Two Minute Tips  

Rotor Bar Analysis Using Electrical Signature Analysis

Mark Koch | Technical Support, ALL-TEST Pro, LLC

A broken or fractured rotor bar fault can occur with excessive starts, large loads, manufacturing processes, etc. When a broken rotor bar occurs, there is no longer a path for current to flow. This creates stress on the neighboring bars in the form of increased current and heat. Eventually these rotor bars fail over time. Electrical Signature Analysis (ESA) evaluates the current and voltage spectrums using a Fast Fourier Transform (FFT), which converts the time waveform to a frequency spectrum. The FFT highlights amplitude and frequencies to identify mechanical faults such as broken or fractured rotor bars.

Generally broken or fractured rotor bars are found as elevated Pole Pass Frequency (PPF) sidebands of line fre- quency (LF). PPF is calculated using the synchronous speed minus the running speed times the number of poles. In this ESA sample there are PPF sideband spacing around LF (3600 RPM or 60 Hz) in the current -1 spectrum.

3 phase AC motor example:

  • 460V, 1200 RPM (synchronous speed), 6 Pole motor, 1 RPM (running speed), 60Hz (LF).
  • 1200 RPM synchronous speed – 1 RPM running speed = 16.9 RPM
  • 9 RPM x 6 (# of poles) = 101.4 RPM or to work in hertz use 101.4RPM / 60 seconds = 1.69 Hz
  • PPF = 101.4 RPM or 1.69 Hz

Vibration and infrared may indicate an initial problem. Using ESA technology, you can pinpoint your motor’s actual problem or verify its condition.


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About the Author

Mark Koch Technical Support, ALL-TEST Pro, LLC

Mark Koch is a member of the Technical Support team at ALL-TEST Pro, LLC (ATP), where he has provided professional support worldwide for users of ATP equipment since 2014. He has 17 years of electrical and reliability maintenance experience. In 2001, he started his career as a construction electrician. During this time, he worked his way up from shop floor apprentice to a foreman in charge of starting and completing remodel and new construction projects. In 2010, he began working at the Metropolitan Sewer District (MSD) of Greater Cincinnati, Ohio, USA, where he learned electrical maintenance best practice and helped implement a predictive maintenance program (PdM). During his service at MSD, his team was the recipient of both the Uptime Magazine’s Best Emerging Maintenance Reliability Program Award and the Uptime Magazine’s Best Asset Condition Management Program Award. At MSD, he learned and provided routine PdM services such as thermography, vibration, lubrication, visual inspection, motor testing, and ultra-sonics.


Education and Certifications:

2001-2006- International Brotherhood of Electrical Workers (IBEW) Apprenticeship Program and the National Electrical Contractors Association (NECA) Inside Wireman Certification.

2013- PdM Visual Testing Level 2 Certification & Level 1 Thermographer Certification; 2014- Vibration Analysis Level 1T Certification.