Re: [CAD_CAM_EDM_DRO] Power Supply Guidance Needed

Larry,

I seen your post this morning and didn't have time to respond. My how times change, a year or so back you would've gotten 10 responses..anyways, I'll offer this...

The servos should be around 10 amps continuous, the 32 amps is undoubtedly the stall current rating. I don't know why they give failure ratings on servos, it's like the car salesman telling someone the engine blows at 9000 rpm. Servos should never be ran anywhere near the peak stall current.

So, working with 10 amps continuous per servo. Some will tell you to build a power supply 10 amps times the number of axis but, I dont agree with this. That much available current is just asking for problems. Crash the machine and it starts frying drives. 3 axis? that would be 30 amps at whatever voltage you wish to use. You wish to use Rutex drives? There was a lengthy discussion recently on the accuracy of their ratings. The drive gurus decided the max rating was more like 20 amps 120 volts, not 40 and 200 as claimed. The jury is actually still out being that Rutex has not rebutted the concerns, that I know of anyway.

Lets assume 20 amps and 100 volts DC. Thats what I would design the power supply for. To get that you will need 70 volts AC from the secondary (1.414 RMS). And 36 amps (factor of 1.8 loss once rectified) or 2.5 kva. So a toroid or two toroids in parallel with 70 volt AC secondaries and a rating or combined rating of 2.5 kva.

The transformer you have sounds like it may fit the bill. Try it, if it's too small, upgrade later. If you measure the core, some of the guys on here can give you an approximate VA rating.

None of the above or below is hardfast, rather a place to start and some basic formulas, plug in the true numbers for actual ratings.

Now for the Capacitors. Take the continuous amperage of all three (30) and multiply that by 80,000 then divide by the DC voltage to arrive at the capacitance required (macrofarad).

So, 30 X 80,000 =2,400,000 / 100 = 24,000 uF. You would need a Cap with a voltage of at least 120 volts DC (20% safety margin)and a Capacitance of 24k uF. These can be multiple in parallel to make up the value if smaller uF values are all thats available. The voltage still needs to be 20% higher regardless of how many are used.

The Bridge Rectifier: Go at least 200 volts and 35 amps.


Hope this helps, Ron






oldpayphones <larry@...> wrote:
Hello Again,
I need some help with PS design. I have a Bridgeport that had
steppers
on it. I found 3 used NEMA 42 SEM servos that have a maximun current
of
145VDC @ 32A so I was wondering what VA rating on the transformer I
would need and also what would be a good rectifier bridge choice? I
was
thinking that about 1500VA on the tranny and voltage around 100 or
so.
Am I in the ballpark? I also want to put a crowbar on the whole thing
operated by table limit switches and E-Stop. I still have the 3
original stepper motor supplies and saturable reactors. Are they good
for anything? I have been unable to locate any information on the
main
transformers (ACME T-68932)so this leads me to believe that they are
built especially for Bridgeport. Does anyone have the specifications
on
them? It seems that they have an output of 70V with 120V on the
primary. Just about right but I think they were designed to run at
about 40V on the primary though. I plan to run Rutex 2020 drives. Any
help would be greatly appreciated.

Cheers,
Larry Olson






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