Gene Vogel
Pump & Vibration Specialist
Electrical Apparatus Service Association
St. Louis, MO
The paper "Isolation Test Bed Design and Construction" by Gene Vogel, presented at the EASA Convention 2017, provides a comprehensive guide to designing and constructing test beds for vibration testing of electric motors and generators. EASA recommends compliance with the vibration standards in NEMA MG1-Part 7 for new and repaired machines, which are tested with no load and uncoupled. Machines can be resilient mounted, rigid mounted, or set flat unsecured for vibration testing. Resilient mounting involves setting the machine on durometer pads to isolate it from the supporting structure, while rigid mounting requires a stable test bed.
Resilient mounting works best for smaller, higher-speed machines, while larger, heavier, and lower-speed machines are better suited to testing on a test bed. The paper explains the concept of natural frequencies and resonance, which are critical for understanding machine mounting for vibration testing. Resonance occurs when an exciting vibratory force is near a natural frequency, resulting in high vibration. Damping reduces the amplitude of vibration and distributes it over a broader frequency range.
Isolation is achieved by mounting the machine on a support with a very low natural frequency, such as durometer pads. The paper provides a table listing the required static deflection to achieve 90% isolation at common machine speeds. For low-speed machines, a test bed offers significant advantages, especially for larger machines.
There are two approaches to test bed design: supporting the test bed on a spring-damper system or using the natural damping characteristics of the subsoil beneath a massive foundation. The latter approach is more practical for most service centers. The design of a test bed foundation requires accurate data on subsoil characteristics, specifications of the machines to be tested, and proper engineering calculations. A massive foundation reduces the ratio of the exciting force to the total mass of the system, providing a stable test environment.
The paper includes case histories of test bed installations at Rotek Service, Inc. and Burford Electric Service, Inc. Rotek Service installed a 10'x12' test bed for testing motors up to 100,000 lbs at 1800 to 3600 rpm. The project involved soil sampling, engineering calculations, and construction of a concrete foundation. Burford Electric re-installed an existing 10'x10' test bed on an isolated foundation with helical pilings for additional support.
The paper emphasizes the importance of planning and engineering to ensure a test bed installation that provides accurate vibration test results. Additional features such as shaft clearance holes for testing vertical solid shaft motors and cover plates for safety can add convenience.
Key Points Covered:
- Compliance with NEMA MG1-Part 7 vibration standards
- Resilient mounting vs. rigid mounting for vibration testing
- Concept of natural frequencies and resonance
- Achieving isolation with durometer pads and test beds
- Two approaches to test bed design
- Importance of accurate data and engineering calculations for test bed foundations
- Case histories of test bed installations at Rotek Service and Burford Electric
- Planning and engineering for effective test bed installations
Key Takeaways:
- Proper mounting is essential for accurate vibration testing of electric motors and generators.
- Resilient mounting works best for smaller, higher-speed machines, while larger, lower-speed machines require test beds.
- Understanding natural frequencies and resonance is critical for machine mounting.
- Isolation can be achieved with durometer pads or test beds with massive foundations.
- Accurate data and engineering calculations are necessary for designing test bed foundations.
- Case histories demonstrate the importance of planning and engineering for successful test bed installations.
- Additional features can enhance convenience and safety in test bed installations.
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