140 deg ambient Motor

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We are moving a small centrifugal pump/motor that normaly runs @ 14-18 hz via a VFD. It will be moved to an area that has at least an ambient of 140 F. Run time is about 6 hours max per cycle.

My local motor supplier did not seem to concerned with the cooling for this motor. Should I be?
 
We are moving a small centrifugal pump/motor that normaly runs @ 14-18 hz via a VFD. It will be moved to an area that has at least an ambient of 140 F. Run time is about 6 hours max per cycle.

My local motor supplier did not seem to concerned with the cooling for this motor. Should I be?


Motors typically rated for 40 Centigrade ambient and that's about 104 Farenheit. Of course you should be concerned. The motor life will be shortened and for trasnformers the rule used to be, you half the life with every 10 Centigrade increase. Of course if the motor is not loaded to its nameplate and it is being run at reduced speed, the reduction will not be that drastic, but it could still be substantial.
 

Ragin Cajun

Senior Member
Location
Upstate S.C.
If the motor is running at reduced load, but full speed, perhaps.

BUT, it's running at a slow speed so the internal fan is about useless for cooling. Hopefully, there is a separate cooling fan?

RC
 

Phil Corso

Senior Member
Laszio, if you don?t mind the informality ?
Continuously operated motors have a rated temperature-rise above ambient! Ambient is ?defined? as the external air enters the cooling air-stream to the motor. ?Typically? the ambient temperature is also listed on the nameplate.

Ptonsparky...

Even at lower speed, the actual T-rise must not be exceeded. If you want a measure of its load, prorate the actual T-rise against it rated T-ise

Regards, Phil Corso

 

Jraef

Moderator, OTD
Staff member
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San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Good question...I will look for the information.
It may not say that specifically, but it will give an 'Insulation Class", which is what tells you what you need to know to start with.

7536.MotorInsulationClass.jpg

140F = 60C. Base temperature is 40C, so at 60C ambient you have already consumed another 20C of rise capability. What you don't know here however will be the rise you are currently expressing on the motor now. Your motor will be raising the temperature of the windings at those slow speeds because of a drop in cooling effects, harmonics in the VFD output etc. That will likely remain the same when you move it, so if you can find out what that is, it will tell you whether or not the move is safe.

Example: lets say your motor is currently in a 40C environment (102F) and you have Class F insulation (very common). But let's say the application is raising the internal winding temperature to 145C, that is a 105C rise over ambient. Class F insulation is 40C ambient base, + 105C rise means it's fine for up to 155C, you are at 145C, so no problem now. But if you move that motor to a 60C environment, then the rise is still 105C over ambient, now you are at 165C so you are exceeding the maximum allowable temperature of the insulation. There is a "hot spot" allowance of 10C (shown as "Thermal Margin" in that chart), but you can expect a shorter motor insulation life now. The "rule of thumb" on that is that for every 10 degrees C rise over maximum, you cut the motor insulation life by 1/2.

Same scenario, but Class H insulation, no problem.

The difficulty is, you may not know what the INTERNAL winding temperature is right now, unless you have performed a Resistance Temperature Test or have some sort of temperature sensor inside. IR scanning from the outside is not a good indicator of the internal temperature, but one thing it can tell you is a MINIMUM temperature. In other words if your IR scanner sees that the case it 105C now, it's a sure bet that your winding temp is AT LEAST 105C now. It's likely 20-30C higher though.

If you do not have internal sensors, the only way to determine the motor internal winding temperature is to do the Resistance Temperature Test, which measures subtle differences in motor winding resistance between cold and hot to calculate the temperature rise. If you look at this link, it provides a decent description of the procedure, about 3/4 of the way down the page.
http://www.leeson.com/TechnicalInformation/hottopic.html
 
Laszio, if you don?t mind the informality ?
Continuously operated motors have a rated temperature-rise above ambient! Ambient is ?defined? as the external air enters the cooling air-stream to the motor. ?Typically? the ambient temperature is also listed on the nameplate.

Yes, and as I said that ambinet teperature is typically 40 Centigrades or 104 Farenheit, so if you intend to operate the motor at a higher teperature than that, 140 Farenheit(?) according to the OP or 60 Centigrades, then yes, the motor HP should be de-rated.
 

mike_kilroy

Senior Member
Location
United States
Another tidbit you can use to help figure if the new ambient will present a problem is to use the simple thermal temp rise equation:

Tnew=Trated*((tmax-tamb1)/(tmax-tamb2)^.5

where Tnew=new torque rating with different temp rise
. Trated=torque rating at given temp rise
. tmax=Max winding temp
. tamb1= one ambient temp
. tamb2= other ambient temp

This will work for torque or HP or currents.

Example:

10nm continuous rated motor at 40C with max temp of 155C. What is new rating in only 25C ambient?

T=10*((155-25)/(155-40))^.5
=10*(130/115)^.5
=10*1.06 or 6.3% higher
=10.6

So at least you can see the increase or derate percentage based on your new ambient temp.
 

mike_kilroy

Senior Member
Location
United States
So I would say you will loose 8% of your nameplate motor rating going to 60C temp from (105/125)^.5= .92. If it normally ran in 25C ambient area, it had about 6% more than nameplate, so total difference will be about 14% less capability after move. And being TENV it is oversized housing so it does not rely on internal fan to cool down to rated 2hz, so no additional problem there. A clamp on ammeter reading at present location would help a lot to know the present load to decide if this additional reduction is an issue or not.
 
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