Effects of altitude on large motor design - need input 2

[ddear]

I work for a company that uses large motors (up to 37,000HP)and know that high altitude affects the motor design. Can someone provide some more detailed information on this topic?

 

[electricuwe]

Two issues that generally apply to electric equipment at high altitude come to my mind:

- increased clearance requirement due to lower dielectic strength of air at low pressure, see IEC standards for this, e.g. IEC 60664-1

- different design for cooling fans to move the required mass of air despite of the different specific density

There might be other more specific issues e.g. with bearings or their lubrication.

 

[cranky108]

I'm not sure of larger motors, but large generators typically are encased and enclosed with a hydrogen blanket (at least the ones we have here). So there is no altitude issues with them. However we do use higher creapage bushings on the GSU's.

What electricuwe said above sounds reasonable for open motors, but I don't believe motors as large as you describe would be open.

 

[ddear]

I think there are cooling issues and I'm pretty sure that more copper has to be added to the windings so the motor basically is oversized for the application apart from the high altitude. I'm wondering at what altitude this becomes a consideration and to what degree are the original parameters (current, air pressure, etc.)affected.

 

[wolf39]

For up to 1000 m altitude IEC standards don't call for special measures. For altitudes above 1000 m clearances for high voltage windings have to be increased.

Motors of the size mentioned usually have closed cooling circuits but the motor as such is still connected with the outside atmosphere, i.e. not completely encapsulated like H2-cooled turbogenerators. The motors in question are equipped with water-to-air heat exchangers. Here standards allow for two options. First, the permissible operating temperatures (standard is 155 degrees C for class F insulation) have to reduced according to altitude. This means more copper to the windings. Second, the standard cooling air inlet temperature (standard is 40 degrees C) has to be reduced according to altitude. Thus max. operating temperatures decrease accordingly.

In general, however, most OEMs would design well below the permissible operating temperatures as defined by IEC. Instead, the operators specify temperature limits which the OEMs then have to guarantee.

Wolf

http://www.hydropower-consult.com

or the standard cooling air inlet temperature (usually 40 degrees C)

 

[ddear]

I should make a clarification to say that the actually motor in question is not the high or medium voltage motor but a auxiliary induction motor of 160kw, 380V, 50Hz.

 

[wolf39]

ddear:

Thank you for confusing the forum by mentioning motor outputs of up to 37000 hp. All forum members are professionals and expect to deal with posts professionally phrased. Now that we know that the motor in question is a standard motor of 160 kW we still are unable to properly address this matter. Be kind enough to let us know the site altitude of the motor you have in mind.

Wolf

http://www.hydropower-consult.com

 

[ddear]

wolf39:

Thank you for your sarcasm! I am a professional working for a company who is the world leader in providing equipment in the pulp& paper/mining industry all over the world however my expertise is in instrumentation and automation, not motors. I thought a professional forum would be the best place to get expert advice on a subject with which I am not so familiar to provide the best information to my customer.

The mention of the potential load, in parentheses, was just to substantiate the scope of the equipment my company supplies (I don't work for Radio Shack)and the information given to this point will still be of much interest and great use to me.

Realizing that my initial inquiry could have been a little misleading, I posted the clarifcation. I apoligize for not having a PHD in professional writing. I will not post in the future!

 

[wolf39]

ddear:

I'm sorry to hear that my sarcastic reply possibly will discourage you to submit other posts in the future. I ask you to re-consider this and also ask you to accept my apologies.

Up to your post dated 16 Dec 10 7:42 I had the feeling that your original post was answered thoroughly. However, had we all known that we are dealing with a 160 kW standard motor our replies would have been different.

I joined the forum some time ago and it is my experience that occasionally questions are raised with little or no useful background information for the prospective respondents. This usually results in exchanging posts back and forth until there are enough data available to comment in a useful manner. This is always a waste of time and the goodwill to help weakens. In case the webmaster of this forum becomes aware of our correspondence he may find a way to remind all members (old or new) to stick to the forum's posting policies.

Again my question about the motor site altitude you have in mind. I'll try to locate in my archive a document dealing with output reduction vs. altitude.

Wolf

http://www.hydropower-consult.com

 

[electricpete]

Wolf and others here have a lot of helpful info to provide.

If your motor is in the NEMA world, altitude is very specifically addressed in NEMA MG-1.

NEMA MG-1-2009, Section 14.2.1 indicates "Usual Service Conditions", which is limited to altitute below 3300 feet (1000 meters).

14.4.1 says that higher altitudes are permissible if ambient is lower in accordance with following:
Maximum Altitude, Feet (Meters) / Max Ambient Temperature, Degrees C
3300 (1000) /40
6600 (2000) /30
9900 (3000) /20

14.4.3 gives a formula for derating the allowable sea-rise temperautre level of motors which are to be used above 1000m at 40C ambient:
TRSL = TRA *[1 - (Alt -3300)/3300]

Where:
TRSL = test temperature rise in degrees C at sea level
TRA = temperature rise in degrees C from the appropriate table in 12.43,12.44, 12.67, 15.41
Alt = altitude above sea level in feet (meters) at which machine is to be operated

Actually my copy of NEMA MG-1 shows 3300 in numerator of the equation and 33000 in the denominator, but logic tells me that must be a typo.


=====================================
(2B)+(2B)' ?

 

[electricpete]

The Alt parameter in the particular equation above would be given in ft, not meters.

=====================================
(2B)+(2B)' ?

 

[ddear]

The altitude at the installation in China is 2000M. The motor supplier is Chinese which is the main reason for my inquiry so I can be sure that they are taking the necessary measures to provide a motor that will run the equipment. Ambient temperatures will range between -30°C and 36°C with an average annual temp. of 7°C.

 

[wolf39]

Pete:

I don't think there is a typo in the NEMA publication. If you insert 6600 ft into your formula the result would be

TRSL = TRA * zero

If a figure of 33000 is used as a denominator you would get as a result

TRSL = TRA * 0.90

which looks okay for 2000 m altitude.

Wolf

http://www.hydropower-consult.com

 

[electricpete]

You are 100% right. Thanks Wolf.

=====================================
(2B)+(2B)' ?

 

[electricpete]

So it should be:
TRSL = TRA *[1 - (Alt -3300)/33000]

=====================================
(2B)+(2B)' ?

 

[wolf39]

The NEMA MG-1 standard does cover motors and generators. The formula given under 14.4.3 tells us that the design temperature rise (at sea level) has to be reduced by 10% for every 3300 ft (1000 m) increase of altitude for altitudes of above 1000 m. This formula has been agreed upon within the NEMA committee but has not been developed scientifically.

The international standards don't call for extra measures for altitudes of between sea level and 1000 m. By application of the international barometric formula, however, for an altitude of 1000 m the cooling air mass (density) decreases by 11.3% already. At 2000 m altitude the reduction is 21.5%, at 3000 m the figure is 30.8% and for 4000 m we get a cooling air mass/density reduction of 39.2%, all figures to be compared with sea level data.

I'm not very familiar with electric motors but I suspect that no slot RTDs are installed in a standard motor of 160 kW. The NEMA formula therefore is of little help when it comes to permissible temperature rise figures. The motor OEM has access to the test field data and he has to be asked for information about motor output reduction vs. altitude, unless slot RTDs are installed.

Temperature rise figures measured with slot RTDs don't only follow the square of the armature current. A motor connected to the system and running idle is already developing windage and iron losses and these can substantially "heat-up" the slot RTDs. Only the temperature fraction above this base temperature follows the short circuit losses, i.e. the square of the armature current.

Wolf

http://www.hydropower-consult.com