Induction Motor data - what is correct

[tomad]

We were involved in an analysis of replacing "normal" efficiency motors by "high" efficiency motors. During the process one issue appeared. Below is a typical example.

One of the motors in question had following data:
(per NEC 2005 - Table 430.250 - typical motors)
Output power: 20 HP ( 14.92 kW)
Voltage: 230V (LL)(Y connection)
Current: 54A
3 phases.
That leads to an input power of 21.5 kVA.
Based on these values, the product of efficiency * power factor = 14.92/21.5 = 0.694
Catalogs indicate for typical "normal" efficiency motors a value of 0.88, which would lead to a power factor of 0.694/0.88 = 0.788.
For typical/average "high" efficiency motors values are 0.92 (effi) and 0.892 (power factor), i.e. a product of 0.82.
If using high efficiency motors, for the same output ( 20 HP = 14.92 kW), the input power would be 18.18 kVA.
That, in turn, would lead to a lower current of 45.6 A, which is basically about 84.5% of the value indicated in NEC. These values have been labeled as "average/typical".
If determining power consumptions, a difference of 15% is significant.
What values would be the most correct one to be used and why ?

 

[electricpete]

Correction in bold"
"And take a look at Tables 50 thru 54 (derived from Table 12-14 of NEMA MG1 full version)"


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[automatic2]

you can bet if you order a motor through a catalogue listing, that's what will be stamped on motor plate, which takes prescedence in the CEC, with table current being the last choice.

 

[rhatcher]

tomad, this document appears to answer your questions:

http://www1.eere.energy.gov/industry/bestpractices/pdfs/mc-0382.pdf

 

[tomad]

Thank you all for the links. They were very helpful.

 

[waross]

What values would be the most correct one to be used and why ?
Motor efficiency varies from zero at no load to a maximum around 65% to 80% load and often dropping slightly at 100% load.
Each motor must be evaluated on the basis of the actual real world loading.
If you are doing a survey or evaluation you may consider using the actual efficiency curves of real motors and superimposing them. Then use this data to construct a curve of expected energy savings at different loadings.
Just look at an efficiency curve of a typical motor and you will see the futility of using a typical loading value for motors.

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

 

[Amotor]

I hope Gunnar and Muthu are correct. Maybe Tomad will educate us.

I wish to encourage this forum as there may be some real world data coming out that will give us valuable information. The Nema charts are interesting. But Tomads numbers, should they be made available, are greatly anticipated.

 

[retred]

Many years ago we would resort to plain old physical data. Pull a power meter off the shelf and measure the input of reactive loads. If muiti-phase caused a problem in motors we would simply ask the source for torque at rated rpm for a motor of rated horsepower. This some times got some interesting curls of smoke. You could smell it over the phone. But it gave you an idea of of the comparative rating of the suppliers.

In truth, we found that labels didn't mean much in performance.

Hope this was not off post.

Great forum.

Retred