Sizing Motors

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jonny1982

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Can someone give me a step by step instruction on properly sizing and protecting a motor? Lets say I have to power up a single motor.

Thanks
 
Jraef

Jraef

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A motor is sized to proved a necessary amount of torque at a given speed, which is defined as HP. Usually the Mechanical people are the ones to size the motor, because they will know the torque requirements and gear/belt ratio issues, as well as the rate at which the load torque increases as it accelerates so as to not exceed the thermal damage curve constraints of the motor, and the recovery rate response for a given step change in load. We just take their HP number (at the motor speed they want) from there. There are formulas that you can apply, but if you are the EC, are you willing to accept the risk if you miss some part of the mechanical selection process?

PS: as the others pointed out first, this is assuming that is what you meant by "sizing motors". In my experience, I have found that often times end users or home experimenters/inventors have no clue about the process and ASSume that it is the job of "the electrical guy" because its an electric motor. I happen to know more about this than most ECs, but I still don't want to take it on from an application risk standpoint, that's not what I do for a living. So I will start asking a bunch of the necessary questions in a rapid fire string, knowing that they will not know the answers, which exposes to them that they need to deal with the mechanical issues FIRST, then come to me with the simpler numbers, namely HP and base speed.
 
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jonny1982

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Jraef said:
A motor is sized to proved a necessary amount of torque at a given speed, which is defined as HP. Usually the Mechanical people are the ones to size the motor, because they will know the torque requirements and gear/belt ratio issues, as well as the rate at which the load torque increases as it accelerates so as to not exceed the thermal damage curve constraints of the motor, and the recovery rate response for a given step change in load. We just take their HP number (at the motor speed they want) from there. There are formulas that you can apply, but if you are the EC, are you willing to accept the risk if you miss some part of the mechanical selection process?

PS: as the others pointed out first, this is assuming that is what you meant by "sizing motors". In my experience, I have found that often times end users or home experimenters/inventors have no clue about the process and ASSume that it is the job of "the electrical guy" because its an electric motor. I happen to know more about this than most ECs, but I still don't want to take it on from an application risk standpoint, that's not what I do for a living. So I will start asking a bunch of the necessary questions in a rapid fire string, knowing that they will not know the answers, which exposes to them that they need to deal with the mechanical issues FIRST, then come to me with the simpler numbers, namely HP and base speed.
Click to expand...

I just meant if I have the nameplate of the equipment, what is the process for determining circuit breaker size, wire size, and the fused disconnect.

thanks
 
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jonny1982

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GoldDigger said:
Do you really need to size the motor for an application? Or do you just need to match the circuit wiring to a known motor?
Click to expand...

I meant if I have the nameplate, how to figure circuit breaker size, wire size and fused disconnect.

Thanks
 
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GoldDigger

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jonny1982 said:
I meant if I have the nameplate, how to figure circuit breaker size, wire size and fused disconnect.

Thanks
Click to expand...
Just be aware, in conjunction with the articles referenced, that for most situations the NEC requires you to combine the rated horsepower from the nameplate with the code tables instead of using the more detailed information from the nameplate.
 
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jonny1982

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GoldDigger said:
Just be aware, in conjunction with the articles referenced, that for most situations the NEC requires you to combine the rated horsepower from the nameplate with the code tables instead of using the more detailed information from the nameplate.
Click to expand...

Thank-you
 
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