Re: [CAD_CAM_EDM_DRO] Power Supply for Gecko 320 and servo motors for 3 axis 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. My guesstimation is total maintained
>current draw for all 3 motors will be around 12 amps (20-25 peak amps
>per motor). So the math works out:
>
>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. I think that 33000 uF is a bit on the low side.

>Is a 50 volt capacitor adequate for the 36 Volts out of the
>transformer? (I think 50 volts is a standard capacitor rating, the
>next lower one being 35 volts)
>
>

Yes, but running the motors on 36 V may provide less that optimal
performance.
Are these the Ametek 900 RPM motors everybody seems to use? I can tell
you (Mariss will scream) that you don't get really tight position control
with these motors on 36 V or less. I think you will get better performance
at a higher voltage. Some of these motors have over 4 Ohms of winding
resistance, so that you can only deliver about 9 A at the full 36 V,
with the
motor standing still. At 900 RPM, you get zero current, because the
generated voltage (back EMF) is 36 V.

>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?)
>
>

They store energy, and also stabilize the voltage. They provide a source
of current for between the peaks of the AC power line. They also supply
current during current demand peaks. And, finally, they can absorb power
coming back from the drives when the motors are decelerated.

>What rating or capacity besides the 100 or so volts do I need in
>order to ensure the bridge rectifier is sized asequately (I really
>don't want this to be the weak link in the circuit, as bridges are so
>cheap).
>
>Is a 100 volt bridge rectifier OK for the 72 volts total between plus
>and minus 36 coming out of the transformer?
>
>What's the functional difference between a toroidal transformer and
>a....well, square? one? Is one better suited than the other for this
>application?
>
>

Magnetically, not much. The actual fabrication of the core and windings
is different.

Jon