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 such as this up to nearly a hundred miles an hour and you again Mary yes draw fire this is a big motor on the other hand in this thimble I have the rotors of over ten thousand electric motors and nobody is going to deny that these are small motors now a linear induction motor like this is an easy thing to understand because it's just like a flowing river this is a riverbed if I dip a piece of wood into the river turn on the water the first thing that happens is that the wood floats it rises up then when the river is flowing nicely over the weir I let go the wood and it sooner attains the same speed as the water if on the other hand I dip into the river a cylinder like this which is free to spin then the river spins it and notice which way it spins the water passing underneath it causes it as it were to roll backwards now let's look at this linear motor which is just like a bit of the baking machine we've just been seeing and this is like a river - in this case I'm going to put in a magnetic piece of wood which is a piece of aluminium I turn on the current which is like turning on the water and the Magnetic wood floats when I let it go it soon attains the speed of the river but of course in this case the speed of the river is much greater than was the speed of the water again if I dip a cylinder this time a copper cylinder into the magnetic River it spins in appropriate direction we can have a shallow river there is a shallow River or turn on some more water there is a deep river when I'm making these comparisons I'm doing what every engineer does almost every day of his life I'm 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 one breaks down if I take a heavy cylinder such as this and place it in the river don't tell on 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 is flowing that way it doesn't matter whether I use a small cylinder or a long cylinder and these are all of copper or whether I use a steel cylinder they all roll backwards a steel ball rolls the same way so there's a steel washer of course you can have a lot of fun with this this all plastic motorcar has had four steel wheels fitted there's the engine when you release it it goes quite nicely now why is this why do all these pieces of metal roll backwards along the field to understand what's going on we need to look at the shape of the field lines over this motor they come from a North Pole into a South like that and the whole field pattern is moving along like the Magnetic River now suppose we could sit at one place over the machine such as in that little square and watch the field lines go by notice the direction of the arrows at the moment the field is to the left now it's upwards to the right downwards left up right down so it's left up right down and the rotation you see his clockwise which is to say rolling backwards so the whole machine behaves like a mechanical rack-and-pinion in which this is the field and this is the rotor and here's the action well of course mechanical things are easy to understand when we make it magnetic it's a little less obvious but we put in a row of horseshoe magnets and the action as you see is very similar indeed this then we call a rack and pinion motor now in the linear motor a row of electromagnets effectively replaces the row of permanent magnets and instead of the many tooth wheel I'm going to use a very simple wheel with just two teeth when I switch on the motor the wheel spins and if I disconnect one of the wires to this motor I can stop this traveling field dead like that and now we can see the shape of it there is the field shape if I draw the wheel slowly along the surface you can see the rack and pinion action in slow motion in the stationary failed now if I reconnect I shall start the field moving again and spin the wheel now the rotary version of this machine is what drives an ordinary electric clock it has a tooth to you which moves inside a set of teeth on the stationary part when I switch it on there is a clock motor motors with steel rotors are purely magnetic machines because they don't depend for their action on any electric current flowing in the moving part unlike for example this copper cylinder when that spins it does so because an electric current is caused to flow in the copper and so I call this an electromagnetic machine now by far the most intriguing magnetic machine for me can be made simply with an ordinary paperclip first i unroll it and then wrap it around a pencil or other non conducting rod and then offer it to the linear motor this spins very well as you see and yet there is clearly no electric circuit so no current can flow in the clip now on to distinguish very clearly between magnetic machines and electromagnetic machines both types are used extensively both in industry and in the home a washing machine for example uses an induction motor which is an electromagnetic machine electric clocks as we've just seen our magnetic machines now magnetic machines get better as they are made smaller whereas electromagnetic machines are better as they are made bigger this last fact I can prove to you in more general terms by using what I call my goodness models every machine has a magnetic circuit which I represent like this the flux in that circuit is made to induce a current into an electric circuit which links it like that 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 electric circuit so flux and current are the things you want lots of the thing which stops you having as much as you'd like 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 electric circuit which is twice as big in every linear dimension is the first then its length will be twice as great but its area is four times as great so the resistance of the big circuit altogether 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 big machine four times as good as the small one so it seems as if 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 the other way around let me show you what I mean these sheets are made of rubber but it's rubber which has been impregnated with a magnetic material and they've been magnetized so that the whole of that surface is a North Pole and the whole of that is a South and yet there doesn't seem to be any measurable force between them they behave as if they were unmagnetized but if I take some scissors and cut two pieces from one of the sheets then we see how those behave there's no noticeable repulsion but at least there's some attraction and if I cut smaller pieces still then at once there's attraction and this time there's even some repulsion put one piece on top of the other with reverse polarity and it just doesn't want to know the other one so magnetic things get better the smaller they are now let's apply this principle of size to our fascinating racking pinion rotors I'm going to put a plastic tray on top of my row of coils and I'm going to roll along its surface some split steel washers a smaller one should go better still very lively now this experiment poses the intriguing question just how small things can I roll can I literally spin an individual iron filing well the answer is that I can and to demonstrate this I'm going to cover the entire surface of the motor with iron filings and switch 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 themselves from each other the spacing between the walls depends on their height so I can comb this lot like it could come my hair all I'm doing now is flattening the filing it down and as I do so they get closer together if you look at an individual wall you'll see that the bulk of the filings in it are actually at rest it's only the ones are on the outside which are moving and they're producing a gradual drift backwards along the field many the experiments we've been doing with iron filings can be done with much simpler apparatus for example all you need is it a horseshoe magnet and the means for rotating it a plastic dish can be clipped over the magnet and then into the dish we put a steel ball when I rotate the magnet slowly the steel ball just goes around opposite one of the poles that isn't very exciting nor is the fact that when I turn more quickly centrifugal force flings out the ball to the sides of the dish but if I rotate more quickly still the steel ball will become a rack and pinion motor there it goes rotating backwards in the travelling field let's put some iron filings in the dish turn the handle and see what happens once we see the walls building up and the backward rotation and something else can you see the spirals of filings running up the hill into the middle the inward movement of these filings implies there must be an outward traveling magnetic field to produce it and if this is true then it's very exciting indeed 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 Lawson shaped solids and these are rolling over and over and spiraling into the middle for the same reason that any rolling object traces out a spiral on a flat surface so we hadn't made a discovery at all but of all the phenomena we observed with the iron filings the most striking perhaps is the building up of the walls it tells us among other things then when we make a rack and pinion motor we shouldn't use a solid cylinder like this we should divide the cylinder into a number of disks these six steel disks have been spaced to match the spacing of the walls in the iron filing experiment they are going to wind up this cord and in doing so lift a two kilogram mass the disks will spin at 3,000 revolutions a minute that is a lot of power to extract from such a small amount of material in fact that rack and pinion motor produces more powerful its size than any other type of hysteresis motor the shape of those disks is very similar to the shape of a coil spring at least it is magnetically the most recent research that I've been doing has been concerned with rack and pinion motors of this kind where we put a very small spring inside a glass tube because this could operate as a self-contained pump a helical pump this spins very nicely and of course smaller ones being magnetic machines operate better still and I can spin this spring about three inches above the surface now if I can develop these to the stage where there will be small enough to operate inside living tissue then there could be very useful to the surgeon during operations if this should be successful then I'm never going to forget that it began with no more than a paperclip and a pencil this then 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's one of the biggest made its rotor is this aluminium disc so it's an induction motor and therefore an electromagnetic machine in spite of the fact that the moving part is rotary it is a test rig for linear machines let's have a look at the driving unit at this row of coils is a linear motor which has been bent slightly to fit the shape of the disc the speed of the field on this motor is nearly 300 miles an hour and motors of this kind are being developed to drive very high speed passenger carrying vehicles of the future now to test such machines on an actual track would cost a very large sum of money you'd have to build a track many miles long to get up to this kind of speed but on this disc we shall be able to get real speeds at the edge of the disc of nearly 300 miles an hour so let's switch it off barrel you you but remember whether it's a big complicated piece of machinery like this or if it's simply a paperclip and a pencil in the world of motors big and small the work you've just seen is only the beginning

Channel: Imperial College London

Views: 1,158,526

Rating: 4.9402714 out of 5

Keywords: Imperial College London, Eric Laithwaite, Magnetism, Electromagnetics, Linear Motors, Linear Induction Motor

Id: oWiYsRi2Dss

Length: 19min 41sec (1181 seconds)

Published: Fri Apr 26 2013

Reddit Comments

[u/Yoh1612]

Now this is very interesting video that you should all watch but the most interesting thing that piqued my interest was at 14:45 it almost looks like prof. created a little galaxy.