# Why can't a superconductor make a DC motor self sustaining?

Superconducting wire can host a low current magnetic field. I do not know if it supports a corresponding electrical field.

Can a superconducting wire that sustains a current accelerate a DC motor? Where is the resistance in a superconducting homopolar motor?

Please explain if I am way off of target. What am I getting wrong?

## 3 Answers

There is no reason why you couldn't build a motor using superconducting magnets, or build a simpler homopolar motor using a length of superconducting wire. There would be no heat dissipated due to electrical resistance, but of course there would still be mechanical resistance due to friction on the moving parts. The energy needed to overcome this, and to drive any load to which the motor is attached, would come from the power source, just as in any other electrical motor.

Thermodynamically, what must happen is that the current drops to zero as the motor accelerates, as the energy in the circulating current gets converted into kinetic energy. Presumably this is because the acceleration causes the charges to experience a force component opposite to their direction of travel through the wire. My electromagnetics is too rusty for me to be confident about the mechanism that causes this - but the key point is that what happens is that the force that stops the current is due to the fact that the charges in the wire are moving through a magnetic field, which is different from electrical resistance.

• Why wouldn't the motor accelerate? Or perform work. If the current models that of a regular homopolar motor, what makes the superconducting one not accelerate. (Overcome risistance). I understand it isn't possible, I just want to know why. Thanks – Vial Dec 13 '12 at 15:31
• Of course it would accelerate, and perform work, in exactly the same way that a normal motor would. If you mean "would it accelerate or perform work without needing a battery or other voltage source" then the answer is no - but in order to adequately explain why I need to have a better understanding of why you think it would. – Nathaniel Dec 13 '12 at 23:26
• When wouldn't it? Does the magnetic field break down or mutate into something unuseable? You know, a superconductor can sustain a zero volt, zero resistance current for quite some time. Nothing changes, the current runs like it would in a regular motor... but I don't see where the actual acceleration of the rotor decreases the current in the superconductor. I hope that makes more sence – Vial Dec 15 '12 at 1:22
• @Velox this is a better question than I thought. Thermodynamically, what must happen is that the current drops to zero as the motor accelerates, as the energy in the circulating current gets converted into kinetic energy. Presumably this is because the acceleration causes the charges to experience a force component opposite to their direction of travel through the wire. However, after sitting down to think about it I've realised my electromagnetics is too rusty for me to be confident about the mechanism that causes this. I hope someone else will supply a good answer. – Nathaniel Dec 15 '12 at 2:24
• ...but however it works, I'm confident that what happens is that the force that stops the current is due to the fact that the charges in the wire are moving through a magnetic field, which is different from electrical resistance. – Nathaniel Dec 15 '12 at 2:27

There are other losses in a motor besides I^2R losses. There are eddy currents in the steel as well as hysteresis losses. There are also losses from friction and wind resistance.

• Haha yeah bloody eddy currents!! As a kid I was convinced that I could spin a generator (with magnetically levitating axis) in a vacuum and it would generate electricity for free. When I read about eddy currents I nearly cried. – Mike S Dec 17 '12 at 1:59
• @MikeS, that is a good reason to cry, physical laws of our universe can be kind of bitchy sometimes – lurscher Dec 17 '12 at 3:52

there's no reason really as to why the superconductivity principle can practically make a dc-motor running as perpetual except during starting only (which needs input first). Perpetual in a sense that it can run indefinitely without requiring input current always as it becomes self-sustaining and able to run with ease even with loads.

but there are sceptics and nay-sayer or other savants and researchers who still say it is not possible ? Why, what's the reason ?

in my view, the resistance per se is actually (unit of which is the ohm) in fact the physical reaction of the material from the passage of the electrons (which constitutes current) per second flowing through the entire lenght of the conductive material which is proportionate to it's longitudinal dimension and inveresely to it's cross section and once this wire becomes superconductive, therefore no resistance at all, then the current flows indefinitely (ad infinitum) without losses. Then if that's the case, what remains is the inductance of the wire only as it is formed into a loop and the more the loop it increases the better the inductance and with this inductance (analogous to inertia) then as the electrical energy (current flow) is stored in the magnetic field and it does in fact supports the storage of the current in magnetic the flux. So, with the help of the permanent magnet's field flux and in reaction with this stored electrical energy in the magnetic flux of the wire loops as in the stator, then one in theory, can make the dc-motor run perpetually, as the rotor rotates (using permanent magnets)!, right ? So, now, why this can not be done or will not work ?

• "Skeptics and nay-sayers" say it's not possible because it isn't possible. You can't get energy out of nothing. What you're trying to do is no different from trying to build a motor out of permanent magnets. Electromagnetism conserves energy, this does not work. – Christopher James Huff Dec 24 '12 at 13:40