Motors run on voltage and current (power), the faster they turn, the more current they use. The higher current causes the motor windings to heat up because of the resistance in the winding wire. If they get really hot, the insulation melts, causing shorts. The shorts cause the motor to draw more current and the condition gets worse until the motor gets so hot that the windings start to burn up. At that point, there is not turning back.
Sooo, based on that, I agree with the others, running motors as slow as you can will not harm them, it will in fact extend their life imeasurably.