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? > > > > > > -- > > AF mailing list > > [email protected] > > http://af.afmug.com/mailman/listinfo/af_af.afmug.com > > -- > AF mailing list > [email protected] > http://af.afmug.com/mailman/listinfo/af_af.afmug.com -- - Forrest -- AF mailing list [email protected] http://af.afmug.com/mailman/listinfo/af_af.afmug.com
