Re: [CAD_CAM_EDM_DRO] Stepper Wire ?

Matt Shaver wrote:

> Hugh & Denise Currin wrote:
>
> > Does anyone have a source (preferably on-line) for shielded wiring?
> >
> > Also, if anyone has thoughts on my scheme below please let me know. :-)
>
> Thanks for bringing up this subject again. I'd meant to reply to a
> couple of other prior posts on this subject, but didn't take the time.
>
> I've been doing a lot of thinking and research on cables to use for cnc
> control applications. There're several different circuit types in a
> typical control:
>
> 1. Drive Electronics -> Motor
> Most modern motor drives, whether stepper or servo, use pulse width
> modulation (PWM) to control the motor current. Basically, the transistor
> switches that connect the power supply to the motor leads are turned on
> and off at some fixed rate (usually in the 2kHz to 20kHz range depending
> on the inductance of the motor windings). The current is increased or
> decreased by increasing or decreasing the ratio of the "on" time to the
> "off" time (this is a somewhat simplified explanation). Since the
> transistor is either fully on or fully off, power dissipation, and thus
> heat generation, in the transistors is minimized. As a consequence of
> this switching, the motor cables carry high currents with a square
> waveform.

Not true. There may be voltages with square corners, but the currents
are very slowly changing triangle waves, due to motor winding inductance.

> While the base frequency is in the low kilohertz range, square
> waves are rich in harmonic content and if they're not carried in a well
> shielded cable, will radiate signals in the RF range with significant
> field strength throughout your shop (this is a bummer if you listen to
> AM radio). So, the main consideration for motor cables is keeping the
> radio frequency interference (RFI) controlled. Since bipolar stepper
> drives require four conductors arranged as two twisted pairs, the cable
> I like best for this application is:
>
> Alpha 5620B1602 (5620B1802 is 18g if you can't do 16g...)
> http://209.208.232.71/pages/322.CFM
>
> or
>
> Manhattan M9746020 (18g = M9748020...)
> http://www.manhattancdt.com/catalog/Instrument121.htm
>
> This cable consists of two 16g twisted pairs, each individually foil
> shielded (with a drain wire) and then the whole cable has an overall
> foil shield (also with its own drain wire). Since most servo
> applications that I've seen carry the tachometer signal in the same
> cable as the motor current, this cable would work for those applications
> as well.

Maybe. These foil shields are not meant for constant flexing, and will
fatigue quickly and deteriorate in a machine tool application. Then, having
the tach shield connect to the motor shield at a number of points will
lead to big trouble.

Jon