Motor Capacity Vs Load

[scsl]

Dear All

I am using 18 KW induction motor which draws 18 amps. at its load. What are the savings that I can gain if I used 11KW motor to run the same load.

Thank you
Sahan

 

[Skogsgurra]

We need, at least, also the motor voltage to be able to have an idea about possible savings. Motor speed would also help - and power factor of present motor.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

Go find the datasheet for both motors and look at the efficiency of each at the expected operating point.

 

[davidbeach]

Given that you allow us to pick the voltage, I'd say that your 12kW load would overload an 11kW motor.

 

[cranky108]

Some motors are designed to handle more than there stated capabilities. This I believe is listed as the SF.
This may not be true for all motors, but some it is.

So if you are under loading your motor, are you having power factor problems also?

 

[jasonf987]

Keep in mind that it also depends on what you are actually driving. For a pump, for example, the pump curve would give you all the info you need in terms of load requirements etc.

 

[willj]

It really comes down to the efficiency of the motor.

Check out your current motor eff. If it is not a newer motor most likely you can save on operating expense by buying a high eff motor.

Of course you want your load to match the most eff curve of the motor.

I tell my operations elec guys to replace any older low effiecent motors (< 20 hp or so) with new higher eff ones, even if they are still working fine.

The size of the motor doesn't greatly effect the power draw. It is motor load determined, as long as they are approx the same size or motor.

By comparing eff you should be able to easily calculate the operational cost savings between two eff ratings.


W

 

[jraef]

On older motors, the efficiency curves were sometimes relatively steep with loading, i.e. the efficiency dropped maybe 10% between full load and half load. But on newer energy efficient motors that is no longer the case, there is barely a few percent difference any more. So as willj said, your first and best opportunity to save is in replacing an older motor with a newer energy efficient one, regardless of size. Keep in mind also that sometimes motors are sized for their ability to provide short-term load increases or survive anomalies such as voltage dips; reducing the size may put your application at risk of overloading.


"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

 

[electricpete]

On older motors, the efficiency curves were sometimes relatively steep with loading, i.e. the efficiency dropped maybe 10% between full load and half load. But on newer energy efficient motors that is no longer the case, there is barely a few percent difference any more

I didn't know that but it kind of makes sense. Someone recently discovered here on the forum that the old thumbrule about undersized motors (let's say drawing 50-80% of full load) being less efficient than fully loaded motors no longer seemed to work. When we tried it out with some typical data sheets (new motors around 10hp as I recall), even the 50% loaded larger motor was more efficient than the 100% loaded smaller motor. For both old and new motors the larger motor is obviously more efficient at 100% load. But apparently (?) there is no longer so much drop between full load and 50%, that would explain the observation about the thumb rule no longer valide. (I realize data sheets are the only way to really answer the question, but it's nice to have a general feel.)

=====================================
Eng-tips forums: The best place on the web for engineering discussions.