Linear Synchronous Motors – The Basics Easily Explained

Wouldn’t it be cool to have a backyard roller coaster with and LSM launch. Well, that is what this video series wants to accomplish. First we will explain what LSM is and then make a prototype.
If enough people watch, maybe we can get some money together to build one to ride

Video Transcript

Hey, Cory here with The Mentored Engineer and I’m excited about this week’s video. We’re going to talk about linear synchronous motors. Woo! And you’re probably saying, what the heck is that? Hey if you like what you see so far, please take a second to subscribe and click that notification bell so I can annoy you at least once a week.

All right so roller coasters today common thing that they’re doing is putting LSM launches on that and LSM means linear synchronous motor. So, when you think of motor, you probably think of rotary motor, a shaft that turns and then work is done off that shaft. Well, this is a linear motor. So, what does that mean? Well, it means it works in a line.

All right. And that line can be definite or somewhat indefinite, depending on how we arrange our stuff. So, for a roller coaster, generally, you don’t have LSMs the whole way. You may have 20 feet of LSM. and you can control the speed and direction of the cart that’s moving on there generally with a roller coaster you’re launching it one way cool ride like pantheon you’re going to go one way then you’re going to go come back the same way and then you’ll come back one more time through it.

So that’s a triple launch you’re using the same LSMs to launch you three times So cool things can be done with LSMs. The last part is synchronous. So, what exactly does that mean? Well, I’m not going to bore you with the technical definition, but what it basically boils down to is we’re going to put out a specific sine wave pattern and we can control the exact positioning of that.

We can accelerate up to a certain speed. We can hold that speed and then decelerate if we wanted to. So, if we were shuttling something from over here to over here, we could accelerate quickly, hold this constant velocity and then decelerate it. And then we could go back. But we can do that using this synchronous technology.

So, I’m kind of getting ahead of myself right now. We’re going to explain all this a lot more in detail later. But what’s going to happen is we’re going to output a sine wave and we’re going to control how far the peaks are apart. So, if we want a constant speed, all the peaks will be the same distance apart. If we want to accelerate, we’re going to have peaks that are far apart

and then they get closer and closer together as time goes on. And then if we decelerate, we’re going to close together and then they’re going to get bigger and bigger apart as we decelerate. So, controlling that sine wave can control the speed of whatever we’re trying to move. In this case, hopefully me.

All right. So, in this series, we’re going to talk about a whole bunch of things related to linear synchronous motor. So, there’s going to be a long series and hopefully we’re going to build a test model. And at some point, hopefully we will build an actual roller coaster with it on it. And I can’t wait.

So, a linear synchronous motor, or LSM for short, uses the principles of magnetics to make motion occur. So, two components of this are, like this one, a permanent bar magnet. This magnet is magnetized through the thickness, the very thin dimension here, so the largest surface on each side is either, in this case, south, and then on the opposite side, north. This is a permanent magnet. It will always be south and north.

It can’t change. The magnitude of the traction cannot change. It’s permanent. On the other hand, we have electromagnets. Electromagnets are cool because you can take a coil of water and put some current through it and you can get an electromagnet.

Now, as you charge them, you’re going to have a north side and a south side, but you can flip the polarity and get, you know, north side over here, the south side over here, and you can keep flipping them back and forth so that your north goes from one side to the other. I used to work in the electric utility business, and we’d go and observe people in the field doing their job, and we noticed that some people were grounding their trucks. And they’d have a spool of wire, I don’t know, between 75 and 100 feet, you know, 50 and 100 feet or so.

And anytime they would take wire off that, they would unroll the whole spool. If they only needed to go 10 feet, they would unroll the whole spool. And I ended up asking them, why you did that? And they said, well, if we don’t and that truck gets energized, that wire wheel will start spinning. And I was like, whoa, you’re right.

It’s a metal wire wheel and it will create a magnetic field with all that wire coiled on itself. So, the practice is to uncoil all the wire and make sure it’s not looped. So that cannot happen. If that thing were to spin at the end it’s going to suddenly you know not have any coils which means there’s no more driving force but it could pick up you know a couple tools in the process and fling them somewhere so you could get hurt just by things going on around that so in a normal motor like a DC rotary motor we would have permanent magnets usually like one north side and one south side and there’d be a stator in the middle which would be like this now imagine that we drilled a hole right through the center here so that this thing could rotate this way and that would be our output shaft as well.

So we put our magnet here and we would energize the coil so that the south pole here and the south pole of the magnet would start repelling each other and it would start rotating now as it comes down here we’re going to have north now and south And that’s going to attract it to itself and it’s just going to kind of stay there. But if at this point, we reversed it, we could do the same thing again. And again.

And you get the point. Alright, so that’s how a DC motor works. So, what I’ve done here is I’ve taken an aluminum C channel, and I’ve glued magnets all the way down. all right I have them alternated too with the sides that are up north south north south north south all the way down so now what I do is I’m going to take this car, and I made it.

And it’s got wheels that ride on the end of the channel. So, it rides right in there. Legos are awesome, by the way. I know I’ve said that before on this channel, but they are. All right.

And then I’ve made a 3D printed electromagnet holder. And I have, I think, 150 coils around here of, I believe, 28-gauge wire. So, I put it there. and it can slide up and down. And now if I go here, I am over a south pole and I can magnetize my electric magnet to be south as well.

So, the south and south will repel and push it up. That’s stupid. I’m not trying to go up. I’m trying to go this way. So that’s problem number one.

Think about that. Wasting energy. All right. However, if I go halfway between the two and I go south, and north on the electromagnet I’m now pushing the two souths away so south and south and then north and south I’ve got a force going this way pushing the cart up and forward and the other ones pulling it down and to the and forward as well so I’ve got the up and down canceling oh nice and motion forward.

And that occurs all the way until I get lined up with the north one perfectly. And then I run into problem number two, which is getting stuck. All right, so problem number one, wasting energy. That’s a lot of energy to waste. It’s not doing anything; it’s just pushing it up.

I don’t need to use that energy. Alright, that’s wasted energy. This is where linear synchronous motors have an advantage over their predecessor, linear induction motors. Both kind of the same principle, but linear induction motors were a lot more wasteful because of this principle. They were not being driven how we’re saying to drive them now.

So, to solve this problem of wasted energy, we want to say that whenever the poles are lined up, either being north, north, south, south or north and south together, we want the voltage there to be zero. So, whenever distance wise, those things line up, when the magnet and the electromagnet line up perfectly, we want zero volts. That’s not doing anything for us. And then what we want to do is have that be maximum when we’re halfway between one or the other. Well, if only there was a trigonomic function that would do that for us.

It’s the sine wave. All right. So, sine wave is going to look like this. When we’re lined up, it’s going to be zero and we’re going to have to time that so that it’s zero. And then we can be 100% of that sine halfway between the magnets.

So, you can see that this exacerbates problem number two, which is getting stuck. If we’re not powering when the poles are aligned, we are definitely not getting any force and we’re relying on momentum to get us through to the next spot where they’re misaligned. Well, I don’t like depending on momentum all that much. I want to positively drive this thing. I’d liken this to your bike.

All right, you’ve got two pedals. You get most of your power when the pedals are even. That’s when you’re pushing with the largest moment arm. And then as you get to the top and bottom of the stroke, you have no power that you can input. So, you can get stuck at the top easily.

But when you’re riding your bike, you have enough momentum to get you over the top. and you can keep going not a problem it is a problem however when you are going up a hill if you have to start your bike up a hill holy cow that’s a tough scenario depending on how steep the hill is you got to like you know pretty much stand on the one pedal while you can get your body weight up and on the seat and then push down with the other one in time and if you don’t make it in time you’re going to stop or fall over or something like that

but that’s not going to be good Well, somebody’s already made the solution to this. And it’s actually been in production and use for over 200 years now. And that is, say hello to my little friend. So, this is a model steam engine.

Yes, I like model railroading too, don’t judge. Model railroads are cool. And basically, up here, there’s a piston. And the piston can move in and out. And it will push on this draw bar here, the main big bar that goes all across and connects all three wheels.

You can see here it’s fully extended. So, it’s at the end of its stroke. It doesn’t have any more moment arm in between the center of the axle and the perpendicular distance to the bar. So, it’s not going to provide any more power. So, it’s stuck.

It’s at the top of the bicycle right there. We’re going to push. We can push all we want. If we’re sitting right there on the top, it doesn’t do anything. We could push with a billion pounds, and it won’t do anything.

Well, Steve mentions don’t have this problem. What they do? Oh, that’s a good question. So, on the other side, we notice that the wheels are 90 degrees difference so that while that on the other side, it is at full stroke. This side, it’s right in the sweet spot of providing power.

So, it can push these wheels back and the other one will start retracting and then it can start using the piston to retract on the opposite side. And soon you’ll get to the full power stroke of that one. So, you have even application of power Now, the problem with this is it’s uneven power. So, going back to our linear synchronous motor, how do we make this work? Well, if we have our one magnet, why don’t we add a second magnet to it?

And about one and a half times the spacing here. So, this is half of the magnet width so that we always have one lined up and one halfway in between the other one. So, while we’re de-energizing this one because it’s lined up, we could be energizing this one because it’s not lined up and it will create the power. Then we have the kind of battling back and forth for who’s getting the power. So, the mathematical way to do this is to take our sine wave for the main electromagnet and then to have this one be ahead of it.

So, it’s going to be leading us if we’re moving this way. It’s going to be the one that gets us there first. We’re going to have that be 90 degrees ahead. So, what will happen is we’ll end up with a cosine wave driving this one and a sine wave driving this one. Now, the cosine function is just a sine wave that’s leading it by 90 degrees.

So, the cosine equals the sine of the angle plus 90 degrees. But as I mentioned, this isn’t going to be smooth, even, nice power. We would actually want to increase this to at least three magnets. We can do four, five, six, seven, twelve, a thousand, a billion. The problem with that is it gets very expensive to drive all those.

You get a lot more components, a lot more wiring, a lot more crap that can go wrong. So generally, most of them use three electric magnet drivers. So, the last thing we’re going to talk about in this is we’re going to have three different output signals, each outputting a sine wave. One will be the base sine wave. The other two will be leading or lagging by 120 degrees.

okay well this is the top level overview of linear synchronous motors we got a lot to talk about we got to talk about the magnetic strength we got to talk about driving them PWM signals there’s just a ton of stuff to cover here so please subscribe and stay tuned because these videos are going to be coming out and there’s a lot to them thank you for watching this video.