From: Jon Elson <jmelson@...>
Tim Goldstein wrote:
That is a good question and I don't know the real answer. From my
experience the input current draw is less than the total of what each
axis
is set for added together (3 axis's set at 5 amp each draws less than 15
amp).
This is because a 'chopping' type stepper motor driver is really a special
case of a 'buck' (or step-down) switching power supply. So, the input
current (at higher voltage) is always less than the output current (at
lower
voltage). So, if you are using a 5 V, 5 A stepper (that's 5 Amps for each
of 2 windings) and are in full step mode where both windings are always
energized, you have 5 x 2 = 10 A x 5 V = 50 VA or Watts out. If the
main power supply is 24 Volts, it draws I = P / V, or 50 / 24, or
approximately 2 Amps. Various losses in the chopper will cause the
current draw to be a bit higher. So, that is how 2 amps in can be turned
into 10 amps out.
Jon,
Thanks for this explanation, I've been trying to figure out what to build my
power supply out of for my stepper Bridgeport project and since I think the
motors are 8 A units I thought I was looking at a 36 V, 48 A supply! This
will save me a lot of money on transformers. Surplus Center has 115V:12V 35A
units for $25/each. I think I'll get two and series the secondaries together
to get the 24Vac I need.
I think I'm going to take some of my savings and order up a sample of that
servo motor that C&H has and send it off to you to eval. Hopefully it will
prove useable and I can get back to the servo world where I belong. Next on
the list for me, probably late summer considering the snail's pace I've been
making lately, is a servo retrofit of my manual BP. I recall you mentioned
that you had some ideas for hardware to implement the "synthetic tach" idea
and I'd love to see the details. Did you also say you had extra servo amp
components left over? I still haven't ordered any parts for the 4 boards I've
got, so I'll happily buy whatever excess you've got towards this end.
Thanks,
Matt
