Dangling Alternator Question

So I'm having problems with my voltage rectifier. In the meantime I'm
leaving it unconnected, and charging my battery at night. My question
is this: because the voltage rectifier is unconnected, the alternator
is now dangling (not outputting into anything). Is this harmful to
the alternator or just plain dangerous?

 

It's a '97 BMW F650. And yes, I made sure that alternator wires were
cared for. Thanks for your response.

 

I think you've got a permanent magnet alternator with
a regulator/rectifier combo. In that setup, the regulator/
rectifier has to dissipate unused current as heat rather
than de-energizing an electromagnetic rotor.

For a permanent magnet setup, I'm not sure about leaving
the R/R completely disconnected. electrexusa.com is a
good resource. I'd also recomend the beemershop in
Scott's Valley, Ca as a pretty good source of advice and
online parts.

 

Do NOT run your engine with a permanent magnet alternator disconnected
from the rectifier regulator, except to test the open circuit voltage.

The insulation on the alternator stator coils is just a thin layer of
plastic meant to withstand 20 or 30 volts and it can be punctured by
open circuit voltage of 90~100+ volts.

 

It may take a while for the voltage to puncture the insulation, but,
given time, temperature and immersion in oil, the windings can short
to the metal core, taking one phase out of use and reducing output
power by half.

Look at all those Suzuki GS stators that went bad.

The plastic connectors that hook the stator and the rectifier
regulator to the main wiring harness would get hot and melt, and
that's almost the same as running the alternator open circuit.

 

So I just spoke to the tech support at electrosport.com, and he stated
that there would be no harm done leaving stator unconnected. Now, I
hear and believe what you are saying, so would a viable solution
(besides actually replacing the bad VR ) be connecting the stator
to the VR, but leave the VR output dangling?

 

Oh, what do the tech support kids know, anyway?

I am assuming that your motorcycle has the typical permanent magnet
alternator, NOT a car type excited field alternator.

If you run an excited field alternator disconnected from a battery, or
from the regulator, nothing bad happens.

A permanent magnet alternator really needs to have a battery in good
condition connected to the regulator DC output.

The battery forces the system to operate somewhere near battery
voltage.

An almost dead battery forces the alternator to run at about 12.0
volts or less.

A fully charged battery in good condition lets the alternator run up
to around
18 volts before gassing and boiling the electrolyte, if the regulator
fails.

The regulating circuit tries to keep the voltage between about 14.5
and 15.5~16.0 volts, depending on whether it's a flooded-cell battery
or a maintenance free AGM battery.

The permanent magnet alternator system uses a three phase full wave
rectifier bridge to change the AC into DC. You will find a .pdf file
that shows a typical rectifier bridge at
http://www.electrosport.com/electrosport_electrical_home.htm when you
click on the technical links.

If you have an ohmmeter, you can easily do the diode check, but you
would need a variable voltage power supply to test the voltage
regulating circuitry in the regulator.

If one of the diodes is blown out, this reduces the power output of
the alternator by half, for half a cycle. Blow out two diodes and the
output could be anywhere from half power to almost nothing, depending
on which of the six diodes is blown.

In order to regulate voltage, this type of shunt regulator has a
silicon control rectifier (SCR) hooked up between one pair of power
diodes and there is a zener diode that senses when voltage rises above
about 14.5 ~ 15 volts *with the regulator conected to the battery*.

If you try to operate this system with the regulator output
disconnected from the battery, the zener diode will continously tell
the SCR to shunt excess current to ground and the regulator will get
very hot when the engine is running.

It's normal for an SCR or a diode to get *somewhat* hot when you run
power through it, though. That's why the regulator is housed in a
finned aluminum heat sink.

But the charging system of a modern motorcycle is really designed to
burn up excess power that isn't charging the battery by lighting the
headlight and taillights, not by continuously shunting ALL the power
to ground via the SCR.

The best solution would be to avoid riding the motorcycle until the
rectifier regulator is replaced.

 

I suppose you're speaking of some sort of electrical "imbalance", not
mechanical imbalance. The magnetic fields inside a permanent magnet
alternator just wouldn't be strong enough to move the windings.

In a three-phase wye-wound stator, the phase rotation is A > B > C,
with
power returning from C > A, etc., via the power diodes.

A blown diode just interrupts the orderly return of power to the
following phase.

The biggest 3-phase generator I ever worked on was one of the 250
megawatt units at Boulder Dam, on the Colorado River.

We secured the generator by isolating it from the circuit so it
couldn't run as a motor, and closed the butterfly valve in the
penstock and chained the wicket gates shut so water couldn't move the
turbine.

Then we crawled around inside the generator, looking for burned
windings and windings that had been dislocated by the powerful
magnetic fields inside the generator while operating.

 

Once again, you use your 'experience' of a particularly crappy design,
that's now 30 years old, and think it applies to modern kit.

You're full of shit.

 

Eat it.

 

Curiousity led me to google up information on the BMW F650.

It seems that it has a remote sensing lead coming from the ignition
switch similar to a Honda.

The extra voltage drop across this long lead causes the voltage
regulator to
think that battery voltage is higher than it actually is.

So some technically minded owners figured a way to shorten the lead by
adding a relay...

http://faq.f650.com/FAQs/Photos/ElecPhotos/wiring1-1.gif

http://faq.f650.com/FAQs/FlayingtheVRFAQ.htm#VR Flay I

 

Right. I was trying to post and cook pasta at the same time and got
distracted. (1)

The battery gets overcharged because the extra connectors and the
ignition switch contacts and about six feet of small gauge wire keep
the zener diode from triggering and the SCR doesn't shunt excess power
to ground.

The diodes also have to put up with excess voltage from continual
charging and they tend to blow out.

The F650 has a $250 OEM regulator...

(1) Write that down, Older Gasbag, so you don't forget all the things
I've said that displesed you.

 

All BMW parts come with those very special
BMW prices.

I took a quick look at my BMW parts CD and
besides the regulator, they show a separate
diode (part #s 61 312 346 432 and 61 312 346 465)

Not sure what's up with that. Nice if you could
really replace them separately. I guess the diode
may not be for the main charging current though.

 

The protective diode is a logic diode that keeps current flowing in
the correct
direction. It's associated with a starter solenoid *enabling* relay,
the neutral
switch and the clutch lever switch.

http://faq.f650.com/FAQs/Photos/ElecPhotos/wiring1-1.gif

In this wiring diagram, green is hot when the ignition switch in in
the ON position
and orange is ground.

The voltage regulator gets its voltage information round-aboutly,
through the
ignition switch, like a Honda VFR...

 

Nice find.

 

Pssssst! Don't tell The Old Gasbag that I found the answer by googling
the web. He thinks that's a sign of weakness...

 

Nice save - a decent design these days should be using a fully electronic
regulator not a simple SCR jobbie, that would have full over and
undervoltage
protection. A DSP that does this is an off-the-shelf part that sells for
maybe
$1 OEM.

I'm amazed the aftermarket hasn't come up with a replacement for that
30-year-old discrete parts design from the Dinosaur age that you described
in your earlier post.

Ted

 

Can you give us part numbers for an American made 3 phase full wave
rectifier bridge with a DSP chip in it? Most motorcycles would
probably be OK with a bridge that can handle about 400 watts. Only a
Gold Wingish land yacht garbage wagon would need more power.

The motorcycle aftermarket thrives on the ignorance of the owner, and
there are often international tariffs on foreign electrical/electronic
parts driving the prices up to about three times what a domestic
manufacturer would charge.

 

Not sure if you were talking about the $250 regulator for
the PM alternator or the complete energized rotor systems
from the earlier BMW boxers.

This place offers a well regarded PM alternator replacement
for the old energized boxer systems. This includes a thyristor
based regulator which I believe costs maybe $125 sold separately.

The whole system is based on one used in Ducs. I've heard
of problems on Ducs, but on elderly boxers it's well thought of.

http://www.euromotoelectrics.com/EDLBMW.html