It is Not Always about Loose Contacts

by Ahmed Osman Mohamed Hamoudy
Electrical PM & PdM Engineer
Cemex, Egypt

In modern industries, there are many predictive maintenance techniques that can be utilized to avoid an unexpected failure. At CEMEX,  we count on those techniques and deploy them in such a way as to get the most use from it, and avoid any undesirable consequences. IR thermography's use has become widespread in cement plants; application areas typically include rotating kilns, insulation, mechanical, and electrical systems.

This article will explore the uses of IR thermography, specifically as part of the Motor Control Center systems predictive maintenance routine, including typical electrical panel failure points (contactor, terminations, feeding breaker, fuses, cables, etc...).

Figure 1. Rotary Kiln

One of the Clinker Rotary Kilns (4,500 Tons per Day) is supposed to be available for operation not less than a year, which is a challenge of course, to guarantee continuous operation for the kiln system for this time without any unexpected shutdown.

Figure2: Controlling DC motor speed

This kiln is driven by two DC motors (320 Kw each) and VSD (Variable Speed Drive) is used for each motor in order to be able to control the kiln speed and to meet the clinker production process requirement.

Figure3: Variable Speed Drive Panel

The electrical distribution system is made up of 6.3Kv and 525v feeding the two kiln DC motors via the VSD panel shown in Fig.3

If we take a close look inside the variable speed drive panel, many failure points (marked in Fig. 3) could put the kiln operation at risk if a loose contact or degradation occurred in one of the electrical components. Because we are dealing with equipment in operation, and with regard to the safety aspects of live work, we incorporate one of the most useful tools available for the PdM team, our IR camera. The camera can easily detect problems associated with heat transfer through a “live” IR inspection on operational electrical systems.

During our bi monthly IR inspections, according to the predictive maintenance schedule, and thanks to our IR camera (FLIR P640) and ThermaCAM Reporter, we found high heat radiation from one of the main contactors in the DC drive feeding Kiln 02 Main Drives as shown in Fig.4 and 5.

Figure 4. VSD contactors IR scan	Figure 5. VSD contactors Digital image

As shown in the analysis table (Fig.6), the thermal radiation difference between the two adjacent phases reached more than 35 °C, is not acceptable, in addition, 88 °C as thermal radiation is not normal in such systems.

Scan Date	28/12/2010
Emissivity	0.78
Atmospheric Temperature	31.3 °C
Ar1 Max. Temperature	88.1 °C
Ar1 Max. Temperature	48.2 °C
Dt1 Value	39.9

Figure 6. Analysis Table

The following parameters were used as references to analyze the thermography results:

1. Feeding voltage and current.
2. Field and Framework voltage.
3. Field and Framework current.
4. Motor speed.
5. Load percent.

All of these parameters were normal. Now it was time to take immediate action and contact the corrective maintenance coordinator on the necessary corrective actions. As a thermography specialist, I needed to provide him with the problem and the recommended actions to be taken.

Figure 7. Magnified IR image

While taking a close look at the IR scan of the contactor, we found that the heating not just from the bolt and connection, but the maximum heat radiation point is under the bolt; also the thermal radiation also affected the contactor bottom terminals.

So we cannot just tell the responsible coordinator to check the bolt tightness and the condition of the connection components, the internal contacts should be checked because it consists of two parts (fixed & moving) contacts, of course we cannot see inside due to the low emissivity protective fiber cover in front of the contacts.

Figure 8: The contact

During the kiln shutdown we arranged for the responsible coordinator to prepare install a new contactor to replace the faulty one. As expected, the contact condition (Fig.8) was very bad. The internal mechanism responsible for the switching process was also in poor condition for this phase, and eventually caused high heat radiation to be emitted from the marked phase.

The heating in electric equipment is an indicator of its operative state, they have normal heating produced by the electric current flow and they also have abnormal heating produced by equipment conditions that increase the resistance to current passage and increases the heat generation in those points. In our case the contact condition was abnormal due to the internal mechanism which needed to be maintained.

But the area coordinator asked me why I did not find this problem earlier since I scan this contactor every two months. I told him that the temperature difference between one and another case can be minimal in incipient problems, and it will increase when the problem becomes bigger. As our perception capability of hot spots is limited to high temperature levels, we cannot always detect abnormal heating problems from their beginnings. Fortunately, by carefully studying the inspected equipment, we can figure out the best routes and inspection frequencies. By good planning, downtime was minimized to the max.

Figure 9. New and old contact

After installing the new contacts (Fig.9), and tightening the screws and bolts, we returned the system to operation, and added this system to the condition monitoring program.  The problem has been eliminated, which proved that all the actions we took were the right ones: this made the corrective coordinator and our BOSS are very happy.

Figure 10 illustrates the temperature pattern after the repair.

Date	05/01/2011
Emissivity	0.78
Atmospheric Temperature	33.5 °C
Ar1 Max. Temperature	44.7 °C
Ar2 Max. Temperature	44.0 °C
Dt1 Value	0.7

Figure10. IR image for the new contactor after operation for a week

Conclusions

By the early detection of the high heat radiation and imbalance found using our best technology on the Predictive maintenance team, many undesired scenarios were avoided:

• Damage to the entire contactor.
• Damage to the other adjacent electrical components in the variable speed drive.
• Emergency kiln stop for not less than 10 hours.
• Losing about 2500 Tons of clinker ($306,000 USD).

The planned thermography schedule (frequency & duration) proved its validity in detecting fault zones early, which will keep us safe from any unbudgeted shutdowns.

It has to be mentioned that thermography reporting and feedback should be accurate and hit the problem in order to take the right corrective actions and to solve the problem completely without any unlikely consequences.

Only one hour of stoppage and losing about 188 tons of clinker ($23,000 USD) , was the only cost paid in this regard.

Ahmed earns 11 ITC certification renewal credits for his article.