Yes, what you said about limitations of static converters is true.
Quote:
Originally Posted by BillC
Except you really need a VFD for each motor for best results. If he gets a mill with only a single motor, then he only needs one. But, if he gets a mill with one or more powered feeds, then he'll need a VFD for each one (or a VFD for the main motor and a static/rotary converter for the others). Whereas a static or rotary converter will feed all of them just fine (assuming it's sized properly).
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Thats what I said. Each 3-ph motor needs its own VFD unless running in a simple open loop V/Hz mode. My radial arm drill has two motors. One for the drill and one for the column lift. 2 motors = 2 VFDs. They both fit in the existing electrical cabinet. FWIW, A 1hp vfd is $150-200 ea brand new. Ebay surplus can be cheaper esp for the bigger ones. FWIW2, you can also get a 120V/1ph input, 230V/3ph output converters up to about 1hp. Super convenient.
Powered feeds are almost always DC since they need to be variable speed. The typical Bridgeport sized "Servo" brand and knockoffs are all DC. I have never seen a 3ph powered one - it would take a pulley stack or a sheave type CVT to change the feed speeds. Even my Kitamura CNC that uses AC servo drives for the three axes just puts a single phase input drive on each of the three phases to balance the load. Easy to move those wires around in the cabinet and run everything off 1 phase.