CNC w/ rotary converters

Again the uneducated have to ask silly questions. Guys I have a 5 HP rotary converter in my shop but I notice that on ebay they sell rotarys that are for CNC machines. Can you tell me the difference between those and a standard converter? I'm gonna end up w/ a CNC yet and don't really want to blow up boards or what not.

A CNC rated rotary converter is one that is well balanced under load with respect to the output voltages between the three phase legs, and which is sized approximately twice the hp rating of the CNC machine to provide a low impedance for power transfer.

Two absolute no-nos for a rotary converter supplying a CNC load machine are automatic restarting capability, and on-line capacitor switching. Both can cause large spikes which can easily fry a CNC control board. These are very expensive, often thousands of dollars.

If your CNC also does regenerative braking, the rotary idler should be 2.5 to 3 times the spindle hp.

I would recomend using one of phase perfects units for a cnc application. I have been using mine for about 6 months now and it works great. I run the entire shop off a 20 hp unit with no problems.
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Take Care
ARB

Bill
I think the CNC units have an extra coating of paint...!
Just kidding ! They do get more $ for the "cnc rated convertors" and I have seen/heard the sales pitch too.
BUT
As Doug said , split off the power to the control and servo transformers, and run a single phase (220v ?) line to them.
Works like a charm. I did it to my Hurco 3 years ago ,and no problems what-so-ever.
My home made rotary drove the spindle motor, until I changed it, to a VFD this year.
Some of the new CNC's run on single phase,because they do the same thing as I just did.

I have only seen one cnc mill (5 inch Boring mill--85,000 pounds) that needed 3 phase for the servo drive(s) , so you are not too likely to run into a 3 phase requirement..

So the CNC rotarys are a waste of money "in my opinion"...which makes a horse race according to Mark Twain!

Badass.

The jets that flew over Madison, WI just flew right over Westfield.

The question I want to ask is how do you tell if a CNC machine needs 208Y or 240 Delta.

The answer is it shouldn't matter, right. Unless the machine uses a neutral for ground, in which case its designers would have to content with zero sequence currents and all that jazz.

Well with that in mind, a machine basically gets what it wants line-to-line which is 220VAC 60HZ 100A in one instance. The tolerance is 3% phase-to-phase and 10% total variation.

I would imagine this machine would run on 240 delta, no? What about a delta situation that came in at more like 230? Is there any sort of drive that would have a problem with certain three phase configurations such as "wild-leg" delta or with bucked 240?

Seems like ARB told me some time back he got the required 208 volts for the control section of one of his machines by taking 208 volts SINGLE phase from the high leg to neutral (ground). It worked.

You must supply the control section(s) of all machines with exactly the voltage they require, AND that should be high quality power. The spindle and drive feed motors are not picky about voltage, due to the operating characteristics inherent to induction motors. Current flow to the windings is governed by impedence changes with changes in mechanical load, and slip; applied voltage is there, but is mainly a spectator in the picture. This is why induction motors are so incredibly robust.

I did the same thing as Rich on my Hurco. Ironically, the KMB-1's original Parajust VFD only took 220V single phase as its input! So converting the machine from 3 phase to single phase was fairly simple. I just wired around the isolation transformer. There were three things, as I recall, that had to be "adjusted"... The control power supply, the servo power supply and the VFD. It wasn't that hard, and I much prefer not having to deal with the converter if I don't have to (don't have a choice on my Deckel manual mill... I have to use a rotary on that because I wanted to keep the electrical box and it has heaters in it).

Alan