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Old Sat 05 December 2009, 21:09
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
The Power Supply needs only to supply 2/3 of the rated motor current

An exchange of posts on the Gecko Yahoo forum in the last few days:

Mariss:

Here's an easy way of looking at the question. 50VDC at 7A is 350 Watts. Divided
four ways, it portions out to 87.5W per axis. Very few NEMA-23 step motors will
deliver more than 75 Watts to a load. Rarer still is having all 4 axis loaded to
the limit and turning at a speed high enough to deliver that amount of power
simultaneously.

If you are interested enough, set your multimeter to the 10A DC current range
and put it in series with your power supply. You will be surprised how little
current is drawn from the supply as you exercise your machine. Four axis worth
of 3.5A motors will have 19.8A sum of total current flowing in their windings.
The power supply current will be under 5A while that happens. Power supply
current is always much less than the sum of the motor winding currents.


Gerald:

Mariss, I have done that measurement and confirm that the actual current drawn
off the power supply is to the order of well below one third the sum of the
motor currents in typical CNC routers. However, the advice for sizing the supply
is to go for 67% of the sum of the currents. Can you give some background on why
you suggest the 67% figure?


Mariss:

The purpose of the "67% rule" is to size a power supply to a limit that will not
ever be exceeded by the drive under any condition.

It (67%) is measured data taken from many dozen motors of various sizes and
ratings. Not emphasized enough is under what conditions this was measured. The
test motors were run at moderate to high speeds (800 RPM to 3,000 RPM) and were
then loaded to stall. The supply current was measured at the instant before
stalling.

The rule is very conservative because maximum supply current draw occurs at the
100% load point at moderate to high speeds. It is less sensible in a 4-axis
system because the likelihood all 4 axis being operated in this fashion
simultaneously is very low.

This current is independent of the supply voltage. This is expected because
motor power output increases proportionally with supply voltage. Assuming
constant motor efficiency, power supply Watts required increases proportionally
with voltage as well. This assumption requires the current to be independent of
voltage.


Gerald

Thanks for the reply Mariss.
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