RE: [CAD_CAM_EDM_DRO] Re: Power Supply for 4-axis CNC stepper driver

Hi Jon,

I think you are confusing the electrical performance of DC Servo Motors with
Stepper Motors. A stepper motor doesn't draw any more power when
accelerating than when sitting still. In fact they are almost the opposite.
Servos draw virtually no power while sitting and steppers are sitting with
full power (depending on step position) running through the windings.
That's why some controllers have a current setback where after x seconds of
no step pulses they reduce current by up to 75%.

As I said before, once the chopper current control is active, the current
through the windings is limited to the set point; 70% of 6 amps in each
winding in our example. But we want to keep our DC power supply at a
particular voltage +/- 5% so that other motors maintain the same
performance. That means we have to have a 60V and (12A * 70%) per motor
supply. If running full step then a full 12A per motor is required. But
since the micro stepping drives rarely suck the full power supply current we
can probably get by with a rule of thumb of 60% of winding currents.

So average case for three micro steppers is 60V * (12A * 60%) * 3 == 1300W.
Remember, this is still for the worst case of all 3 motors stopping on one
of the two micro-step locations where both windings are energized to the
70.7% of nameplate winding current. At ten micro steps per step I believe
the probability is 0.05 but there are 3 motors so the probability drops down
to 0.000125 or 0.0125%. That's why an 800VA to 1000VA transformer runs the
system without too much effort or heating.

Just a reminder too why we want to keep our supply voltage within 5% of our
chosen point. When we tune the system, we usually adjust acceleration and
top speed on one motor at a time. If the other two motors are sitting at
idle or even in low current mode, then the motor being set up will have full
rail voltage which will help top speed for fast traversing - rarely do we
mill at that speed.

So now with all three motors set up, imagine the XY axis are traversing
toward the start point at full slew rate while the Z axis moves from home
down to a point just above the working surface. All three motors run
through the high current point and the XY motors skip a couple of steps
because the motor rail voltage dropped down to 45V for 1/8 second. Oops.
We're out of position.

Not repeatable. Next time the program runs everything works. Frustrating
but that sort of problem can be due to power supply issues; just hard to
prove. So my suggestion is build the supply for worst case unless you are
selling 100+ systems per year and you want the extra profit. After all if
you are milling castings that cost you $75 each and you ruin two because the
system had a hiccup the $150 could have been put to the power supply. Or in
the course of a day if you lose 1 hour milling time due to slower fast
traversals and you are billing $50/hr machine time that's $50 per day lost.

And if you are a hobbyist, drive that beater one more month before buying a
new car and save one month payment for the power supply.

John Dammeyer

John Dammeyer

> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Jon Elson
> The 14.4 W (plus some iron losses in the motor and switching
> losses in the driver) is the power dissipation when standing
> STILL, only. When
> accelerating,
> the power requirement (that's not all loss, the motor's shaft is
> delivering real
> power) goes up dramatically. It can rise to about 60 * 7 *
> .66 = 277 W per motor. Except under exceptional conditions,
> it is pretty hard for a
> machine
> to require this kind of power for very long, however. So, a power
> supply that can
> deliver that power (X number of motors) for only an instant
> will do OK.
> Bulk,
> unregulated power supplies are easily capable of delivering
> short bursts of power above their continuous ratings.
>
> So, the worst case load would be somewhere around 1100 W (277
> * 4 axes), but a power supply with a conservative 600 - 800 W
> rating should be
> completely
> adequate for a milling machine. If you were building a high-speed
> router, it
> might be better to plan for 1000 W rating, as these machines
> tend to really keep the motors spinning fast.
>
> >
> >
> Jon
>
>