Re: [CAD_CAM_EDM_DRO] LARGE Gantry table: Geckos on large servos.

Kim Lux wrote:

>I don't know if a Gecko will run a really large servo motor.
>
>We've got a 3HP servo motor, depowered by running at 80V, on a large
>table of a special machine. We recently tested it with a Gecko 320 and
>blew it up. It would operate fine at lower speeds, but the drive ran
>warm. At higher speeds, the drive failed. It seemed to have problems
>getting the damping/gain correct for such a large motor.
>
>We sent the drive back to Gecko. They looked at it but have not
>commented on what exactly caused it to fail.
>
>I suspect that the motor would draw more than the rated max drive
>current (ie 20Amps) during acceleration.
>
>

The Gecko doesn't have output filters, which causes 80 V (or whatever the
DC supply voltage is) square waves to be applied to the motor's terminals
at all times. This may cause high circulating currents at 25 KHz to flow
through the transistors. It may also cause high currents to flow just to
the windings if the capacitance to ground is low, or if there are EMI
filters in the motor. It might be necessary to put an inductor in series
with the motor.

Finally, if the Gecko failed when stopping, the problem is due to the
back EMF of the spinning motor PLUS the extra voltage created by
currents in the motor inductance produces a DC supply voltage GREATER
than the DC supply normally produces. In other words, during deceleration
of large motors and heavy inertial loads, the bus voltage can rise to
dangerous
levels.

Jon