Re: Power Supplies

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Dave Shiels <dshiels@e...>
wrote:

> Ah the plight of the newbie!
>
> I am no electronics expert but from my reading and from what you

are

> telling me
> really all I need to do is find a transformer that puts out the

correct

> voltage with
> adequate amperage, add a rectifier bridge and a capacitor with a

bleed

> resistor
> and I am in business. That has to be fairly low cost.
>
> So let's say for argument sake, I am using Gecko 201, I have motors

with

> a 3amp
> rating X 3 axis. that would be 9 amps OK call it 10amps. I find a
> transformer that
> outputs 75v @ 10 A add a rectifier bridge say a 400V 10 amp model

and a

> 20,000 uF 75VDC Capacitor and a bleed resistor. Does this sound
> correct? Are
> there formulas some place for sizing these components?
>
> Is it better to limit the input side of the equation by controlling

the

> transformer
> size in amps?

Couple of things here.

Transformers only work on AC and AC is by nature a sine wave. Ergo,
we measure it 'somewhere' the place that is picked is at the root
mean square or RMS, and NOT the top of the peaks.

When you rectify it and then us a capacitor, the DC voltage is
charged up to or every near the peaks. This is 1.414 times the RMS
voltage.

Your 75V transformer will see about a 1.2 volt loss thru the bridge
rectifier and a 1.414 time voltage gain.

75-1.2 = 73.8 volts times 1.414 = 104.35

Geckos are rated for a max of 80 volts.


Assume you use a 48 volt transfomer, and again a 1.2 volt loss from
the rectification.

48-1.2=46.8 46.8x1.414=66 volts.

You can find 48 and 50 and even 55 volt transformes on E-bay.

Next, look at the current. your selection was for 9 amps.

Say you get a 48 volt, .5kw transformer.

500 watts / 48 = 10.6 amps. AT 48 volts.

since this will be run at 66 volts, we can divide 500 watts by 66
volts and we get 7.575 amps.

As Ballendo said, Geckos are designed for a power supply that is 2/3
the sum of the motors. the 9 amp would be fine with a 6 amp power
supply. 7.75 amps in not overkill.

As for the capacitor, the higher the voltage the lower the required
rating. Use the formula in the Gecko white paper and note that for
our 50 cycle friends it is 100.000 and not 80.000.

Dave