Select an A/C Motor for ON/OFF Control from AutomationDirect
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Learn how to quickly select an AC motor for On/Off control (as opposed to variable speed control).
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Selecting a motor can seem a little daunting but if you break it down it's really not that bad at all. The first thing everyone wants to know is what horse power do you need. That's a hard question because most of us don't have a natural feel for Horsepower. A much better question is how much torque do you need? That's something we can understand and measure. For example, suppose I have a conveyor, and I load it down with the heaviest load I expect to see, and I use a spring scale to measure how much force it takes to move the conveyor by itself with no motor or gear box. Torque is just that force times the distance from the pivot point. Suppose I needed 200 lbs. of force to move the conveyor and this radius is a quarter of a foot. Then the torque required is 50 ft-lbs. If we have a 25 to one gear box between the motor and the conveyor then the motor needs to provide 2 ft-lbs of torque. Assume we have a motor turning at 1800 rpm, that's 30 revolutions per second. And we know that 1 revolution - or 1 circumference - is 2 pi radians so that gives us 188.5 radians per second. And since 1 horse power is defined to be 550 ft-lbs per second. And we have 188.5 times 2 ft-lbs, the power we need is: .685 horse power. So a ¾ horse power motor should be enough to do the job. Me, I like to shoot for a little margin, so I might opt for a 1 horse power motor for this application. The bottom line is: don't think in terms of Horse Power -- think in terms of Torque. Get that in ft-lbs, multiply by the motor RPM and then divide by this scaling factor -- which is just all of this stuff combined into one number - and you have horse power. Maybe add in a little margin, and you can now answer the question -- what size motor do I need? Could we have measured the torque required right at the input to the gear box? Sure, assuming you have an easy way to attached the torque wrench to the shaft, of course. Then you would just drop that number right in the formula and you are done! Again, torque is something that's easy for us to get a handle on and to measure. So get in the habit of thinking in terms of torque, not horse power. We're talking about AC motors in this video, but if you find you need a lot of torque at low speeds, then take a look at DC motors. That's where they really shine. One of the leading causes of motor failure is heat. If you run a motor at full load, it will get hot -- that's ok it's designed to handle it. But if you put a fully loaded motor in a hot environment then you are compounding the problem. If you use an open motor with ventilation holes, and junk gets in there, then the motor has to work harder, which generates more heat. It always comes down to heat. And heat kills motors. So keep that in mind when sizing your motor -- you may not want to spec a motor that always runs at or near its maximum load if you know it is going to be in a hot or dirty environment. Of course, if you have a dirty environment, then you'll probably want to use a totally enclosed fan cooled motor -- or TEFC motor - as opposed to an open frame drip proof -- or ODP - motor. Enclosed motors cost a little more up front, but they save you money in the long run because they won't get fouled with dirt and grime that can lead to premature failure. An ODP motor on the other hand is designed to have cooling air run through it and is drip proof in that it is designed to route water away from the motors internals when mounted in the proper orientation -- usually with a horizontal shaft. It's important to understand though, that ODP motors are not designed for wet environments, they are really just setup to handle the occasional drip or accumulation of condensation. Can you use an "Inverter duty" motor in an on/off application? Of course. But, beware that Inverter Duty Motors are optimized for Variable Frequency Drives which is different than a motor which is optimized for use in a switched on/off application. So if you need to get the best possible performance out of an A/C motor in an on/off application, with the best torque, the best efficiency to save you money on operating costs, you'll want to stay away from Inverter duty motors and stick with the premium efficiency motors from AutomationDirect. They really are different animals designed for different applications. Check out the videos on using motors and inverters in variable speed applications to learn more about that. We're only talking about on/off control in this video. How do you know which kind of motor to buy? If you have a wash-down application like a food processing plants or something like that, then you definitely want Stainless Steel. Or if you just like spending money on shiny objects -- I guess that works too... Otherwise a rolled steel or cast iron motor is what you want. The rolled steel motors are the least expensive and probably the best choice for most general purpose applications up to around 2 horse power. The Cast Iron motors from AutomationDirect are still general purpose motors, but they can handle the larger horse power applications because they have things like bigger bearings, cooling fins that are part of the case so you get better thermal transfer, lots of little things like that designed specifically to get you the best possible performance which saves you money on operating costs. So if you want more performance and durability, then definitely consider the cast iron motors from AutomationDirect. There are a few Cast Aluminum motors in the AutomationDirect catalog. Those are primarily for inverter duty applications because they are so good at removing heat. Some motors have built in encoders. You can add an encoder to any motorized system, but that usually takes special adapters or additional idler pulleys, things like that. The motors with built in encoders save you the time and effort and parts cost of doing all of that yourself, plus, since the encoder is part of the motor it takes less space, isn't exposed to potential damage and it will be more reliable since there are fewer parts in the overall system -- no need for extra idler pulleys for example. Why would you want an encoder? There are a number of reasons why you might want a direct measurement of the shaft position and speed, but, if you are using a motor with a Variable Frequency Drive and want the absolute best system performance, then feeding encoder data back to the VFD is the way to do it. It gives you better low speed performance, tighter speed regulation, better control over the torque, etc. Check out the videos on using Variable Frequency Drives for more on using encoders to enhance performance. Motors typically come in 1200, 1800 and 3600 RPM. Which should you use? The 1800 RPM motor is your general purpose go to motor. When would you use a 1200 RPM motor? Well, remember what we said earlier -- it's all about TORQUE -- right? And since Horse power is Torque times speed ... if we lower the RPM, we get MORE torque for the same horse power! So if you need more torque but don't want to spend the money on a larger horse power motor -- and you can handle the lower RPM -- then use this little trick of using a lower RPM motor to get more torque for the same horse power! Also, keep in mind that with the 1200 rpm motor, you may be able to get away with using inexpensive belts and pulleys instead of expensive gearboxes. And since belts and pulleys are more efficient than gearboxes you save on operating costs every day. When would you need the higher 3600 RPM shaft speed motors? These are great for fan and pump applications -- they save you the cost of having to gear them up to speed. AC motors operate on 115/208/230/460 and 575 volts. Which do you use? You usually don't have control over that. You just buy the motor that works with what you have available. At home that will be 115 or 230. In industry, that could be 208, 230, and 460. The 575 VAC motors AutomationDirect has are primarily used in Canada, though there are still some older mills in the US that use 575Volts. We covered a lot here, and it might still seem a little overwhelming, especially since AutomationDirect has over 200 motors to choose from, but the good news is the AutomationDirect website makes it easy to select a motor. Just click on Motors, then over here select the horse power you want -- we'll use 1 horse power -- the voltage you will be operating at -- we'll use this 208 230 460 guy -- the speed -- we'll use 1800 -- and lets say we want to use it with an invertor -- a Variable Frequency Drive -- and boom, just like that we have narrowed our search down from over 200 motors to just one. Easy. If you need help, please don't hesitate to call AutomationDirect's free, award winning tech support during regular business hours. They will be happy to help you out. And don't forget the forums -- there are lots of folks there that love to share their years of experience -- just don't post any questions to support there -- they don't monitor the forums on a regular basis. Check out the other videos to learn more about how to use the the automation products at AutomationDirect.
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