Servo power supply help --Please

[turbotoys]

I am building a machine with 4 DC servo motors and i'm trying to figure out what i need for a power supply the specs of the motors are here -

- maximum operating speed: 4200 rpm

- continuous torque: 270 oz/in - torque constant (KT): .1757 N. m/am

- peak torque: 850 oz/in - voltage constant (KE): 15.71 volts/100rpm

- terminal voltage: 72 VDC - resistance: 85 ohm

- continuous current: 8.5A - inductance: 2.35 mH

- peak current: 38A

Now, i did some research here by searching all over and i think i know what i need but the torrodial i am looking for cant be found.... Here is what i think i need let me know if i am wrong.

4motors x 8.5amps a piece = 34amps total. with an input of 115VAC and an output of 72VDC ??? so i need a big ass torrodial correct? or maybe 2 of them ? also would i be better off running this off of 230VAC? oh I have the Gecko G340's - not that that matters much... if my calculations are correct where would i get something like that in the USA? i have tried the few links i have seen on here and from what is pre-made it looks as if everything is way too small. Hmm, now if i remember right from school wouldent i actually need a 50vac output so once i rectify the output it would give me the 71-72vdc?? 1.414 right??? Any input would be greatly appreciated ... Thanks everyone.

[Al_The_Man]

In all probabillity you are not going to run all 4 servo's at rated current all at the same time!
So you can apply a derating value of say 60%. Look for Toroids on Ebay, there is one seller that produces them in all sizes, You can aim for slightly higher voltage as the drive should regulate the max torque via the current limit setting, The PWM drives I use, recommend a supply anywhere between 110% and 150% of max motor rating.
The DC voltage of the supply, although delivered to the motor via the drive, is not of a continuous nature (is PWM in nature) which results in a mean current (torque), depending on the demand.
Do a search here of toroid or power supply, there have been many previous posts.
Al.

[turbotoys]

thank you for the input so far- I am really lost now . ok here is my setup, i am running a 3 axis machine but have 4 motors one will be slaved - would i still want a derating of 60%? I also have been looking on eBay at the toroids and see nothing with a high amp rating of 20amps (60% derating applied).
Also, are you saying i should go above the 72vdc? Are you saying anywhere between 80vdc - 108vdc would work for me?

[Al_The_Man]

The guy on ebay goes under a company of Antek, usually the largest he makes is 1.5Kva, which is probabally going to be ample in your case.
As far as maximum voltage goes, you have to consult the drive manuf. specs. to confirm how high you can go.
Al.

[NC Cams]

I dunno if you HAVE to have a toroid. Want? yes, Need? I dunno

My Bridgeport has 3 servo's rated at 19 in/lb torque, 24 amp max and 140 volts and it uses a conventional (non toroid) transformer. Granted, these are/were custom wound for Bridgeport but it goes to show that you don't have to use a toroid.

The ONLY time you should ever "see" peak current is if/when you go locked rotor. The chances of two or more motors going locked (peak current) simultaneously is rather low. THis is why you can derate the P/S rather than simply summing up the total peak that could/should occur.

[Al_The_Man]

NC is right, any configuration will do as long as it is sized right, the problem is that EI configuration types are hard to get the exact Voltage/current secondary.
The beauty I find in the Toroid type is the ease that it can be modified by increasing/decreasing turns, also you can easily add a small auxilliary 5~24v winding if needed.
I have done this to quit a few EI style but they are a royal pain, usually, unless you are decreasing the winding.
Al.

[turbotoys]

Al - and NC thanx for the input - I found antek on eBay and only on a few postings does he list his website - but when i found the link i did find the 1.5Kva. I consulted the gecko manual and it looks as if i have to stay at 80vdc or under so there is my answer.. Thank you for the help - I really appreciate it.