Re: Check my math and reasoning....

I will probably NEVER have all axes running full cap! If using the
4th axis, then the Y axis will just be sitting there on the
brake...So realistically, no more than 3 will EVER run at the highest
rating & that would be a rapid move anyway with next to no load on
the motors.

Thanks again!
-Brady

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Les Newell <lesnewell@h...>
wrote:

> 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
> >
> >