RE: [CAD_CAM_EDM_DRO] Re:low cost stepper driver

Adrian,

Rarely does a connector or a switch disconnect immediately.

Also, a stepper motor has two windings which makes it a form of
transformer. If there is lots of current flowing through one of the
windings and you only disconnect that one the collapsing magnetic field
generates a whopping big voltage spike in the second winding which is
still coupled to the driver transistors. No visible spark will be seen.

Again, there's no guarantee that both windings are disconnected at exactly
the same time (we're talking micro-seconds here) when there is power
flowing through them.

John


"ELS! The Solution"
Automation Artisans Inc.
http://www.autoartisans.com/ELS/
Ph. 1 250 544 4950

> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of artgartg
> Sent: Tuesday, October 02, 2012 12:00 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Re:low cost stepper driver
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "John Dammeyer"
> <johnd@...> wrote:
> >
> > You cannot guarantee that when you unplug a powered stepper
> motor that the
> > connector or switch won't have the contacts bounce on/off for a few
> > micro-seconds or even milli-seconds. During that time the
> stepper motor
> > windings behave just like the ignition coil and the
> resultant high voltage
> > spike will kill the drives.
> >
>
>
> This is what I was wondering. I appreciate that you get a
> back-EMF spike when it's disconnected, but it's
> *DISCONNECTED* - it's not the driver transistor switching off
> so it shouldn't reach the driver. But intermittent
> connections (or a spark that jumps the gap, as Tony
> suggested) is possibly the reason.
>
> I note that some good quality drivers have tranzorbs which
> can presumably absorb more energy than the internal diodes
> (and also don't dump it into the power supply).
>
>
> -adrian
>
>
>
>
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