Fried alternator rotor?

[j_ds_au]

Hi folks.

A couple of days ago, my Battery indicator lamp came on during a short drive home. It was getting late, so I decided to investigate this later. So a day or so later, I discovered that the battery (which is only a few months old) was down to about 3.5V. Delving deeper, I determined that my alternator was faulty, loading down B+ even with the ignition off. Delving deeper still, I found that the RE72 regulator had a short-circuit between the F- carbon brush terminal and ground. As I believe that the F+ carbon brush terminal is connected directly to B+ within the alternator, this means that the rotor field coil was effectively connected between B+ and ground, which is why the battery had discharged so quickly and unexpectedly.

Now what I need to figure out, is whether the rotor field coil could have survived being so connected between B+ and ground, for however many hours it took to flatten the battery? I can measure 2.0 ohms between the slip rings, but don't have any specification for what the rotor field coil resistance should be for this particular alternator, type 92094625 (Bosch F 005 A00 033 = BXH1333), with a current rating of 40-100A. If anyone knows what this resistance should be, that would be appreciated (I've found figures mentioned of between 2.8 and 5.5 ohms, but these were for unspecified alternators, not this particular type). Incidentally, there is no continuity between the slip rings and ground, which at least is some good sign.

Joe.

[In-Tech]

I am unfamilar with your exact situation. Most often it is a diode failure. It will almost always work when charging and drain when not.

[pman92]

Does it look like it has been hot at all? If it looks fine and isn't shorted to ground it is probably fine.

If the rotor had full current while driving it would of been charging flat out. If it was only a short drive it might be fine. But usually you can tell when its not straight away when everything is black and it smells burnt

[j_ds_au]

I am unfamilar with your exact situation. Most often it is a diode failure. It will almost always work when charging and drain when not.

Yeah, maybe it's an uncommon fault. The heavy drain on the battery disappears when I just pull the regulator out, so there doesn't seem to be anything wrong with the diodes. The transistor within the regulator, that drives the rotor field coil, is definitely shorted-out though.

Joe.

[j_ds_au]

Does it look like it has been hot at all? If it looks fine and isn't shorted to ground it is probably fine.

If the rotor had full current while driving it would of been charging flat out. If it was only a short drive it might be fine. But usually you can tell when its not straight away when everything is black and it smells burnt

Yep, the alternator would have been charging flat out during the short drive home, with the voltage moderated only by the battery. Fortunately, I was driving quite moderately, so the current out of the alternator would have been closer to 40A than 100A.

However, the excessive rotor current would have continued to flow from the battery after switch off, hence the flat battery the next day.

I can't really tell if anything looks burnt as such, but there's no burnt smell, so that's a good sign I guess. However, the 2.0 ohms rotor field coil resistance seems a bit low to me, but as I don't have a spec., that's just my guesswork. If we take the battery voltage as 12V, then 6A would have been flowing through the rotor field coil after switch-off, so 72W, without the aid of the alternator internal fan spinning.

Joe.

PS. Actually, the above 72W calculation is incorrect, because it only considers the rotor field coil resistance. However, the whole circuit has a total of about 1.0 ohms of additional resistance, which firstly reduces the fault current by a factor of 2/3 and secondly, means that only 2/3 of the power is dissipated in the rotor field coil itself. So the revised figure for the rotor field coil power dissipation (with a shorted regulator transistor) is 32W.

[antus]

It doesn't sound blown to me. Im not an electical guru but my thought are if it had blown, I think you'd have the burnt look and smell, and you'd have no connection now. You'd also have a battery that's still at operating voltage because something carrying too much current would have melted and broken the circuit before it drained that low. 2 ohms sounds fine to me because your measuring DC not AC.

[Gareth]

We bench test by applying 12v to the rotor coils and observe current draw which should be 4-6ohms

The regulator sounds like your fault in this case, for what it's worth bin it for a new genuine one.

[j_ds_au]

It doesn't sound blown to me. Im not an electical guru but my thought are if it had blown, I think you'd have the burnt look and smell, and you'd have no connection now. You'd also have a battery that's still at operating voltage because something carrying too much current would have melted and broken the circuit before it drained that low. 2 ohms sounds fine to me because your measuring DC not AC.

Well, I agree that the lack of obvious burning and that smell are encouraging, however, the facts of the failure and (what seems to me to be) a rather low rotor field coil resistance give me concern. I realize that mine was a DC measurement, but so were the other resistance measurements I found on the web for other alternators. And with the regulator transistor that drives the rotor being shorted out, DC is indeed what the rotor would have experienced. Now as far as "too much current" fusing something and breaking the circuit, I don't see that either the alternator output (while the engine was running; say 40 to 100A), nor the rotor field coil current (say 6 to 8A), would be sufficient to do that. Yet running say 6A at 12V (72W) for several hours through the rotor, without the air cooling of the internal alternator fan spinning, would have caused some serious heating of that rotor.

Joe.

PS. The above 72W power dissipation figure for the rotor field coil would actually be 32W, due to the additional 1.0 ohms around the circuit (as explained/corrected in my previous post, above).

[j_ds_au]

We bench test by applying 12v to the rotor coils and observe current draw which should be 4-6ohms

The regulator sounds like your fault in this case, for what it's worth bin it for a new genuine one.

Thanks, Gareth.

So is that 4-6 ohms, as calculated from the observed current draw using Ohm's Law, or did you mean to type a current draw of 4-6 amps?

Joe.

PS. Gareth has now confirmed that it's actually 4-6 amps, which then means about 2-3 ohms for the rotor field coil.

[Charlescrown]

From my experience little goes wrong with the rotor. I have seen one go short to ground easily picked up by measuring resistance from a slip ring to the rotor shaft. Usually around 3-4 ohms resistance between slip rings for a lot of alternators. Even if the regulator is faulty (which is my guess) the rotors resistance will controll the current and prevent damage occuring. As prevoiusly mentioned a look at the colour and a smell should tell you if there is a problem. I did have a regulator that only went short to ground after the engine was turned off and that was obvious by a hot regulator when it should have been cool.