David90,Why does a 3 phase AC motor need an HP rating and a SF HP rating? Why not replace the HP rating with the SF HP rating since the SF HP rating is the maximum permissible load that the motor can operate at?
David90,
Your question implies a view of the world where everything is black or white, with no shades of gray. What I mean that in my many years of engineering experience, I have come to believe that many devices in the real world cannot be easily divided into nice neat categories. That is true of motors. How much horsepower any one motor can produce is dependent on how long the motor will run without burning out the winding insulation. In other words, a 2 horsepower motor will not suddenly stop running if the load jumps up to 2.1 horsepower. That 2 HP motor might easily produce 3 HP - for a few minutes! It is not a cut-and-dried number, so therefore the Service Factor ratings were developed to measure the amount of reduction in life if the basic horsepower rating is exceeded.
Therefore, if someone wants a motor that can be overloaded by 15% and still not have its life-time reduced, then they need a 1.15 SF motor in the same HP rating.
Why even bother with SF HP rating at all since it's safer to design using a motor's HP rating. I am right?
Yes, but the effect for the user is what? Does the typical motor user care about the cooling capacity, or about the extra power that the motor can produce due to the extra cooling?A motor with a 1.15 service factor has 15% extra cooling capacity at full load. That's it! Nothing else.
Yes, but the effect for the user is what? Does the typical motor user care about the cooling capacity, or about the extra power that the motor can produce due to the extra cooling?
Think of it like going to Burger King. Do you want regular, or do you want it super-sized? It is still basically the same product.
Thank, Dick. I know that, and have used 1.0 in most cases, and occasionly 1.15 if there is a chance of an overloaded conveyor or some other type of unexpected event. In fertilizer plants, they like 1.15 because some humid days can cause the fertilizer to set up like concrete on conveyors and chutes and it takes more power to move it. I don't think I have ever used a 1.4 SF.Lancie, if you have an application for a motor that never encounters longterm overload, longterm overtemp, or any other unexpected similar disturbance, then, whether you buy a 1.0, 1.15, or 1.40 service factor makes no difference. It will be of no benefit to overspecify the motor.