Amperage regulation

[Huw]

Here is the situation, I have a 55amp alternator from a car, which is connected to a battery {45ah}. Occationally the load is over 55amps. This has caused one alternator to burn out. I have been racking my brains, to think of a way to regulate the amperage produced, when an excessive load is applied. Anyone got any ideas that dont cost a fortune?.

 

[MikeMl]

What kind of alternator? The Ford Prestolite alternators on my airplanes are "impedance protected" in that they can use core saturation to limit the output current even into a short. When spinning at a constant RPM, an alternator is a "current-amplifier", where output current = k*Field current, k is a constant like 20. If you limit the current to the field windings, that in turn will limit the output current.

The other possibility; install a circuit-breaker in the output lead, or get rid of the equipment causing the "overload". You probably need to install a circuit breaker in your acoustic nerve, too.

 

[Huw]

Thanks for the input, I'll give you the full story. I have a small FIAT Cinquecento, with a marelli 55amp alternator, due to the fact that FIAT saw fit to make the alternator a specific fittment it is difficult to retrofit the engine with a bigger alternator. I have all the usuall current draws off the system, but the main problem is the addition of a larger sound system. I am not a boy racer, it's just I like my music. No thumping subwoofers or anything like that. With the sounds on and the lights etc, she pulls just over 55amps, and on a constant draw, the alternator gets a bit hot. There are only two leads off the back, the large feed to the battery, and the feed to the warning light. Is there something like a shunt that would limit the call on the alternator, no worries abought flatning the battery as that is easily upgraded. I am a bit wet behind the ears, so mike's earlier comment will take me a while to desypher. P.S like the plane.

 

[MikeMl]

Ok, sounds like the regulator is inside the alternator. My comments about limiting the field current would require modifying the regulator, which is difficult to do with an internal regulator. So two possibilities: mount a cooling fan to get the heat out of the alternator, or use a circuit-breaker to protect the alternator from overloads.

 

[kchriste]

use a circuit-breaker to protect the alternator from overloads.

Be careful with that. If the alternator is unloaded by the breaker popping, the output voltage could spike and take out the diodes and regulator.
Either turn down the volume or get a beefier alternator.

 

[MikeMl]

Be careful with that. If the alternator is unloaded by the breaker popping, the output voltage could spike and take out the diodes and regulator.
...

Good point. Question about the small wire to the alternator? Are you sure that it is an idiot light output? It could be a key-switch input to the alternator which shuts down the alternator during cranking. If so, by breaking that wire, that will shut down the alternator. So you would have to sense the output current (look up a ZXCT1009), and break that wire if the output current exceeds say 50A.

 

[Diver300]

The alternator in a Fiat Cinquecento has an internal regulator. The small wire is for the charge warning light, so the alternator will continue to work if that wire is disconnected, although it may not start up if the current from the warning light isn't there.

The alternator has a 3 phase winding and 9 diodes. 3 large ones connect the 3 phases to earth. 3 more large ones, the other way round, connect the 3 phases to the battery. 3 small diodes connect the 3 phases to the warning light connection. The field winding is run from the warning light connection, via the voltage regulator.

I thought that alternators were current limited by their own inductance, and we never burned one out in the 16 years we had the Cinquecento. The brush / regulator pack can fail and can be changed without removing the alternator from the car. I also had to fix one of the small diodes in the diode pack.

You can shut down an alternator like that by shorting the warning light connection to ground. However, that can overload the diodes, because the full alternator current flows though the little diodes for half a second or so until the field current decays. You could also disconnect the regulator, but that would need opening up the alternator to find the feed to the regulator.

If the alternator had been overloaded and burned out due to overheating, the insulation on the main windings would have burned and flaked off, and would have had a distinctive look and smell. I think that the brushes of the OP's alternator were getting a bit old and all it needed was a new brush / regulator pack. Of course, most garages will change the whole alternator as it costs more money.

 

[Huw]

Wow, I am not worthy. Major thanks for the input guys. Yes the insulation on the old alternator has flaked off a little, either due to age or overload. Bearings are dry as a camels ear lobe also, this would sugest it was passed it's best, and my tunes were the final nail so to speak. I have no mains electric in my garage, so have to run batteries and an inverter, I am a mechanic in a tool hire shop and through scrounging have put together a small honda engine, running a 45amp Lucas [old mini style] alternator and an ammmeter. If I have this running to charge the batteries and put a heavier load on it the amps can go beond 60+, hence my trepidation with the car. I did retrofit the ammeter to the car lastnite and switch everything on, lo and behold the amps went up to 50, and the stereo wasn't quite up to 11 [spinal tap]. I definatley think that I should be either investing time and money on a bigger unit, or just leave out the tunes till it's light and I can switch off the headlights. Shame because the theory sounds workable, something to limit the flow to 55amps regardless of the call of the battery. Engines and electrics I am ok with, but electronics come up a bit short.
In mechanical terms it is like resticting a pipe with a choke to limit the flow, as per my earlier question would an ammeter shunt do that?. Or would the call just cause it to get hot and blow?. Sorry for seeming a bit clueless. Yes you are right, I am ex motor trade, we just used to change them as a unit, service exchange. But none of the motorfactors round us do service exchange. They just want to sell you a new one, for £80+.
If it wasn't just coinsidence that the old alternator was on it's way out, it could get an expensive hobby.

Thanks guys, you've given me alot to think about.