Broken Bars - Case History 2
Compressor Drive - Offshore Production Platform

EXPERT ANALYSIS & RECOMMENDATION

Faulty Rotor Assembly

Aluminium die-cast rotor, no visible damage.

Analysis of the Motor Current Spectra in Figures (b-3) and (b-4).

Figure b-3 shows Motor (A);
No ±2sf1 sidebands, a perfectly healthy rotor

Fig b-4 shows Motor (B);
There are ±2sf1 sidebands due to a faulty rotor winding.

The sidebands are 34.35 dB (average) down on the supply frequency component (f1) and this corresponds to an unhealthy rotor winding.

Corrective action had to be taken but several factors need to be considered in the decision making process.

Since this was a die cast aluminium rotor the chance of secondary damage to the stator winding is considerably less than with a copper fabricated rotor (see Industrial Case History - 1).

Consideration has to be given to the effects of the forces and subsequent vibration
caused by the rotor winding problem.

The pulsating vibration on the bearings, gearbox, and drive shaft system can result in accelerated mechanical wear and damage.

The process demand had to be considered and in this case the average load demand could be provided by the healthy unit.


The RECOMENDATION was that Motor "B" could be used as a standby and on light load
and that Motor "A" should take the load.

A new rotor should be immediately ordered and installed upon delivery.


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