Re: VFD and inverter duty 3phase motors

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Lance Hopper"
<snaggletto@c...> wrote:

> Hello,
> I've got a 5HP Automation Direct GS2 AC (0-400Hz) drive. I'm
> looking to buy a 3-5HP 220VAC 3phase ~3500RPM motor for a small

lathe

> application. I've been doing some looking and it seems that there
> are motors that are "inverter duty" rated. The inverter duty

motors

> seem to have a rated speed like 1725RPM @60Hz, and then have

a 'max

> safe speed' usually rated much higher like 3450+RPM. I'm guessing
> this max speed is at 120Hz. Also it seems that inverter duty

motors

> have a much better spread of torque than standard motors made to
> operate at or close to the rated RPM only.
>
> Should I just get a regular ~3450RPM @60Hz motor, and limit my

VFD

> to 60Hz, or should I go for an inverter duty motor where I can run
> much higher frequencies/speeds. Any comments would be appreciated.

For my lathe I initially installed a 3hp 1750rpm motor running off
a 5hp VFD with 0 to 400 hz. My belt drive from motor to spindle is
1:1 ratio. I limited the motor rpm to around 4000 by setting the
hertz limit in the VFD.

No problems with this setup except in turning aluminum around 5"
diameter. It didn't have the torque needed for chip breaking which
requires a fair depth of cut in aluminum.

I did a simple change over to a motor rated 1150rpm at 60hz and now
have plenty of torque for turning and tapping.

Both these motors were purchased used and are conventional, older
large frame three phase (not inverter duty). The 1150rpm motors are
a little harder to find, I bought mine from S&S motors in Washington
state, $180 with new bearings and 90 day warranty. Been running
hard for 5 years with no problems.

Incidently, Baldor told me their new motors can safely be run up to
5000 rpm. Not sure I'd do that with an older motor though.