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GMR Electric Case Studies
Case Study: Rotor Porosity in New Motor Causes Motor Failure – Cast Aluminum

A new 350hp motor was purchased by our customer and installed. The winding had failed within a few weeks of operation. The motor was sent in to our shop for an emergency overhaul. We rewound the motor over the weekend to meet the customers rush needs. The motor appeared new when it arrived at our shop, but it appeared that the winding may have been overheated. The customer was sure that they did not overload the motor in service. At this time all "normal" test procedures were done on the motor. None of the standard procedures revealed a problem with the rotor at this time. The rotor appeared to be in good condition as well, from a visual inspection. Due to this failure, and our shops previous experiences, we advised the customer to allow us to full load test this motor on our dynamometer, before it was delivered back to site.

The motor was aligned to the dyno, and ran at full load (350hp) and analyzed using our PdMA MCE/Max tester and vibration analysis. Here, it was crystal clear that the rotor was indeed the problem. At full load, we can do a High and Low Resolution capture to test the rotor. (See Test results of before and after). We then told the factory that the rotor had to be replaced and asked them if we could cut it apart for analysis and they allowed us to do this. We set the rotor up in the lathe and parted the end-rings off. This revealed the huge air pockets that were trapped in the casting, and some of the bars were not attached to the end-rings, due to these huge air pockets. (see pictures). We recieved a new rotor from the factory and then re-tested the motor. This completely cured the problem. This problem is apparent in a lot of new motors and was undetectable until we got the technology to test it. We can detect even minor porosity problems in rotors with this technology.



This image reveals the air pockets that were in the casting due to a factory defect, causing the open rotor. The rotor bars did not contact the end-rings where ever there were air pockets.



The dark ends of the rotor bars are where the airpockets in the endrings were. You can clearly see that there was no connection to the endring.



Another image of the endring as it was machined off the rotor.



Current Signature Analysis FFT of motor with New Rotor.



Demod Spectrum of Damaged Rotor



Low Resolution Spectrum showing Rotor bar problem at Rotor Bar Pass Frequency (Shaft Speed)



High Resolution Spectrum showing Bad rotor.



The Hi Resolution Spectrum taken on the motor after the rotor was replaced. The marker is on Shaft Speed, showing the Rotor Bar Pass Frequency.