De-rating AC electric motors for ambient and altitude
De-rating AC electric motors for ambient and altitude
(OP)
I am working on a spread sheet the will de-rate an electric motor if used in high elevations or high ambient conditions. Is there a rule of thumb for de-rating electric motors for use in elevations above 3300' or for use in ambient temps above 104F (40C)? Does it have to be a combo of both before you de-rate? Do I have to de-rate if my ambient is 110F or 115F if I am at 1500' elevation? Likewise do I have to de-rate if I am at 5000' but the ambient is 95F? Any help would be appreciated.
Thanks!
Thanks!





RE: De-rating AC electric motors for ambient and altitude
RE: De-rating AC electric motors for ambient and altitude
RE: De-rating AC electric motors for ambient and altitude
RE: De-rating AC electric motors for ambient and altitude
I also think that NEMA allows for an increase in temperature rise for ambients less that 40C and, alternately, requires a decrease in temperature rise for ambient temperatures greater than 40C. I am not certain because I may be confusing the NEMA standard with the API specification.
In either case, the general rule is that the combination of ambient temperature and allowable temperature rise should equal 120C (Class B) unless otherwise specified by the customer. For example; the standard ambient of 40C plus the standard temperature rise of 80C, the standard marine ambient of 50C plus a temperature rise of 70C, and the occasional odd (I have seen this) ambient temperature of 46C plus a temperature rise of 74C.
Of course, the information on allowable temperature rise is useful from the design standpoint but it may not be useful to you if you are interested in determining how to derate existing NEMA rated motors for non-standard ambient temperatures and altitudes.
If you let me know which case you are looking at (new design or existing) I will review MG-1 on Monday and respond with an answer after that.
RE: De-rating AC electric motors for ambient and altitude
RE: De-rating AC electric motors for ambient and altitude
Fan HP = 34.93
Motor HP = 40
Ambient = 110
(((1-(((110))-104)* 00277= .98338 (Since our ambient is over 104 the motor hp is derated by this amount)
40*.98338 = 39.3352 hp
We then calculate the HP draw of the fan to the derated motor hp
34.93 / 39.3352 = .888 so the fan is drawing 88% of the available derated HP. if this number was to be above 90$ we would try to lower the hp draw from the fan or more than likely go up in hp on the motor.
I don't even know if this is the correct way to derate the motor, the company I work for has been doing this forever but that doesn't make it right.
I have no idea where the constant .00277 in the formula came from.
RE: De-rating AC electric motors for ambient and altitude
RE: De-rating AC electric motors for ambient and altitude
RE: De-rating AC electric motors for ambient and altitude
Your post is offering a "homerun" explanation. Enjoy the "Prussia Dortmund vs. Bavaria Munich" game.
Wolf