Motor Rating
- Thread starter Anthony_S
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
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.
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.
weressl
Esteemed Member
The 10% oversizing is far less than what actually happens in industry. Motors driving variable loads like pumps and agitators and even conveyor belts are subject to overloading.
Sizing to the end-of-curve performance of a centrifugal pump is necessary to accomodate operational changes in the process and often results in motors running at 75% or lower.
Phase voltage imballance, high harmonic contents originating outside of the drive train, low voltage can all effect the motro's efficiency, therefor reducing the actual mechanical power delivered on the shaft, be it expressed in kW or HP. (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.)
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.
If you use SI units (and much of the world besides the EU does) you wouldn't actually need that pesky conversion factor....
But, more seriously, SI units make many engineering calculations so much simpler. I say this from the perspective of being conversant with, and having used, both Imperial and SI.
Well, that depends on a number of factors.
This the manufacturer's data for a motor where we provided the starter.
Sure it's a fairly big machine and I don't have measured values or equivalent circuit parameters so I can't confirm that the performance was achieved.
So, I have looked at a much smaller drive where we had better detail.
Efficiency was calculated as 95.8% at full load and 94.5% at half load.
We had to guarantee and demonstrate efficiency figures for the motor/drive combination. Financial penalties for not meeting that meant getting them wrong was not an option.
Sounds harsh maybe, but typical of many of the similar projects we undertake. They are often capitalised over 20 years. The most significant cost is total energy consumption over that period.
weressl
Esteemed Member
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.
I hate when somebody brags about the superiority
of the SI system....I lived with both.:smile:
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This is the original post:
I still don't think it makes sense.
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.
IMHO the business end of a motor produces torque at RPM, and the electrical end is KVA and KW.
I can't think of any motor load that would not need to be considered in terms of torque and RPM at some point.
Are motors available in 10% increments? ... either KW or HP?
I still don't think it makes sense.
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.
IMHO the business end of a motor produces torque at RPM, and the electrical end is KVA and KW.
I can't think of any motor load that would not need to be considered in terms of torque and RPM at some point.
Are motors available in 10% increments? ... either KW or HP?
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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 superiority
It is common in many parts of the world - outside of the USA that is.
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.
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