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When using a VFD, it's common to run the motor very slow for some operations, and the spindle motor's cooling fan, turning slow, provides very little cooling air. Running a constant speed cooling blower keeps the motor cool regardless of speed... and I also program my VFDs to go to at LEAST 120hz (twice motor speed). High speed on the motor causes those fans to howl like a fire engine siren... ;-)
Find out what that panel has in it!
Possible that most or all of the stuff in the panel can be replaced by the VFD so the request for what is inside is spot on.
Also the low speed cooling is on but usually only for long duty and such machines often only see very short spindle run times.
The twice speed (120HZ) can be a problem on lower cost motors. The first is balance and vibration. The second is bearing wear which is a square of the speed so twice as fast is 1/4 the life.
In high gear or top speed you can not run the spindles in such machines at 2X normal. The spindle bearings will not like this for any amount of time.
The bigger one is the guts letting go due to centrifugal force. Once you create such an explosion you will not forget the lesson learned.
I now worry at anything above 125% the design speed, will do 150 if tested, 200% means serious looking at the engineering.
Many use a VFD in place of a rotary phase convertor on the outside or input side to do the single phase to three thing.
But if the control panel has pull in relays or other things it controls like a coolant pump you have to keep it set a 50-60hz which wastes a lot of the VFD capabilities.
So yes it's about what is inside the panel and why it is there.
You can rewire the spindle start stop buttons to the VFD. Hard current limits such as fuses or breakers should probably stay in place so this is not a 5 year explanation or task.
Good VFDs come with manuals. If you are not a controls engineer reading them is so very hard.
Just a really rough type end. If your VFD will light up with single phase input then yes you can set it for 60hz and put it on the main machine inputs and use it as 3phase power from the electric company.
A VFD takes incoming AC power, turns it into DC (straight line) and the chops this up on it's output side to sort of make the new AC or the sine waves needed.
Why do you see overate power levels as a standard recommendation? A single phase input only gets to use a part of the input circuit so that limits how much power can be carried before things go up in smoke when you make that DC bus.
I don't know how to make this simple.
Assuming the machine has only a spindle to run run you can live fine with all sorts of options.
I have never tried the second motor VFD in front of the mains one. I would think ok but wonder.
Anybody cascade these things down a chain? You go back to DC but is the second a happy camper?
Bob