So we changed out the motor in this exhaust fan. It is a Loren Cook 100R2B. We replaced the motor with the exact one that was removed (Dayton 5K601). It is 1/3 hp 115v 1725 RPM. The amp draw was within the limits when we installed the new motor. So today that new motor won't run, its only humming and then shutting off. It was drawing something like 40 amps. This is about 2 weeks after we originally replaced it. So we went back and threw in a brand new Dayton 6K778 that we had laying around, basically the same motor as the 5K601. It has full load amps of 6.6, but no matter what we do, we can't get it to draw less than 7.1 amps or so. The bearings, shaft and wheel of the fan all seem to be in good condition. The belt is brand new, tension is correct, and is aligned straight. There are no restrictions in the duct at all. We adjusted the pulley on the motor to slow it down as much as possible. But it still draws over 7 amps.
Is there anything else we can check? Change the motor pulley size? Any other ideas?
We've changed countless motors on exhaust fans and have never had any issue like this.