In the "olden days" it was common for an ME to put in a 20% "fudge factor" on the HP rating of a motor, rounded up to the next standard size. So if he determined that the required shaft power was 36BHP, he selected a 50HP motor. As times got leaner and energy efficiency became more of a concern, the more common practice is to size the motor as close as possible to the required shaft power. So that 36HP load would get a 40HP motor (or if it's a cheap OEM, maybe a 30HP 1.25SF motor run into the Service Factor). But 10% is still reasonably close. An AC motor will pull only as much power as it needs but will run at maximum efficiency starting at around 90% of full load. Beyond that the efficiency curve remains relatively flat. It's still good at 80%, but starts to drop off more steeply below75% loading.
Keep in mind though that motor sizing has more than one parameter to think about. Acceleration torque can be just as critical in selecting a motor as final shaft power. Sometimes if you select a motor too close to the bone, it can't rovide enough acceleration torque to get to full speed before it overloads.
It is fairly common for motor output power to be given in kW rather than in HP. As was noted in the post prior to this, there is a conversion factor from one to the other.No Comprendo.:-?
HP is torque and RPM.
Horsepower = Torque * RPM/5252
KW is V x A x PF
When you are talking about oversizing the KW by adding 10% to the shaft power, what do you mean?
If you use SI units (and much of the world besides the EU does) you wouldn't actually need that pesky conversion factor....(Although we commonly refer to kW as the electrical energy required by a given motor in EU the kW is used as an expression of mechanical energy on the shaft as well, doing away with the pesky 0.746 additonal multiplier.)
Well, that depends on a number of factors.An AC motor will pull only as much power as it needs but will run at maximum efficiency starting at around 90% of full load. Beyond that the efficiency curve remains relatively flat. It's still good at 80%, but starts to drop off more steeply below75% loading.
It is fairly common for motor output power to be given in kW rather than in HP. As was noted in the post prior to this, there is a conversion factor from one to the other.
What are the advantages and disadvantages of oversizing the motor kW rating say adding 10% to the calculated shaft power.
I have never known it to be anything other than output rating. If I buy a 22kW motor, I expect it to be able to deliver 22 kW.So how do you know when someone talks about a 22 kW motor if that is the input or the output characteristic of the motor? Maybe that little ANSI usage has more than an annoying purpose.![]()
Not a brag - just a simple statement of fact, old bean.I hate when somebody brags about the superiorityof the SI system....I lived with both.:smile:
It is common in many parts of the world - outside of the USA that is.I am no engineer, nor very sophisticated, but in over thirty years of being involved with motors, I have never once heard anyone refer to the output shaft of a motor in terms of KW... until this thread.
I agree. Output power is torque times rotational speed. To me, it seems entirely logical to express that power in the same units as the input power.I can't think of any motor load that would not need to be considered in terms of torque and RPM at some point.