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Old 06-06-2012, 10:08 PM   #215
Beastly Adventurer
JoelWisman's Avatar
Joined: Apr 2009
Location: STL, MO, USA
Oddometer: 1,392
Depending on priorities...

The only technical part I disagree with above is where the heat ends up.

There are 3 fairly different kinds of voltage regulator for PM alternators.

1: SCR shunt. This regulator contains diodes to rectify the AC on the back side and 1 to 3 SCRs to shunt excess current to ground. The diodes and SCRs do have some resistance and therefore do heat up when there is a significant excess charging capacity from the alternator, EG at high RPM with few electrical loads switched on. HOWEVER, the greatest resistance in the system is the stator and that is where the great majority of heat is created.

2: MOSFET shunt. This type of regulator is exactly the same as above except for the use of MOSFETs in place of the SCRs above. MOSFETS have extremely low resistance and very little switching heat so the only appreciable heat generated is from the rectifying diodes themselves. Note, a wound rotor alternator like an airhead has, has rectifying diodes as well.

3: series PM regulators. This type of regulator uses MOSFETs to switch the stator open to regulate voltage and creates no greater heat then a wound rotor alternator, but like a wound rotor alternator is not quite as efficient as a shunt regulated alternator owing to losses flowing in series through the MOSFETs

NOW, lets consider the components in each common system (series PMs are uncommon)

WOUND ROTOR ALTERNATORS like an airhead has.
1: a wound rotor that can and does fail.
2: sliprings to transmit power to the wound rotor. These wear out or can short.
3: 2 brushes both of which wear out and can also break or burn up or frequently melt their holders and jam.
4: Separate bearings for the rotor on many applications including some modern BMW's such as the R1200GS or K bikes. Oh yes these bearings fail.
5: A belt driving the alternator if separate which of course wears out and will break instantly if it picks up the right rock.
6: air cooling which can plug.
7: A stator which of course can fail.
8: A diode bridge to rectify the AC which can be reliable but if oe is most unreliable on BMW's
9: A voltage regulating circuit that controls current to the wound rotor.
10: A circuit to energize and de-energize the voltage regulating circuit.

Practical experience, I dealt with airheads that had a failure in some part of this charging system causing the bike to cease charging and strand the rider at least weekly which is significant considering there aren't a great deal of air heads on the road.

1: A flywheel with permanent magnets that never breaks. It needn't be included in this list because in 24 years I have honestly never seen one fail.
2: A stator. Some are designed well, some aren't. The stator in the BMW F800GS fails regularly between 20k and 50k miles, but has since been redesigned. Most stators are VERY reliable with a few exceptions of poorly designed units.
3: A single component that does the rectification and voltage regulation and is nearly universally adaptable from one bike to the next if capacity is at all similar though there is nothing wrong with using one that has a higher capacity. It is usually the diodes that go and usually because battery terminals were left loose in which case the system fails open and voltage goes low. On rare occasion the voltage does go high just the same as it can with a wound rotor alternator.

Thats it for the PM alternator, 3 components 1 of which never fails. The remaining 2 components will fit as spares under the seat and weigh less then 3 pounds.

The wound rotor alternator has between 4 and 7 more components all of which have a high incidence of failure and weighs about 4x to cary as spares to say nothing of the increased difficulty in diagnosis and field repair.

On an adventure, i'd go with a PM alternator every time.
Owned to date. Honda Aero 50, Honda Elite 80, Honda Elite 250x2, Suzuki Katana, Suzuki RF600, Yamaha YZF1000R, Kymco Xciting 500, Suzuki GS500, Suzuki Burgman 650, BMW F800GSx2, BMW S1000RR, Aprilia Scarabeo 200, Aprilia Caponord, Aprilia Sportcity 250
I love and miss you Jeneca and I'm sorry.
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