Message Board Thread - "Thermal Overload Heater in MCC severity criteria"

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Thermal Overload Heater in MCC severity criteria Scott Willits 4/3/2004
I am working in an older hog fuel cogen plant that has what the electrician refers to as 1940's technology in most of the MCC's -- they are using Furnas overload heaters that really do heat up when under significant loading. In many cases I am finding contacts up to 100°F over ambient. With indirect readings close to that on the surface of the covers, I'm concerned that these things are approaching critical. This is their first thermal scan so we don't have a baseline to work with, but they don't seem overly concerned.

I'm not sure how to interpret this. I think I should stick with the NETA severity criteria for conductors themselves, but how do I determine what's really a problem with this kind of componentry? Any discussion at all will be useful and appreciated.

Scott Willits
 
Re:Thermal Overload Heater in MCC severity criteria Carl M 4/4/2004
Thermal overload elements are supposed to be hot, that's how they work. For a three phase motor all three phase components should have a similar thermal signature and the same current measurement. The conductors and terminals must be within their rating, which could be either 60ºC or 75ºC. If the ratings are exceded, you have a problem. If there is a considerable difference of temperature between phases, you have a problem. In this application if you are going to use temperature differential for determining severity it would be the differential between similar targets, for example, the reference temperature could be a baseline of normal operating temperature for that target or another phase terminal of the same component, not ambient temperature.
 
Re:Thermal Overload Heater in MCC severity criteria Scott Willits 4/5/2004
Thanks for the response, Carl.

I'm aware I need to worry most about imbalances between phase legs and conductors outside their ratings...that's the easy part. But what I can't seem to locate is information on the rated upper limit temperatures the heaters themselves are supposed to operate at. It's difficult to establish a baseline since we've got a whole bunch of these things and the loads on them vary quite a bit. We did current checks on the hottest ones and the electrician didn't seem too concerned, but I'm just looking for some threshold value where I can say, "That one's too dang hot."

Are people still seeing a lot of these things out there, or is this particular plant just more stubborn than most?
 
Re:Thermal Overload Heater in MCC severity criteria Carl M 4/6/2004
When they get "too dang hot" they trip the overload relay. Sorry I don't know the temperature range these devices operate at but I imagine that it varies with design and manufacturer. They (manufacturers) may have the information you're looking for.
 
Re:Thermal Overload Heater in MCC severity criteria Thermasearch 5/5/2004
I would not get excited about your readings just yet and would suggest that if you do use NETA, that you use it in comparison to a similar component under similaar load. Use of NETA values to ambient might be a "cry wolf " situation. Equipment produced in the 50's and earlier were built to a rougher set of requirements than those interpreted as additional to standards like the MG1(motor generator 1)of today. This does not mean that you are seeing a normal occurance, but based on my experience with similar systems in the field, you may be.

I might suggest establishing a baseline if you have not done so and continue to monitor closely for a defined period. If there is no significant variations that can be attibuted to equipment(associated with the parts in question)use you may not be seeing a fault and the data collected will be valuable in future scan reporting of suspected faults in similar systems.

Please keep us posted on results and thank you for sharing your concern with the rest of us.

Mike
 
Re:Thermal Overload Heater in MCC severity criteria Carl M 5/5/2004
The image in my last post appears to be a loose fastener on the bottom of the right phase overload heater. Although the temperature was not critical at this time(140ºF) there was a rise over the temperature of the same spot on the other two phases (105ºF). The overload did not trip as the motor's load was not close enough to full capacity and or the motor was not run long enough to reach temperature equalibrium. These are a few of the problems with comparitive analysis between similar pieces of equipment. (Actual load, duty cycle, and running time)

In this report I recommended that the conductors and heaters be removed and inspected. This would show any loose or stripped fasteners as well as damage that could have been caused to the conductor, terminals or overload unit by the heat.
 
Re:Thermal Overload Heater in MCC severity criteria jvoitl 5/5/2004
Several years ago I attempted to determine "normal" temperature of heaters. The Allen-Bradley heater I tried operates in much the same manner as others. In it, there is a small hollow shaft with a rod soldered inside. When the heater warms the solder enough to melt the shaft turns and trips the overload contact. The melting point of the solder in these units is 278 degrees F. The heating coil around the tube has a constant I.D. but the O.D. varies according to the amperage rating of the heater. Thicker wire for higher currents. The heating coil is inside a bakelite case of the same size regardless of current rating, so in the higher rated heaters the coil is closer to the outside case and the case will get hotter. I concluded there is no real way to determine a proper temperature. I ignore them except to see if all three are about the same temperature. I have found motor current imbalance and also where someone made a mistake and the heaters were not all the same size.
If they get too hot they simply open the contacts and shut the motor off. If for somereason they fail, nothing but the heater is damaged and it is only a couple of dollars and two screws to replace.
 
Re:Thermal Overload Heater in MCC severity criteria Big Bob 5/7/2004
One, you could contact the manufacture to see if they have any criteria on the operational levels of O/L heater.
Two, check your voltage Drop readings across each heater should be less than 2.5 volts
Three, what is the currrent readings on the circuits.
 


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