How To Size Electric Motors for Any Project: A Beginners Guide #085

it doesn't matter if i'm making an electric go-kart with my kids or even building an insanely powerful bat swinging machine there's one question i have to answer over and over again in my projects what size motor do i need to get the job done now there are many parameters that go into selecting the right type of motor today we're going to be focused on how much power do you need it doesn't matter if you're buying your motor off the shelf or you've pulled a motor out of a no broken appliance somewhere on that motor is going to be a power rating and that number is really important because it tells you how much work the motor can do and it tells you how quickly the motor can do the work to illustrate this i've subpoenaed my kids to move some bricks for me to calculate the power output of my kids we need to know three things how much force they're applying to the bricks how far they move the bricks and how long it took for them to move the bricks put all those numbers in the formula and you get power in this case if i'm not satisfied with the power output i can always input some motivation to get better results yes yes but something interesting happens when you go from moving in a straight line to moving in a circle in fact if i take these two pulleys to do some work it matters a great deal which one i put on the motor to illustrate that we're going to use a seesaw many of us have experienced the sensation of sitting on a seesaw with someone whose weight doesn't equal ours even kids without thinking about it intuitively adjust their position on the seesaw to try to balance out their weights what you're actually doing is balancing out the torques torque is a combination of the force you're applying in this case your body weight multiplied by the length of your lever or the distance to the center of rotation these two variables are equally important in this example you can see we've got twice the weight on one side being balanced out by a lever that's twice as long now i want you to think of this pulley as a lever with the length of the lever being the radius of the circle here the motor has a fixed amount of torque that it can put out and just like we saw in the example with the seesaw if you increase the length of the lever on one side you're going to need to decrease the force in order for them to be equal so using different size pulleys will give you different amounts of force output but there's an interesting trade-off because the arc length here is much greater on this one i've given up some force but i've increased in speed in fact they're directly proportional putting this one on the motor will give me a lower speed but more force output putting this one on the motor will give me a much higher speed but a lot less force now that we have that foundation let's take a closer look at the power formula here in the us the horsepower is the customary unit but i'll be happy to show you what's on the screen here as well since pretty much everyone else in the world uses that we're simply multiplying torque times speed and dividing it by a constant you can also back calculate whatever variables you need let's take this motor for example we have both the horsepower and the speed but we want to figure out how much torque this motor has we can rearrange this formula a little bit and you can calculate how much torque the motor is producing at that speed there's one more scenario i want to mention and that is what if you have a motor with a power rating on it but there's no speed under those circumstances you're going to need to get an rpm gauge and actually measure the speed of the motor and with that knowledge we can get to work for this first example let's look at how i selected motors to automate my table saw to size the motors here there are two basic questions we need to answer one how much torque does it take to rotate this handle and two how fast do i want to rotate the handle let's start with speed that's something that's entirely made up by you right i could have this go from 45 degrees to zero degrees in a fraction of a second that would be ultra fast spinning of the handle and require a lot more power of course you can select the speed however you like but i decided completely arbitrarily to have the speed be about 50 faster than what it would be if i was rotating it by hand i figured that out by counting the number of rotations it takes to go from one extreme to the other and then i timed myself going from one extreme to the other putting those two together will give you the rpm that you desire now let's take a look at the torque requirements for that you're going to need a scale so i bought this on amazon but any old fish scale will do we've got to set the pounds and you're going to hook this on there we go and then pull on it there's a couple things to think about you don't want it to be down like this i mean this scale doesn't weigh very much but in this orientation you are technically also including the weight of the scale and we don't want that the scale needs to be tangent to the circle that you're pulling another way to think of that would be if you drew a straight line from the center of the wheel to the center of the handle you want your scale to be perpendicular to that line you just drew we're going to clear the scale out make sure it's at zero and then you're going to start pulling four one four two there we go so we're going to say about 4.2 pounds to get the handle to start to rotate once the handle starts moving the force required to move it will go down a little bit you're primarily interested in that initial peak number what's the highest amount of force required to get it going from a dead stop so now we know the force required we just need to measure the distance at which that force is being applied i'm going to measure from the center of the handle to the center of the wheel which is 45 millimeters well that gives us all the raw data we've got our rpm our torque so now we can calculate power and we can select a motor here's the formula for kilowatts as well as horsepower and the first thing i want you to do is pay attention to your units make sure if you measured in centimeters for example that you convert that to meters and kilograms and newtons are not equal so i'm going to make up some numbers because i don't actually remember what i just measured in the shop let's say i want 100 rpm and we need a torque which includes 0.1 feet multiplied by 3 pounds we're going to divide that by 52.52 so in this totally made up example the horsepower requirements are quite small but we're not done we need to add something called a safety factor safety factor is a margin that we add to the number we calculated in order to account for temporary overloads of the motor and a few other things now safety factor can be a really deep well in fact it's related to another number found on many motor labels called service factor but i'm going to put a pin in both of those topics for now and i'll come back to that at the end of the video in this particular case because we're selecting a stepper motor i'm going to use a safety factor of 2 but that's pretty high for most other motor types so again we'll come back to that a little bit later now that we have our safety factor let's multiply that by the horsepower we calculated a minute ago 0.0114 or about 8 watts in terms of how much power we need this is it all we need to do now is go and select a motor that matches these power and speed requirements with stepper motors though i want to go one step further regarding the power because most of the time with the specs that you get from a stepper motor show you what's called pull out torque so let's look at a chart from one of the motors i'm using on my cnc table saw to explain what i mean i want you to notice a couple things first it says this is pull out torque this is basically the torque where your performance starts to drop the motor is going to start to lose steps so we don't want to be at that limit the other thing i want you to notice is that the units are in newton centimeters the final thing i want you to notice is that as the speed increases the torque goes down we know we're looking for something a little over 40 we're just going to say just barely under 50 newton centimeters and we're at 100 rpm so that definitely puts us in a safe margin here and you can see that even if we increase the speed quite a bit we're still way down in a very safe zone but let's say we needed more torque than that let's say that we were up around 200 newton centimeters and the speed was also higher at about 330. in this situation there's a couple things to consider one you've already added a safety factor so technically this should still work if it was me i would still try to inch away from that line a little bit but i'd also be weighing that against things like is this a motor i already have am i buying a new one anyway and how much difference does the cost if i get one that's just a little bit bigger so i have to leave those details up to you but what about examples where you don't know all the loads in advance in this particular case when i was building my bandsaw or even my disk sander i didn't just need to know the power to move the disc or the blade in this case i needed to know how much power it takes to actually saw the wood or saw steel which is also something i wanted to be able to do on this bandsaw in those particular cases i use what i call the experience method i simply look at what other manufacturers have done in similar applications like looking at disk sanders or in this case looking at band saws or even steel cutting band saws to get an idea of how much power they've decided they need in order for their machine to work properly and then that gives you a really good starting point now you might be tempted to add a safety factor to that but keep in mind that they've probably already added a safety factor so unless you know that tool to be underpowered for your particular applications i probably wouldn't add a safety factor to that number kind of feel like this goes without saying but you could also just simply experiment through trial and error which is what i did with this miter saw build here but there's a whole video dedicated to this project so if you want to know more about what happened with this build you can click on the link in the description what i've shown you so far are the easiest methods for getting through most diy projects if that wasn't quite sufficient for you that means you're going to need to do a little bit more math there are lots of formulas online available for calculating what's called rotational inertia it's basically a measurement of how difficult it is to get something to rotate under those circumstances you'll figure that out for each part of your system add them together and that'll allow you to calculate how much force is needed to get everything spinning that information is available in the description and i'll also put some more video links down there for you okay i want to be the first to admit there are a whole bunch of other topics that need to be covered along with this one when it comes to selecting a motor for example what kind of duty cycle do you need is this thing going to be running 24 hours a day or is it going to be running for five minutes at a time so that's duty cycle number two what's the environment like is it super hot super cold is your motor going to be under water all of those affect what type of motor you select and what type of enclosure you select do you need speed control for your motor that's going to affect what type of motor you select does your motor need to be mobile in other words does it need to be powered by a battery in those circumstances you're going to need either special controls or it's going to need to be a dc motor and what about all those other numbers that we see on the motors label like service factor what do those things mean and which ones matter to my situation fortunately for you i've covered a lot of these in previous videos so instead of making this one an hour and a half long i'm going to put links in the description so that you can go to those other videos and fill in some of those details thanks for watching you

Channel: Jeremy Fielding

Views: 638,764

Keywords: Electric Motors, motor selection, calculate horsepower, calculate power, estimate power needs

Id: 64Ky4mJE7bU

Length: 11min 21sec (681 seconds)

Published: Sun Oct 11 2020