Motor Sizing

Which type of motor?

I had a similar problem Mr. Heath. I needed to determine the torque of an AC fractional HP motor to drive a chain mechanism which was to move 4 metal bins along a length of 6 feet. The bins rolled on casters (sort of). So, I have a digital fish scale ( you know the type where you hang the fish from a hook on the scale and it reads the weight). I hooked that scale to the bin on the end of the train, with the entire mechanism hooked up (less the motor) and pulled the bins until I recorded the highest weight. I also pulled that same bin quickly, at first, (jerked it) to determine the highest starting weight. With that weight information, I contacted motor manufacturers, gave them the weights the motor would be pulling and they calculated the torque for me. Remamber, that in using this method, all frictional cosiderations are already accounted for. Good luck, Max

Believe it or not, calculating masses, how much you are moving how far at what rate, throwing in a conservative safety factor, and then using the nearest (larger) motor is the traditional way.

Our company specializes in variable speed dc / ac drives. We have sized everything from paper machine drives accelerating large inertias and taking minutes to get up to top speed, Unwind stands that have relatively high inertia compared to the sheet tension, Cranes, veneer lathes, and conveyors, all from the physics behind it tempered with "experience". Many of the applications have tables published with the recommended hp. That is a huge source of experience.

Sometimes with relatively new applications you can make some basic physics calculations, then verify with a torque wrench or scale of some sort and size the motor accordingly. Hydraulic systems can be calculated from flow and pressures.

In many applications where there are fractional HP or small HP requirements oversizing is so inexpensive you don't think twice (1/4Hp or 1/2 hp, one off, who cares?). As the HP requirement grows it gets costly and more care is taken. For special applications or with large volume applications more testing and verification may be warranted. (A space shuttle motor actuator would be very carefully calculated with large penalties paid for inadequate under sizing or weight and power penalties for oversizing.) In some applications for very fast response systems there is actually a point of no return for increasing HP. The larger motor has more inertia, so no net time savings is achieved. (Consider a sports car versus a transport truck. There is less mass in the car and less total hp, but faster response. But the car can't haul 50 tons. The truck has far more HP, but even if the truck has no load the car is quicker.)

So, what you end up with is an "engineering" problem in the true sense of the profession. Applied mathematics tempered with experience. Engineering is part math and part art. We are practitioners of applied science!

As an easy practical cheap method.

If you had spare a larger motor, you may use, temporarily fit this then you can measure the start and run currents, this will tell you the actual dynamic loading, rather than static, you may then choose a more suitable motor. Hope this helps.

Gentlemen:

These were great comments, and very helpful. I learned that solving engineering problems is a little uncomfortable if you are use to "the" correct answer, and the method I am using is a reasonable to solving this type of problem.

In the future I will use an analytical method and an experimental method along with the appropriate saftey factors. I guess I have to wait for the "experience" to kick in.

Thanks

--Mr. Heath

If you have a specific application, design requirements, machine geometries, (inertia or rotating parts dimensions and material (steel or Aluminum etc) I am sure we (collectively) would be fascinated at giving "a kick at the cat" for you. Then you could draw on some of our more detailed experience.

Reliance Electric used to have a manual TM-1800-A, "Solid State Drive Fundamenatls", that was an excellent overview of the techniques and equations for selection. You may want to check the Reliance or Rockwell sites and see if you can download or order. My copy is about 30 years old.