Question on switched reluctance generators

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billvon   1 MW

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Question on switched reluctance generators

Post by billvon » Jan 03 2019 12:59pm

A question for all you electric-machines people:

So I think I get how an SR motor works. In motoring operation, phases are driven so that current is driven through a phase winding before the salient pole of the rotating core aligns with that phase winding. The energy in the coil is given by 1/2LI^2 (standard induction.) As the rotor turns to align with the phase winding the average permeability of the magnetic circuit goes up, and so inductance goes up as well. Torque comes about as the rotor attempts to minimize the magnetic flux distance.

When the salient pole is perfectly aligned with the winding, torque is zero. Current to the winding is then switched off, and the pole is free to continue to rotate towards the next winding.

(Needless to say, details of when the winding is turned off and on, and what its waveform looks like, will vary a bit.)

For an SR generator, you apply current when the salient pole is aligned with the winding. As the pole pulls away from the winding, inductance goes down. Since energy (1/2LI^2) is conserved it has to go somewhere, and it appears in the form of higher voltage - and that voltage is conducted back through the inverter to the generator's output.

Right so far?

Now if the rotor is slightly magnetized (intentionally or unintentionally) it's easy to start up this generator, since the moving magnetic fields generate small currents in the windings, and thus provide a synchronization signal.

But how do you start up an SR generator _without_ that field? Do you always need a position sensor of some sort? Or is there an excitation that gives you information on the rotor position so you can start up such a generator without explicitly knowing the rotor position?
--bill von

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