Re: [CAD_CAM_EDM_DRO] Help, continuing to burn drives

Keith Clark wrote:

>Hi Guys, My bridgeport retrofit continues to burn Gecko drives. It
>happened just a few minutes ago when I powered up the machine and
>computer. The powersupply was plugged in but the relay controlling
>the powersupply was off. When I hit the button in the software to
>turn on the relay the Gecko immediately burned as did the protecting
>fuse. It always happens on the y axis when I first start up. It
>never seems to happen while running the machine. This is at least the
>fourth time this has occurred. Each time I think I have found the
>problem but alas, no. The last time was early Feb and then again
>last week. A new Gecko was installed thursday. It makes no sense
>why the thing would run for a month with no trouble. It think
>everything is installed correctly but there must be a powersurge from
>the powersupply when the relay first turns on. Is this a possibility
>and is there a way to protect from this?
>
>

This is sure wierd! I seem to remember some discussion about this in the
past. Well, you've got to find out what is DIFFERENT about the Y axis.
I think after all this craziness, I'd swap the X and Y motors, to completely
prove it is or isn't the motor. I might also try to "meg out" the motor.
If you're not too scared of electricity, I might build a low voltage version
of a megger (insulation tester). Get a neon lamp, and a 120 K Ohm resistor.
Connect these in series to the wall socket , and make sure the neon lamp
lights. Then, ground the motor case, connect the hot side of the line to
the resistor first, then the lamp in series, and then the other end of
the lamp
to each motor terminal. Assuming the capacitance of the motor windings
to the case are relatively low, the lamp should not light at all. If the
capacitance is a lot, it might glow dimly. You are applying up to 170 volts
peak from the windings to the frame. These motors should handle this with
no problem whatsoever. If the lamp glows brightly, you have broken-down
motor insulation. That motor needs to be replaced. If no lighting of the
lamp is observed, try moving the motor shaft. If you move it fast, the
generated
voltage could light the lamp, but you shouldn't see more than the
faintest flicker,
and again, it should be totally dark when the shaft is not moving. If
still no
light, tap on the motor fairly hard with the handle of a screwdriver.
If the
light light then, there may be metal debris in the motor, but it may not be
easy to fix.

Another thing is, do these drives ever get hot when running? You need a lot
of heat sink and airflow to keep them cool when running motors at max rated
current. If they run hot, you need bigger heat sinks or more airflow.
Also, the
thermal coupling between the drive and the heat sink needs some thermal
coupling material. metal against metal provides poor heat transfer in
most cases
due to surface irregularities. The metal really only touches at a few
high spots.

What is the normal power supply voltage? You really don't want to run
it much
over 70 V unless you have good systems to control voltage surges, such as
from power line variations and returned energy while decelerating the
motors.

Jon