Introduction
All motors are designed to work within the limits of some given environmental parameters. For example, Parker’s motor engineering team rates the servo motors to 25 C ambient temperature at near sea level. Operating a motor in environments that are outside of these confines can greatly affect the capabilities and life expectancy of that motor.
Two of the greatest environmental factors to consider when sizing and selecting your frameless kit motor are ambient temperature and pressure.
Vacuum environment
A vacuum environment is simply a contained area that has a pressure lower than that of standard atmospheric pressure and is often measured in terms of the absolute scale of Torr. Commonly specified vacuum pressures are 10-4 Torr, 10-7 Torr, and 10-9 Torr.
Outgassing
One of the most prevalent problems is outgassing which occurs when material particles (commonly measured in terms of total mass loss (TML) and collected volatile condensable materials (CVCM)) vaporize in a low-pressure environment. These particles can float throughout the vacuum chamber and potentially contaminate the application. Even though outgassing cannot be eliminated, it’s still important to select materials that have a lower tendency to outgas. Common materials that are prone to outgassing are silicon, winding insulation, and many types of glue.
The primary materials used in standard construction of our kit motors that are susceptible to outgassing are RTV silicon and wire insulation. Wire insulation for vacuum environments is then specified to be Teflon which is highly resistant to outgassing. RTV silicon is an adhesive used to bond the hall commutation board to the kit motor stator. Since the silicon-based adhesive is prone to outgassing, we do not include the board as an option on vacuum rated kit motors.
Thermal dissipation
Motors dissipate thermal build up through various means, but the most prevalent method is natural convection. In other words, the heat is transferred to the ambient air. However, “low pressure” also means “low air quantity.” With less air, the motor can’t get rid of the heat as easily as it normally would. Therefore, the motor must rely on other methods. Since thermal conduction is the next best method to efficiently expel the heat, kit motor users must ensure they install the stator with ample thermal conductors to extract the heat. If cooling becomes an excessively difficult hurdle to overcome, one solution may be for users to consider a hermetic water jacket.
Extreme temperature environment
Torque capacity of any given motor is directly correlated to the motor’s thermal capacity. Heat builds up as current is run through the winding wires, and motors are rated to only handle up to a specific temperature. It is rather intuitive to understand that a lower ambient temperature can more easily pull heat away from a motor than a higher temperature environment.
For this reason, the torque capability of any motor would be reduced the higher the ambient temperature rises above the standard 25 C. See the torque curve below that compares the performance of a sample motor at various ambient temperatures. If the temperature rises above 80 C then Parker engineering must investigate each component to make sure they’re rated appropriately.
Effect of various ambient temperatures on the performance of a sample Parker servo motor
The customer needs to verify with their distributor whether the high temperature derating will influence the motor selection.
