Permanent Magnet Motors. Question = Types/differences/advantages/disadvantages/etc.

[Freemasm]

OK. I was cluttering up someone else's thread; a superpower of mine. Assumption = Permanent Magnet motors had constaint counter-torque. Response provided below.

Nope. The MZ just runs the 3-phase generator coils open-circuit when the regulator is not enabled, so no mechanical load. Yes, the generator is making whatever dozens of volts thanks to being permanent magnet, but because there's nothing attached across the coils, no amps are flowing, and therefore zero work being done (watts or HP, same thing).

In the case of this MZ-60 TR, according to your logic, we'd have 800-ish watts, which is over 1 HP of parasitic loads. On my iron bird with the 1-cyl gas engine, 1 HP is a significant load and would be obvious in the engine sound if the generators were pulling that. When connecting a 60A load as I did in testing, you can definitely tell that the generators are pulling engine power, but no drag when the regulators are turned off.

IIRC, 'Old school' PM generator regulators would regulate by varying a shunt across the output, so then you have parasitic load all the time as you say. Bill J's regulator design is the secret sauce that enables the no parasitic load when turned off.

Also, you can demonstrate your "counter-torque" by taking one of Bill's generators without the regulator attached and spin the shaft by hand. Then tie all 3 generator wires together and try to turn the shaft. You won't be able to.

So the MZ gens have constant counter-torque loads once enabled? This is different from my assumed and witnessed characteristics of the PM motors we utilize in our industry which don't require any electrical input and have a constaint torque/counter-torque balance. That loading/losses are constaint regardless of duty/load. They are required for starting power plants (critical systems, back-up starting generators, etc.) when no power from the grid is available; important because Texas tries hard to collapse their grid every couple of years.

So, is the definition of a PM motor wider than assumed? Do the rotating PMs induce a poles in the stator (see comment above about zero parasitic load)? Is it self exciting? Would seem to be work counter to efficiency. Can anyone with knowledge of the various PM motor types share such?

Be insulted that an ME is stepping blindly into your briar patch. Please inform.

 

[Mich48041]

Bill Judge, owner and designer of Monkworkz, had a permanent magnet 3 phase motor at an OSH forum. No rectifier-voltage regulator or electronics were connected. He invited members of the audience to spin the motor. The motor spun easily and coasted to a stop. But when the 3 motor wires were connected together, the motor shaft was difficult to turn. I concluded that the heavier the load (lower electrical resistance), the more force required to turn the shaft.

 

[bjdecker]

Some effects to consider --
1. A magnetic field (flux) moving adjacent to a conductor (wire) will create current flow in that conductor
2. Current flowing in a conductor will create a magnetic field (flux) proportional to the amount of current (more current, more flux)

3. A field/flux changing in polar orientation will create current changing in direction (see 1)
4. Current changing direction will create a field/flux that changes in polar orientation. (see 2)

Putting it together --
The resistance (or Load) in the conductor/circuit will determine the amount of current that can flow. And since we confuse ourselves what a "Load" means in relationship to Resistance (Ohms), some clarity: Large Load or Heavily Loaded means very small resistance (close to 0), Small Load or Lightly Loaded means very large resistance (far from 0), No Load means Infinitely Large Ohms or open circuit.

A 0 ohm load (really "big" load) will allow more current to flow in the conductor and thus create a larger field/flux and create more resistance to the spinning magnet.
A 100k ohm load (really "small" load) will allow less current to flow in the conductor and thus create create a smaller intensity field/flux

Here's a video to demonstrate the above effects. Magnet in Tube The copper tube in this video is a closed circuit of nearly 0 ohms, similar to a coil of wire, with each of the ends tied together, or maybe with an incandescent light bulb in between.

If you were to split the copper tube lengthwise down the middle, the magnet would fall through, unimpeded.

A spinning (or moving) magnet (N-S, S-N, etc.) will create sympathetic current flow (AC) in a conductor/circuit (adding the term circuit here...it's important)

So the MZ gens have constant counter-torque loads once enabled?

No, once enabled the "counter torque" must increase with the amount of current produced.

So, is the definition of a PM motor wider than assumed? Do the rotating PMs induce a poles in the stator (see comment above about zero parasitic load)?

PM motors make great generators...Especially when you don't want them to be This is why the "weird" circuit introduced with the Skytec 149 PM-LS, XLT starters and Van's Drawing OP-10 (Zone E-6) - Lightweight Starter Circuit.

Is it self exciting? Would seem to be work counter to efficiency. Can anyone with knowledge of the various PM motor types share such?

Self exciting insomuch that they'll make current flow once the circuit is "closed" - Yes.

Be insulted that an ME is stepping blindly into your briar patch. Please inform.

I love my ME bretheren -- without them we wouldn't have Magnetos, Fuel Injection, Margarita machines

 

[Freemasm]

...No, once enabled the "counter torque" must increase with the amount of current produced.

This is the part where the wheels come off for me. Yes, conservation of energy dictates that counter torque and wattage will balance. So how is t his happening? There's no VR controlling the field strength -> device output; RPM being an non-controlled boundary condition. So, how is the force balance controlled?

 

[bjdecker]

This is the part where the wheels come off for me. Yes, conservation of energy dictates that counter torque and wattage will balance. So how is t his happening? There's no VR controlling the field strength -> device output; RPM being an non-controlled boundary condition. So, how is the force balance controlled?

Only if the transfer/transform was 100% efficient.

Look again at that video - the magnet is still falling despite the eddy (or counter torque) force.

Now I'll venture outside of my lane -- think of magnetic flux as a gas, and not a fluid; it's compressible.

 

[danny]

The technical term, for me, is PFM.
danny

 

[CF86301]

Describe how a three-phase permanent magnet generator generates counter-torque. Include an explanation of back EMF.

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Then I asked about regulating the output of a three-phase permanent magnet generator, and there was some hallucination on the part of the AI, so I tried again...

I think this answer is incorrect, because the voltage regulator for a three-phase permanent magnet generator does not vary any excitation voltage to the gene...

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I don't know how Monkworkz does it, but the active rectifier concept is pretty interesting.

 

[terrye]

PM motors make great generators...Especially when you don't want them to be This is why the "weird" circuit introduced with the Skytec 149 PM-LS, XLT starters and Van's Drawing OP-10 (Zone E-10) - Lightweight Starter Circuit.

My OP10 R6 shows this circuit in Zone E5-6.

 

[hgerhardt]

To pull this together, it sounds like @Freemasm is thinking the PM generator works like this:

2.3. Simple shunt/dump‑load regulation (common in small systems)​

• The three‑phase output is rectified to DC.
• A comparator monitors the DC voltage.
• When the voltage exceeds a threshold, a dump load (e.g., resistor bank) is switched in (often via PWM).
• This increases electrical load on the generator, increasing counter‑torque and clamping the DC voltage around a setpoint.

Key idea: Regulation by controlled loading, not by field control. You “burn off” excess power as heat or useful load to keep voltage from rising too high as speed increases.

But, based on my testing, I'd say the MZ works like this:

3. Active rectifier–based regulation​

Instead of a passive diode bridge, some systems use an active three‑phase rectifier (MOSFETs/IGBTs in a full bridge):

• The rectifier is PWM‑controlled to shape the generator currents and control the DC bus voltage directly.
• The controller measures DC bus voltage and generator currents and adjusts the duty cycles to maintain a reference voltage, while also optimizing efficiency, power factor, or torque ripple.

In this case, the “voltage regulator” is essentially the control algorithm driving the active rectifier. Again, no rotor excitation is changed; all control happens via switching and current control on the stator side.

The "counter-torque" (seen as mechanical drag or load) is only happening while the MOSFETs are turned on within the PWM duty cycle. While the MOSFETs are turned off within the PWM duty cycle, the generator is open-circuit and has no drag. That's how the drag varies based on output power.

 

[lr172]

I have a home made generator, copied fron the SD-8. It has magnets on the outter spinning cup. When the output circuit is open and you spin the shaft, you can feel the magnets resistance and you have to force it past that resistance and it then snaps to the other side of that pole. The force is pretty minor, but very obvious. Definately not free spinning. you can clearly feel those "notches as you turn it. Is that normal and expected? is the monk different in that regard? Clearly not robbing any real power, as it would once a load is introduced, but not what i would call free spinning.

 

[hgerhardt]

........you have to force it past that resistance and it then snaps to the other side .......

Think of a hill: you push your bike uphill, but then the bike pulls you downhill. So the net force is cancelled out.
And the MZ does that too.

 

[lr172]

Think of a hill: you push your bike uphill, but then the bike pulls you downhill. So the net force is cancelled out.
And the MZ does that too.

AH HAH! Thanks for that. Wasn't thinking of the counter acting forces netting a 0 sum. Was only thinking of the initial resistance.

 

[Freemasm]

To pull this together, it sounds like @Freemasm is thinking the PM generator works like this:


But, based on my testing, I'd say the MZ works like this:

The "counter-torque" (seen as mechanical drag or load) is only happening while the MOSFETs are turned on within the PWM duty cycle. While the MOSFETs are turned off within the PWM duty cycle, the generator is open-circuit and has no drag. That's how the drag varies based on output power.

Thx. This makes some sense for the specific applications I’m familiar with. When energizing emergency systems, especially when trying to cascade power to larger generators, efficiency is not really a concern. Stone simple reliability is paramount.

I’ll state the fog has lifted a bit but ultimately, I don’t have any real talent in this art. My brief thought of a double major decades ago ended with signals. I could get the right answers via the math but ultimately, I had no real intuition for the physics involved. Having know a LOT of EEs over the years, my situation was probably no different than theirs.

Thx. Probably won’t be the last of my electrical misconceptions so I’ll apologize in advance.

Edit = “Circuits class was fun and easy. How bad can the rest be?” thought a very naive person long ago.

 

[skylor]

Think of a hill: you push your bike uphill, but then the bike pulls you downhill. So the net force is cancelled out.
And the MZ does that too.

The net force is not completely cancelled out because the "uphill" side of the magnetic "cogging" generates some heat which is not recoverable on the "downhill" side. The loss may be small but it nevertheless is still a loss that doesn't exist in a wound-field alternator when the field is de-energized. The question is whether or not that loss is greater than the friction loss of the brushes in the wound field alternator...

Skylor

 

[bjdecker]

My OP10 R6 shows this circuit in Zone E5-6.

Yeah - that's where it is; I updated the reply...

 

[bjdecker]

I have a home made generator, copied fron the SD-8. It has magnets on the outter spinning cup. When the output circuit is open and you spin the shaft, you can feel the magnets resistance and you have to force it past that resistance and it then snaps to the other side of that pole. The force is pretty minor, but very obvious. Definately not free spinning. you can clearly feel those "notches as you turn it. Is that normal and expected? is the monk different in that regard? Clearly not robbing any real power, as it would once a load is introduced, but not what i would call free spinning.

The "chug" will be introduced in & by a closed circuit coil AND/OR ferromagnetic material (iron cores, nickel, mu-metal (meow metal? meow??) Maybe there's some Fe in the mix?

 

[mstrauss]

Some effects to consider --
1. A magnetic field (flux) moving adjacent to a conductor (wire) will create current flow in that conductor

So, how does the flux-capacitor factor into this?

 

[Jjackh10]

So, how does the flux-capacitor factor into this?

It stores 1.21 gigawatts of flux to be released at 88mph obviously.

 

[monkworkz]

OK. I was cluttering up someone else's thread; a superpower of mine. Assumption = Permanent Magnet motors had constaint counter-torque. Response provided below.

So the MZ gens have constant counter-torque loads once enabled? This is different from my assumed and witnessed characteristics of the PM motors we utilize in our industry which don't require any electrical input and have a constaint torque/counter-torque balance. That loading/losses are constaint regardless of duty/load. They are required for starting power plants (critical systems, back-up starting generators, etc.) when no power from the grid is available; important because Texas tries hard to collapse their grid every couple of years.

So, is the definition of a PM motor wider than assumed? Do the rotating PMs induce a poles in the stator (see comment above about zero parasitic load)? Is it self exciting? Would seem to be work counter to efficiency. Can anyone with knowledge of the various PM motor types share such?

Be insulted that an ME is stepping blindly into your briar patch. Please inform.

Slow on the uptake here as always but as folks that have done the spinny spinny demo, spinning the generator with and without the leads shorted together, know:

torque on the motor shaft is NOT constant.

I think the easiest way to think about this is that

when current flows in the windings the movement of the magnets is resisted

When the regulator lets more current flow, the torque goes up and this happens at the speed of light(corrected for the medium/velocity factor).

Any explanation beyond that and you're getting neck deep in electromagnetic fields, one of the more challenging topics in electrical engineering.

There is also the thermodynamics perspective, the power has to be coming from somewhere, and that power is coming from the engine via the power/torque equation.

The torque can be calculated by taking power out of the regulator and correcting for efficiency, and then running the torque/power equation.

At any fixed power level, run the generator faster and the torque goes down.

As some have seen, no load, basically no torque.

Bill