Magnetizing Airhead Alternator Rotor

[Bob_Roller]

Just saw this on the Euro Motoelectrics website .

Anyone ever hear of this before ?


https://www.euromotoelectrics.com/v/vspfiles/assets/images/permanently%20magnetizing%20your%20bosch%20alternator%20rotor.pdf

[montmil]

  Anyone ever hear of this before ?

Nope, not me.

And this "tip" would likely cause me to think long n' hard:

"1. With a DC adjustable power supply, solder 2 wires to the commutator slip rings on the rotor near the wound wire connections."

I would really want a little more confirmation from whomever wrote this before trying it. So many other things to check/confirm within the charging system before breaking out the soldering gun... "Hello. Rick Jones?"

[nhmaf]

"Permanent magnetization" of simple iron is not really permanent - enough heating cycles and strong vibrations or shocks will result in non homogenous polar realignment.  Depending upon the  severity of these things, it could take a few seconds, or months, before it reverted back to its earlier, less magnetically polarized state.   One could argue that the collective months of operation with smaller currents tends to maintain a weak magnetic polarization of the ferrous core materials in the rotor anyhow, and it really is the amount of current through the rotor windings that does >98% of the flux generation anyway.  

So, while I agree that -in theory- this process could improve charging system function by some percent for some indeterminant length of time, I don't believe that it is really a permanent fix, nor a viable solution to try if you aren't getting proper charging voltage.   IF you are the sort who just like to tinker with things for hours for the sake of tinkering, it might be an interesting use of a weekend.

[Barry]

I've heard of magnetizing rotors before but only on a new alternator to make it start charging. Could it be that's what they are getting at ?

There would usually be some residual magnetization on a used alternator. Isn't that why charging can still kick in at high revs even if the charge lamp is open circuit.

I was confused by the outcomes of magnetizing the rotor as stated in the last paragraph.  Mine behaves just like that anyway.

The more I read it the more I think it's instructions for installing a new alternator or perhaps a remedial process for a new alternator that is not performing as it should.

[nhmaf]

There usually is a little bit of magnetization that persists and it can be what might enable some amount of charging to begin if the charge lamp is open circuit, but it is a slight chance that it would be enough to get you home even keeping the RPMs high.

One needs a constant current source mode power supply to do the charging procedure outlined.. I'd consider doing the procedure on a brand new rotor if it didn't already have permanent magnets isntalled.

[Bob_W]

Whoa! Read Snowbum about alternators. His explanation is far better than mine, but BMW uses an alternator with the rotor wound with wire. Current is passed through the wire which creates an electromagnet. This magnet spins and the wires in the stator cut the flux lines creating electrical current.
The rotor is magnetized initially by a small current flow through the generator lamp, through the brushes to the slip rings and thus starts producing power.
A different type of generator can have a permanent magnet rotor and will start generating without introducing a current. Even an electromagnetic alternator may have enough residual magnetism to actually start the current flow, but not likely.
There is basically nothing in the rotor but wire and a core to magnetize. Yes-the method suggested may produce some residual magnetism, but with ignition switch on and generator lamp lit, the rotor is magnetized and current will start to flow.
Snowbum has far more knowledge than I and a read of the appropriate section of his site should explain totally the function of an alternator-late BMW motorcycle, modern cars or even heavy equipment.
Generators produce DC, require no diode board and are a different animal and primarily belonged to a different era.
Bob



 

[Bob_W]

I error, EME sells an aftermarket system with a permanent magnet rotor. It may need to be magnetized prior to operation. This does not apply to standard airhead charging systems.
Bob

[Luca]

Current is passed through the wire which creates an electromagnet. This magnet spins and the wires in the stator cut the flux lines creating electrical current.  

You're starting to sound like Snowbum himself!  Yep, the initial exciting current for the rotor is provided by the battery.  No gen lamp with the ignition on and the bike not running means no excitement.

A different type of generator can have a permanent magnet rotor and will start generating without introducing a current. Even an electromagnetic alternator may have enough residual magnetism to actually start the current flow, but not likely.  

Yes, that magnetism might be enough to produce some juice at higher rpm's, but, as nhmaf pointed out, it is not reliable.  Think of magnetizing a screwdriver with a permanent magnet, then dropping it on a hard surface to remove the magnetism.

You can have an alternator, not just a generator, with a permanent magnet rotor.  The VN750 I used to ride had such a setup, as do many older Hondas.  They tend not to last as long as a wound rotor, especially with the modern resin coatings used charging system windings.

There should be no need for magnetizing a wound rotor.  The whole point of them is that they do not rely on permanent magnets.  A better, less time consuming, $40 solution to the low rpm drain on boxer charging systems is to install a high output voltage regulator.  It will not increase the overall charging capacity, but it will make the charging system plateau at peak output lower on the rpm band