Motor circuit analysis

During routine operation of a coal-fired steam plant within a corn milling facility, a 100-hp motor that drives one of the two coal pulverizers was tripping offline shortly after start-up.

12/14/2011


During routine operation of a coal-fired steam plant within a corn milling facility, a 100-hp motor that drives one of the two coal pulverizers was tripping offline shortly after start-up. As a result:

  • The inability to run the No. 2 coal pulverizer placed the plant in a state of reduced reliability.
  • The motor was tripping offline without material loaded into the pulverizer.
  • The initial 30-sec inrush set of data was inconclusive without a baseline inrush set of data with which to compare it.
  • Vibration analysis was detecting increased vibration levels; however, due to the configuration and construction of the equipment, a source could not be isolated.

Figure 1: The load current is captured at the beginning of the run cycle. This capture was at 3:28 p.m. Source: Allied Reliability

Using Motor Circuit Analysis technology, the load current was captured during the first few minutes of operation (see Figures 1 and 2). An initial 10-A oscillating load current was discovered that gradually increased to 70 A before it tripped the motor overloads at 168.9 A. Peak-to-peak time measurements of these oscillations were very uniform at 0.51 ±0.01 sec.

Figure 2: The same load capture was reviewed again 6 min later at 3:34 p.m. to begin to determine where the offline trip was originating. Source: Allied Reliability.

The motor was restarted and a demodulated spectrum captured to further narrow down whether or not this issue could be load related or a rotor issue.

At 1.934 Hz, there was a 2.8 dB peak with a mechanical speed of 116 rpm. The 116 rpm correlated with the 0.51 ±0.01 sec load oscillations captured during the inrush current spectrum. After conferring with the maintenance and repair department, it was learned that the pulverizer’s main shaft rotated close to 2 times per sec. The recommendation was then made for the maintenance and repair department to look into gear misalignment or rubbing associated with the equipment’s main shaft.

Still not convinced that the equipment’s problem was due to a mechanical issue, the client asked to have the load separated from the motor and the motor retested. An additional inrush was captured, which helped the company understand that the problem was a “loading” issue. This confirmed the mechanical issue initially observed.

Armed with useful data, the maintenance and repair department was able to focus its search towards the pulverizer’s main shaft and associated gear reducing section.

What they discovered

The pulverizer and its main gearbox were disassembled. It was found that the final beveled gear was not properly meshed with the main shaft gear, and a significant amount of backlash was evident. This, coupled with a binding skid plate, was causing an oscillation equal to the revolution of its final gear. The gear reduction was 5:1, putting the main shaft at 116 rpm.

Conclusion

Motor Circuit Analysis technology is a nondestructive testing technology that measures inherent characteristics within a motor circuit. The demodulation spectrum is normally used as a trending tool; however, in this case we were able to utilize corresponding data and, through direct communication with knowledgeable personnel, refer our client towards the appropriate section of a complicated piece of equipment to make the needed repairs.

Griesmer is the IR/MCA analyst for Allied Reliability.



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