# How do you drive a variable reluctance stepper/SRM as a generator?

I think I understand the principles, even if not the details, of operating a VR/SRM as a motor. But I don't understand, and can't find any good explanation online of, the principles for how to use one as a generator, just statements that it can be done. What kind of signal would you drive it with, and what kind of output would you expect, operating this kind of motor as a generator?

• In Reluctance motor on Wikipedia you can find Flux switching alternator under the See also header. - If there are things you do not understand from that Wiki page then you can edit your question to be more precise. – Harry Svensson Apr 9 at 2:10
• Stepper motors can be used as generators with a full bridge rectifier ( diode or FET) for each isolated coil. Then they can be made in parallel or series for a split supply with centre common. V out is proportional to RPM and current is limited by coil DCR. – Sunnyskyguy EE75 Apr 9 at 2:20

I'm sorta guessing, sorta going on basic physics. Here's one way that pretty much has to work -- but is almost guaranteed to not be the best way. Drive it with a constant-current source, so that it'll generate an AC voltage impressed on whatever the $$\I R\$$ drop of its coils make. Pick off that AC voltage, and run that to your load (shown as a resistor). Turn the motor; enjoy.

Making it generate power efficiently would take, like, thought.

simulate this circuit – Schematic created using CircuitLab

• Thanks. I didn't specify in the question, but I'm interested in both the basic concepts and how you'd do it efficiently. My (very very) high level intuition says that if you drive it right, it should be very efficient (and that agrees in principle with SRM use in EVs), but I have no idea what "right" would be. – R.. Apr 9 at 1:22

In motor operation, the phase of a SRM is energized until the rotor and stator teeth are aligned for that pole, producing driving tourque. In generator operation, the phase is energized when the teeth are aligned, thus producing braking tourqe and generating power.

To actually use that, you would need current controllers for the phases to energize them at the right rotor angle

• +1 Perhaps put more simply as: motor operation is letting teeth snap together, generator operation is ripping them apart. – Jon Apr 9 at 11:50

Shameless plug

Essentially, all electrical machines can be made to motor or generate. Some this comes inherently (PMSM, DC-motors), some you need todo a bit more work (induction machines).

SR machines are the latter. For an SR to generate you actually need some form of power source, This powersource is energized when the rotor and stator are aligned to build up flux. The switching is then turned off as the rotor is forced around and the change in reluctance permits current flow

• Interesting paper. My understanding was that SRM cannot provide the same power density as PM motor/generators. Surely in an aviation environment this would prohibit their usage, as the benefits of lower mechanical cost and simpler drive circuit would not as important as weight. Also is there a reason it has to be placed at the exhaust end of the engine? – Jon Apr 17 at 13:40
• @jon you are correct about the density but that was all part of the research project. SR come into their own in very harsh environments and that is why this generator was placed where it was. The was also a PM generator placed in a less harsh environment – JonRB Apr 17 at 15:28

Motors generate a back EMF that reduces torque with rising speed.

When used as a generator they generator voltage with rising speed and the electrical load can draw more current if you turn it with adequate torque.

simulate this circuit – Schematic created using CircuitLab

Mini-Stepper with PM magnet motors have as many as 200 magnetic poles per revolution.

• A VR Stepping Motor may have gears and fewer poles per rev. They do not have as much torque as PM stepper but offer a wider speed range with high efficiency.

If you use the motor as a brake to charge a battery, it does not return all the energy you used to get up to speed, only the kinetic energy that is reduced by generating current to slow down the generator.

But since the voltage generated is only equal at no load when you slow down it becomes lower than the battery voltage, so a boost Voltage current PWM LC regulator must be used to charge the battery and draw power from the motor as a generator.