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Oh yeah. Those metric hertz will get you every time. What the
heck was wrong with Imperial hertz anyway?
Get off my lawn!
bp
<part15sbs{at}gmail{dot}com>
On 3/22/2019 8:06 AM, Chuck McCown
wrote:
It is a pretty high quality gearmotor. I think it is
made by oriental motor. I have the fan guard off and will
manually count the gear reduction. Not sure what else it
could be other than the VFD reporting Hz wrong. Maybe they
are using metric Hz...
Sent: Friday, March 22, 2019 8:54 AM
Subject: Re: [AFMUG] OT VFD speed vs hz
Are you sure the gear reduction box is a true
15:1 or is there some fudge factor there?
On Thu, Mar 21, 2019 at
7:46 PM Forrest Christian (List Account) < [email protected]>
wrote:
Well that's irritating.
Assuming the VFD is putting out the frequency you think
it is, and
that's a 4 pole motor, and the gearbox ratio is what you
think it is,
my math would agree with yours .... it should be putting
out 2 RPM per
Hertz.
I mention the "assuming" part of all of the above
because back in my
industrial automation days I've seen nameplates not
match reality on
both gearboxes and motors. I don't remember what was
different on the
motor, but I've definitely seen a gearbox which wasn't
the stamped
ratio as I specifically remember hand cranking a gearbox
and counting
the input:output ratio and finding it different than the
stamped
ratio.
I just remembered something else.... have you verified
the motor is
wired correctly? Sometimes a mis-wired or
not-connected winding can
do weird stuff like this. Depending on the motor it
may also need
to be wired differently for wye vs delta, etc.
There is also always a bit of slip between the
calculated RPM and the
actual running RPM - induction motors have to run slower
in order to
develop torque., But this seems far enough out that
this probably
isn't it.
On Thu, Mar 21, 2019 at 8:26 PM Chuck McCown <[email protected]>
wrote:
>
> It has 4 poles printed on the nameplate.
>
> Sent from my iPhone
>
> > On Mar 21, 2019, at 6:15 PM, Forrest Christian
(List Account) <[email protected]>
wrote:
> >
> > What is the nameplate RPM and Hertz on this
motor?
> >
> > Assuming a 4 pole motor, you'll have
(frequency*120)/poles = RPM
> >
> > So if you have a 4 pole motor running at 10Hz,
you'll find that it runs at 10*120/4 = 300 RPM After
the 15:1 gearbox, you'd have 20 RPM at 10 Hz.
> >
> > It's linear so 2 RPM per HZ. So at 5 Hz you
should get 10RPM. So you've got something off.
> >
> > Let's assume that you are just wrong with the
poles. 26 RPM * 15 = 390RPM. Then,
(10*120)/poles = 390. 1200/poles=390. Looks like
it might be a 3 pole motor.
> >
> > Back the other way:
> >
> > 3 Pole Motor @ 10 hz = 10*120/3 = 400 rpm /
15:1 = 26.666 RPM. That's in the right range.
> >
> > Would 26.666RPM be in the range of your
measurement/VFD accuracy?
> >
> >
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>
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