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I'm working on a coffee roaster that uses a repurposed hot-air popcorn popper whose motor is 20.4V. The closest power supply I could find was 24V. The motor will be driven through an Arduino board using PWM control. The algorithm never drives the motor at full speed. Do you see any problems with this setup? Suggestions?

At these low voltages with a straight dc motor about the only thing to worry about is speed when you go high on voltage. In your case the potential overspeed is about 17% but you are using pwm control so can easily limit the speed to 100% by limiting the pwm so I don't see anything to worry about. Note these comments apply only to real dc, not "brushless dc"

Thanks Foxx! I'm assuming that a cheap, air popper would use the lower cost brushed DC. Does that seem like a reasonable assumption? The motor has a sticker on the side that says Model HR 355QU 20.5VDC 1.78A Max, Wah Ming Elec Ltd.

In my experience if it's "brushless dc" the nameplate or sticker says so but this may not be true all the time. A "brushless dc" motor is really an ac motor with an inverter built in to turn the dc supply to ac. If you can, try turning the shaft by hand. If you can feel a bit of friction it's probably the brushes rubbing on the commutator and so is true dc. If there is little or no friction and the shaft turns in pulses or shows a preferred stopping position it is probably an ac motor. It may become a case of try it and see what happens. If it's "brushless dc" the inverter may very well work at the bit of overvoltage you have; after all it's only 17% ov