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Does anyone know the term for these artifacts in a B-H curve and their cause? I have no idea what to use to look up anything about them.

These were obtained from the terminals of running a generator with various, sometimes switched, loads, and sometimes with current being injected back into the generator, like a motor I guess. Not by injecting a sine-wave into a static core.

These plots were obtained with voltage and current probes connected to the terminals of the generator running under load and plotting the integral of the terminal voltage against the current in an XY plot.

These should be thought less as B-H curves for the core material and more curves attempting to track the magnetic activity of the core while the generator runs.

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DKNguyen
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  • This looks like a resonance. Were there any capacitors present somewhere in the circuit? (Could also be a mechanical resonance.) – Jonathan S. Jan 06 '22 at 21:20
  • @JonathanS. Yes. Both graphs are RC loads in one way or another. The first graph is unswitched. The second graph is switched in a particular way. – DKNguyen Jan 06 '22 at 21:23
  • How did you determine the magnetic field and magnetic induction of the generator core while it was running ? – tobalt Jan 06 '22 at 21:40
  • @tobalt An oscilloscope was hooked up across the terminals of the generator and the integral of the voltage was plotted against the current in an XY plot. So there were no explicit magnetic sensors. I wish there were, but there aren't any. – DKNguyen Jan 06 '22 at 21:43
  • Why do you call them BH curves then say they shouldn't be thought of as BH curves? – Andy aka Jan 07 '22 at 08:32
  • @Andyaka Not sure what else to call them. – DKNguyen Jan 07 '22 at 14:19

2 Answers2

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This simply doesn't work as long as the generator is running.

A generator applies a changing magnetic field to a coil with an iron core. In an ideal generator, this magnetic field can't be influenced by the load current: If the current rises, you draw energy from the magnetic field, which is then replenished by the mechanical power source attached to it. (Otherwise the generator wouldn't work.)

The way the magnetic field is generated doesn't really matter for the (magnetics-only) operation of the generator. We could just as well generate it with another winding on the magnetic core, turning it into a simple transformer. The primary of that transformer is connected to a fixed AC voltage source, which is equivalent to a rotor rotating at constant RPM (which is the case in almost all generators).

Now that we know that a generator at fixed RPM is equivalent to a transformer with a fixed AC input voltage, we can do further analysis. An ideal transformer just steps various electrical quantities up or down: Voltage as N, current as 1/N, and impedance as N². In particular, this means that the output of our ideal transformer (and therefore generator) is just another ideal voltage source. With a non-ideal transformer, we also get a leakage inductance and copper resistance in series with the voltage source, as well as a magnetizing inductance in parallel to the latter. The fact that the magnetizing inductance is in parallel to the voltage source means that no matter what you do at the secondary of the transformer, the current through the magnetizing inductance doesn't change. The load current simply doesn't flow through it. The same applies to the generator: As long as it's spinning, the magnetizing inductance of its coils is permanently connected to an (imaginary) voltage source. The magnetic flux through the coils doesn't change with the load current, just like in a transformer.

If you want to measure the BH curve of a transformer, you first have to disconnect it from mains, otherwise the secondary current just gets transformed into an equivalent primary current and has nothing to do with the magnetic field in the transformer's core. The same applies to the generator by the analogy above: If you want to measure its BH curve, you have to stop its rotation, otherwise the current at its output just gets transformed into a mechanical force acting on its rotating shaft - and therefore it has nothing to do with the magnetic field in its core.

What you've been measuring is just a Lissajous curve of the load current vs. the integrated (phase-shifted) AC voltage. The fact that you get these loops simply means that you have reactance (i.e. capacitors) in the load. The flat tops indicate that there's a rectifier somewhere. (A PFC circuit in the load, maybe?)

Jonathan S.
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  • I don't want to measure the BH curve of the material. I want to trace the flux activity in the generator core as it's running. (There's an encoder so I can correlate the rotor position to different points in the plot). – DKNguyen Jan 07 '22 at 00:01
  • What do you want to infer from the flux activity? – Jonathan S. Jan 07 '22 at 00:14
  • One of the things we're looking for is how quickly you can get the domains to flip inside the generator. I can't really get into it too much. – DKNguyen Jan 07 '22 at 00:16
  • I doubt you'll find a way to measure this that doesn't involve placing a magnetic sensor in the generator, unfortunately. The external current simply doesn't correlate in any appreciable way with the magnetic conditions in the generator, it's exclusively determined by the load after all. The voltage does correlate but I'm not sure whether you can read much from that alone. – Jonathan S. Jan 07 '22 at 00:49
  • Man, that pretty much involves getting a custom generator made with slots to put sensors into. – DKNguyen Jan 07 '22 at 01:29
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It is probably something to do with parasitics in the circuit. Change the frequency of the input waveform and see if it goes away. If it does go away and you can still make the measurements you need to then use the new frequency. If you have to measure at that frequency then change some of the other values in the circuit (if you can).

Voltage Spike
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