Running a motor at FLA more effecient? 6

[BrainEJ]

My boss here at our plant (Maintenance manager) was challenging me yesterday saying that a motor runs best when ran AT it's FLA, vs running it a bit under, say at like 80% of its FLA. I felt that is wrong, but can't find any data to back it up. Can anyone help on this topic?

 

[BrianE22]

Some considerations:

1) More amps mean more (I**2)R losses
2) If higher amps produces higher speeds then there would be more air losses.

 

[waross]

Do some googling of motor efficiency curves.
You may find that the peak efficiency of most motors is around 70% to 80%.


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

 

[waross]

THIS GRAPHIC SHOWS TWO THINGS.
1> The efficiency of this typical motor peaks at about 75%.
2> The efficiency curve above about 40% loading is quite flat. You only drop about 1/2 of a percentage point from 75% load to 100% load.

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

 

[Strong]

"a motor runs best when ran AT it's FLA, vs running it a bit under, say at like 80% of its FLA."
Ask your boss what he means by "best" such as output, economy, or reliability; and just the motor or the entire machine?
For example driving a centrifugal pump beyond its BEP with motor at FLA is not efficient. It is best to consider both the driven machine/load and the motor performance curves.

Walt

 

[waross]

On the other hand, curtailing production 25% to pick up 1/2% of motor efficiency may be false economy.

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

 

[LittleInch]

I agree. What the motor is doing is totally irrelevant. It's what it's driving that is important. Motor efficiency looks to be pretty flat.

Who cares if your motor is running at 50A when it's FLA is 60?

Matching as best you can the motor rating to the power required is a good thing, but a pump e.g. will have the motor rated for end of curve operation but the pump itself is at BEP probably 15 to 20% below max power/FLA.

Your manager sounds like he's referring to some very specific case and then enlarging that to all motors.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jraef]

It also is dependent on the size of the motor you are discussing. Smaller motors have less flat curves.

Looking at just the motor efficiency though (because yes, what it is driving is more important), there were several studies done by the Dept. of Energy years ago, resulting in programs to increase efficiency in electric motors. This paper, geared toward energy efficient motors, corroborates the other information about the peak efficiency being around 75-80% load, but you are really splitting hairs because at 100% load it's only off by a small fraction.

https://www.energy.gov/sites/prod/files/2014/04/f15/amo_motors_handbook_web.pdf

Look at the chart on the top of the page in Chapter 4 page 13.
But also notice that a 1HP motor does NOT follow that rule.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[edison123]

"If higher amps produces higher speeds then there would be more air losses."

Higher amps do not produce higher speed. The speed is solely determined by the supply frequency.

As others have said, the motor efficiency peaks around 75% to 85% of rated load depending on the motor design and the rated load efficiency falls off slightly.

Muthu

http://www.edison.co.in

 

[BrainEJ]

HEy- just want to thank everyone for all of the speedy replies!! Didnt expect to see this much support so soon!

So, the debate started regarding A.) Him saying bluntly and generically that motors run "best" at FLA, and B.) he knows this because he was told by Atlas Copco Air Compressor company to make sure to run the plant air compressors at 100%$ FLA or else it voids the warranty (something like that, it's been a few days since the conversation so don't direct quote me, lol).

It seems though, through reading these comments, that the most important thing is the device that the motor is running, and maybe in this case, the compressor requires to be ran at 100%?? I know the conversation/debate threw me because in my experience in industrial manufacturing maintenance as an electrician, it's never a good thing to see a motor running at 100% FLA, primarily due to the fact that it's borderline tripping the O/L, and a slight process variation will send it to trip status. I also saw this scenario similar to an internal combustion engine- yeah, it makes the most power at redline WOT, but are the bearings going to last as long when I'm running it that hard/fast over long periods of time?

 

[jraef]

The compressor overall efficiency, i.e. input power to work performed, may be best when the compressor is fully loaded, and they have sized the motor to run at full load for that operating point, so the statement could be true, FOR THAT MACHINE.

To address something else you said though, running at 100% of rated load is not going to be "borderline tripping the O/L". Most O/L relays have a "pick-up" point of 115-125% of the motor FLA value, meaning the O/L does NOT enter the trip curve at 100% of nameplate FLC. At the pick-up point (depending on the motor type), the trip time will be at least 2 hours, but the trip time decreases at the square of the increase in current (called an I²t curve), the end of the curve being the trip time at 600% of FLC, either 10 seconds or 20 seconds depending on the motor design "class" (IEC motors are Class 10 for 10 seconds, NEMA motors are typically Class 20 for 20 seconds). You should be able to run your motor at 100% FLC forever (or until the bearings wear out).


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LittleInch]

Many more questions than answers after that revalation. Large air compressors tend to run loaded or unloaded to avoid multiple starts.

So how you can run a system at FLA when it doesn't want to is beyond me.

A little knowledge is a dangerous thing....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[waross]

I think that what your manager may have meant was that the compressor runs best at full load.
He may not have been aware that that is what he meant to say.

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

 

[Strong]

Since when is compressor performance based on motor FLA? Perhaps something got lost in the translation!

Walt

 

[FacEngrPE]

Centrifugal compressors have a controller that adjusts massflow (with an inlet modulating valve) through the compressor based on motor power consumption. It is not unusual for the factory to set the power controller to 105% FLA (using up all of the service factor).

Screw compressors (positive displacement) have an unloading valve, but they do not do capacity control with inlet modulation.