Re: [CAD_CAM_EDM_DRO] Dead servo

>You might want to check for a short to ground on that bad motor.

Unfortunately, to keep the price down, the Gecko and Rutex drives don't have
high-side current sensing, and so will self-destruct if there is a short to
ground.

>(My servo amps DO have high-side sensing which will cause the amp to fault

if excessive current develops.)

>Jon

Hi Jon,



I love to test one of your drives.



Strictly speaking, it does not matter if the current sensing is in the
high-side or low-side as long as the power supply is floating. It does
exactly the same job. The Regulations of many countries require power
supply to be floating - that's why most of the OEMs prefer to use floating
power supply. And then, simple ground leakage detector can detect short to
the ground in the motor before the drive is damaged.



Even if you put a current sensing in high-side as well as in low side (or in
each leg of the motor) in the drive delivering kilowatts of peak power at
voltages close to 100V or above, then the drive still might not handle a
solid short circuit. You can pay thousands of dollars for the top-brand
commercial drive and it will still require minimum inductance on the output
(motor inductance). It is simply not justified to overcompensate the
circuitry in the drive to handle that. There is simply too much power. It
is like asking a car manufacturer for the guarantee that you will not crush,
if you loose a wheel.



Having an inductor in series with motor might not be a best choice as well.
A small inductor (in micro-henries) only decreases the RFI, although the
closely twisted motor wires do about the same job. Large enough inductor (in
hundreds of micro-henries) dramatically increases the electrical time
constant of the motors and even worse - it stores the energy and keeps
driving the motor when the drive is not. It can totally upset the PID
filter of the drive. Secondly, and more importantly, the inductor can
quickly saturate at high currents. You would have to use physically very
large transformer-like inductor - not a PCB mount ferrite coil.



Cheers



vh