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generator regulation


Alexander50
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There is a three brush 6 Volt generator in my car. These generators are regulated by interupting the field which is fed by the third brush. This third brush is between the two main brushes and is on a voltage of about half the generatorvoltage. The voltage on the third brush reduces when the current is increasing. A fully charged battery though leads to a high outputvoltage (up to 8 Volt) so the chargecurrent is too high and the battery is producing a lot of gas. The life of the battery and the bulbs is shortened. All connactions in the car are OK and the third brush is moved as far as possible to the earthed mainbrush, so it is working as intended within the limitations of this way of cargeregulation. I am not satisfied with this and want to regulate the generator with a normal voltageregulator, but connecting the filed to the normal regulator is not possible. The field of a generator that is intended to use with a normal voltageregulator is internally connected to the outputbrush. The field armature of the three brush generator has a much lower resistance and is internally connected to earth. I want to keep the generator as original as possible and dont want to rewind the fieldarmature or change the connections in the generator. Has anyone an idea how to use a regular voltageregulator without changing the generator ?

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You can use an electronic regulator. I can mail you a shematic diagram if you like. It can be made small and live inside the generator. I have it on my cars and it works well. Many of these old generators can not charge with enough current for the headlamps and tail lamps if you use stronger light than original. The specification for my Buick 1932 is maximum 20 A when cold and 12 A when warm (they don't tell at what temperature!). So if I also have the heater on it will discharge 2 or 3 A.

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What car do you have? I went to my competitor recently,and wanting to start an argument I told him"They never made a GOOD three brush generator" He agreed,much to my surprise. The previous gentleman had good advice if you are locked in on generator location and design. If you have a belt drive unit.we replace them with 40 up two brush units and a Delco 201 or 301 regulator

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  • 4 weeks later...

Thanks for your help. I considered the electronic regulator, but was not fully satisfied because (1) there is no limit to the current the generator delivers and (2) I expected the regulator to get pretty hot because of the amount of power that is dissipated. So I used a mechanical voltageregulator as used on the 6 volt VW-beetle. Because I want to be able to convert it to original anytime, I did not change the groundconnection of the fieldcoil and left the third brush where it was. I used a powertransitor to invert the voltage on the fieldcoil. This transistor does not dissipate any power because it switches at the speed of the mechanical contact of the regulator. That contact hardly conducts any current (only the current to the basis of the transistor which is less than 1 Amp) so it will live forever. Because a third brush generator delivers only about -4 volt to build up the needed magnetic field, I used a resistor (about 1 ohm/5 Watt) to reduce the fieldvoltage from 6 volt to 4 volt. The transistor is connected to the ignitioncontact so the field gets full power even at idle. All these additional parts are fitted under the regulator so you can't see them. This solution limits the current from the generator at a safe level if the battery is exhausted. After starting the engine the generator starts immediately to charge even at idlespeed. The current is limited to approx 30 amps (which is the longtime limit for this gen) at low battery and reduces to approx 2 amps with a full battery. The transistor stays cold.The voltage is stabalized nicely on approx 7 volts at all speeds with a fully charged battery. I never could reach that situation with the third brush mode.

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There seems to be a minor misunderstanding here. A three-brush generator has the rather odd property that above a minimum speed it delivers more or less a constant current (albeit somewhat dependent on temperature), or sometimes a choice of two more or less constant currents (my Rover's dynamo delivers a constant 8 or 9 amps). If the third brush is left functioning as originally intended, adding a voltage regulator cannot increase the maximum output current; it will merely reduce it when the output voltage would otherwise rise excessively. Thus the third brush performs very much the same task as the current sensing part of the circuit of an electronic regulator or as the current sensing winding on a mechanical regulator.

Hence Alexander50's first objection should not apply, unless of course in adding a regulator he proposed to disable the third brush. He is quite right of course that adding a regulator must not be allowed to increase the current delivered much above that which the generator delivered as designed, because it might well overheat.

Ken G, 1925 Rover 16/50 (San Francisco)

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Guffin,

I tried a an Anod in the cut out on my 1928 Buick and I am not getting good results.

I copied the set up that they sell for early Ford V8s. The Ford with the positive ground uses a Diod. I figured that with the negative ground, I needed an Anod so I bought one with the same value as the Ford Diod.

My car worked fine with the cut out relay but when it went out and they wanted $75.00 for a replacement one, I tried the modification.

Do you have any ideas that will help me on this?

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I don't know what you mean by an Anod. Just turn the diode the other way. It conducts in only one direction. For best result you should use a Schottky diode. They have a lower voltage drop. There are such diodes with 0.48 V at 20 A. The cost around $10. This is about the same voltage drop as over the contacts of the cut out relay at 20 A. But the relay contacts are resistive so the voltage drop will depend of the current. The diode has nearly a constant voltage drop and if you adjust the generator voltage to 7.5 V you will get a very stable output voltage of 7 V after the Shottky diode.

The power disipation is also lower in the Shottky diod, approximately 10 W instead of 20 W or more for an ordinary diod. Both type of diods have to be mounted on an heat sink to keep it cool.

If you put an electronic voltage regulator on your third brush generator your cut out may fail. This is because the cut out for such generators are adjusted to activte at 8 V or more. You have to adjust the cut out to activate at a lower voltage.

Jan

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  • 4 weeks later...

A thicker third brush will reduce your output. You may have too thin a brush installed currently. I once only had a thinner brush to install while on a tour in my third brush position. This thinner brush caused my normal output of 12 amps to go off my 20 amp gauge. I ran with my lights on for the rest of the tour to protect things. Once home, installing the correct thickness third brush corrected the problem. You will see the same thing happen with a new brush that has not 'set-in' and worn to the curvature of the commutator and you are only running on a tangent of the brush.

Concerning output. The third brush regulation reduces output at higher speeds to reduce overcharging. That's about the only 'regulation' it really provides other than your manual adjustment of it.

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