RE: stepper's crosslinked? - also posting problems

Two attempts to post this via the web interface
over last 16 hours have not show up. Please forgive
me if this message eventually shows up multiple
times.

Steve Stallings

--- In CAD_CAM_EDM_DRO@y..., batwings@i... wrote:
> I just noticed when I crank the X axis manually, the Y axis will
step one
> way or the other, and that direction also depends on X axis
direction.
> Nothing powered up when this happens. What does this mean in
electrical
> terms, and how should I eliminate it? Is this a factor in the
explosion of
> a chip on the Y axis a few weeks ago? Thanx for all help.
>
> Regards, Hoyt

Manually turning stepper motors that are connected
to a stepper driver will subject them to the voltage
generated by the motor. This can be a significant
amount of voltage. If the drive does not have protection
diodes in the output stage, then all bets are off as
to what will happen to the driver. The application notes
for the L298 show how to use diodes. Harmful voltages
can also occur when a motor is pushed by the inertia
of the mechanical load as it tries to slow down, and
also as a simple result of the motor inductance trying
to keep the current flowing as the driver tries to
shut if off. Many modern drivers use MOSFETs in the
output and these typically have intrinsic diodes that
will help to protect them, but drives based on regular
bipolar semiconductors will not have these. Even with
MOSFETs, turning the motor when power is off is a bad
idea as the drivers rely on recirculating currents
going through a controlled path that is always created
by the driver circuits. Likewise it is a bad idea to
disconnect or connect motors while power is applied
as the effect of interrupting current in the motor is
to generate high voltages that tend to arc and damage
semiconductors.

As a guess to what is happening, the motor you are
turning is generating power that is effectively
rectified and fed back into the power supply and thus
to the other drive. This is likely the case if the
protection diodes are present. Without active control
signals you somehow end up with a situation that causes
the other driver to sometimes take a step using this
rectified power. If you want to run the machine with
manual controls, I suggest that you provide an
electrical disconnect between the motors and the
drivers while running manually.

Regards,
Steve Stallings