power supply sizing vs. servo rating

[Erick Davidson]

Greetings All,
Please excuse my electrical ignorance, I am a mechanical engineer and most of this seems like voodoo to me. I am building a 5' x 10' plasma table with the possibility of router use now and again. I am from the school of more is better and have been sizing things accordingly. I have purchased the MP1000 from candcnc, as well as my gecko 320s with the intent to drive my 1100 oz peak 90V nema34 MCG ID33004 servo motors from camtronics. My last question refers to power supply sizing. I am still not certain wether I will try to build or buy, but the logitrol 700W Power Supply 75vdc interests me. As I understand it, the higher the volts, the higher the rpm? I wanted to run these motors at a higher rpm then reduce down. I was suggested to get a 36vdc power supply and direct drive them as they were strong enough. I wasn't worried about strength as much as efficiency and accuracy. So I guess my question is if cost isn't the driving factor, what would you recommend?
Cheers, Erick Davidson

[andy55]

If you want an accurate system and your motors have enough torque for direct drive then I would go with that.
Only if your motors don't have enough torque do you want to gear down. But that will introduce more or less backlash since the encoder usually sits on the motor and not the screw.
This is ofcourse assuming that your machine is mechanically accurate (linear guides with no slop, ballscrews without backlash etc.)

For the powersupply you need to find out what current your motors will draw when they are stalled. Then you need to think how many motors are going typically to be working flat out, for a plasma table maybe only two axes at a time, so you need to be able to deliver twice the stall current for the motors.
To determine the power of the transformer, P=UI, where P=Power, U=voltage and I=current. It's a good idea to oversize it a little bit.
If you want 90VDC (can the geckos handle that?) you could go with a transformer that has two 32VAC secondaries and wire them in series. that will give you ca 1,4*(32+32) = 89V
Try googling for simple psu designs, it's not that hard to build your own.

another take on the direct drive vs. gear down:
The motors will be delivering full power when they are running at full rpm (that requires about 90V if they are rated at 90V) and when they are loaded to full torque. Then take the rapid feed that you are aiming for and calculate what ballscrew rpm that corresponds to. Then gear your motors so that max rpm equals the highest rapid feed you want...