All:
For a given minimum motor surface temperature, are there any guidelines for estimating the minimum winding temperature for the purpose of comparing the value with motor insulation class?

http://www.usmotors.com/default_service.htm

Select FAQ, Then Motor Frame Surface Temperatures, and you will see:

"Question: What is considered an average motor frame surface temperature?

Answer: Table 3 gives average winding temperature rises for various motors. Frame surface temperatures are typically 15 - 20 degrees centigrade less than the average winding temperature, depending on the size and type of motor along with standard manufacturing variation. For example, a motor designated class F with a 1.15 service factor has an allowable average winding temperature rise of 115 degrees centigrade. This motor has a total temperature of 155 degrees centigrade with the inclusion of the 40 degrees centigrade maximum allowable ambient temperature. Therefore, this motor frame surface temperature could reach 135 - 140 degrees centigrade, as affected by ambient and load conditions. "

Thanks Pete, Check out the Thermographs attached

EF1104_Exhaust_Fan_Motor_Thermographic_Reportpost.doc (601 Kb, 49 downloads)

Check out http://www.motorsanddrives.com they have a very good article listed in their Cowan Papers at http://www.motorsanddrives.com/cowern/motorterms4.html

Thanks Bob.

Gentlemen:

Great information. However, caution must be taken.

The US Motors information assumes the temperatures at contact points between the core and frame. There are, however, a great number of things that effect the surface temperature of an electric motor:

1. Above about 50% load, the temperature rise (winding temperature) will increase almost linearly with load;

2. The surface temperature will also depend on the material (ie: cast iron, rolled steel or aluminum) and where, how and how much of the material is in contact with the surface.

3. Airflow due to fans, etc. will cause a change to the surface temperature.

The best recommendation, if relying upon infrared to test motors, is to trend motor temperature and investigate faults further in motors that show significant changes in spot or overall temperature. I would recommend that you do not rely upon surface temperature readings in order to determine the winding temperature.

Howard

Second comment:

In the infrared report, it is noted that the 'high temperature at the center of the motor is typical of a motor that is heavily loaded.'

Actually, this is a rolled-steel, open drip proof (ODP) motor manufactured by US Electric (see previous note from ElectricPete). The stator core is located directly against the frame and airflow is maintained within the stator. The infrared image that you see is the exact location of the stator core and appears to be evenly loaded.

Before reporting that this motor is overloaded, I would recommend taking current readings from the control.

Also, I would recommend a second infrared scan of the connections in question. It appears that you may be receiving a false reading due to the angle of the contact surface. With the high temp, I would expect to see the incoming wire, at least, to show high temperature as well.

Howard

It seems that using one tool/technology to detect a "potential" fault might need other tools/technologies/tests to substantiate or confirm that a fault exists. How far do you want to "stick your neck out" with just an IR temperature measurement when recommending corrective actions?

The only time data from a quick scan of a motor can be used is where it applies to the bearings and even then findings need to be confirmed with another technology. I totally agree that motors need to be trended, and that external temperature alone is not sufficent to make conclusions about motors. Sources like http://www.usmotors.com/default_service.htm and http://www.motorsanddrives.com/ do provide very valuable information about motor temperatures and construction in general and give us an understanding of what we are looking at, and this is very important for thermographers.

We should also NEVER use the normal severity criteria applied to other electrical components, as these are not intended for use with motors.

Finally, recommendations with motors should always require the use of other technologies for confirmation.

Thermal Vision
bob@thermalvision.ie

The text on the thermographic scans indicate current was close to service factor loading at the time of the scanes. This seems consistent with the temperatures.

Don't know the duty cycle or the insulation class and as pointed out any conclusions about winding temp are a rough estimate.

The overloads heaters appeared to show no abnormal patterns.

According to NEMA standards, if we operate FLA for sustained periods, we may expect reduced life. For small motors it seems a relatively common practice to operate abobve FLA, perhaps due to assumptions that there is a large thermal margin built into small motors, and small motors are not as critical as large motors.

Sorry guys.. I left out some information:

The insulation class is "B" 130 C. S.F. 1.15.
The load was at the limit of the service factor.

My recommendation was to do a motor circuit evaluation to determine the condition of the winding, and to shed some load if possible.
The docs you see were not the final report.

They also have a back up motor that they switched to.. we will also do therm on that notor and the connections, and an MCE.

MMM

Darn.. that stray finger I have..."Notor??" MOTOR!!!

Howard:
I too found it suprizing how little heat was transferred to the conductors from the heater elements. Especially considering that from the thermograph it appears that the "Hot Spot" is metalic with presumably low emmissivity. That would put the true temp much higher and thus increase the expectation of how much heat would be conducted through the conductors.

What I am trying to understand from your comment is how the angle could give a reading higher than true temp. A reflection would have to be from some source, right? I could not find a 90c potential source for a reflection.

Could you (Howard) explain a little further what you meant? I'm getting that feeling I'm missing something important!
I don't think this is a black body cavity, although it is hard to be certain.

I don't see any problems on the overload heaters. I think you are seeing the cavity effect and the 97C is close to the true temperature of the heaters (they are heaters after all). (actually you might be reading a little bit higher than actual since the cavity may approach 1.0 and you are set to 0.092). There may be some low-emissivity materials close by which are also hot but not showing it. By the time you get to the cable you probably are reading roughly correct temp again. So it goes from 97C to 40C in that short distance. I agree it's a little strange but looks like other ones that I have seen. The heat from the heaters doesn't usually reach all the way to the cable.

Well that was my next question.. how hot do heaters get? What is "normal"

ElectricPete:

Thanks for the comment/response to Martin. Basically the same comment I would make.

Also, as there have been a number of comments on multi-technology (in this thread and others) I am posting the article on the "Multi-Technology Approach to Motor Diagnostics" that was assembled early in 2004. I have changed the contact information to my present info.

If there are any questions, please feel free to email: howard@motordoc.net

Howard

The_Multi_technology_approach_to_motor_diagnostics.pdf (154 Kb, 29 downloads) Multi-Technology

Martin – you asked about normal temperatures for overload heaters. I have noticed often the cavity in the heater is the hottest appearing thing in the picture (partially due to cavity effect and partially due to high actual temp), ,but I don’t pay attention to it because it is not a connection.

After your question I went back and took a look at some old images we have saved. I found two where the heater cavity was ~ 60C and two where the heater cavity is ~ 100C. I’m sure it depends on loading and other factors. Also some of my images may have been too far away.

Again, I don’t try to judge anything based on heater cavity temperature, but I am attaching two of the images where we saw 100C for your info. (The region-of-interest boxes in these images are not on the heaters)

This message has been edited. Last edited by: electricpete,

HeaterShots.ppt (700 Kb, 24 downloads)

Thanks again Pete and Howard, I do want you to know I follow-up on these.