RE: [CAD_CAM_EDM_DRO] Re: VFD and inverter duty 3phase motors

Standard NEMA specifications allow 150%, to 2700 RPM or about 90 hz. Most
motor companies are actually OK with 200%, to 120 hz. I have run cheap
Chinese 3 phase motors up around 200 hz, no explosions but all the available
torque was used up by bearing and belt friction. It couldn't do much but
make a lot of noise and overheat. There are lots of VFDs that will put out
400 hz if needed.

As the frequency rises it becomes impossible to switch the current because
of the inductance, so the current decreases at higher frequencies. The
result is that the speed torque curve is pretty much constant torque below
the rated frequency and constant hp above the rated frequency. Constant
torque below the frequency is the result of current limitations in the motor
(gets too hot) and induction limitations at higher speeds.

If you want to get to very high speeds, the best bet is a spindle motor.
They have smaller rotor diameters so the centripetal forces (rotor bursting)
are much lower. Forces increase linear with radius and to the square of
rotational speed.

The most common limitation I have run into when trying to get the speed
range up in CNC is not the induction motor problem, but rather the spindle
bearings. If you always run at high speed you can select high speed
bearings and depend on the dynamic stiffness of the bearing. If you
sometimes run at low speeds but also need high speeds, the preloaded
bearings needed in the low speed spindle will overheat at higher speeds.
High end machine tools will vary bearing preload on the fly to get over this
problem.

G. Jackson

> -----Original Message-----
> From: cnc002@... [mailto:cnc002@...]
> Sent: Saturday, July 31, 2004 10:22 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] Re: VFD and inverter duty 3phase motors
>
>
>
>
> In a message dated 7/31/2004 12:14:07 AM Eastern Daylight Time,
> DougR@... writes:
>
> Hmmmm......what you guys are saying is contrary to the info Baldor
> is giving me. They tell me it's ok to run their newer non-inverter
> duty 60 hz rated motors up to 5000 rpm.
>
> With an 1800 rpm 60hz motor that would involve programming the max
> hertz output of the VFD to 166 hz.
>
>
>
> I hate to tell you this but it was a Baldor motor that I saw come
> apart.
> Now, the guy was trying to run it at 12,000 rpm. Also, I was under the
> impression you were talking about a motor that ran at 3450 rpm at
> 60Hz and then
> running the frequency up to 400Hz. Did Baldor tell you that
> their 1800 rpm motor
> was the one that would operate at 5000 rpm? I have run quite a few 60Hz
> motors at 70Hz, ones that were rated at 3450rpm or there abouts
> and had no
> problem that translates to about 4025 rpm at 70 Hz. Also keep
> in mind that if the
> motor is wound for slower speeds it is probably not going to
> have the torque
> at higher frequencies.
>
> Randy Abernathy
> 4626 Old Stilesboro Road NW
> Acworth, GA 30101-4066
> Phone / Fax: 770-974-5295
> Cell: 678-772-4113
> E-mail: Cnc002@...
>
> I furnish technical support, repair, and other related services
> for your
> industrial woodworking machinery. My background as Senior Service
> Engineer for
> the SCMI Group for nearly fifteen years with factory training,
> combines with
> my extensive background in electronics, mechanics, pneumatics,
> electrical and
> CNC machinery to offer you needed support for your machinery.
>
>
> [Non-text portions of this message have been removed]
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