On Thu, Jan 7, 2016, at 08:29 PM, Steve Traugott wrote:
> On Thu, Jan 7, 2016 at 5:14 PM, Brian Morel <brianmore...@gmail.com> wrote:
>
> > As far as the current issue, you need to be careful using old motors with
> > a vfd. The insulation ratings of the older motors were not designed to
> > handle the voltage spikes that can occur with running motors fairly
> > deviated from their original hz designs.
> >
>
> This sounds to me like a likely culprit. The high-frequency, high-amperage
> switching on the output side of a VFD causes high voltage transients which,
> if the motor isn't rated for VFD duty, can pierce the winding insulation
> and cause internal arcing and carbon tracks. You may be seeing higher
> current simply because the windings are now shorting together in a few
> spots.
>
> I remember spending some time searching for a good 5 HP motor with the
> right insulation rating to run on a VFD -- they cost a bit more, and are
> harder to find. I don't now remember what the magic designation is, but a
> quick google search just now tells me it may have been "class H"
> insulation. In several subsequent years of running said motor and VFD,
> I've never had any issues.
>
> This looks like a pretty good overview:
> http://energy.gov/sites/prod/files/2014/04/f15/motor_tip_sheet14.pdf
>
> Steve
If you are doing 400V (EU) or 480V (USA), this is definitely an issue to
think about when retrofitting a VFD to an older motor. But many LinuxCNC
retrofits are on smaller machines with 240V motors. Including Andy's
machine that started this thread. On a 240V motor the insulation system
is usually a non-issue, unless the motor is really ancient.
>From the paper that Steve referenced above:
The insulation system on a 208/230-V motor is identical to that
of a 460-V motor. Thus, voltage spikes produced by inverters on
208- or 230-V systems are unlikely to cause insulation damage
at any cable length or drive carrier frequency.
--
John Kasunich
jmkasun...@fastmail.fm
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