While servo motors and drives are prevalent in a wide variety of industrial applications, what should be considered when they operate in extreme or hazardous environments? In many cases, the motor itself operates in the extreme environment, while the drive electronics are protected from hazards. This blog addresses the servo motor performance challenges and the design considerations needed when a motor operates in extreme environments.
Types of environments:
Temperature Extremes – The typical servo motor is rated for an industrial environment that is based on an ambient temperature of 40° C. Operating temperatures will often range from a low of -10° C to a high ambient of 50° C. In applications adjacent to an oven or operations in a downhole environment, temperatures can range much higher. In refrigeration, cold storage, cryogenic systems, or at the south pole, temperatures of -40° C and lower may be present.
Vacuum – Standard motors are designed to operate within a limited range of atmospheric pressure. Certain machining applications or critical assemblies may require operation within a vacuum for a given process to limit effects caused by atmospheric pressure. High altitude and space related applications also operate within increased vacuum related conditions.
Clean Room – Often seen hand in hand with industrial vacuum applications, clean rooms have strict limitations regarding contaminates introduced into the environment from outgassing.
Water – Electric motors and water do not mix well. Standard motors are rated for levels of dust and moisture sealing based on an IP rating, with most designs established at IP54. Wet environments require ratings of IP65 and higher. In the food processing industry, it is common to find a variety of conditions where protection is needed against water spray and corrosion. High pressure wash downs or periods of submersion may also be required. Submersible applications not only need protection against water ingress, but also need consideration for higher pressure requirements.
Hygienic – A subset of food processing requirements, hygienic environments not only require high pressure and high temperature washdowns, but also need to prevent pathogen growth on motor surfaces. Motors designed using aluminum or steel housings work well in most industrial environments but are unacceptable for use in hygienic conditions due to oxidation and surface irregularities that may encourage pathogens.
Explosive – Applications in grain elevators, flour mills, textile mills, or other facilities that contain combustible gases, dust, vapors, or fibers require motor designs particularly suited for these specific hazards.
Radiation – Motor applications in close proximity to a nuclear reactor or in certain outer space environments where high levels of radiation is present presents a challenge in the selection of motor materials to minimize degradation over time.
High Vibration and Shock – Standard servo motors are designed to handle vibration in excess of typical industrial environments, however, for high repeating vibration levels or sudden impact vibrations, additional considerations are required.
Specific Considerations Based on Environment
Each environment highlighted above represents deviations from the typical industrial environment that the motor designer must accommodate. Combinations of these environmental conditions (such as an extreme cold environment and an extreme hot environment) are even more of a challenge to mitigate.
Temperature Extremes– All motors are rated based on the temperature rise in the motor coils vs. a specific ambient temperature. The temperature rise heats the coils to a maximum allowable temperature rating based on the motor insulation system, which is then assigned a specific Class rating (F, H, etc.). In a hot ambient condition, the motor is limited in torque production by its specific ability to dissipate losses to avoid overheating the motor coils. One adaptation for this environment is to derate the motor based on the difference in rated vs. ambient temperature. Another consideration is to improve the insulation system with a higher rated Class insulation, allowing for a higher temperature rise. Other methods to deal with increased temperature is to cool the motor by moving air over or through the motor or by liquid cooling that injects fluid through a customized motor housing. Cold environments affect the motor in other ways, such as how well the bearing grease performs or even how brittle the motor material may become (such as lead wire). Careful selection of motor materials and bearing lubrication can satisfy cold temperature environment concerns.
Vacuum – In a vacuum there are limited convection capabilities to dissipate motor heat so, much like with high temperature concerns, the motor either needs to be derated or a means to increase conduction of heat away from the motor must be employed. Motor material and bearing selection can also be critical due to the outgassing properties of motor materials and bearing lubrication.
Clean Room – Material selection is critical for a motor used in a clean room environment. Depending on the clean room class level, materials used in motor construction must be scrutinized for outgassing properties based on temperature and atmospheric pressure levels that will be present in the clean room. Materials such as lead wire, bearing grease, and shaft seals are very susceptible to outgassing molecular particulates that can contaminate the clean room environment.
Water – Sealing methods prevent unwanted water seepage that may contact internal materials subject to corrosion. Simply adding seals may not result in a robust water-resistant design. A motor that is considered well-sealed may actually have an inherent issue related to internal pressure coupled with natural motor heating and cooling cycles. As the motor temperature increases, the resulting internal pressure pushes against the seals. As the motor cools, the reduction in internal pressure pulls the seal inward. The continual seal flexing will eventually cause the seal to fail. If the motor is allowed to breathe as its temperature rises, the internal pressure will not increase to create wear on the seals. Submersible electric motor designs incorporate an internal, non-corrosive fluid and a pressure bladder that accommodates pressure changes when the motor is submerged to increased depths.
Hygienic – Similar to wet environments, sealing is important in hygienic applications. Additionally, to prevent pathogen growth in cracks and crevasses, stainless steel is required on all exposed motor surfaces. The motor housing should have rounded edges, no joints or connection hardware that can collect liquids, and any flat areas are sloped relative to mounting position. Sealing materials and cables are specified based on food grade requirements in food industry applications. The typical sealing requirement is IP69K (high pressure, high temperature with caustic chemicals).
Explosive – Explosive environment concerns require mitigation based on the specific gas, vapor, fiber, or dust in the environment that could cause an explosion. The motor is designed so that if windings short and create an internal explosion, the motor housing would limit and contain potential flame paths at each motor joint. The specific explosion proof ratings are governed by UL in the United States, ATEX in Europe, or the CCC in China. The rating specifies the specific explosive hazard and severity.
Radiation – Radiation environments are measured by total integrated dose, which identifies the level of radiation over a specific time period. Motor material selection is based on this metric. Standard motor materials will degrade over a short period of time in high levels of radiation. However, a radiation hardened motor with material selection tolerant of the total integrated dose levels will endure for extended periods.
Vibration and Shock – the direction of the shock and frequency of the vibration will determine the best options for mitigating potential motor damages. A common solution involves choice of the bearing system and feedback device. In a high shock environment, a robust feedback device, like a resolver is a better choice than a fragile glass scale encoder. Different types or sizes of bearings may be appropriate depending on the shock and vibration levels.
Though each of these conditions pose challenges to the motor designer, numerous motors have successfully operated within these types of environments. Designers should examine successful applications in similar environments to help choose the right motor materials and mechanical construction suitable for the hazard or extreme environment.