Topic: Hey, TwoTired... can you explain something for me?

[Jonesy]

I'm curious to know how the alternator works on the CB750 (and the other SOHC/4's). I know that you put DC current (governed by the regulator) in via the field coil and get AC current out from the stator (in turn rectified to DC), but what I want to know is how does the rotor play a role in this? Do the wavy cutouts in the rotor cause interruptions in the magnetic field from the field coil and is this what causes the current to alternate? Do the shape of the cutouts influence the sinusoidal waveform of the AC output? I don't quite understand the magnetic principles involved.

[malcolmgb]

http://en.wikipedia.org/wiki/Alternator if in doubt go to Wikipedia is my motto.

[ChopperCharles]

Hey, not to but in but I have a question too... how reliable are the charging systems on the SOHC 4's? (Specifically the 550K). I owned a DOHC CB750 a while back, and the charging system was garbage. I replaced the rotor, stator, brushes, and voltage regulator, rewired all of the harness between all the component parts, rewired the headlight on its own circuit with a relay, and installed a brand new freshly charged battery, and the damn thing would still discharge the battery in stop and go traffic or if sitting at a long light. And once the battery was below a certain point the bike would stall. I had to leave the thing on a charger when I wasn't riding it to make sure that the battery was topped off completely at all times, just in case I got stuck in traffic or construction or something. At night at a long light I could watch my headlight getting dimmer and dimmer and dimmer.

Oh, and it ate charging rotors every 12,000 miles without fail. I had a puller and a spare rotor in my saddlebags on long trips.

I'm hoping the SOHC is better....

Charles.

[Jonesy]

I believe the DOHC alternator has an internal field coil fed power via brushes and slip rings (the field coil is encased inside the metal rotor). This is supposed to be a more efficient alternator design, but if the brushes fail or the slip rings become corroded (among other numerous things that can happen) it can effect the alternator's performance.

The SOHC alternator takes a different approach. The rotor is "dished" to clear the field coil which is attached to the alternator cover. Since the field coil does not spin along with the rotor, it can be hard-wired to the charging circuit. Not as efficient as the DOHC-type, but they worked very well.



Huh.. I guess I answered my own question with the help of the above drawing. Since the windings of the coil cause one end to have an "N" pole and the other an "S" pole, the alternating "fingers" of the rotor make a constant N-S-N-S-N-S field over the stator.

[Kevin400F]

Actually, our bikes have three-phase alternating-current generators, not alternators.

I'll take a stab at explaining how it works, and those who are more knowledgeable than me can follow up as needed:

The wavy cutouts in the iron rotor are necessary in order for the field coil to create multiple magnetic fields within the rotor itself (as opposed to the rotor needing to have permanent magnets attached to it).

The magnetic fields of the rotor, moving inside the stator coil, is what produces the three-phase AC current.  The voltage regulator controls the field coil, which in turn creates greater or weaker magnetism in the rotor and therefore more or less electrical output from the stator coil.  The rectifier changes the stator coil AC output into DC.

Kevin in TX

[bryanj]

Kevin is close.
The stator (inner) produces a steady magnetic field that varies in intensity depending on the voltage fed to it (changed by the regulator)
The rotor with its wavy groove causes the magnetic field to move about
This movement , cutting accross the stator (outer) windings, produces electricity of an AC type, due to the fluctuating field in 3 seperate windings which is rectified to DC by the rectifier.

On a seperate point ALL generators produce AC voltage its just that dynamos rectify it to DC via the commutator and brushgear, whilst alternators use a silicon diode type rectifier, BIG advantage of Alternators is that any brushgear only passes a small current and so is smaller in area and more reliable than commutators and big brushes

[Kevin400F]

Thanks for the feedback, Bryan. 

If I were to mock up a rotor and field coil on a bench and energize the field coil, would I find that the rotor has been magnetized (actually six sets of magnets), with the inner and outer "fingers" being opposite polarity?  Would iron stuff actually stick to the rotor itself?

[TwoTired]

Huh.. I guess I answered my own question with the help of the above drawing. Since the windings of the coil cause one end to have an "N" pole and the other an "S" pole, the alternating "fingers" of the rotor make a constant N-S-N-S-N-S field over the stator.

Yup. BryanJ gave a pretty darn good explanation, too.

If I were to mock up a rotor and field coil on a bench and energize the field coil, would I find that the rotor has been magnetized (actually six sets of magnets), with the inner and outer "fingers" being opposite polarity?  Would iron stuff actually stick to the rotor itself?

Yes, the rotor core must be of a permeable metal.  It should remain magnetized so long as the field coil is creating a magnetic field, in which it resides.  Although the rotor "fingers" do become polarized north or south, I'm not certain a given finger assumes one polarity for a run cycle.  Because the field coil is fixed and the rotor is moving, it is likely that the rotor fingers may also switch polarity as it rotates.  Regardless of whether the polarities in the core actually change or if the pole strength simply varies in intensity, both actions further the goal of creating crossing lines of magnetic flux across the windings of the stator, causing current to flow there.  Moving lines of flux create current in winding wires, static flux lines do not.

Cheers,

[Bodi]

Most of the SOHC4 engines have "induced field" alternators:
If you look at the field coil, you'll see two steel end caps and the coil wound in between. The end caps become magnetized when there's current flowing in the coil - one end piece will be "North" and the other "South", depending on which way the coil wires are connected... it really doesn't matter which way it is connected.
The rotor has a non-magnetic (aluminum alloy) web with 2 iron/steel pieces cast in. The two steel pieces are pretty similar, rings with fingers sticking out one side basically. They are positioned with the fingers together and interlocking, so that around the outside edge of the complete rotor there will be one finger connected to the ring on one end then a finger connected to the ring on the other end. The rings are located around the field coil end pieces when all together, with a minimal gap between the field coil poles and the rotor rings.
So the rotor takes the magnetic poles N and S at opposite ends of the field coil and bends them around so you get alternating poles N-S-N-S on the outside edge of the rotor cylinder.
These sweep over the stator coils when the rotor turns which produces the output voltage.
There are also engines (650 and 500/550?) with powered rotors that have the coils wound right on, powered through slip rings and brushes. This type is more efficient - you lose magnetic field strength in the gap between the field coil and rotor and in the bending pole assemblies with the induced field type - but is prone to trouble with the brushes and sliprings plus the coils tend to fail (work harden and break) from the stress of rotating on the crankshaft end with vibration from power pulses.

[ChopperCharles]

So basically, the charging system is reliable then? Cool.

Personally, I prefer the permanent magnet rotor spinning inside a stator, with the excess voltage shunted to ground by the voltage regulator. Simple, easy, and ultra reliable. My V65 Magna has this type of stator, and it's been operating without any problems for 62,000 miles so far.

Charles.

[dustyc]

Charles, since going through and cleaning up my connections mine has become alot more reliable.  I was having problems when I first got it.  Also I've been alot more conscious about keeping the RPM's up and cutting my headlight if I'm just idling at startup.

I'd love a system that charges at idle, but I'm willing to live with it.