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I am running tests on a stepper motor used as a generator. The stepper motor is directly coupled to a motor, which I can control with an ECS. In the ESC's software, I can choose to drive the motor with constant speed or constant current, whereas I use constant current, which I think will put a constant mechanical power (torque vs rpm) at the input of the generator. My goal is to get as much power across the load as possible for a given power input. I will try to explain my understanding of the system:

The circuit equivalent of the generator is (considering one phase only)

enter image description here

where Rm=1.8ohm, Lm=2.5mH, and I have measured the voltage constant Ke to 39 V/kRPM (RMS). Let's assume that the input mechanical power is 10W, and that the generator speed is 1000 RPM. Let's also ignore Lm for simplicity. As the input power is 10W, I would think that the total power dissipated by the circuit is 10W as well. Then I would think that an infinite load resistance would assure that all of the input power is dissipated by the load, and not by the source. Is this correct? I assume there are some practical limitations here.

Bonus question: How can I simulate a voltage source with constant power with SPICE/LTspice?

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Is this correct?

No, the only time you can get maximum power transfer is when Rm=Rl (with AC you also need to include Lm which will change the current phasor)

An infinite resistance (which would mean open) won't transfer any current.

If you meant zero resistance (short) then that will cause maximum current, not necessarily power transfer. In addition if you short, in the real world a mechanical generator will have a loading curve and to get maximum power, you have to find the RPM\torque combination that will give you maximum power, if you short a motor like this, it's likely to slow\stop or break (overheat).

As far a simulating in spice, you can use constant power loads and sources:

Bn P=f(...) Arbitrary Power Sink/source where f is a constant or is an arbitrary function of any valid node voltage, branch current, etc. as with standard b-sources (note: power is sourced when f is negative). In order to avoid large currents at voltages near zero, the arbitrary power sink/sources foldbacks to resistive behavior when the absolute value of voltage across the device falls below a default value of 1 volt. The foldback point may be modified by specifying a VprXover parameter for the device, e.g. VprXover=50mV.
Source: https://ltwiki.org/?title=Undocumented_LTspice#B-Sources

But if you are trying to simulate generator mechanics (torque/load), then you'll need mechanical dynamics (if that's what you are going for).

Also, steppers look like this: enter image description here

So you will need multiple coils in your simulation. The best thing you could do is hook up a stepper to a large motor and do a test where you transfer mechanical energy into the stepper

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  • \$\begingroup\$ Okay so when Rm=Rl, Rl dissipates only half of the total power in the circuit (10 W). If I increase Rl to say 5.4 ohms, while the mechanical input power is kept at 20 W, then the power dissipated by Rl is 75 % of the total power in the circuit, so 15 W. It doesn't make sense to me.. \$\endgroup\$ Commented Jul 12, 2024 at 7:54

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