Home Brew PMA

[MacMcMacmac]

Looking at an XS stator on eBay, a thought occurred to me, instead of powering the windings in the stator to create a magnetic field to induce current in the rotor windings to generate electricity, why not machine a rotor to replace the original and embed magnets in it to induce a current in the stator windings and feed it to a regulator/rectifier unit and power the bike that way. It would eliminate the need for brushes and allow goodies like series regulators to be used. There are a wide variety of super magnets available which could "easily" be imbedded in the rotor which have a strong magnetic field. The only issues I see would be with the relatively scant amount of wires in the stator, and calculating how much power you could generate with this setup. It seems so simple, there must be a catch to it. Anybody care to explain to me why it would not work? Designed correctly, it would seem to be possible to build a rotor much lighter than the OEM unit, freeing up some mechanical energy to boot.

 

[TwoManyXS1Bs]

This has come up before. A google search of "rare earth magnet rotor" will bring up some threads like:

http://www.xs650.com/forum/showthread.php?t=12408

I like the idea of imbedding some rare earth magnets in the rotor fingers, yet retain the original regulator. Have it put out just barely enuff power to bootstrap the charging system and ignition, such that it won't overcharge, and the regulator would supply extra power to the rotor as demand requires.

One of the main problems is polarity. On the 70-79 systems, the inner ring is grounded, 80-up systems supply power to the inner ring. So, the magnetic fields are reversed depending on which type of charging system the rotor is installed, and the imbedded magnets would need to be fitted accordingly...

 

[retiredgentleman]

There seems to be a general theme that the stock type alternator must be replaced with a PMA, or else risk electrical diaster. Its my experience that the stock alternator works just fine. 8 years running my stock alternator, and using the same brushes. If someone can point out why brushes are a problem, I'm all ears.

If you want to use a PMA, then go ahead and buy one that's already on the market.

Sure you can design and build a home brew PMA, but expect higher cost than buying one, and a long period to develop it, so its reliable. Do you really think you can compete with the labour costs in China and Taiwan?

 

[fredintoon]

- - - Do you really think you can compete with the labour costs in China and Taiwan?

Hi rtgt,
sure I can, as can anyone who is tinkering in his home workshop instead of vegging out in front of a TV.
Competing with professional electrical engineers or even knowing WTF I'm doing? Not so much.

 

[jamesgs4]

There seems to be a general theme that the stock type alternator must be replaced with a PMA, or else risk electrical diaster.

I've noticed that a lot of people start on project bikes that come with a bad charging system. Instead of spending hundreds on stock system replacement parts, they just upgrade to a more powerful aftermarket system for a little bit more cash. My 30+ year old stock system worked perfectly fine until the day it died

 

[retiredgentleman]

I've noticed that a lot of people start on project bikes that come with a bad charging system. Instead of spending hundreds on stock system replacement parts, they just upgrade to a more powerful aftermarket system for a little bit more cash. My 30+ year old stock system worked perfectly fine until the day it died

There's no question that a new alternator, of any type, should be much more reliable than a 30+ years old alternator. I keep expecting my stock alternator to fail, but it just won't quit, so I continue to use it.

"More powerful aftermarket system"...................I think you're mistaken on that. The PMA's that I've seen for sale are rated at 200 watts, which would be a downgrade. The stock alternator on my bike is rated at 224 watts.

 

[mrriggs]

"More powerful aftermarket system"...................I think you're mistaken on that. The PMA's that I've seen for sale are rated at 200 watts, which would be a downgrade. The stock alternator on my bike is rated at 224 watts.

Yes, but how many watts is the field coil consuming? Is 224 a gross output or net output after subtracting the field current?

I'm not trying to be argumentative, I honestly don't know the answer.

 

[MacMcMacmac]

It might be a fun project?

It might be a cheap way to get a bike up and running?

It might be a way to express one's mechanical creativity?

You'd also keep the stock timing marks and tci pickup.

Ditching the brushes would be a bonus, and if you are handy rewinding the stator, you may be able to build something quite a bit more powerful for less money than you'd blow on an upgrade kit.

 

[retiredgentleman]

Yes, but how many watts is the field coil consuming? Is 224 a gross output or net output after subtracting the field current?

I'm not trying to be argumentative, I honestly don't know the answer.

Rotor uses about 1 amp, once the rpm goes up to 1800 to 2000 rpm or greater. So, 1 amp at 14 volts = 14 watts. 224 less 14 = 210 watts available.

 

[TwoManyXS1Bs]

On mine, the 5.x ohm rotor will pull over 2 amps at full demand, for around 30 watts. The old alternators were rated for 115 watts, and I've seen the uprating published for the later models, like RG's 224 watts, and wondered how they got it's rating upped that far. As far as I can tell (for now), the only significant difference between RG's (in stock configuration '78) charging system and mine might be a better rectifier. Using a solid-state regulator (or RG's vr-115) should actually derate the alternator, due to the Vf voltage drop of the driver transistor, versus the direct contact of the mechanical contacts of the original regulator.

All the single and twin cylinder Hondas I worked on had a fixed-magnet rotor, surrounded by the stator, just like Mac's proposed system. Some just used 2 of the 3 legs of the 3-phase for daytime charging, the 3rd leg would connect thru the light switch. I wonder if a Honda CB-450 rotor is close enuff in size to fit in there. If so, would just need to recut the taper and keyway. Maybe others would fit...