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

You should be really careful over revving a motor with a pulley on it or
even worse the driven pulley. At 200% the centrificual forces are
tr45emenous. I hate to see a pulley explode and the damage or injuries it
could cause.
Dan

----- Original Message -----
From: "Greg Jackson" <greg@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, July 31, 2004 9:41 AM
Subject: 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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> OFF Topic POSTS: General Machining
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aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if
you have trouble.
> http://www.metalworking.com/news_servers.html
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> http://groups.yahoo.com/group/jobshophomeshop I consider this to be a
sister site to the CCED group, as many of the same members are there, for OT
subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM.
DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> bill
> List Mom
> List Owner
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