Adverse Effects of Running DC Motor at Lower than Rated Voltages?

None. I use a little gear motor to measure coefficient of friction and I run it from about 0.5 V up to 6 V with no noticeable ill effects. The inertia of the gearbox is a help.

BTW, I paid $.99 for this motor from a lady in Shanghai (Suzy's House of Dumpster Recycling, I think). Ain't mass production grand?

Efficiency will drop, and it will likely produce extra heat. All -in-all it's probably just fine.

About the only time you will see adverse effects from running a motor slower than rated speed is when it is cooled by an integral fan. At the slower RPM, the fan may not move sufficient air to properly cool the motor. For the hobby motor you described, I expect there will be no problems.

Use your senses... common, touch, sight, and smell. If the motor doesn't become too hot to safely touch, you don't see smoke, or smell the insulation burning off the copper windings, then your motor should be fine. Good luck with your project.

Thanks for the input. This info is exactly what I needed.