Understanding hysteresis brakes

[BrianE22]

I've enclosed a cut-away of a hysteresis brake. I think I understand how it works:

1) When the poles align a strong flux density flows and drives the cup into saturation.
2) The flux goes low when the alignment of the poles is minimal.

If that's true, then I believe you are working only in the first quadrant of the hysteresis loop, i.e. there is no flux (or H) reversal (assuming a DC coil), correct? Also, you would want a wide hysteresis loop for the cup, correct? If I wanted to operate at high rotational speeds I would probably be limited to steels for the cup material, correct?

 

[MagBen]

I would agree that the rotor (drag cup) is working in the first quadrant, no flux reversal. Actually in your assemble, the flux as a whole will not change if the current at the poles do not change because the magnetic field between the stator teeth is determined only by the current applied.

For a specific local point in the cup, there is an alignment issue, this point "see" a stronger field and a weaker field alternatively as it rotates.

It is not true that the wider the loop, the better the performance. Wide loop only means the material is hard to reverse. In your design, there is not even a reversal. I feel even a soft magnetic material works for the rotor.

 

[BrianE22]

Thanks for the reply MagBen. I haven't seen many hysteresis loops - other than a few basic ones in text books and vendor sites. Do you know of a good source for plots of hysteresis loops for different materials?

I'm struggling a bit with seeing how there is much movement on the loop when the cup rotates and the poles align and then don't align. Even at max-unalignment there will still be H and B passing through the cup.

I'm dissecting a unit I bought off Ebay and I'm going to look at the gaps and poles and amp-turns and then model it with my (not-very-powerful) 2d magnetics software. Hopefully I'll gain more insight in to how B and H varies in the cup.

 

[MagBen]

can you tell what kind of material is the cup made up of? It is said that the cup is a hard magnetic material which resists changes in magnetization from the air gap. So the cup must be radially pre-magnetized, and it works in the second quadrant, instead of in the first quadrant.
Braking torque is created by forcing like poles of the rotor (induced pole) & stator together, and opposite poles apart (rotor's original poles).
the wider the loop of rotor, the more difficult to create an induce pole, and so the material should not be too magnetically hard, or not too soft.

If the cup is soft material, say carbon steel, the attracting force between rotor and stator should create a baking torque. However, this simple mode might not work as well as using hysteresis material.

 

[MagBen]

I do not understand why not apply an AC field to the coil, so the stator magnetize the rotor, then reverse. The hysteresis characteristics of the rotor resist the magnetization reversal to create baking torque?? something is missing here.

 

[IRstuff]

The impression I get from the descriptions is that there is no "alignment" per se, i.e., the rotor has no field at all. Otherwise, if the coil is unpowered, the rotor would "cog" against the stator teeth, but most datasheets describe that case as the rotor "freely rotates," which should mean there's no intentional field in the rotor cup itself. The stator is arranged so its field alternates, and any single point on the cup must continually match its magnetic alignment with the nearest tooth. It's this constant field flipping that creates the drag that slows down the rotor. Most articles discuss cogging only from residual fields that result from abrupting turning off the coil, but that supposedly can be mitigated by applying a decreasing sinusoidal to the coil on turn-off to essentially demagnetize the cup.

http://machinedesign.com/mechanical-drives/electromagnetic-brakes-and-clutches

http://www.designworldonline.com/the-development-of-a-new-magnetic-brake/

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[BrianE22]

Sorry for not getting back to you guys. The cup is not magnetized. I don't know what king of material it is. I'd be nice if I could measure a BH loop for it. I see there are methods out there that use the sample as a core for a "transformer" and measure the input current (H) and integrate the output voltage (B) to come up with the BH curve. I want to look at doing that.

The sample I have is designed for a DC input voltage but I'm going to try an AC voltage and see how it acts. If my understanding is correct then I would think the AC excitation would be able to absorb quite a bit more energy per revolution of the cup.

We also have a dyno for testing our motors that uses a larger hysteresis brake. For that I have removed the cogging by ramping down the current and spinning the rotor. The cogging will happen if I suddenly stop rotation while the coil current is high.

 

[Compositepro]

An AC voltage will heat the rotor like it is turning at high speed. Also DC and AC coils are usually designed differently.

 

[IRstuff]

As described in links I cited, you do need to run AC into the coils once it's stopped to demagnetize the cup. However, prior to that, you should be using DC, because it's the alternation in field when going from an outer stator tooth to an inner stator tooth that generates the field flipping and hysteresis. An AC voltage applied to the stator would have the effect of potentially no braking, every time the rotational speed of the cup between alternating teeth corresponds to the alternation of the AC.

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