Standard motors are rated for operating ambient temperatures between 50°C and -20°C, which is adequate for most purposes, and some applications require motors to operate at higher or lower temperature ranges than standard motors allow, outside of these temperature ranges Operating a standard motor anywhere will result in poor performance. Typically, ambient temperatures drop to approximately -20°C [4°F] without any modification. For environments below this temperature, the first consideration is lubrication, and for grease lubricated bearings, make sure the grease is suitable for the surrounding environment. When the ambient temperature drops below -25°C [-13°F], additional consideration must be given to the type of material used in the manufacturing process.
The most common material changes are fans, shafts, frames and end brackets, fans may need to be changed from plastic to steel or phosphor bronze. Shaft materials may change from standard materials such as AISI1040 to high-strength alloys such as AISI4140. Due to the brittleness of gray iron at low ambient conditions, cast iron frames and end brackets are often changed from gray iron to ductile iron, which requires a temperature range of -25°C and -60°C [-13°C and -60°C, depending on the standard material and application. These material changes were performed between different ambient ranges of °C.Cryogenic Motor SelectionLow temperatures and vacuum conditions make motor selection a challenge, there are two main options: stepper motors or DC servo motors, stepper motors have a certain number of phases, usually from 2 to 5, when the controller gives the different phases in a coordinated manner When the combination of phases is energized, they move in steps. This produces a semi-continuous motion of the motor shaft. There is only one unique sync speed that will allow continued movement. Such motors can provide positional feedback using step signals from the controller, but cannot provide positional feedback directly from the actual position of the motor.
Stepper motors are widely used in low temperature field, there are some manufacturers who specialize in making motors for vacuum and cryogenic conditions, they are very expensive, some instrument developers buy high quality stepper motors and prepare them for low temperature operation Replace the casting parts with polished metal surface to prevent volatilization, and replace the bearings with special dry low temperature bearings, the cost reduction can be reduced to one tenth of the original.Cryogenic Motor DriversThere are currently two main types of drivers used at cryogenic temperatures, one that has a drive source outside the cryogenic environment. It has high output power, but the actuator system requires a lot of space, which makes it difficult to sustain extreme environmental conditions. Another type of actuator is small and located entirely in a low-temperature environment. It is used in micropositioning stages, but its application is limited.A DC servo motor basically rotates at a speed that is a function of the voltage applied by the controller. In this case, the position is unknown and requires an optical encoder or other positioning device to determine the position of the drive. This is especially difficult at low temperatures because the optical Encoders and inductive sensors do not work well in this environment.
Another important factor to consider is that the torque of a stepper motor is a function of operating speed and temperature, the faster the motor is run, the less torque it will deliver. In addition, cold operation reduces the torque of the motor. A loss of phase on the controller or motor causes torque to decrease from nominal A, resulting in a loss of steps despite a well calibrated drive, thus causing a wrong positioning of the step counts. A stepper motor is combined with a feedback resolver to meet the unique requirements of the application, and both units are housed in an exotic nickel-chromium steel alloy frame for thermal stress resistance and dimensional stability. Due to the similarity of resolver components to motor components, resolver technology was chosen for the feedback system.Evaporation of lubricating and insulating materialsThe first problem is the volatilization of lubricating and insulating materials at low pressure, and the second problem is related to the performance of motor materials at extremely low temperatures, large temperature changes, low temperature brittleness and different material shrinkage stresses can be reduced by traditional materials. the structural integrity of the motor. In order to reduce low temperature volatilization, the insulation material is made of selected polymers, the magnet and lead wire materials are carefully specified to avoid volatilization or fracture, and the adhesives usually used in the manufacture of electric motors are bonded by a thermal expansion coefficient close to that of adjacent steel members agent instead.The choice of AlNiCo (aluminum-nickel-cobalt) magnets favors the rare earth combination because AlNiCo retains its magnetism better at low temperatures, and the use of dry film lubricated stainless steel ball bearings is also for the same reason. All machined metal parts are stress relieved, and the result is a design that can operate at cryogenic temperatures, within the confines of a vacuum chamber, without vaporizing the motor's material.
Shrinkage of metal parts and hardening of non-metallic parts are the two main factors that prevent stepper motors from working at low temperatures. In the case of shrinkage, if motor components with critical dimensions shrink at different rates, this can lead to motor lockup. The resulting stresses can cause cracks in metal parts that are overcooled and embrittled.In order to overcome these effects, special alloys must be selected for cryogenic motors, and all metal parts must have comparable thermal expansion coefficients. Cable insulation and bearing greases both tend to harden at low temperatures and may require dry lubrication, and insulating polymers must be carefully selected to maintain molecular integrity through low temperature cycling.in conclusionBy using higher grade, specially formulated materials, motors can be designed for extreme temperatures, well beyond what standard motors can handle. Other factors that affect motor usage under extreme temperature conditions include the frequency at which the load is applied. Most standard motors are rated for continuous duty, but if the duty cycle specification is lower than continuous, it will affect the motor application process.
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