How To Size a Motor from AutomationDirect
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It can be easy to size motors with the right approach. This video covers key topics on how to size an electric motor. AutomationDirect carries motors from brands like: IronHorse, Leeson, Marathon and WEG.
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AutomationDirect.com sells an incredible selection of motors from a wide range of brands to fit most any application. So, you need to buy a motor and have to figure out what horsepower you need. But what is horsepower? The power of a horse? 746 watts? Or, the practical definition for sizing a motor is 550 lb-ft per second. This allows us to convert torque, which we can measure into horsepower. Torque is just the force times the distance from the pivot point. A quick note on torque, at least the US version: length and force are often interchanged. It may be shown as or said as foot-pound or pound-foot. In this conveyor example, we’d measure with maximum load applied. We can measure torque with a torque wrench at the end of a roller shaft, or with some lever attached and a spring scale, or we can measure the force required to move the conveyor. If we use a lever, we would measure the force on the scale and multiply it by the length of the lever in feet. If we measure the force of moving the conveyor, we need to multiply that by the radius of the roller in feet. So, in all cases, we would measure 50 ft-lbs. Next, if there are any mechanical speed reducers they must be accounted for. We simply divide the shaft torque by the reduction. So with this 25:1 gearbox, we’d divide the 50 ft-lbs by 25 which gives us 2 ft-lbs at the gearbox input. Since horsepower equals torque times RPM divided by 5252 we can see that with a 1800 rpm motor, we’d need a minimum of 0.685hp or with a 1200 rpm motor, we’d need 0.457hp. Notice that as speed decreases so does the horsepower requirement. So, if we were using a VFD we would size the motor at its full speed and allow constant torque to keep the conveyor running at lower speeds since typically motor horsepower will decrease as speed decreases but torque remains the same. So, our example shows we need 0.685hp at 1800 rpm. Keep in mind that we always must round that up to a standard horsepower so the minimum would be ¾hp. I’d probably select a 1hp motor especially if it is in a hot area since oversizing the motor will allow it to run cooler. So, we head over to the store, pick motors, pick AC motors, select 1hp, 1800 rpm, I’ll go with 480V, and now we can buy the right motor for our application. AutomationDirect is providing the products and support you need to make us your common sense way to buy industrial controls. If you need more help picking a motor please see our tech support options here. We have many other motor videos please click here to view all. Click here to subscribe to our YouTube channel for upcoming products and solutions.
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