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Thread: Old Jeep Voltage Regulators; An Introduction: Section II

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    Super Moderator LarrBeard's Avatar
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    Ft. Wayne, IN

    Old Jeep Voltage Regulators; An Introduction: Section II

    Since I got too windy to put all of this in one section, here is Section II

    Current Limiter Relay

    If you look back at the picture of the voltage regulator with the cover removed, the middle of the three relays is the current limiter relay. Unlike the first of the relays we will discuss, this one is clearly and accurately named. Its function is to limit the current provided by the generator to its rated value to protect armature windings, brushes and the commutator bars from being damaged by delivering excessive current. Most 6-volt Jeep generators are rated at 35-amps (there may be a 50-amp unit out there) and the current limiter does a good job of holding them to no more than 35-amps. I have added the current limiter relay to our partial schematic of the regulator to show how it relates to the cutout relay we discussed earlier.

    See attached Figure 3. Forum protocols do not support embedded pictures

    Voltage Regulator Partial; Schematic, Cutout and Current Limiter Relays

    The partial schematic I have shown above represents the state of the voltage regulator just as the cutout relay closes at engine start up. Let’s assume it took a bit to crank that 134 cubic inches on a cold morning, so the battery voltage is down a bit and will accept all the charge we can drive into it. The battery is connected to the charging system through the cutout relay contacts, the current limiter relay contacts (single-pole, normally closed contacts) are closed as are the contacts on the voltage regulator. (Don’t worry about them; we’ll talk about them later).

    Just for information, the field coils of the generator have about 3 or 4 ohms resistance. In the condition we show above, the battery will push about 2-amps of field current into the coils. That is more field current than the generator needs to provide 35-amps to the battery. The entire generator current is going through the current limiter coil (which on the VR-2 regulators is about 7 turns of # 10 copper wire). From the arithmetic we did earlier, that’s 245 amp-turns which create enough magnetic force to overcome the spring tension that hold the current limiter relay contacts closed. When the relay contacts open, field current provided by the battery flows through resistor R1 and this cuts field current back to about 1/5 of an ampere. The generator is still producing a current output, but that’s not enough field current to drive the generator output to 35 amps. Since there is no longer enough magnetic force to hold the contacts open, they close again and the cycle repeats.

    A couple of things to note here:

    1. The relay contacts only open a few thousandths of an inch – there is no big CLUNK as they open and close.
    2. They open and close a couple of hundred times per second; more like a buzzer than what we think of as a relay. If you take the cover off the regulator and feel the base of the unit, you will feel the “buzz” of the contacts cycling.
    3. While this is happening, the ammeter shows full charge rate. Even though we have just described an ON-OFF condition with the relay contacts, the switching happens so fast that the mechanical meter hangs in there at 35-amp charge.

    Now, let’s add the last relay to the picture.

    Voltage Regulator Relay

    See attached Figure 4. Forum protocols do not support embedded pictures

    Voltage Regulator Schematic; Cutout, Current Limiter and Voltage Regulator Relays

    OK, here is the same picture with the last relay added. The voltage regulator is another single-pole, single-throw, normally closed relay. It differs from the current regulator in that it is a voltage sensitive relay. Remember that the current limiter relay had a coil consisting of 7-turms of # 10 wire connected in series with the generator; the voltage regulator coil is a whole lot of turns of what looks to be about #30 wire. The voltage regulator coil is connected across the battery voltage to ground. The contacts are held closed by spring tension. As the battery charges, the current through the coil increases to the point that the magnetic field in the solenoid overcomes the spring force holding the contacts closed. The contacts will open only a few thousandths, but this will break the circuit for field current which reduces the voltage produced by the generator. When this happens, generator voltage drops, the contacts close and the cycle repeats. These contacts open and close a bit slower than the current limit relay contacts because the battery acts as a bit of a buffer, but they can still cycle a hundred times a second – again more like a buzzer than the CHUNK-CLUNK action we expect from a relay.

    You notice R2, a 10-ohm resistor hanging out there on the FIELD terminal of the regulator to ground. When either set of contacts opens (especially the voltage regulator contacts) the current in the field coils changes. When the current changes, the magnetic field collapses and a high voltage spike is created. High voltage spikes make sparks and arcs – something we want for plugs, but not regulator contacts. This high voltage spike would cause considerable arcing at the contact points, so R2 is added to dissipate this energy and reduce point arcing.

    After the Jeep has been running for a while, the battery is approaching a full charge state. The generator is delivering a modest amount of current, so the current limit relay contacts are closed and generator control is left to the voltage regulator. According to some Buick service notes from 1950, the current limit and voltage limit relays are not active at the same time, and I see this mentioned elsewhere as well.

    How do we understand the three charge states we all see on our ammeters? Just after start, the high charge rate is controlled by the current limit relay – battery voltage is low enough that the voltage regulator is not active. Easy to understand.

    After a while, going down the road as the battery approaches full charge, the current limit isn’t active and the voltage regulator is tickling the battery every so often, keeping just an average of a few amps going into it. Again, fairly clear.

    But, what about that medium rate charge we see in between those two conditions? No one talks about that. Here is a theory - I’ve not gone out with meters and oscilloscope to verify this – I’m not all that curious. In the medium charge state, the battery will still take enough charge to activate the current limit relay. If we hit the battery with a pulse of 35-amp charge current, the current limit relay will open up, leaving the resistor to set field current. But, the battery is getting close to the voltage regulator set point, so that pulse of current will push battery voltage high enough to trip the voltage regulator. When the voltage regulator contacts open, the generator basically shuts down (no field current). The current relay points close, voltage regulator points close and the two relays kind of ping-pong back and forth to establish that mid-rate charge. After a while, the battery voltage stabilizes and the voltage regulator has complete control. That’s my story and I’ll stick to it until someone comes up with a better one!


    I promised at the start to go back and describe the differences between a regulator for the Type B generators used on the MB and GPW and the Type A generators used on civilian Jeeps. I will answer the question now – “No, I don’t know why.” Since most post-war cars and trucks used Type A generators, there had to be some good reason, but it is probably lost in the fog of time.

    Here is the schematic of the regulator and generator set up for a Type A system, the civilian version:

    See attached Figure 5. Forum protocols do not support embedded pictures

    The changes are simple. In the generator, the field winding is connected internally to the armature which provides a source of field current. In the regulator itself, the voltage regulator and current regulator contacts are returned to ground instead of to the battery. Everything works just like it did in the Type B description.

    Exotic Variations

    The regulator I have described is today’s one-size-fits-all unit. If you read old manuals, you will find variations that engineers used to fine tune voltage regulator performance for specific reasons.

    You will find diagrams that have additional windings on the current limit and voltage regulator coils. Most commonly there is a voltage winding added to the current regulator and a current coil added to the voltage regulator. As best I can determine, these coils were called “frequency coils” and they were added to speed up the action of their respective relays. Again, this was a fine tuning to get the absolute maximum performance from the unit.

    It was also not uncommon for there to be a resistor in series with the voltage regulator coil. This would affect the voltage trip point of the relay. Last of all, in some even more rare cases, there was a resistor across the contacts of the voltage regulator. Instead of just cutting off field current, the voltage regulator would reduce it much like the current limiter.
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    Last edited by LarrBeard; 03-24-2019 at 04:13 PM.

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