Motor Efficiency Loss 4

[EEtheEE]

Hi All,

We have a 460 V, 100 HP compressor motor that is running at about 50% of rated load. What kind of efficiency losses are typical when running at this low of a load?

Please let me know if you need more information.

Thanks.

 

[cranky108]

Have you tried your post in electric motors & motor controls engineering?

 

[dpc]

For a 100 hp motor in the 3600 - 1200 rpm range, I'd expect the efficiency at 1/2 load to be at least as high as the full load efficiency. Sometimes it's a little higher.

Seems like max efficiency in this size range generally occurs around 3/4 load.

 

[gepman]

dpc is correct (see table 1 of attachment). I have attached a paper on determing when to replace oversized or underloaded motors. You can also download MotorMaster 4.0+ at

http://www1.eere.energy.gov/industry/bestpractices/software.html

It is very useful.

 

[dpc]

Thanks for the links, gepman - very useful.

 

[waross]

The power factor will be lower. Correcting the power factor may not make a noticeable improvement in the efficiency of the system feeding the motor but may make a noticeable reduction in PF penalties.
respectfully

 

[jraef]

Excellent bit of info there gepman. Taken in a slightly different direction, it shows the big gaping hole in the arguments for Nola "energy savers". They always start out by stating how inefficient motors are when underloaded. This shows quite the contrary on larger motors.

 

[stevenal]

Underloaded motors may not be inefficient in themselves, but they contribute to overall system ineffiency due to low pf. Surprising the DOE publication makes no mention of the power factor issue Waross brought up. VArs must be supplied to these motors, taking up capacity and increasing losses all the way from VAr source to motor. May be insignificant downstream of the meter, but when overall source to load efficiency is concerned, they have an effect.

Efficiency if defined in terms of real power, but in the end most of us don't just want efficient use of watts, but efficient use of money. You must consider pf penalties to get there.

 

[gepman]

stevenal The amount of energy lost in distribution systems due to low power factor is relatively small compared to other inefficient practices. Remember that all of the copper in the facility should be sized for the larger motor which will actually decrease (I haven't fully checked this statement out but I believe the NEC conductor sizing is based upon heat generation, I2R losses) the losses compared to a fully loaded motor on copper sized for the smaller motor. The DOE is not advocating oversizing motors but if you look at the payback in energy savings in replacing an existing 100hp motor due to loading it at 50% there will not be a big incentive. You can of course correct the power factor at the service to eliminate the penalties from the power company (you will still have power factor problems inside your plant). You can also correct the power factor at the motor which would then solve this problem at the source.

Compressor motors (as this case is) are sized based upon the worst anticipated operating conditions where they may not always run at these conditions. I have attached another paper on motor efficiencies. Take a look at Figure 3 which gives motor power factor versus loading.

Again to summarize, do not intentionally oversize motors but if the motor loading is 50% or over then you will most likely find a better way to spend your money. EEtheEE should make certain that he measured loading in kW not in amps although this paper indicates about 50% amps at 50% load.

 

[EEtheEE]

Thanks everyone for the helpful information.

 

[ozmosis]

EEtheEE
Attached is a section from a Siemens standard motor catalogue (M11) indicating efficiency and PF at full, 3/4,1/2,1/4 and also 5/4 load. It should give some indication as a guide for others.