Re: [CAD_CAM_EDM_DRO] Treadmill motors and interference

Tim Goldstein wrote:

> I have tried putting a 1 uf cap in the treadmill motor power leads and
> it
> seems to stop the problem. Looking at the design of the speed control
> there
> is no caps downstream of the rectifier, so the motor is getting very
> dirty
> DC from what I can figure. I am surmising that the addition of the
> little
> cap is just enough to keep the ripple from creating enough
> interference to
> cause ghost signals to appear on the step and direction lines.
>
> I guess the question is, should I accept the cap as being a realistic
> and
> reliable solution?

Well, it depends on the design of the treadmill speed control. It
really should have
an output filter on the speed control, anyway. That rectifier, is in a
SCR?
That would explain some of the problem, as the sudden cutoff of current
when the SCR drops out of conduction will cause a transient on the long
motor wires. Or, it could be brush noise coming FROM the motor. In
that case, the
capacitors will help. It depends on what type of rectifier you are
using, also.
But, I would imagine an SCR speed control would be a big noise source.

Anyway, putting a capacitor across the output of an SCR speed control
is probably not a good idea. When the SCR turns on, it would be
expected
to deliver a large current trying to charge the capacitor instantly to
the input
voltage. There are a couple of ways to prevent that current surge. One
way,
knowing nothing about the speed control, is to put a resistor in series
with the capacitor. Depending on the supply voltage, something between
10 and 250 ohms could be about right. It is also possible (likely,
even) that this
is a fast transient problem, and that a MUCH smaller capacitor (perhaps
.001 uF) would be plenty of capacitance. That would produce a much
smaller current spike.

Jon