Professor Eric Laithwaite: Motors Big and Small - 1971

Ready Mary, yes, four, three, two, one fire, this is a linear induction motor, it uses about 100 kilowatts and it can accelerate four pound missiles like this up to almost a hundred miles per hour and you again, Mary, yes, shoot, this is a big engine. On the other hand, in this thimble I have the rotors of more than ten thousand electric

motors

and no one is going to deny that these are

small

motors

. Now a linear induction motor like this is something easy to understand because it is like a flowing river, it is a bed. of the river, if I submerge a piece of wood in the river, I turn on the water, the first thing that happens is that the wood floats, it then rises when the river flows well over the weir, I release the wood and before it reaches the same speed as the water, if for On the other hand, I immerse a cylinder like this one in the river that can rotate freely, then the river makes it rotate and observe in which direction it rotates, the water passing underneath makes it, so to speak, roll backwards, now let's look at this linear motor which is like a part of the baking machine that we just saw and which is like a river;

In this case I am going to place a magnetic piece of wood, which is a piece of aluminum that I light. the current which is like spinning the water and the magnetic wood floats when I let it go soon reaches the speed of the river but of course in this case the speed of the river is much greater than the speed of the water again if This time I submerge a cylinder, a copper cylinder in the magnetic river, rotate in the appropriate direction, we can have a shallow river, there is a shallow river, or open a little more water, there is a deep river, when I am making these comparisons, I am doing what every engineer does it almost every day of his life.

I'm using an analogy. Now a good engineer knows when to use an analogy and where it fails, and I'll show you where this one fails if I take a strong Cylinder like this and put it in the river, don't tell the water, you see the flow rolls the cylinder downstream. If I now place the same cylinder in the magnetic river and turn on the magnetic water, it rolls upstream because you see the river. It flows that way, it doesn't matter if I use a

small

cylinder or a long one and these are all copper or if I use a steel cylinder, they all roll backwards, a steel ball rolls the same way, so of course , there is a steel washer.

You can have a lot of fun with this all-plastic car that has four steel wheels installed. The engine is there when you let it go. It works quite well. Why is it like this? Why are all these pieces of metal rolling backwards across the field to understand what's going on? Next we have to observe the shape of the field lines on this motor, they come from a north pole to a south pole like this and the entire field pattern moves like the magnetic river. Now suppose we could sit in a place on the machine like in that little square and watch the lines of the field go by, watch the direction of the arrows right now the field is to the left, now it's up, to the right , down, left, up, right, down, then it is left, up, right, down and rotation, see its clockwise, that is, rolling backwards so that the whole machine is behave like a mechanical rack and pinion where this is the field and this is the rotor and here is the action.

Well, of course, mechanical things are easy to understand when we make them magnetic. a little less obvious, but we put in a row of horseshoe magnets and the action, as you can see, is very similar. In fact, we call this a rack and pinion motor. Now, in the linear motor, a row of electromagnets effectively replaces the row of permanent magnets and takes its place. of the wheel with many teeth I am going to use a very simple wheel with only two teeth when I turn on the motor, the wheel rotates and if I disconnect one of the wires from this motor I can stop this moving field like this and now we can see its shape, there it is the shape of the field, if I draw the wheel slowly along the surface you can see the action of the rack and pinion in slow motion in the steady state it failed now, if I reconnect I will start moving the field again and turn the wheel now the rotating version of this machine is what drives a common electric clock it has a tooth that moves within a set of teeth on the stationary part when I turn it on there is a clock motor motors with steel rotors are purely magnetic machines because they do not depend for their action on any electric current flowing in the moving part, unlike, for example, this copper cylinder, when it rotates, it does so because an electric current is made to flow in the copper and that is why I now call it electromagnetic machine.

By far the most intriguing magnetic machine to me can be made simply with an ordinary paper clip, first I unroll it and then wrap it around a pencil or other non-conductive rod and then offer it to the linear motor, this spins very well as you see and still. There is clearly no electrical circuit so no current can flow in the clip. Now we are going to distinguish very clearly between magnetic machines and electromagnetic machines. Both types are widely used in both industry and home. A washing machine, for example, uses an induction motor which is an electromagnetic machines electric watches as we just saw our magnetic machines now magnetic machines get better as they get smaller while electromagnetic machines get better as they get bigger I can demonstrate this last fact in more general terms using what I call My God, each machine has a magnetic circuit that I represent like this, the flow in that circuit is made to induce a current in an electric circuit that links it in a way that the force that can be obtained from a machine is proportional only to the product of the flux in the magnetic circuit and the current in the electrical circuit, so flux and current are the things you want, many of the things you want. prevent you from having as much as you want, of course, is the resistance in the case of the electric circuit and the Magnetic resistance or reluctance as we call it in the magnetic circuit, the resistance of an electric circuit is proportional to its length and inversely proportional to its area , so if I take a second electrical circuit that is twice as large in every linear dimension as the first, then its length will be twice as large but its area is four times as large, so the resistance of the large circuit in total is only half the resistance of the small circuit and when we put the two circuits together to make our machine, the combined effect of the two is to make the large machine four times better than the small one, so it seems that the rule for electromagnetic machines is that the bigger they are, the better they are now with purely magnetic motors the rules seem to work backwards, let me To show you what I mean, these sheets are made of rubber, but it is rubber that has been impregnated with a magnetic material and they have been magnetized so that all that surface is a North Pole and all that is a South Pole and yet there doesn't seem to be any measurable force between them, they behave as if they weren't magnetized, but if I take a pair of scissors and cut two pieces of one of the leaves, we see how they behave, there is no noticeable repulsion, but at least there is some attraction and If I still cut smaller pieces, there is immediately attraction and this time even some repulsion.

I put one piece on top of the other with reverse polarity and it just doesn't want to know the other, so magnetic things get better the smaller they are. Now let's apply this sizing principle to our fascinating rack and pinion rotors. I'm going to put a plastic tray on top of my row of coils and roll some split steel washers along its surface, a smaller one should work even better. very lively now this experiment raises the intriguing question: how little things can I roll? Can I literally spin a single iron file right? The answer is that I can and to demonstrate it I am going to cover the entire surface of the engine with iron filings. and turn it on and see what happens well, I don't know what you expected to happen, I know the first time I tried this I certainly didn't get the result I expected, look at the way the walls are forming walls of filings. separating each other, the space between the walls depends on their height, so I can style it like my hair.

All I'm doing now is flattening it and filing it down and as I do that they get closer to each other if you look. On an individual wall you will see that most of the filings it contains are at rest, only those on the outside move and produce a gradual backward movement across the field in many of the experiments we have performed. Making with iron filings can be done with much simpler apparatus, for example all you need is a horseshoe magnet and the means to rotate it, you can hold a plastic plate over the magnet and then inside the plate we place a steel ball when I spin the magnet slowly the steel ball just spins in front of one of the poles which is not very exciting nor is the fact that when I spin faster the centrifugal force throws the ball on the sides of the plate, but if I spin faster, the steel still spins. the ball will become a rack and pinion motor there it goes rotating backwards on the moving field, let's put some iron filings on the plate, turn the handle and see what happens once we see the walls build and the backward rotation and something else, can you see the spirals of filings going up the hill towards the center, the inward movement of these filings implies that there must be a magnetic field traveling outwards to produce it and if this is true then it is very exciting because magnetism centrifugal, which is what it would be, is unknown in the present, but if we turn the handle more slowly and watch what happens in slow motion, we can see that the filings form into Lawson-shaped solids and these roll over and over and They form spirals towards the center for the same reason that any rolling object traces a spiral on a flat surface, so we had not made any discoveries, but of all the phenomena we observed with iron filings, the most striking is perhaps the formation of the walls.

It tells us among other things then when we make a rack and pinion motor we should not use a solid cylinder like this we should divide the cylinder into several disks these six steel disks have been spaced to match the space of the walls in the experiment filing iron are going to wind this cable and By doing so, lift a mass of two kilograms, the discs will rotate at 3000 revolutions per minute, which is a lot of power to extract from such a small amount of material, in fact, the pinion motor and zipper produces a more powerful size than any other type of hysteresis.

Motor, the shape of those discs is very similar to the shape of a coil spring, at least magnetically. The most recent research I've been doing has focused on rack and pinion motors of this type where we put a very small spring inside. a glass tube because it could work as a self-contained pump, a screw pump that spins very well and of course the smaller ones which are magnetic machines work even better and now I can spin this spring about three inches from the surface if I can develop them to the stage where there will be living tissue small enough to operate on then it could be very useful to the surgeon during operations, if this is successful I will never forget that this started with nothing more than a clip and a pencil.

So it's the world of small engines. Now I don't have to tell you that this is a big engine. In fact, in terms of damage, it is one of the largest. Its rotor is an aluminum disc, so it is an induction motor and therefore an electromagnetic machine. Although the moving part is rotating, it is a test bench for linear machines, let's take a look at the driving unit in this row of coils, it is a linear motor that has been bent slightly to fit the shape of the disc, the speed. The field speed of this engine is almost 300 miles per hour and engines of this type are being developed to propel the passenger transport vehicles of the future at very high speed.

Now, testing such machines on a real track would cost a very large sum of money. You'd have to build a track many miles long to reach this kind of speed, but on this drive we'll be able to get real speeds at the edge of the drive of almost 300 miles per hour, so let's turn it off. Remember whether it's a big, complicated machine like this or whether it's just a paperclip and a pencil in the world of engines, big and small, the work you just saw is just the beginning.