# Professor Eric Laithwaite: Motors Big and Small - 1971

Feb 27, 2020
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

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and nobody is going to deny that these are

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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

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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.