Professor Eric Laithwaite: Motors Big and Small - 1971

ready mary yes four three two one fire this is a linear induction motor that uses about 100 kilowatts and can accelerate four pound missiles like this up to almost a hundred miles an hour and you again mary yes it shoots fire this is a big motor on fire on the other hand, in this thimble I have the rotors of more than ten thousand electric

motors

and nobody 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 this is a bed of the river if I dip a piece of wood in the river I open the water the first thing that happens is that the wood floats and rises then when the river flows well over the spillway I let go of the wood and soon it reaches the same speed as the water if in change i submerge in the river a cylinder like this that is free to rotate so the river spins it and look at which direction it rotates the water that passes under it makes it as if it rolls backwards now let's see this linear motor that is like a part of the baking machine that we just saw and this is like a river - in this case I'm going to put a magnetic piece of wood which is a piece of aluminum that I turn on the current which is like turning on water and the magnetic wood floats when I leave it go, it 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 I immerse a cylinder this time a copper cylinder in the magnetic river rotates in the appropriate direction we can have a shallow river there is a shallow river or turn on a little more water there is a deep river when i am making these comparisons i am doing what every engineer does almost every day of his life i am using an analogy now a good engineer knows when to use an analogy and where it breaks down and I'm going to show you where this breaks down if I take a strong Cylinder like this and put it in the river.

Don't tell the water, you see the flow roll the cylinder down the river. If I now put the same cylinder in the magnetic river and turn on the magnetic water, it rolls upstream because you see the river. it's flowing 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 of course there is a steel washer you can have a lot of fun with this this all plastic car has four steel wheels fitted there is the engine when you drop it it runs pretty good now why is this why all these metal pieces roll backwards across the field to understand what is happening Now we have to look at the shape of the field lines on this motor.

They come from such a North to South Pole and the whole field pattern moves like the Magnetic River. Now suppose we can sit somewhere on the machine like in that little square and watch the field lines go by, watch the direction of the arrows at the time the field is to the left now it's up to the right down left up right down so it's left up right down and the rotation you see is your clockwise ie rolling backwards so the whole machine behaves like a mechanical rack and pinion where this is the field and this is the rotor and here is the action, 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 as you can see the action is very similar in fact we call this a rack and pinion motor now in the linear motor one row of electromagnets effectively replaces the row of permanent magnets and instead of the many toothed wheel I am going to use a very simple wheel with only two teeth when I turn on the motor the wheel spins 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 is the shape of the field if i draw the wheel slowly along the surface you can see the action of the pinion and the rack in slow motion on the hover failed now if i reconnect i will start moving the field again and i'll spin the wheel now the rotary version of this machine is what drives an ordinary electric clock it has a tooth that moves inside a set of teeth in the stationary part when i turn it on there is a clock motor motors with rotors of steel 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 now i call it an electromagnetic machine by far the most intriguing magnetic machine to me can be made simply out of an ordinary paper clip, i first unroll it and then wrap it around a pencil or other non-conductive rod and then offer it to the linear motor, as you can see it rotates very well and yet there is clearly no electrical circuit so no current can flow in the clip now on to distinguish very clearly between magnetic machines and electromagnetic machines both types are widely used both in industry and at home a washing machine, for example, uses an induction motor which is an electromagnetic machines electric clocks as we have just seen our magnetic machines now magnetic machines get better as they get smaller while electromagnetic machines get better as they get smaller they get bigger this last fact i can prove in more general terms using what i call omg it models that every machine has a magnetic circuit which i represent like this the flux in that circuit is made to induce a current in an electrical circuit that You tie it like this, the force you can get 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 that prevent you from having everything you want of course is resistance in the case of electrical circuit and magnetic resistance or reluctance as we call it in magnetic circuit the resistance of an electrical 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 each linear dimension is the first then its length will be double but its area is four times as large so the resistance of the large circuit in total is just the 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 big machine four times better than the small one, so it seems that the rule for electromagnetic machines is that the bigger the better now with pure magnetic motors the rules seem to work backwards let me show you what I mean these blades are made of rubber but it is rubber that has been impregnated with a magnetic material and they have been magnetized so all of that surface is a North Pole and all of that is a South and yet there doesn't seem to be any measurable force between them, they behave as if they're not magnetized, but if I take a pair of scissors and cut two pieces from one of the leaves, we see how they behave, there is no noticeable repulsion, but at least there is some attraction and if I cut smaller pieces, then immediately there is attraction and this time there is even some repulsion.

I put one piece on top of the other with reverse polarity and it just doesn't want to meet 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 across it, a smaller one should work even better. very lively now this experiment raises the intriguing question of how little things can i get rolling can i literally get an individual iron filings spinning well the answer is i can and to demonstrate this i am going to coat the entire surface of the motor with iron filings and turn it on and you'll 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. spacing each other, the space between the walls depends on their height, so i can style it like it's my hair, all i'm doing now is flattening it, filing it down, and as i do it, it gets more together if you look. on an individual wall you will see that most of the filings on it are actually at rest, only the ones on the outside are moving and gradually drifting backwards across the field many of the experiments we have done doing with iron filings can be done with a much simpler apparatus for example all you need is a horseshoe magnet and the means to rotate it, a plastic plate can be clamped over the magnet and then on the plate we put a steel ball when I spin the magnet slowly the steel ball just spins in front of one of the poles which isn't very exciting and neither is the fact that when I spin faster the centrifugal force throws the ball to the sides of the plate but if I turn the steel still faster the ball will become a rack and pinion motor there it goes spinning backwards in the displacement field let's put some iron filings on the plate turn the handle and see what happens once we see that the walls build up and the backward rotation and something else you can see the spirals of filings going up the hill towards the center, the inward movement of these filings implies that there must be an outward traveling magnetic field to produce it and if this is true, so it's very exciting because centrifugal magnetism, which is what this would be, is unknown at present, but if we turn the handle more slowly and watch what happens in slow motion, we can see that the filings form into solids Lawson-shaped and these roll over and over again and spiral towards the center for the same reason that any rolling object spirals on a flat surface, so we hadn't made any discoveries, but of all the phenomena we observed with the iron filings, the most surprising perhaps is the construction of the walls, it tells us among other things that when we make a rack and pinion motor we should not use a solid cylinder like this we should divide the cylinder into several discs these six steel discs have been spaced to match the spacing of the walls in the iron filing experiment they are going to wind this wire and as you do so lift a two kilogram mass the discs will spin at 3,000 revolutions per minute that's a lot of power to extract from such a small amount of material, in fact, the rack and pinion motor produces more power for its 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 it's magnetically, the most recent research I've been doing is on rack and pinion motors of this type, where we put a very small spring inside a glass tube because this could work as a self-contained pump a helical pump this spins very well and of course the smaller ones are magnetic machines which work even better and I can spin this spring about three inches above the surface now if i can develop them to the stage where it will be small enough to operate inside living tissue then it could be very useful to the surgeon during operations if this is successful then i will never forget that it started with nothing more than a paper clip and pencil this later there is the world of small motors now i don't have to tell you that this is a big motor in fact in terms of damage it is one of the biggest in fact its rotor is this aluminum disc so it is an induction motor and therefore an electromagnetic machine despite the fact that the moving part is rotary it is a test stand for linear machines let's take a look at the drive unit in this row of coils it is a linear motor that has been slightly bent to fit the shape of the drive field speed on this engine is nearly 300 miles per hour and engines of this type are being developed to drive very high speed passenger transport vehicles in the future now to test such machines on a real track would cost a 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 actual speeds at the edge of the drive of almost 300 miles per hour, so let's turn it off. remember if it's a big complicated piece of machinery like this or if it's just a paper clip and pencil in the world of motors big and small the job you just saw is just the beginning