Oversized Motor - Poor Power Factor
Oversized Motor - Poor Power Factor
(OP)
All,
First let me preface this post by stating that I am in no way an electrical engineer. My experience with motors and electricity is limited; so please respond in a way that typical engineer could understand.
I have a very oversized motor driving an reactor agitator. The agigitators were changed back in the 1970's, but the project manager chose not to replace the motors. The new agitators require a 75HP motor; the existing motors are 150HP. I grabbed my I&E tech and hooked up a portable power meter with very disturbing results:
HIGH-SPEED
POWER
kW 17
kVAR 17
kVA 71
PF 0.216
VOLTAGE
V AN 283
V BN 283
V CN 283
V AB 485
V BC 485
V CA 485
AMPERAGE
A N 162
A A 83
A B 84
A C 85
Notice the power factor of ~0.20. As everyone knows, energy reduction is a hot topic in industry right now. My question is, if I replace these motors with more appropriately sized ones, am I going to realize any savings due to decreased reactive load? Obviously there is some savings to be had by upgrading to a new high efficiency motor, but I am asking specifically about reactive load/power factor improvement. Thanks!
First let me preface this post by stating that I am in no way an electrical engineer. My experience with motors and electricity is limited; so please respond in a way that typical engineer could understand.
I have a very oversized motor driving an reactor agitator. The agigitators were changed back in the 1970's, but the project manager chose not to replace the motors. The new agitators require a 75HP motor; the existing motors are 150HP. I grabbed my I&E tech and hooked up a portable power meter with very disturbing results:
HIGH-SPEED
POWER
kW 17
kVAR 17
kVA 71
PF 0.216
VOLTAGE
V AN 283
V BN 283
V CN 283
V AB 485
V BC 485
V CA 485
AMPERAGE
A N 162
A A 83
A B 84
A C 85
Notice the power factor of ~0.20. As everyone knows, energy reduction is a hot topic in industry right now. My question is, if I replace these motors with more appropriately sized ones, am I going to realize any savings due to decreased reactive load? Obviously there is some savings to be had by upgrading to a new high efficiency motor, but I am asking specifically about reactive load/power factor improvement. Thanks!






RE: Oversized Motor - Poor Power Factor
But direct energy savings by improving power factor is not as great as often expected.
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
RE: Oversized Motor - Poor Power Factor
Also 70 Kva is in the ball park for the load on a 75 HP motor.
Check your files for information on the 150HP motors or, if possible, call the manufacturer. There are usually graphs which will allow you to estimate the motor efficiency.
RE: Oversized Motor - Poor Power Factor
Don't know what you are using for the data, but if it is giving you the 20% PF you should be smelling a rat!
Having said all of that, my experience with motors tells me that to get a 20% PF, the motor will have to be idleing on line with absolutely no load. Have seen it on sewing machines in textile factories back when we used to have them in the southeast US.
Alan
Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin
RE: Oversized Motor - Poor Power Factor
Alan
Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin
RE: Oversized Motor - Poor Power Factor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Oversized Motor - Poor Power Factor
RE: Oversized Motor - Poor Power Factor
What is happening however is that the motors are not as efficient as they could be here, because the magnetizing current is higher due to the larger frame, therefore the percentage of useful work compared to total power consumed is lower. You would benefit greatly from replacing those motors with energy efficient 75HP versions, and I'd venture to say that many utilities here in the US would provide some kind of financial incentive to do so. it's definitely worth looking in to.
But just correcting the pf is not likely to be worth much in actual energy savings.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Oversized Motor - Poor Power Factor
Alan
Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin
RE: Oversized Motor - Poor Power Factor
He should also contact his utility. Without the proper metering and a corresponding rate structure, improving his pf may have little or no effect on his bill. As Alan points out, the meter may never see it.
If his interest is with "energy reduction", he should see a professional who can evaluate and prioritize improvements to his entire facility.
DC
RE: Oversized Motor - Poor Power Factor
As others have said, low power factor is usually not a problem with a low load. It depends on your utility rate structure. The conductors are also probably oversized for the new agitators, so I²R losses are probably low even with the low power factor.
RE: Oversized Motor - Poor Power Factor
Thanks for you responses. The data reported is what I was provided with; I recognize that it doesn't make much sense. Attached is the motor curve for the original 150HP motor. You'll notice it says 150/37.5; that is because this is a two speed motor, but I am only concerned with the high speed.
Scratch the power readings; assuming the amperage readings are correct at 85amps/phase; I think I am looking at a motor output of 57%, a power factor of about 0.64, and an efficiency of 91% based on the motor curves. What is the proper way to calculate my REAL power consumption?
KW = %outHP * MaxHP * Conversion / Efficiency
70.1 = 0.57 * 150 * 0.746 / 0.91
RE: Oversized Motor - Poor Power Factor
RE: Oversized Motor - Poor Power Factor
Yeah you are right; don't know where that came from.
Other than the typo; is the method correct if I can accurately measure the amperes per phase?
RE: Oversized Motor - Poor Power Factor
It seems you need a 60hp motor.
RE: Oversized Motor - Poor Power Factor
I am a bit troubled by the nature of the motor duty. What is the agitated substance? If the substance is homogenous, motor could be overzized, but then if the substance is like a slurry, you could expect "sanding" problems which will require designing motor drives to overcome high starting torques to come up to speed; hence a larger or slower motor.
Just asking.