General Operation Considerations for UHV-Compatible Motors

It is recommended that motors and mechanisms are operated in air during early commissioning. This has the advantage that more cooling is available and that the operator can see and hear that the motor is stepping.

1. Temperature Rise and Run Times

   The maximum recommended operating temperature of AML motors is 200 °C, as measured by the embedded type K 
   thermocouple.
   
   The motor should be run at the minimum phase current consistent with the requirements of the load. This will reduce the 
   maximum temperature of the motor and outgassing from the motor. Resistive heating losses in the winding resistance, R, are 
   given by I2R. The winding resistance is approximately proportional to absolute temperature so even small reductions in phase current, I, produce worthwhile reductions in temperature rise and outgassing. For phase currents down to about 50% of maximum the output torque is reduced roughly in proportion to phase current.
   
   The minimum practical phase current is determined by the load friction and inertia, and the required acceleration and 
   maximum speed. It is best found by experiment. A reasonable margin of safety should be allowed for any expected increase 
   in load friction, which might occur after bakeout.
   
   Vacuum stepper motors achieve maximum efficiency at full-step rates between 500 Hz and 2 kHz.
   AML SMD drives allow the reduction of the phase current dynamically during each step at low step rates, with separate control of initial and final currents and transition times. Use of this technique can dramatically reduce the power dissipated in some applications. The provision of heatsinking will improve the performance.
   
   Irreversible deterioration of the winding insulation will occur at 230 °C and the motor will subsequently produce large amounts of gas, even at lower temperatures.

2. Outgassing and Bakeout

   A newly installed motor will outgas in vacuum, mainly due to water-vapour retention in the polyimide. As this material is microporous the water is released rapidly and the rate will subside after a few hours. The rate may be accelerated by running the motor to self-heat it.
3. Rotating Mechanisms – Holding Torque