Re: [CAD_CAM_EDM_DRO] stepper amperage question

In a message dated 07-Mar-02 19:05:11 Central Standard Time,
scoob22@... writes:

> when you have a unipolar stepper motor and a unipolar stepper
> controller the motor is rated at say 5v. and 3. amps if you want to
> increase the motor voltage to say 12v. does the amperage also double?
> lets say i want to increase the voltage to 24v. what would the
> amperage need to be ? is the a formula that someone could tell me?
>

Kevin: The 5V./3 A. example you cite is max. steady-state. When you begin
to STEP a stepper, the inductive reactance goes UP, so, to keep the current
at that desired 3 A., you must INCREASE the effective voltage to the
windings. The old-fashioned "L/R drive" does this poorly but "sorta" with a
high-wattage, low-ohm resistor in series with the +common to the windings.
The value of that resistor is selected so that the current at standstill is
about 3 A. for your example. This will go down a bit, but the voltage across
the windings, being "shared" with the resistor, goes up, relative to the
motor. The newer and much, much better way is "chopper-drive" (see Gecko!),
where the DC to the windings is "chopped", and the "duty-cycle" of that
pulsed-DC is varied (made wider!) when the step-rate goes up. This is a MUCH
better system, as the current "used" from the power-supply is not NEARLY that
the windings in the motor "think they are getting".

Mariss has some "white papers" on this, I think. I'm not sure. ASK him! If
you increase your voltage to 24 volts, you MUST have either a "chopper drive"
(like the Gecko 201 or 210), or the old-fashioned inefficient "L/R drive"
with the power-wasting resistors (usually waste MUCH more than the useable
motor-power!)

Jan Rowland


[Non-text portions of this message have been removed]