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Voltage Regulators

High Output Voltage Regulators and
How Mechanical Regulators Work

From (Brian Curry) Thu Jun 6 23:55:59 1996

At 01:17 PM 10/13/95 PDT, 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

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