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Could a Lorentz engine (like the ones that move the write head inside of hardrives) be used to run car ? Would it be possable to get it producing 60hp, and a decent amount of tourque, if so how large would it have to be / what would its power consumption be ?

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I couldn't find any reference to a "lorentz" engine. I found lorenz engines (jdeklerck.tripod.com/engines_e.htm#Lorenz) but I doubt those are what you are referring to. Can you perhaps provide a link to an example of such an engine? –  Benjamin Horowitz Jun 26 '12 at 17:23
    
Drive heads are moved by voice coils - since they use a lorentz force I suppose they are lorentz engines? –  Martin Beckett Jun 26 '12 at 17:28
    
sure, the sort of engine that moves the read/write head inside of a hard drive. - explained here in this video (skip to 1:10) youtube.com/watch?v=Wiy_eHdj8kg –  sam Jun 26 '12 at 17:46
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Every electric motor runs using the Lorentz force, so there is no difference in principle between the motor in the hard drive and an electric car motor. There are commercial 60 hp electric motors that run electric cars, so the answer is yes. It's power consumption is, well, 60 hp. That's 45000 watts, plus a little more for heating the engine and so on, so say an even 50,000 watts. This randomly selected 60hp electric motor is about 18 inches across: (look for 364T in this link).

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Yes, such an engine would be possible. For a circular motion you would need to modify the setup a little bit but not significantly.

While Ron is absolutely right that there is no difference in principle a big engineering/economical problem is that an engine with such a design would need very large and powerful permanent magnets. NdFeB magnets in a size enough to create 50kW would be quite heavy and expensive.

A standard electrical motor such as the one shown below from a vacuum cleaner does not require a large amount of rare earth materials. Instead all fields are created by coils wound around a high permeability material (iron alloys or for high end motors alloys with some amount of Nd).

enter image description here

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I didn't distinguish between permanent magnets and induction magnets, since I am not sure that the hard-drive magnets are permanent--- if so the motor would lose precision if the magnets demagnetized a little. They are probably placed inside coils to keep them magnetized, so the result is effectively the same. Even if not, I don't think OP cared if the magnets were permanent or not. –  Ron Maimon Jun 27 '12 at 17:27
    
@RonMaimon: Modern hard drives use permanent magnets, as far as I know without any additional coils. You can see in the linked video the large magnet that is removed at 1:08. Other than that a standard electric motor does not differ a lot from the actuator in a hard drive. So I can only guess that the OP wanted to emphasize this fact. –  Alexander Jun 27 '12 at 19:33
    
Permanent magnets are quite common in the electric motors used in cars (see p. 10 of this document for an example). –  mmc Jun 27 '12 at 19:45
    
@RonMaimon As HDD voice coil motors use (quite sophisticated) closed-loop controllers, they are quite insensitive to variations in the magnetic field strength. –  mmc Jun 27 '12 at 19:49
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