OT car alternator excitor (2023 Update)

The excitor got stuck on when the car was off and the alternator would pull 4 amps and kill the battery. The battery light didn't go on. Now the excitor is stuck off, the battery light stays on and the alternator doesn't pull any current. I'm wondering if I need to replace the whole alternator or just the regulator. I'm having trouble wrapping my brain about what is going on. I'm thinking the excitor signal somehow connects the alternator output to the exciter coil input and that the exciter signal wire doesn't really power any coils. But I'm not sure. If the coil is gone, then I need a whole new alternator. But it might be just some semiconductor that can be replaced. I wish they had schematics for these things.

Most car alternators have a rotating field, an electromagnet that spins and pushes flux into the output stator windings. The field coil needs 0 to 12 volts, depending on how much output current one needs.

A voltage regulator or engine control computer adjusts the field current as needed.

The AC output of the stator windings is rectified by a bunch of diodes. Such a rig can't be back-driven to act as a motor. Maybe some hybrids use one gadget as both generator and motor.

Small engines and some cars have rotating permanent-magnet fields. Their output is regulated by shorting.

I did a lot of research on PM alternators.

onsdag den 19. oktober 2022 kl. 23.14.52 UTC+2 skrev snipped-for-privacy@noplace.com:

a car from this century?

The alternator could have shorted diodes. But more likely the regulator is trashed. Disconnect the field wire and see if the current drain goes away. 4 amps might be about what the field uses.

What do you mean by "excitor coil?"

It might be useful to search for "diode trio" online. IIRC Delco alternators used two sets of rectifiers: The main full-wave set, for the charging output, and a second half-wave "trio" that provides DC power to run the voltage regulator. This allows a switched DC input to drive the field which is isolated from the battery and relatively isolated from output loading once up to speed.

Perhaps a reference to a separate stator winding, additional to the "diode trio" that provides excitation power that's more isolated from the influence of load on the main output. Not used in auto alternators AFAIK, but common in AC mains type alternators.

I do have a Delco schematic from an old manual. If there's interest I could scan and post it.

hth,

bob prohaska

Here's wikipedia

This is why I call it an exciter coil, though I keep spelling exciter wrong.

I guess you refer to it as a field coil. I think the alternator in the car looks something like this.

with the addition of a rectifier/regulator between the M and the plus and minus outputs.

I think in old cars the 'f' field coil went directly to the battery light and the ignition, but when I unplugged the exciter wire, the exciter stayed stuck on. Now that short has burned out and the exciter is stuck off but the battery light is on so something is there grounding the light bulb.

So my question is: Does the exciter wire directly power the field coil or does the exciter wire go to a transistor or something that turns the field coil on? Is there a transistor or diode I can replace?

Yes there are schematics on the internet but I have not found any that go beyond black boxes for the regulator and the alternator. Basically they're just wiring diagrams for the car and not circuit diagrams.

It's an old car, A 1997 VW GTI VR6. If it was a new car I would buy a new alternator. But in an old car, you have to consider how much money to put in to it before the next thing breaks and when do you give up?

the typical circuit powers the regulator through the alternator warning lamp, and through an auxillary positive tap on the main rectifier, if you're seeing 4A flowing into the regulator it's busted. (and probably the lamp too)

If you had 4A flowing through the field winding with the motor stopped the alternator is cooked, (also busted)

You're probably right, but ...

The exciter got stuck on and pulled 4 amps with the car off. With the car on, the alternator worked normally. Produced around 14.5V regardless of the RPMs on the engine, so the regulator was working. I disconnected the exciter wire and the exciter stayed on and the car still worked. I'm a clever engineer so I put a big hundred amp shottkey diode between the starter motor and the alternator to stop the reverse current flow. But that turned the exciter off and the exciter wire wouldn't turn it on. Removing the diode turning the exciter back on and the car worked again. I would have to disconnect the battery when the car was off but when it was running the car was fine. Then one day I connected the battery, tried to start the car and the battery lamp came on. The exciter was off, the alternator no longer worked but the exciter does connect to ground to cause the lamp to go on and the lamp works. So I think the field coil is not directly powered by the exciter wire. I can imagine a circuit with a transistor. The exciter wire connects to the base, the emitter connects to the field coil and the collector to the output of the regulator.

The real problem is that this alternator is buried deep in the engine and I have to dismantle half the car to get there. I don't have a garage and winter is coming. I'm a slow mechanic. Things usually go sideways for me and I need to wait a week for a part or tool to fix some unexpected problem. So I'm going to wait until next summer. I think the car is sidelined by a burnt out $5 transistor deep inside the engine, but you might be right and I need a whole new alternator.

I'm just frustrated and posting stupid questions on usenet.

Interesting. What does the exciter wire do?

The Alternator uses a 6 diode 3 phase bridge to rectify the 3 phase voltage produced to charge the battery. This blocks any reverse current such as trying to power the alternator as a motor.

The alternator acts as a current source but with sufficient voltage at an idle RPM to charge the battery. This is controlled by a variable current to ground to the exciter winding which as V+ on the other end internally. Think of this like a current controlled magnetic coupler from stator to rotor to couple more output current if the engine is running and the demand current is sensed by a battery voltage less than the internal regulator reference of 14.2V (typ.)

The alternator has this magnetic current gain then from exciter field winding current to battery output current, that might be from 4A in to

120 A out depending on the rating of the alternator.

That's how it works.

It powers an elecromagnet in the rotor of the altenator. that makes the rotating magnetic field that generates the EMF that produces the electric current.