I have a VFD and induction motor on my mill. It is a good quality Mitsubishi drive but just the standard Bridgeport 2HP motor so I dont know if its an inverter duty motor. I have heard that the inverter duty motors dont drop off so much in torque but how good they are I dont know. What I do know is with my VFD/motor combination I lose too much torque if I go down below 50% of the motors rated frequency. I still use the vari speed pulleys on the mill to get to the rough speed range and then fine tune by the VFD (external potentiometer). I was considering hooking the VFD to Mach but after using the mill for a while I decided there was no point as the drop off in torque would be no use for the varying types of work I do.
Hood,
Saw this post and it made me wonder. Could you not permanently set the pulleys transmission to say 4:1 reduction and thus have a low speed of 370RPM (assuming a 2 pole motor) - which is at 50% rated frequency like you said - 30Hz. Then for higher speeds, good inverters can output frequencies as high as 400Hz, which would mean a top speed of almost 5000RPM with the same pulleys. Or did you figure the motor wouldn't last long at 20kRPM. But if it were a 4 pole motor then I think it could handle 10kRPM no problem.
I am asking this because I'm considering doing this - using a 3 phase, 4 pole motor (or even better a 6 pole motor if I can find such) with an inverter to give speeds from 1500RPM (50% rated frequency for a 2 pole motor) to about 10,000RPM (at about 160Hz). What do you think?
Daniel
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