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My circuits class is teaching that in AC power, there are two kinds of loads. One is a resistive load, with absorbs power at all times. The other is a reactive load, which absorbs zero average power.

I'm trying to understand this concept in relation to motors, which are inductive.

A superconducting motor has no resistance. Yet it will have an inductance. If the superconducting motor is turning something, it must be receiving power. Therefore it must be resitive. However, the coils themselves have no resistance. What is going on here?

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Electric power that is converted to mechanical power by a motor is real power. The motor appears to be mostly resistive to the AC power supply, but that is the effect of the power conversion not actual resistance in the motor.

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  • \$\begingroup\$ I believe that you are correct. However, I'd like a resource that I can cite about this. Where can I find that? \$\endgroup\$ – axsvl77 Oct 18 at 2:25
  • \$\begingroup\$ Any electric machinery text or lecture notes will present an equivalent circuit for the machine. That will show the internal resistance and inductance of the machine and either a variable resistance or an opposing voltage source that represents the load. The equivalent circuit will vary according to the type of motor. Variable resistance is used for induction motors and an opposing voltage source is used for DC and synchronous motors. It appears the synchronous motors are used for superconductive motors. \$\endgroup\$ – Charles Cowie Oct 18 at 2:41
  • \$\begingroup\$ Perhaps this will help: electricalacademia.com/synchronous-machines/… \$\endgroup\$ – Charles Cowie Oct 18 at 2:45
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Someone correct me if I am wrong.

Let’s start with just a superconducting inductor. It is a purely reactive load so you will lose no energy in it and receive it all back eventually. The energy is stored as a magnetic field and will be converted back to a current when allowed to.

If you have a normal inductor with both resistive and reactive qualities, then you will lose energy to the resistance as heat and regain the reactive energy stored as a magnetic field.

Now let’s look at a normal motor. It is an inductor but it is also spinning a mass that experiences momentum and friction. Now you are losing energy as heat due to resistance and friction, but the motor is also storing energy as a magnetic field and momentum. This energy can be recovered.

So if you have a superconducting motor turning a mass in a vacuum with frictionless bearings, it will be purely reactive.

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  • \$\begingroup\$ As requested :-) : IF you deliver energy to a load that requires real power. The example rotating mass may lose minimal energy once rotating BUT requires energy to accelerate it to speed and energy to brake it (either explicit braking or from losses.) \$\endgroup\$ – Russell McMahon Oct 18 at 8:53
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It would perform better than motor without superconducting windings as it won't heat. The resistance is the pure loss, all the equations in electrical engineering are valid also in case of superconducting material, just insert R=0;

The electrical machines are devices that convert electrical power into mechanical and vice versa. The power (current & voltage) is dictated by load, not by resistance. The resistance is just a parameter that is related to the loss of the electrical machine.

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