>From bmwbrian@omni.voicenet.com  Tue Nov  7 10:52:52 1995
From: bmwbrian@omni.voicenet.com (Brian Curry) (by way of bmwbrian@omni.voicenet.com (Brian Curry))
Date: Tue, 7 Nov 1995 11:53:22 -0500
To: davet@ncsa.uiuc.edu
Subject: Re: BMW: high output voltage regulator 

At 01:17 PM 10/13/95 PDT, joe@advamedi.com wrote:
>On Fri, 13 Oct 1995 13:52:11 -0400  Brian Curry wrote:
>
>>The high output voltage regulator provides full output sooner, at lower rpm 
>>than a mechanical regulator will.  The maximum output rate remains the same. 
>>Your battery will be happier for the use of it.
>>
>Mechanical regulator?  How does this work?  

Looking at this, I recognized that I could have led the non-electrical 
people off in the woods with no way out. Sorry.  It is "mechanical" because 
it has moving parts.  The high-ouput model is all electronic, which is also 
why it can provide more output sooner.

A mechanical voltage regulator is basically a voltage sensitive relay.  The 
relay has one contact and two positions.  One position applies 12 VDC to the 
alternator field.  The other position shorts the alternator field to ground. 
 The 12 volts to the regulator is provided either from the diode board, 
(Those three undersized looking diodes) or from the battery through the 
indicator lamp.  The indicator lamp provides enough current that on startup, 
the alternator will provide enough additional current, that the rotor field 
can be taken to maximum magnetic field strength.  (It cannot from the 
current from the indicator/idiot lamp.)  This is called "bootstraping."

The alternator output voltage is the product of the rotor magnetic field 
strength and the rotor speed.  If a voltage regulator was not installed the 
output would go up and down with engine speed.  (Triumphs, Nortons, and Jap 
bikes all have used such a scheme.  They used Zener diodes to limit the 
maximum system voltage effectively turning excess output into heat.  If the 
diode opened up the system voltage went real high, boiling batteries and 
causing very bright head lights for short periods of time followed by 
darkness when the bulb burned out.)

The regulator contact flips back and forth applying full voltage to the 
field and then taking it off.  If alternator output voltage gets too high 
the regulator shorts the field to ground to reduce the magnetic strength 
right now.  It takes some time for the magnetic field to build up and die 
down so the regulator can control it.  It does this so fast you do not 
notice the flipping. It basically buzzes between the two contacts.  At low 
rpms it buzzes more towards the the diode board output contact applying more 
current to the alternator field, and at high rpms buzzes more towards the 
grounded contact applying less current.

This is also why a voltage regulator bypass switch can destroy the 
regulator. If 12 volts is applied directly to the alternator field, at high 
rpms the regulator will be trying to reduce the field strength by shorting 
the field to ground.  It is shorted through a resistor under the regulator.  
The resistor is not designed for continuous duty.  Thus, if it has 12 volts 
applied long enough, it will overheat and fail.  This will take minutes not 
seconds, but take care when testing for a bad regulator/rotor.

The voltage sensitive relay contacts must be far enough apart to prevent the 
moving contact from not opening due to electrical arcing.  This would result 
in the relay having both contacts "closed" at the same time.  If everything 
is closed at the same time, the regulator is fried.  This gap causes a "dead 
band" beween the start of charging voltage and the maximum allowed voltage.

The electronic regulator has no moving parts.  It electronically compares 
the output voltage to the desired voltage, and the maximum voltage, and 
controls rotor magnetic field to stay in between them.  Since nothing is 
moving, these voltages can be real close together.  With a higher set 
voltage, the alternator rotor has a higher voltage applied to it, with 
resultant higher current and magnetic field strength.  Due to this, the 
alternator provides more output at lower rpms.  While this is good for 
keeping the battery charged, when the maximum alternator output is reached, 
the alternator voltage/current/magnetic field is limited and the battery is 
not overcharged.

The electronic regulator provides more output at lower rpms and protects the 
battery from overcharging better.  Another one of the miracles of modern 
electronics.

Brian Curry
K75RTs both coasts
K-Whiner #21