Altitude question

Hi, got a question about electric motors and altitude. I work for a water company and we have pump stations at elevations from 6200ft to 7400ft. I have been told that when sizing electric motors one has to take elevation into account because they lose a set amount of power per thousand feet. I can understand this with internal combustion engines, but have no idea if it's true for electric motors, (though it seems to be) and the biggest thing bothering me is WHY???? I haven't figured a logical reason an electric motor should lose HP at altitude. Any ideas??

Eric

Altitude derating 4% per 1000 ft elevation above 500 ft. There is reduced ability for heat dissipation in surrounding ambient with increasing elevation.

--s falke

Cooling? Air gets less dense the higher you go, and most of those motors are air cooled. TTYL

I agree with you that air gets less dense as you go higher.

But going up the mountain I live on my old car used to overheat till I got to about 3000 feet and then it would run cooler . Above 3000 feet, paticularly between 3500 and 4500 the road was much steeper. Can you explain this. . . I DO NOT FOLLOW MANY OF THESE NEWS GROUPS To answere me address mail to snipped-for-privacy@aol.com

Ahh.. the temperature switch on your radiator's thermo-fans is set too high??

Cameron:-)

Your power output of the engine dropped due to the increase in altitude. In addition, the fuel air ratio changed (hint on older car, EFI would compensate). A richer fuel air ratio burns cooler.

Makes sense.

BUT were it possible to place an upper limit on the ambient temperature, is it possible to reclaim the power you paid for?

AT 10k feet, you are 40% down!

No I hard wired the fan on. . . I DO NOT FOLLOW MANY OF THESE NEWS GROUPS To answere me address mail to snipped-for-privacy@aol.com

on an 86 subaru? . . I DO NOT FOLLOW MANY OF THESE NEWS GROUPS To answere me address mail to snipped-for-privacy@aol.com

altitude.

Well, I would assume an 86 subaru must have had a oxygen sensor and thus compensated for the altitude. I assumed you meant OLD, not old.

Then it must have just been the reduction in power due to the decrease in air density.

I could never figure it out. All I know was that at 4000 feet the car still ran hot but it did not overheat. I might have to stop a couple of time to let it cool off till 4000 feet.

But the solace was, it never overheated running down hill ;-) . . I DO NOT FOLLOW MANY OF THESE NEWS GROUPS To answere me address mail to snipped-for-privacy@aol.com

You mean the capacity. That would make sense, since the motor's rating depends on dissipation at some maximum ambient temperature.

While dissipation of the heat (stator, rotor and bearing) is related to ambient temperature, it is also directly linked to air flow and air density. Just changing the ambient isn't enough. Sure, you might dissipate slightly more with a lower ambient, but increasing the cooling aiflow (both externally and internally) can have a greater effect. Most standard motors are rated for minus 20 to plus 40 Deg C ambient, up to 1000 m elevation.

Also keep in mind that most industrial motors are wound with one class of insulation, but to increase operating life, actually run at the next lower insulation class's temperature rise (ie: F insulation with a B temp rise).

Sometimes this cushion is used to accomodate the extra elevation.

But the end result is still the same: hotter running motors have lower operating life.

If you are purchasing a new motor for the application, it is possible for the manufacturer to take into consideration the elevation, and make a unit that is not de-rated. I'd been involved in many pieces of equipment going to the copper mines of South America at 11,000 feet.

(Also, higher rotor and stator temperatures can cause premature bearing failure since the heat reduces the effectiveness of the lubricant)

tg

Can't they use a teflon or some other high temperature lubricant? . . I DO NOT FOLLOW MANY OF THESE NEWS GROUPS To answere me address mail to snipped-for-privacy@aol.com

Higher temperature lubricant is a band-aid. Bearing life is related to operating temperature and it's better to keep the bearings cool.

Bearing heat is developed not only through the friction of rotating, but also picks up heat from the rotor (via the shaft), and the stator (via the circulating air, and/or the frame). IEEE-841 has a paragraph specifically relating to allowable bearing temperature.

Also note that the shaft elongates with heat, and the "slop" in the bearings may be taken up by too much heat and put them in a bind. Another contributor of early failure.

tg

Aren't there high temp motors or are they too expensive.

And what about air cooling?

I have seen motors with built in fans to keep them cool. . . I DO NOT FOLLOW MANY OF THESE NEWS GROUPS To answere me address mail to snipped-for-privacy@aol.com

Liquid cooling could easily solve this problem?

Of course you have. This is exactly the type of motor most are talking about. But the density of air is an important factor in the cooling capability. And the maximum temperature achieved internally in the windings/insulation needs to be limited for maintaining longevity of the equipment.

Now, make the conductors out of a better conductor than copper, or provide more intricate cooling system to remove the heat and you can even 'uprate' the equipment.

Take the time/effort to enclose the existing motor in a larger casing, filled with hydrogen. Strengthen this casing and pressurize the hydrogen to something like 60 psi. Add coolers to cool the hydrogen with a water source and you can more than double the rating (be sure to upgrade the leads supplying the motor). This is the sort of thing done with large electrical generators. But is it practical for all the motors you want to operate at high altitudes?? Probably not ;-)

daestrom