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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