Re: [CAD_CAM_EDM_DRO] gecko and power sources

whollid1@... wrote:

> I have just recieved 2 servo motors off e-bay, and the prelim tests
> suggest that 10 amps of current at 50-90 volts should be about right.
> My problem is that I seem to be having difficulty finding a power
> supply that can deliver. I would like to use g340 amps but cant seem
> to find a transformer that can deliver that sort of performance
> easily. There are 2 designs I am running around in my head ( the
> system is a cnc sherline with emc/parallel interface )

Wait a minute! 90 V at 10 A is 900 W, or 1.2 HP! Do you need
1.2 HP to move the sherline table? I would suspect you could
break the leadscrew with that kind of power! I'm using 69 V at 6 A
(peak) and that will deliver over 1000 Lbs of linear force to the
table of my Bridgeport, AT 100 IPM. Since the Sherline usually has a 20 TPI

screw, that could deliver 2+ TONS of linear force on the table, which
would do about as much damage as running over the Sherline with
a bulldozer!

> System 1 is
> a ~ 60 volt transformer, which can deliver 10 amps, running through
> a rectifier with a big capacitor to yeild ~90 vdc,

I believe the Gecko drives are limited to 80 V max.

> The second idea is
> a 60 volt but cheap, 2 amp, transformer which charges a nimh/lead-
> acid battery which supplys the current for short times of more than 2
> amp draw.
> So the question is : does anyone know of a cheap source for 10 amp
> 60volt secondary transformers, or does the battery/low current
> transformer sound viable?

Note that the Gecko and all other PWM (switching) drives convert
power to power - that means that if they are delivering 10 A to a slowly
moving motor at 5 V, they only draw 50 W from the power supply.
If the power supply was 50 V DC, then it would only draw 1 Amp
from the power supply, even though it is delivering 10 Amps to the
motor. I know this is hard for people not familiar with switching
power supplies to understand, but it is true. Also, unregulated
DC supplies can usually provide more current than they are rated for
for short intervals. With the Sherline, these intervals would be
short, INDEED! Do you really need to accelerate your machine
table at 100 G? If it wasn't for the weight of these motors, themselves,
I think you'd have little trouble getting there. Do these motors weigh
more than the machine?

Jon