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Old January 10th, 2019, 03:43 PM   #1
Timeismoney08
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-- Specifying motors - Torque or speed too high --

I have a question regarding some calculations I'm doing...

I find that most of our motors are around 1/2HP, so for some applications we may install one and gear it down to fit our needs.

What I have a question regarding, is how much over on torque or speed is acceptable. I can run the numbers sometimes and get 100% speed match, but way over on torque, or 100% on torque and high on speed. I would average the two and go in the middle of each as long as the application will work with it.

Obviously this is a improper motor selected for the application, I'm just curious what is considered normal values.




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Old January 10th, 2019, 04:54 PM   #2
jraef
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A motor only supplies the torque necessary to move the load (and it's own mass), so the "too much" aspect only comes into play when something goes wrong. Even then, TOO MUCH torque is a problem if you have more available torque than the mechanical components can stand up to, i.e. twisting off shafts, breaking keys, damaging bearings etc., when there is a jam (or at initial start-up).
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Old January 10th, 2019, 05:05 PM   #3
Timeismoney08
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Quote:
Originally Posted by jraef View Post
A motor only supplies the torque necessary to move the load (and it's own mass), so the "too much" aspect only comes into play when something goes wrong. Even then, TOO MUCH torque is a problem if you have more available torque than the mechanical components can stand up to, i.e. twisting off shafts, breaking keys, damaging bearings etc., when there is a jam (or at initial start-up).
I understand the problems that can occur and much prefer utilizing a new motor that is properly specified for the application, but I'm just curious how much people oversize motors for some applications.

I think most people will double whatever is needed for safety factor, but I would like to know when it becomes "too much".. Is 10 times the torque considered acceptable at times if it achieves the speed someone was looking for?

I guess this is a bad question to ask because it falls under one of those "it depends" scenarios.. I'm just curious and trying to learn.


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Old January 10th, 2019, 05:13 PM   #4
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Ultimately there is no free lunch. Power is power and if the application requires more of that than the motor you have you are out of luck.

First of all I am assuming this is a VFD application since you can't go "too fast" feeding the motor across the line. We use Marathon Black Max motors around here. The whole family is rated for constant rated torque from zero to base speed and constant rated horsepower from base speed to twice base speed. Constant horsepower with increasing speed means decreasing torque. So from a motor standpoint running above base speed is not an issue, within reason.

The rest comes down to how you are doing your calculations. If you are gearing the application to get the motor torque down to rated torque but the motor will be running above base speed to do it you have an issue. You only get rated torque to base speed. However, if your application is more of a constant horsepower application, where the torque requirement decreases as the machine speed increases, you can play games with gearing and motor speed to get you better utilization of the available motor horsepower.

Keith
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Old January 10th, 2019, 05:31 PM   #5
kvogel
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I chose a motor & gearbox that gives the output rpm I'm looking for and lets the motor run at an appropriate rpm (depending on motor type not too slow especially for fan cooled motor).

Then pick the smallest motor possible that will give the needed torque. (with safety factor)

Sometimes your limited by the gearbox on motor selection so pick whatever works to get you best balance of factors as long as you meet the power speed combo to accomplish the task.

Ken
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Old January 10th, 2019, 05:44 PM   #6
jraef
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In days of yore when I started, a 20% "fudge factor" on sizing a motor was common practice. More recently that has gone away, especially with OEMs, who don't want to buy anything larger than absolutely necessary. But if you are the end user, that +20% is prudent as a minimum.

Double is over kill in my opinion, that can put you into some energy efficiency problems. But there is also something to be said for making things all the same so that spare parts are "universal donors". So if you need 1/4HP, 1/3HP and 1/2HP motors, there is something to be said for just using 1/2HP on everything.
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Old January 11th, 2019, 05:52 AM   #7
Gene Bond
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+1

I remember a time when the Automotive Industry never used anything smaller than a 140 frame motor, which meant a 1hp motor minimum....

Sure efficiency suffered, but rules were rules!
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