De-rating AC electric motors for ambient and altitude 1

[JB160]

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!

 

[dpc]

Altitude derating is covered in NEMA MG-1. It is temperature-dependent, but the maximum ambient is 40 deg C as I recall. The derating reduces the allowable temperature rise at sea level as the altitude is increased above 1000 meters, IIRC.

 

[micalbrch]

I know from European motor manufacturers that 5 °C higher ambient temperatur than 40 °C reduces the nominal power by 5 %. That's valid up to 60 °C. At 40 °C ambient temperature the nominal power will be reduced by approx. 8 % for every additional 1000 m altitude up to max. 4000 m. For isolation class B motors full nominal power at altitudes of 2000, 3000 and 4000 m is available at ambient temperatures of 32, 24 respectively 16 °C on these altitudes.

 

[dpc]

NEMA MG-1 has a graph showing loss of life of the motor as winding temperature increases. Ambient temperature by itself is not really predictive since it is the actual winding temperature that is the concern. It is possible to purchase motors nameplated for ambient temperatures above 40 deg C. I assume this is just a larger motor, but there may be other modifications as well.

 

[rhatcher]

The standard ratings are for 40C and 3300ft. I do not think that NEMA MG-1 provides for an increased temperature rise at altitudes less than 3300' but I may be wrong. They do require derating of allowable temperature rise at altitudes greater than 3300'. I do not remember the formula off the top of my head but can provide it.

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.

 

[JB160]

I'll elaborate a little more on what it is I'm working on and what it's for. I work for an air-cooled heat exchanger company and some of our units require electric motor drives. We currently have an excel checklist we use to verify/check certain items like design pressure, design temperature etc. One of the checks is for the electric motor, however we have only been considering the temperature when doing this check. I am redesigning / updating this check list and through some research it appears that we need to give more consideration to the altitude as apposed to the ambient design temperature. We factor in the hp draw from the fan, ambient & elevation. If needed we derate the motor or in some cased lower the pitch on the fan to get the hp low enough so the motor hp will be ok. I would like to be able to write a formula that would calculate the motor derate based on these conditions: motor hp, fan hp, ambient & elevation. Hope this makes sense and I appreciate all the help.

 

[JB160]

We currently use the following formula to determine if a re-rate of the electric motor is needed.

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.

 

[JB160]

That's supposed to be .00277 in the formula not 00277

 

[ScottyUK]

The de-rating as altitude increases is due to the lower air density, which means that the shaft-driven fan is able to extract less heat from the motor due to the smaller mass of air being moved across its fins. This problem is almost certainly balanced by the reduction in work done by the motor in driving the fan pushing air through the fin-fan cooler, because the motor will run cooler when doing less work. Unless you intend to modify the fan itself to compensate for the lower air density at high altitude then you may be compensating for a problem which doesn't exist in this application.

 

[wolf39]

Scotty:

Your post is offering a "homerun" explanation. Enjoy the "Prussia Dortmund vs. Bavaria Munich" game.

Wolf