Stepper Motor Design Considerations for High-Temperature Applications

Stepper motors are known for their precision and control, making them a popular choice in many industrial and robotic applications. A great example is HVAC systems, where they’re often utilized in controlling the dampers and valve mechanisms. However, using these miniature motors in high-temperature environments presents unique challenges, given that refrigerant temperature and outdoor units exposed to direct sunlight demand that steppers be rated for high temperatures. Here we explore stepper motor design considerations that will ensure optimal HVAC system performance.

Challenges of Operating at High Temperatures

Operating motors at high ambient temperatures creates several challenges that need to be addressed to ensure performance and reliability. These key challenges include:

• Wire Insulation Degradation: Insulation materials used over magnet wires and lead wires can easily degrade when exposed to temperatures greater than their rated temperature. Degradation in wire insulation can lead to breakdowns, short circuits, and ultimately motor failure.
• Material Degradation: The material system of a stepper motor consists of different materials such as adhesives, plastics, metals, and magnets. These materials can degrade faster or lose their strength when exposed to high temperatures, affecting the motor’s performance and reliability.
• Reduction in Motor Performance: As temperature increases, the resistance of the coil increases, leading to a decrease in current. Additionally, higher temperatures can reduce magnet strength. Increased coil resistance and reduced magnet strength result in decreased motor performance.

Considering Ambient Temperature in Motor Design

When a motor is supplied with voltage and current, heat is generated due to copper losses. Even if the motor operates at higher temperatures, the heat generated from these losses will further increase the motor’s temperature; this means that ambient temperature alone cannot be used to design and decide motor part ratings. To determine the design temperature, thermal calculations must be performed to estimate the motor temperature for the required duty cycle in the given ambient temperature. A factor of safety should also be considered above the design temperature.

Overcoming High-Temperature Challenges

To overcome the challenges of high temperatures, the right selection of materials is crucial:

• High-Temperature Wire Insulation: One of the primary considerations when designing a high-temperature stepper motor is the temperature rating of the magnet wire insulation. Depending on the desired working temperature, wire insulation rated for high temperatures, such as Class 155 or Class 180, can be selected.
• High-Temperature Magnets: For high operating temperatures, magnets with a high Curie temperature must be selected. The Curie temperature is the temperature above which the magnet loses its magnetic properties. Materials like Samarium Cobalt (SmCo) or Neodymium Iron Boron (NdFeB) are commonly used because they have higher Curie temperatures. NdFeB magnets are stronger than SmCo in terms of magnetic strength but have a lower maximum working temperature compared to SmCo.
• High-Temperature Lubrication: Selecting the right lubricant for motors operating at high temperatures is essential to ensure performance and reliability. The lubricants used in motors and bearings should be rated for high temperatures. The viscosity grade of the lubricant should be chosen to provide proper lubrication at high temperatures without thinning out.
• High-Temperature Adhesives: Adhesives used inside the motor should be reviewed and selected to ensure they can withstand the maximum design temperature.

       Stepper motor cross-section

Design Validation Through Testing

Once the motors are designed, they need to be validated through testing at high temperatures to ensure performance and reliability. Portescap can assist with the following in-house testing for high-temperature motor validation:

• Testing rotor assemblies at accelerated temperatures and thermal shock conditions in environmental chambers and ovens.
• Conducting axial and torsion force tests to determine rotor mechanical strength at specific temperature requirements.
• Performing accelerated life testing to predict a motor’s service life in its application.

Final validation must occur in the end application, where the motor is simultaneously exposed to refrigerant, high temperatures, temperature gradients, humidity changes, and actual load conditions.

Conclusion

By addressing the challenges posed by high ambient temperatures and selecting appropriate materials, it is possible to ensure the performance and reliability of stepper motors in demanding environments. By leveraging Portescap’s extensive experience in designing high-temperature motors for HVAC systems, you can confidently implement stepper motors tailored to meet your application’s specific needs. For more information or to start collaborating, reach out to Portescap here – we’ll be happy to help.