Switched Reluctance Generator Question

[nuckythompson]

Hi there, I am reading about wind generation and trying to learn about switched reluctance generators.

I can picture the rotor poles aligning with the stator concentrated windings (phases) to allow a path for the magnetic flux but how does this induce a voltage in the stator? I am assuming this is a basic principle that I am not understanding.

Any help or literature/websites that could help would be much appreciated.

Thanks

 

[winnie]

I also find this difficult to wrap my head around, and honestly don't fully understand it. Here is my stab at a hopefully helpful analogy:

First an analogy. Consider a variable capacitor. You set the plates close together, and charge the capacitor up. Now you disconnect the circuit from the plates, and mechanically pull the plates apart. What happens to the voltage on the plates?

Since the capacitor is disconnected from the circuit, the charge cannot change.

Since the plates are being pulled apart, the capacitance has to go down.

To store the same charge in a smaller value capacitor, the voltage must go up! And since the charge is the same, the energy stored in the capacitor also has to go up.

No magic here; you are doing mechanical work to pull the plates apart, and that mechanical work is being stored as electrical energy in the capacitor.

Now back to your switched reluctance generator. You have current flowing in the coils, and are creating magnetic flux. As the rotor moves, the reluctance of the magnetic path is increasing. The same magnetic flux flowing in a higher reluctance path means greater current must flow. So the mechanical work of the rotor spinning, increasing the reluctance of the magnetic path, is pumping energy into the stator current.

Hope this stimulates some discussion.

Jonathan

 

[nuckythompson]

I also find this difficult to wrap my head around, and honestly don't fully understand it. Here is my stab at a hopefully helpful analogy:

First an analogy. Consider a variable capacitor. You set the plates close together, and charge the capacitor up. Now you disconnect the circuit from the plates, and mechanically pull the plates apart. What happens to the voltage on the plates?

Since the capacitor is disconnected from the circuit, the charge cannot change.

Since the plates are being pulled apart, the capacitance has to go down.

To store the same charge in a smaller value capacitor, the voltage must go up! And since the charge is the same, the energy stored in the capacitor also has to go up.

No magic here; you are doing mechanical work to pull the plates apart, and that mechanical work is being stored as electrical energy in the capacitor.

Now back to your switched reluctance generator. You have current flowing in the coils, and are creating magnetic flux. As the rotor moves, the reluctance of the magnetic path is increasing. The same magnetic flux flowing in a higher reluctance path means greater current must flow. So the mechanical work of the rotor spinning, increasing the reluctance of the magnetic path, is pumping energy into the stator current.

Hope this stimulates some discussion.

Jonathan

Thank you for this analogy.

I am still wondering how the magnetic field is first created. Is there any excitation current on the stator? I am not sure how this would work because the stator would be the output and where the voltage would be induced...

 

[nuckythompson]

I just read that the stator windings of a phase are only energized when the rotor is aligned with that phase, this would be part of a control circuit. So, in that case, like asynchronous generators, an external power source would be required for the generator to operate. I was confused because google was saying initially there is not excitation required, which now makes sense.

So now I am wondering what the advantages are. No rotor windings, no risk of phase to phase shorts due to concentrated windings, what else. I thought that they could operate without a gear box, but couldn't any genertaor operate without a gearbox with the right converter control system?