Re: [CAD_CAM_EDM_DRO] Power supply question

In a message dated 2/19/2003 10:43:59 AM Central Standard Time,
joachim.reif@... writes:

> What kind of quality current does a stepper motor need?

Joachim: The motor itself "would not care" if the "quality" of the DC was
5% or even 10% ripple. I imagine some drive-circuits might not function well
if TOO much ripple, but most of these have MUCH lower voltage for the "logic
circuits", so ripple for that part of the circuit is easy to filter.

A good rule of thumb is to have about 1000 µF per ampere. E.g., if you need
a 5-amp supply, a 4700 µF capacitor should do.

There is no need to "regulate" the DC for a stepper-motor! All you need is a
transformer, rectifier-diodes or "bridge", and a filter-capacitor. The
voltage under load should be just-below the "rating" of the driver. E.g:
Suppose your driver-circuit is rated for 35 volts DC, max. With all
connections made, the voltage of the DC supply should be NO MORE than 35.00
volts (I'd shoot for 32 or 33 VDC!).

An important detail to know: If you are using a "chopper drive" (not L/R
ballast-resistors in series with the +common center-tap of both windings!),
often, your power-supply need NOT supply more than about 33% of the "rated
current" of the motor(s)! The power-supply must supply only the WATTS
required by the motor at whatever full-speed. Say your motor draws 3 amps at
3 volts (at standstill, or very slow stepping-rate). There are TWO such
coils. Thus, that motor wants 18 watts, total. Thus, if your supply is a 30
VDC supply, it need only supply a max. of 18/30=0.6 amp (per motor)! If,
however, you have a UNIpolar motor and resistors, you must supply the FULL 6
amps per motor, "all the time". Wasteful!

I hope I have not confused you more! Jan Rowland


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