Re: [CAD_CAM_EDM_DRO] Power Supply for Gecko 320 and servo motors for 3axis router

plastiguy wrote:

>
> Hi folks -
> I'm putting together the power supply for a 3 axis router using 3
> Gecko 320 drives with 3 DC Brush servo motors at 360 oz-in peak
> torque. The motors are rated at 36 vdc. I want to make sure I'm
> sizing everything correctly.
>
> 80,000x12amps/36 VDC = 33,333 microfarads for the filter cap.
>
> Queries:
>
> Is it OK to use a filter capacitor between 33,333 microfarads and
> 100,000 microfarads? (I think rule of thumb is 1 to 3 times required).

Yes. The bigger the better, but necessarily more expensive.

> Is a 50 volt capacitor adequate for the 36 Volts out of the
> transformer?

Marginally no, 36 * 1.414 = 50.9VDC and while a 50V capacitor might last long
enough at 51 volts, its life will be shortened considerably once you
take any back EMF voltage surges (from decelerating motors) into account.
Also there is no safety margin, if your local AC power migrates above nominal
voltages.

> Though I understand what a capacitor is and does, what's the function
> of these filter capacitors (e.g. what would happen if I didn't use
> them?)

Do you mean the local external 1000uF capacitors that Gecko recommends for
each of its drives? Yes those are filter capacitors to lessen the strain
on the miniature components within the small Gecko case.

> What rating or capacity besides the 100 or so volts do I need in
> order to ensure the bridge rectifier is sized adequately?

Anything above 100V should be fine. You'll want to make sure you have
adequate amperage ratings on the bridge rectifier.

> Is a 100 volt bridge rectifier OK for the 72 volts total between plus
> and minus 36 coming out of the transformer?

100 volts refers to 'above zero', so theoretically you'd only need a 36V
rated bridge to handle +/- 36V swings.

> What's the functional difference between a toroidal transformer and
> a....well, square? one?

The square one is properly called an 'EI' transformer. Toroids have every
advantage over EI transformers, except for one: higher cost. See my website
for a list of these many advantages (including lower cost-of operation...
toroids are more efficient and therefore use less power... and an idling toroid
uses VERY MUCH less power than an idling EI transformer....).

> Any great clues on finding good prices for quality components like
> bridges, capacitors and transformers would also be used and
> appreciated.
>
> Powerless,
> Scott

All of the issues you raise above, are the reason I've made my monster
capacitor/bridge kit available ($97 on my website):

- 68,000uF capacitor 80VDC continuous, 100VDC surge
- 1000V 35 Amp bridge rectifier
- four 1000uF 100V local Gecko filter capacitors
- four pair solid core capacitor-to-Gecko wires (crimped ring terminals on the cap end)
- packet of thermal grease for under the bridge & Gecko-to-heatsink

- all prewired and ready to go (4 insulated terminals on the bridge)

Part of the difficulty for the consumer is actually buying a single big capacitor like
this. If you are lucky enough to find one in a catalog, they invariably have no stock.

Judging by the backorder delays I encounter for these (ordered 50 at a time), I'm convinced
they start up the production line just for me! Also, large electrolytics have a limited
shelf life of 10-15 years, so don't be tempted to use one you've been keeping in a box for
5 years, scavenged from a machine many years older than that!

Hope this helps to answer your questions...

Cheers
Doug Fortune
http://www.cncKITS.com