to determine if the
motor
> is bad
> by comparing resistance readings of the two windings against a
known good
> motor. If one of the windings is way lower resistance, that could
be a
> clue.
> If not, you can use a single D cell to put current through each
winding,
> one at a time. Compare holding torque with a good motor. If one
of the
> windings has much weaker holding torque, it is very likely bad
(shorted
> turns). This will fry a Gecko drive.
>
> Jon