kW Savings From Replacing Oversized Motor 1

[broc028]

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!!

 

[edison123]

Efficiency of induction motors start falling down steeply generally below 50%. The no-load losses (which is constant) in a 10 hp motor is more than a 6 hp motor. So, a 6 hp motor running a 5 hp load will be less lossy as compared to a 10 hp motor.

On the other hand, a 10 hp motor will run lot cooler and hence will have a longer life.

 

[edison123]

And of course, there is the issue of frame size, which would involve modifications to the old bed.

Make sure the 10 HP motor was not provided in the first place to overcome starting issues.

 

[broc028]

Thanks edison123.

What I am really looking for however is a method of calculating the kW savings after replacing the motor, if thats possible...

Thanks again!

 

[edison123]

Put an energy meter and measure the KW-hours.

Calculating the losses and efficiency of an induction motor is not a simple one.

 

[rbulsara]

Calculate kwatt-hr consume before and after the change and take the difference between the two. Multiply with the cost/kwatt-hr.

Only you have the data of load, motor nameplate and run time etc.

If not hire an engineer who knows.

 

[edison123]

If you can get hold OEM's test certificates for both the motors, you can get some numbers (not all) like no-load and load losses and do the math. That would just be a starting point.

 

[starkopete]

Hi,
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

 

[electricpete]

Great point starkopete.

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[electricpete]

The general trend: each motor has its own highest efficiency in the neighborhood of 75% (give or take). But for higher horsepower motors the curve is generally shifted up compared to lower horesepower motors.

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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[electricpete]

last comment - as edision mentioned you have to match frame size for interchangeability. These two particular motors have different frame size and wouldn't be mechanically interchangeable even if you wanted.

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[edison123]

Good catch starkopete.

 

[waross]

Don't compare efficiency percentages with different size motors.
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

 

[edison123]

Bill

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

 

[edison123]

Another point that bugs me why the motor designers design for maximum efficiency at 75% and not at 100%. Is it because the end users tend to over spec the motor HP requirement ?

 

[waross]

They could do that by calling a 10 Hp motor a 7.5 Hp motor. That would put it at the most efficient operating point, but like so many design questions, the best answer is often a good compromise.
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

 

[electricpete]

Don't compare efficiency percentages with different size motors.
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

I don't think you have captured starkopete's point.
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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[electricpete]

Not to beat a dead horse, but I think an earlier comment was incorrect:

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

should be:

10 HP motor at 50% load - Efficiency 92.3% - Loss - 0.77/2 HP=0.385HP
5 HP motor at 100% load - Efficiency 89.5% - Loss - 0.525 HP

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[edison123]

pete

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.

 

[electricpete]

It is 92.3 % efficient at 50% load (5hp). That means the output is 5hp when the input is 5hp/0.923 (satisfies efficiency = output/input). We don't use 10hp in the calculation of efficiency at the 5hp operating point of a 10 hp 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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