Re: [CAD_CAM_EDM_DRO] Check my math and reasoning....

Hi Brady,

3 x 12000uF @ 100V sounds fine. The calculation is only an approximation
anyway, these things don't need to be precise.

In fact your power supply will only be rated at 22A or less with 2x
800VA. 22 * 70V = 1540W. According to a transformer company I spoke to a
while back, transformers should be derated by about 20% when used with
rectifiers. This is because the rated output is achieved with a purely
resistive load. A rectifier pulls current as a series of spikes so the
windings heat up more. To add to the confusion some transformers are
'rectifier rated' which means they are overbuilt to provide full rated
output into a rectifier. Confused yet?

However all is not lost. You will only be needing the full 29A very
rarely and for a short time. Let's face it, how often will you have all
4 axes running at full acceleration and full speed together?
Transformers can take quite heavy overloads for short periods of time.
This allows you to use a slightly undersized power supply without any
problems.

Les

braidmeister wrote:

>OK gang,
>I am trying to piece together my power supply and have a few
>questions regarding the rated voltage for a capacitor. I am finding
>that most use 50v caps and they are cheap and available...I am
>wondering at this point if I am missing something...here's where I am
>and where I want to go with the PS:
>
>I am running Sigma N42 motors on the Bridgeport and the manual says
>that they are 6.8A. I would like to build a power supply with enough
>juice to power these 3 steppers AND an 11A Servo for the 4th axis
>down the road. I will eventually need to run 4 of these 11A servos @
>70v ~ so I am thinking why not just build one supply and be done with
>it?
>
>Right now I have 2 50v (what decal says on each...but advertised as
>55v when I bought them) 800VA torid transformers. I bought 2 at the
>time so that I would have enough amperage to run the 4 servos. Now
>comes the question of capacitors...I need caps are rated by voltage
>after being rectified, right?...In this case at least 80v...not 50v,
>right??? I searched around on here and didn't see any reference to
>this.
>
>So...If my math is correct, I will need (using the servo motor draw
>as a guideline) Assuming 50v rectified to 70v by the 2 torids:
>
>44Amps * 67% (from Campbell's pdf) = 29.5A
>
>C= (80,000 * I) / V)
>(80000 * 29.5) / 70 = 33691 uF
>
>Could I get away with 3 P10653-ND from DigiKey? They are 12,000 uF @
>100v +/-20%.
>
>Did I do my math right...or am I missing something???
>
>Thanks!!!
>-Brady
>
>