Tom Bishop, PE
Senior Technical Support Specialist
EASA
St. Louis, MO
The paper "Selecting Replacement DC and 3-Phase Squirrel Cage Motors" by Tom Bishop, presented at the EASA Convention 2018, provides a comprehensive guide to selecting replacement motors, focusing primarily on the electrical aspects and considerations for speed and torque. The paper addresses various scenarios, including replacing DC motors with DC motors, DC motors with 3-phase squirrel cage motors, and AC 3-phase motors with 3-phase squirrel cage motors. The goal is to ensure that the replacement motor provides the required performance and reliability.
When replacing DC motors with DC motors, it is crucial to match key characteristics such as horsepower, speed range, armature voltage, field current, and type of field (series, shunt, compound). The paper discusses the challenges of replacing a compound wound motor with a shunt wound motor, highlighting the differences in performance due to the lack of a series field in the shunt motor. Adding a series field to a shunt motor can change its speed-torque characteristics, potentially making it unsuitable for the application. The paper provides formulas for calculating speed and torque to illustrate these changes.
For DC motor to 3-phase squirrel cage motor replacements, the paper emphasizes the importance of using a variable frequency drive (VFD) to achieve variable speed capabilities. The VFD varies the frequency and voltage to maintain constant torque up to the rated line frequency, allowing the motor to operate above rated line frequency at constant power with reduced torque. The paper provides guidelines for selecting the appropriate VFD based on the required speed range and application type.
When replacing AC 3-phase motors with 3-phase squirrel cage motors, the paper addresses the replacement of synchronous motors and wound rotor motors. Synchronous motors are known for their constant speed and ability to adjust power factor, making them challenging to replace with squirrel cage motors due to inherent slip. The paper suggests using a vector type VFD with closed-loop control to maintain fixed speed with varying load. For wound rotor motors, the paper discusses the benefits of eliminating maintenance associated with slip rings, brushes, and external resistance by using a squirrel cage motor and VFD.
The paper also covers potential issues with speed-torque and speed-current characteristics when replacing squirrel cage motors with other squirrel cage motors. Differences in design, such as NEMA design letter codes, can affect motor performance. The paper provides speed-torque curves and guidelines for selecting the most suitable motor design for various applications.
Key Points Covered:
- Matching key characteristics when replacing DC motors with DC motors
- Challenges of replacing compound wound motors with shunt wound motors
- Importance of using VFDs for DC to 3-phase squirrel cage motor replacements
- Guidelines for selecting VFDs based on speed range and application type
- Challenges of replacing synchronous motors with squirrel cage motors
- Benefits of replacing wound rotor motors with squirrel cage motors and VFDs
- Potential issues with speed-torque and speed-current characteristics when replacing squirrel cage motors
Key Takeaways:
- Ensuring replacement motors match key characteristics is crucial for performance and reliability.
- Adding a series field to a shunt motor can change its speed-torque characteristics.
- VFDs are essential for achieving variable speed capabilities in 3-phase squirrel cage motors.
- Vector type VFDs with closed-loop control can maintain fixed speed with varying load.
- Replacing wound rotor motors with squirrel cage motors and VFDs can eliminate maintenance issues.
- Differences in motor design can affect performance, requiring careful selection based on application needs.
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