Ambient temperature for motors

[OMNENG]

Hello Experts,

If a pump motor is designed for 40 C ambient temperature was put in a place where temperate is always above 40 c for more than three months each year;what you think will happen? will the motor get burned? will its performance be affected?

I will appreciate your opinions and comments.

Regards

 

[electricpete]

Unknown. In most cases it is no problem. But to understand it better you would need to give more details:
loading level of the motor
insulation class, rated temperature rise
voltage deviation from nameplate +/- may play a small role in considering the whole problem.
any historical winding temperatures measured in similar application, along with associated loading and ambient temperature information.


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(2B)+(2B)' ?

 

[jraef]

Motor insulation class defines the maximum allowable temperature rise over ambient (plus an allowance for a "hot spot"), but also by using that with the rated ambient (always 40C), you can get the maximum temperature as a hard number to use when operating above the ambient. So for example if you have Class F insulation, that's 105C over a 40C ambient with a 10C hot spot, so the total max temperature is 155C. If your ambient is 60C, then the max rise can only be 95C at a hot spot or 85C overall. So did that motor ever exceed an 85C rise? No way to know without some sort of real temperature sensor in the motor.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[electricpete]

So for example if you have Class F insulation, that's 105C over a 40C ambient with a 10C hot spot, so the total max temperature is 155C. If your ambient is 60C, then the max rise can only be 95C at a hot spot or 85C overall. So did that motor ever exceed an 85C rise? No way to know without some sort of real temperature sensor in the motor

If you knew the loading you could make an educated guess (hence my request for info). You'd have to make some assumptsion. First have to make assumption about ratio of no-load losses to full-load load-related losses (1:1 seems in the ballpark, test data sheet would sharpen the estimate). Assume the load-related losses vary proportional to current squared and that the temperature rises are proportional to the losses. For conservative assumption, assume the "hot spot rise" is constant.

For your example above we can estimate
dT/dTrated = 85/105 >= 0.5 + X^2 where X is fraction of full load current
X <= sqrt(85/105 - 0.5) = 0.55
If motor operates below 55% FLA, I wouldn't be worried….
Constant hotspot rise is a conservative assumption (it would decrease at lower load)
Temperature rise proportional to losses is accurate for conduction temperature drop (through the depth of the insulation) but I think it's conservative for convection.
If there is service factor specified we could sharpen the pencil to get a little higher limit.

There's a lot of assumptions in there, but would get you a rough idea. And for benefit of op, none of this is an exact science anyway… it's not like the motor is guaranteed to operate reliably at 1 degree below rated and fail at 1 degree above the rated temperature. For older thermoplastic insulation systems (asphaltic) the aging was predicted by Arhennius… 10C above rated temperature only means the insulation system thermally ages twice as fast as it otherwise would during the period of that overload (so short duration overloads are not huge concern). Newer thermosetting insulation systems (epoxy and polyester) tend to have more of a threshhold temperature where cured resin chracteristics changes so there is not as much basis for predicting capability beyond rated without knowing something about the insulation system.


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[waross]

How much above 40 C and for how man hours per day?
What is the loading profile of the motor?

Some thoughts for correction by others.
40 C = 233 K
The heat radiation from the motor is related to the 4th power of the temperature.
Motor heating is related to the square of the current.
If the motor current is 90% of rated current, the heat generated will be approximately 80% of the allowable heat.
Over sizing the motor may allow operation in an ambient higher than 40 C without exceeding the allowable hot-spot temperature.
We do not have any data as to the load profile nor the actual ambient temperature expected, and so at this time can make only general statements.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I would say heat conduction associated with the temperature rise is dominated by conduction and convection. Heat is conducted from hot copper across the electrical insulation (which is thermally insulating) to the core/frame. It is removed from the core/frame primarily by convection. Radiation would typically not play a significant role.

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