Published efficiency of NEMA design motors

[eeprom]

Hello,
I'm looking for a NEMA publication which states the efficiencies of each motor NEMA design type and horsepower. I have not been able to find this on the NEMA website. Can someone send me a link to such a document?

thanks,
EE

 

[dpc]

http://www.nema.org/stds/complimentary-docs/upload/MG1premium.pdf

Google NEMA Premium Efficiency.

This only applies to motors specified as "Premium Efficiency". Also note that these efficiencies are NOMINAL, not guaranteed minimums. Variations from the nominal are allowed within a specified range.

I don't believe there was a required NEMA efficiency for the old standard efficiency motors.

If you can let us know specifically what you are interested in, we can probably provide more information.

 

[eeprom]

DPC,
Thanks. I have that document. Maybe if I tell you what I'm trying to do, I will be clearer.

I will be doing an energy inventory of a power plant, and I will be looking at all of the major loads (motors of 100 Hp or more). For each of these loads, I will determine the generated mechanical Hp and then decide whether or not the motor is properly sized. If it is sized properly, then I will look at the cost benefit of changing the motor to a high efficiency motor. But I cannot establish a cost benefit unless I know the efficiency of the existing motor. Most of these motors are nema B. So, I need a NEMA document which states the efficiency of non-premium motors.

thanks
EE

 

[dpc]

I would use typical data from a common motor manufacturer for standard efficiency motors. The NEMA standards on efficiency evolved over time and were generally very loose up until about 10 years ago.

I have some typical efficiency data for older motors somewhere. If you are interested, I can try to find it and scan it in.

 

[eeprom]

I won't have time on site to look up motor manufacturer's. Do you know if it would be accurate to make an assumption of motor efficiency at 90%. That seems a bit high. No need to scan your data. But thank you for offering.

EE

 

[electricpete]

NEMA MG-1 (2009), has 3 tables for nominal and minimum efficienc of NEMA Design B motors 1-400hp.
Table 12-10 standard
Table 12-11 energy efficient
Table 12-12 premium

Of course if your motors were purchased according to this standard,they would have the efficiency stamped on the nameplate. You might have to look at older versions of MG1 to see what it said (if anything), depending on the vintage of your motors.


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[electricpete]

Just to clarify, the requirement to stamp efficiency on the nameplate was added recently. I think there were standards for efficiency included in MG1 before there was a requirement to stamp efficiency on the nameplate.

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[davidbeach]

Figuring out the mechanical power is far more difficult than direct measurement of the electrical power. Why guess at efficiency when you have mechanical power? Usually efficiency is used to estimate mechanical power from known electrical power.

 

[eeprom]

Thanks for the reference to MG-1 eletricpete. I'm pretty sure I have that in my library.

davidbeach,
There are two parts to my study. The first part is to determine if the motor is over or under sized. For that I will need to look at the estimated mechanical power and then determine the ideal motor size for that mechanical load. If the motor is grossly over sized, such as a 50 Hp pump which is throttled back to the point of running a 20 Hp load, then I can consider the efficiency loss based on a partially loaded motor compared to a fully loaded motor. A 20 Hp motor driving a 20 Hp load costs less to run than a 50 Hp motor running the same load.

The second part of the study is to look at the large motors and consider how much the efficiency comes into the overall cost. If an existing 1000 Hp fan motor is 88% efficient, and a new premium motor is 95% efficient, well that's a 7% loss. In dollars, that's over $30k/year, based on a kwh rate of 9 cents.

EE

 

[ScottyUK]

"A 20 Hp motor driving a 20 Hp load costs less to run than a 50 Hp motor running the same load."

Maybe. A 25 Hp motor would likely be more efficient because motors are seldom at their peak efficiency at full load.


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If we learn from our mistakes I'm getting a great education!

 

[eeprom]

Yes, I agree. As I said, I am looking for grossly oversized or undersized.

 

[electricpete]

Just to add to Scotty's comment, the proper procedure is of course not to compare nameplate full-load efficiencies among motors of different sizes, but to attempt compare efficiencies at expected operating load. That may be easier said than done - NEMA only gives the full load efficiencies if I recall correctly.

For new candidate replacement motors, you can get that info from the OEM. For existing motors operating at less than full load, it may be tough to reconstruct without info from OEM or other measurements.

A little bit of a sidetrack - Reliance has a widget that purports to estimate operating motor "efficiency" based only on input electrical measurements and speed measurements and some standard nameplate data. Basically it builds a model of the motor and computes the efficiency based on the model. Some people's reaction may be to dismiss such an approach out of hand because it doesn't directly measure output power. No-one will argue that inference from a model is inferior to direct measurement. However, when direct measurement is not available we sometimes use the info available to make predictions and then and try to estimate the degree of accuracy of our predictions. The last part is the tough part for this thing imo.

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[electricpete]

such as a 50 Hp pump which is throttled back to the point of running a 20 Hp load

In that case I would be much more interested in right-sizing the pump. Can sometimes be done relatively easily by trimming the impeller. Efficiency of pump operating far below BEP can be very very low and that is much lower hanging fruit than the motor, which may not be fruitful at all.

I'm just mentioning this in passing (I suspect even if I didn't, someone else would). I realize it may be outside your scope. If you already know what I've said anywhere, I apologize in advance.

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[electricpete]

In addition to trimming impeller, sometimes a stage can be removed from multistage pump (destaging). Both destaging and impeller trimming have the effect of moving BEP lower (toward the operating range).

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