Broken Bars - Case History 2
Compressor Drive - Offshore Production Platform
Test Objective
In this case history the current spectrum of one compressor motor is compared to another which exhibited excessive vibrations.
Nameplate details
3-phase, SCIM, 134 kW/180 h.p.
440 V, 210 A, 1485 r.p.m., 50 Hz, 0.88 p.f.
star connected.
Additional Information
Aluminium die cast rotor,
single cage design
58 rotor slots.

Full-Load Slip = 0.01 or 1%
Click within this image for in-depth
analysis of faulty Motor B

Figure b-3
Motor A spectrum - Healthy
Click within this image for in-depth
analysis of faulty Motor B

Current Spectrum
Figure b-4
Motor B spectrum - Unhealthy
Test Conditions
MCSA FFT Current Spectrum
Dynamic Range = 80 dB
Frequency Resolution = 15.6 milliHertz/line
Test Conditions
The normal duty cycle of the motors was 15 minutes ON and 15 OFF and they were started "direct-on-line (D.O.L)".

During the MCSA tests Motor "A" was operating normally. and the problem Motor "B" was on for 10 minutes and off for 1 hour.

General Symptoms
Prior to MCSA being applied the client reported that Motor (B) was producing pulsating vibrations and an audible beating noise. Motor (A) was operating normally.

Vibration analysis of Motor (B) by the operator suggested that the motor bearings were possibly faulty. The motor was removed from the installation and new bearings were installed but the motor was only tested on no-load.

The client was told the motor was now healthy but if it were a broken rotor bar problem it would not be detectable on no-load since the rotor current is small and the fault characteristics would have been negligible. The motor was reinstalled with the new bearings but exactly the same pulsating vibration and audible noise were present.

There was considerable debate between the mechanical and electrical maintenance staff concerning the real problem. For example, was it broken rotor bars, abnormal airgap eccentricity, or a compressor/gearbox problem?

The current spectrum in Figure b-3 shows a healthy rotor winding

  • This current spectrum shows there are no ±2sf1 sidebands

  • Note that the supply frequency was 50.156 Hz and not 50 Hz (isolated offshore power system). Cannot use the nameplate frequency to predict the ±2sf1 sidebands - the actual f1 from the measured current spectrum must be used.

  • Due to the gearbox in the drive there are current components due to the mechanical drive characteristics - different shaft speeds, etc.

  • The motor was operating at 120 A, ie 57% of the full-load nameplate current of 210 A.
    Note this does not mean the motor was delivering 57% of the rated full-load nameplate output power.

  • The full-load rated slip (deduced from the full-load nameplate rotor speed and supply frequency) sf.l.= 0.01 or 1%.

  • On this reduced load the slip will be less than 1%.

  • If ±2sf1 sidebands were present due to broken rotor bars then they would be less than ±1 Hz around f1 - no sidebands exist.
Click here for analysis of Motor B
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