mycom UD Series Instruction Manual page 27

 If operation at partial load, which is not greater than 30% of the indicated load, is
continued for a long time except when starting up the machine, abnormal noises
or vibration may be generated. So avoid such operation.
 Repeated startup and stop in a short period is harmful not for the startup devices
and electric machinery but also for the compressor itself. For information on the
start/stop limitations, refer to each instruction manual. Wait at least 15 minutes
after stopping the compressor before restarting it.
UD-series Screw Compressors
Chapter 2 Compressor Specifications and Structure
2.3 Compressor Specifications
2-6