kW Savings From Replacing Oversized Motor
kW Savings From Replacing Oversized Motor
(OP)
Could anyone explain to me the kW savings from replacing an oversized motor.
I have calculated the kW Input from
(V x I x 1.73 x P.F.)/1000
and calculated load from:
Load = kw Input/(Rated H.P. x 0.746)/F.L. Efficiency.
If I have a 10hp motor running at 50% load, how do I justify replacing it with, for example a 6hp motor.
Thanks!!
I have calculated the kW Input from
(V x I x 1.73 x P.F.)/1000
and calculated load from:
Load = kw Input/(Rated H.P. x 0.746)/F.L. Efficiency.
If I have a 10hp motor running at 50% load, how do I justify replacing it with, for example a 6hp motor.
Thanks!!





RE: kW Savings From Replacing Oversized Motor
On the other hand, a 10 hp motor will run lot cooler and hence will have a longer life.
RE: kW Savings From Replacing Oversized Motor
Make sure the 10 HP motor was not provided in the first place to overcome starting issues.
RE: kW Savings From Replacing Oversized Motor
What I am really looking for however is a method of calculating the kW savings after replacing the motor, if thats possible...
Thanks again!
RE: kW Savings From Replacing Oversized Motor
Calculating the losses and efficiency of an induction motor is not a simple one.
RE: kW Savings From Replacing Oversized Motor
Only you have the data of load, motor nameplate and run time etc.
If not hire an engineer who knows.
RE: kW Savings From Replacing Oversized Motor
RE: kW Savings From Replacing Oversized Motor
The attached is from MotorMaster (US DOE). I went with Reliance Premium Efficiency, NEMA Design B, general purpose motor. As you can see the 10 HP has a higher efficiency at 50% load than the 5 HP has at 100% load.
Thanks
RE: kW Savings From Replacing Oversized Motor
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RE: kW Savings From Replacing Oversized Motor
Even if you should find a motor with marginally better efficiency, considering you have already invested in the large motor, you will need a lot of efficiency gains to justify purchasing a new motor. In contrast if you have no motor and choosing between two motors, it is often easier to justify a small increase price for higher efficiency motor.
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RE: kW Savings From Replacing Oversized Motor
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RE: kW Savings From Replacing Oversized Motor
RE: kW Savings From Replacing Oversized Motor
As an example,
10 HP motor, 90% efficient, about 1 HP losses.
5 HP motor, 85% efficient, about .75 HP losses.
Which motor will be cheaper to run?
Calculate the actual losses in HP or kW before comparing the efficiency of different size motors.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: kW Savings From Replacing Oversized Motor
Good point too.
Per starkopete's motor master catalog
10 HP motor at 50% load - Efficiency 92.3% - Loss - 0.77 HP
5 HP motor at 100% load - Efficiency 89.5% - Loss - 0.525 HP
RE: kW Savings From Replacing Oversized Motor
RE: kW Savings From Replacing Oversized Motor
Now, consider that even though the motor is running at best efficiency at 75% loading, the efficiency of a 7.5 Hp motor fully loaded may not be much different. Also, many motors tend to operate at less than full load anyway. If you have an 8 Hp load, you will buy a 10 Hp motor and it will be running close to maximum efficiency.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: kW Savings From Replacing Oversized Motor
10 hp motor carrying 5hp load - 92.3% efficienct
5 hp motor carrying 5hp load - 89.5% efficienct
Which is cheaper to run when carrying 5hp load?
Answer: The 10hp motor.
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RE: kW Savings From Replacing Oversized Motor
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RE: kW Savings From Replacing Oversized Motor
If you are calculating 92.3% efficiency of the 10 HP motor on a 5 HP output basis, it's not right. Note the no-load loss (iron loss + windage + other mechanical losses) are higher in a 10 HP motor.
RE: kW Savings From Replacing Oversized Motor
Let's do it a little more precisely:
10 HP motor at 50% load: Efficiency= 92.3%: Losses=5*(1-0.923)/0.923 = 0.42 hp
5 HP motor at 100% load: Efficiency 89.5%: Losses=5*(1-0.895)/0.895 = 0.59 hp
If the 10hp motor is more efficient at 5hp then the 5hp is at 5hp, then.... the 10hp motor is more efficient at 5hp then the 5hp is at 5hp! (i.e. the 10hp motor consumes less power driving this 5hp load).
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RE: kW Savings From Replacing Oversized Motor
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RE: kW Savings From Replacing Oversized Motor
I believe that I made a blunder. I will defer to you on this.
edison123;
The no load losses are less on the 5 HP than on the 10 HP, but the load losses follow a square law and the I2R is greater on the smaller motor for a given current.
Although the no load losses are less for the 5HP motor, the load losses for a 10 HP motor running 50% loaded will be about 1/4 of the full load losses. That more than compensates for the greater no load losses.
(Did I get it right this time Pete?)
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: kW Savings From Replacing Oversized Motor
RE: kW Savings From Replacing Oversized Motor
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RE: kW Savings From Replacing Oversized Motor
You're right. I forgot what efficiency meant. :)
RE: kW Savings From Replacing Oversized Motor
Bill I think you are right that in general going from 10hp motor driving 5hp load down to 5hp motor driving 5hp load, the no-load losses increase but the load losses decrease.
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RE: kW Savings From Replacing Oversized Motor
Going from 10 HP to 5 HP, I would recommend running the numbers.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: kW Savings From Replacing Oversized Motor
Where's my Nola.
Keith Cress
kcress - http://www.flaminsystems.com
RE: kW Savings From Replacing Oversized Motor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: kW Savings From Replacing Oversized Motor
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RE: kW Savings From Replacing Oversized Motor
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RE: kW Savings From Replacing Oversized Motor
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RE: kW Savings From Replacing Oversized Motor
But the scammers still try to capitalize on the basic instinct people have to believe that.
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RE: kW Savings From Replacing Oversized Motor
This is right up there with wye-delta starters suck, VFDs can make three phases from a single phase, and centrifugal pumps work less with outlets restricted - not more.
Now I can add: over sized motors are not necessarily less efficient.
Thanks folks!
Keith Cress
kcress - http://www.flaminsystems.com