Duty cycle generally exists because of two things :
- Temperature (motors emit quite a bit of heat)
- Wearing of the bearings/gears/whatever with regard to the rated life of the device (i.e. if the device is rated 5 years at duty cycle 20%, it'll only be 1 year at 100%, assuming you can cool the device properly)
Yes, running a motor at less than the given load will increase the acceptable duty cycle. However, please note that the temperatures on the playa are high, and I think it will be good to provide shade and proper cooling and/or heatsinking to the motor. Some motors have an integrated fan.
Always use a mechanical thermal cut-off switch, in addition to any other security device you may have in place. Always make sure that the temperature of the windings NEVER EVER exceed 150°C / 300°F or windings will be damaged. This translates to a much lower case temperature for the motor; I'd get a 100°C / 260°F thermal fuse (such as http://search.digikey.com/us/en/product ... ND/1014759
). Be careful, this is a fuse, once it's melt it's over, so get a few spares and add a temperature cut-off software switch in your adruino (put a digital temperature sensor on the motor case and switch off the motor when temperature is close to the one of the fuse).
Some motors have an integrated thermal fuse, take a look.
Hope this helps