VFD calculation.

chris@cnc wrote:

For my surprise, it was 1.5KW Fuji motor and 3,7KW Fuji VFD. 
This works since more than 10 years with millions of tool change.
I try to get some rules how to put motor and VFD together but i am now back on start line.
I don't understand why this system works. It must be somthing more as put motor and VFD power together. 
First note to me, Source power and driver must be equal.
Open question, How to select motor and VDF power?

The VFD has to have at least the same power rating as the motor. Larger is okay. You then program the VFD with the motor parameters (max current, speed etc) so that the VFD does not overload the motor.

(Besides, if the motor is correctly dimensioned for its task, it will simply not draw more current than it can handle in the first place. Well, unless it is ran at overvoltage or overspeed.)

You caught me, I never can remember it all, numbers get jumbled around and reused in the wrong places without really thinking them through. I should have said 120/208v.

There is a common 3-phase system in the US, called 120/208 Wye. This system provides 3 separate 120 V single-phase hots to ordinary office loads, plus a common neutral. Also, you have 208 V between any two hots. Most 240 V 3-phase motors will run OK on this 208V 3-phase power, but with slightly increased current. This system is commonly found in apartment buildings and offices.

Another system seen sometimes is center-grounded open delta, where one center-tapped residential transformer is combined with one 240 V transformer to provide center-neutral 120/240V power, plus a 3rd hot wire for true 240 V 3-phase power.

Another system I have run into in old buildings is corner-grounded open delta. There, two 240 v transformers are connected together to provide 240 V 3-phase power, but since one wire is grounded, there are only two "hots". This allows the use of two-pole breakers and a 2-phase breaker panel. I'm pretty sure there will be no new installations of this setup.

Jon