Re: [CAD_CAM_EDM_DRO] More VFD questions (was Re: VFD recommendations for Bridgeport...)

Steven Ciciora wrote:

>As long as there has been a lot of VFD questions lately... the last thing
>I need to do to make my series II Bridgeport useable is get the 3 phase
>motor situation under control. I have a home made rotary converter, but
>it's not well assembled (yet). I also have a brand new Baldor BDI1100
>220VAC in single phase 2 HP VFD, and a 4 HP motor on my Bridgeport. What
>would be the problem with using it to run my 4 HP? It has an over current
>fault output (a similar manual but only says 1 HP is located here:
>http://www.baldor.com/pdf/manuals/71100-1296.pdf). Would the 4 HP motor
>have too low of an inductance for the 2 HP VFD?
>

Yes, that is possible. The makers of VFDs strongly advise against this.
I will also admit I blew up a 1/2 Hp VFD trying to run a 1 Hp Bridgeport
motor.

> If so, what if I wired up
>the 4 HP motor for 440 VAC (the 3 pairs of windings in series rather than
>in parallel) and only ran the frequency to 40 or so Hz for 220VAC
>out? This should have 4 times the inductance then when wound in parallel
>for 220VAC, and keep the volts/hz ratio about correct..
>

Maybe, but I think you'd get a large power loss above 30 Hz.

> O.K., now here's
>the biggest question: can I run my 4 HP motor in "half winding" mode (think
>8 wire steppers)? I believe this will have the same inductance of a 2 HP
>motor, and would put 220VAC across the half winding, just like it would
>expect to see.
>

Unlike steppers, induction motors are not designed to be run this way.
If you make one winding carry all the load, it will draw too much
current, and
I think even at only 2 Hp load, it will eventually fry the winding.

> The only problem I can see is what if the two half windings
>per phase were located at different positions, rather than
>overlapping.
>

They CAN'T be (I think). There are some really odd dual-speed motors
that use
exotic tricks like this to switch between 2 and 4 poles, but I haven't
seen this
done on dual VOLTAGE motors.

Jon