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Vibration Analysis report of Broken/Cracked Rotor Bar

Muhammad Trendo
Muhammad Trendo | Condition Monitoring Supervisor, Indah Kiat Pulp & Paper Perawang

Vibration Report: Motor Turbo Blower

Overall Vibration – Turbo Blower 2

Dominant vibration at motor

Dominant vibration at motor - CBM CONNECT

Dominant vibration at motor - Rotor Bar - Vibration Analysis

Motor Spec

Brand = ABB

Power = 800 kW

Voltage = 6.3 kV

Number of poles = 4

Frequency = 50 Hz

Background

  • 18 Mar 2016 vibration normal but motor temperature increased.
  • 28 Mar 2016 Motor Turbo Blower running rough with periodic noise.
  • We checked vibration, motor vibration increased, multiple harmonic of 1x motor running speed; this seems to be a rotating looseness problem.
  • We checked vibration with higher resolution and we saw sideband Fp (pole pass freq).
  • Periodic noise frequency equal to pole pass frequency (0.46 Hz).

Waterfall Plot Vibration Motor Turbo Blower (NDE)

Waterfall Plot Vibration Motor Turbo Blower (NDE)

Waterfall Plot Vibration Motor Turbo Blower (DE)

Waterfall Plot Vibration Motor Turbo Blower - Rotor Bar - CBM

Spectrum Vib Motor Turbo Blower (Normal Resolution)

Spectrum Vib Motor Turbo Blower - Rotor Bar - CBM CONNECT

Low resolution data – seems to be rotating looseness problem.

Spectrum Vib Motor Turbo Blower (High Resolution)

Spectrum Vib Motor Turbo Blower (High Resolution)

By using a high-resolution spectrum, we can see sidebands around harmonic of 1x running speed.

Spectrum Vib Motor Turbo Blower (High Resolution)

Spectrum Vib Motor Turbo Blower (High Resolution) - Rotor Bar - CBM CONNECT

Vibration Analysis

  • From the spectrum we can see 1x harmonic running speed with pole pass freq sideband; this indicates a rotor bar problem (broken or cracked rotor).
    • Pole pass freq calculation
      • Sync speed = 1495
      • Slip freq = Sync speed – Rpm actual = 1495 – 1488 = 7rpm or 0.1167 Hz
      • Pole pass freq = Slip freq * Number of poles = 0.1167 x 4 = 0.467 Hz (Match with sideband 0.46 Hz)
  • Rotor bar frequency appears with 2FL (line frequency) sideband

Recommendation

  • Prepare spare motor
  • Replace motor as soon as possible

Corrective Action

  • Electric & mechanic team replaced motor on April 6th, 2016.
  • We did re-alignment with thermal growth 0.25mm in the vertical level (motor set to +0.25 mm).

e-alignment with thermal growth - Cracked Rotor Bar - Vibration Analysis

Radial Blower - Vibration Analysis - CBM CONNECT

Result: Vibration OK!

Vibration Analysis - CBM CONNECT

Overall Vibration Value

Overall Vibration Value - CBM CONNECT - Vibration Analysis

Vibration back to normal - Rotor Bar - Vibration Report

Vibration back to normal

Vibration Spectrum

Vibration Spectrum Analysis

After motor changing, there is no Fp sideband appearing around 1x harmonic running speed.

Rotor Bar Cracked

Cracked Rotor Bar - Vibration Analysis - CBM CONNECT

After the motor was dismantled, we inspected the rotor and found the rotor was cracked.

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TARAPADA PYNE
TARAPADA PYNE
1 year ago

Mr Mohammed,
Like your analysis, sequence presentation in knowing the unknown frequencies.

Secondly, normally we get PPF, RBPF etc in many motors and also with additional inputs from MCSA. But identifying faults in rotor bars correctly need serious analysis involvement. Nice and all the best for your next predictions. Now there is a need to go for RCA.

Ian
Ian
1 year ago

nice one!

Mika Savolainen
Mika Savolainen
1 year ago

How many rotor bar is this motor? Did you see that freqeuncy?

Ahmad Alfakeer
Ahmad Alfakeer
1 year ago

NICE JOB Muhammad and good analysis

About the Author

Muhammad Trendo
Muhammad Trendo Condition Monitoring Supervisor, Indah Kiat Pulp & Paper Perawang

He was graduated from Universitas Islam Indonesia, a well known reputable University in Indonesia. His background was bachelor degree of Informatics Engineering (IT). He started his career not as IT Engineer but as Production Analyst of Paper Machine, he discovered and learned something new, production analysis, cost analysis, the whole process of paper making. After 2 years he moved to Condition Monitoring Section, he was given challenge to enter new career path, move forward, learn mechanical & rotating equipment and finally found his passion in Machine Condition Monitoring. In 2013 he passed Mobius Institute Vibration Analyst CAT I. He found that Mobius Institute Cristal Clear learning system was very helpful, easy to understand with many animations and interactive learning system. Then in 2016, He passed Mobius Institute Vibration Analyst CAT II with score 90-100% exam result. He also active as Vibration & Field Balancing Trainer in APP Academy. His recentachievement is Mill Best Employee 2018 for Supervisor category. His dream is to become professional in Condition Monitoring & Reliability.