Re: [CAD_CAM_EDM_DRO] Here we go again - more trouble

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