Is VFD Good or Bad for Old 3 Phase Motors

I guess I disagree with that assessment, but that's based more on industrial experience versus my own. I do have a VFD on my lathe, a GE industrial model, that services the Baldor motor and it's been problem free. But when I had a day job we had VFDs on quite a few motors and any problem with those motors was always traceable to some other circumstance, like frequent stop/start, etc. But if you've ever visited the OWWM forum, those guys swear a 3 phase motor with a VFD is hugely preferable to a single phase; and I don't recall ever seeing a complaint/problem about that type of setup over there. Maybe what you heard was complaints about using a static phase convertor, those have some limitations and generally only give you 2/3's or less of the rated HP.
 
first of all a a little history the motor you show is a Leeson motor and not really what i would consider an OLD motor.The company was founded in the 70's by Lee Doerr's sons Lee was the son of Emil Doerr one of the founders of Baldor motors which was founded in 1920. All Leeson motors are of the newer design and are what we who can remember that far back as "T" from motors and are typically designed with class "F" insulation. I mention this because one of the problems with early application of inverters had to do with the older "U" frame motors (Prior to the 1960's) It was believed that the class (type) of insulation used on the older motors did not hold up the the PWM (pulse width modulation) used to synthesize frequency in the inverters. The motors should run ok on the inverter.Remember that you will not develope full horsepower at frequencies less than 60 cycles but that a whole nother subject. OK I'll go back into retirement now.
 
Since you are dealing with a three phase motor, you should not have issues with starting torque, but you may have cooling issues when running at slower speeds.
 
Danny,
The Leeson motor your link shows is fine for use with a VFD. The problem with using VFD's on induction motors has been related to the voltage that the motor experiences. The issue is the PWM that the VFD uses to commutate the motor and the wiring from the VFD to the motor. What happens is you actually get a reflected voltage (transmission line) that is related to cable length and the rise/fall time of the semiconductor (IGBT's)used to generate the PWM. In theory the motor will experience up to about twice the applied voltage, caused by the reflected wave. This specific motor is "Inverter rated", so it basically has better insulation on the windings, and is designed to be run with a VFD. If you look at the manual for the VFD, it will likely specify some maximum cable length.
 
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