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.



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
Strategic outsourcing delivers efficiency; Sleeve bearing clearance; Causes of water hammer; Improve air quality; Maintenance safety; GAMS preview
World-class maintenance: The three keys to success - Deploy people, process and technology; 2016 Lubrication Guide; Why hydraulic systems get hot
Flexible offshore fire protection; Big Data's impact on operations; Bridging the skills gap; Identifying security risks
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me