Re: Volvo 24v alternator exciter
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- The exciter relay coil is powered by 12V from the ignition switch, and when energized applies 24V from the house bank (via the indicator bulb) to the alternator.
- If the alternator has been recently used, there may be enough residual magnetism for it to self-excite. The symptoms are exactly as you describe -- it will start working at higher RPM, and keep working even if you lower the RPM again. But if left unused for long enough, the magnetism will fade and you'll have a dead alternator.
- I'm not sure of the exact wiring, but I was under the impression that this is simply a light bulb in series with the exciter wire. There may also be a diode in there so that it will turn off when the output voltage rises.
- I believe so, but I think the exact implementation depends on your alternator model.
Ryan and Kelly
SM 233 Iteration
Boston, MA, USA
Thanks for this info and pictures.
Operationally at this point, at startup we are not getting 24v alternator output until about 2000-2200 rpm. Then backing down rpm continue with 24v output until about 1200 rpm, and below that not. I have not yet tracked the behavior of the fault light on the 24v panel in these various conditions.
I am still trying to understand the circuit logic for the exciter. I cannot find any circuit diagrams for this and I am not at the boat to trace the circuitry. The internet is not providing much relative to exciter theory or engines with both 12v and 24v systems. Any insights you have on the following would be helpful.
- Since the 24v alternator is not producing 24v at startup, is the 24v alternator exciter circuit initially powered by the 12v battery (simultaneous with the 12v alternator), or by the 12v alternator after it is producing its 13.5v? There could be a design theory that the 24v alternator is excited by and only AFTER the 12v alternator is producing 13+ volts thereby reducing startup load slightly.
- Is it the case that the 24 alternator would be “self-exciting” after generating 24v? If so, is that “self-exciting” dependent on rpm? And if so would this relay drop out the external 12v power for the exciter?
- I have noticed that the alternator fault light on the 24v panel historically stayed on after starting the engine while at low rpm, but generally went out after a small increase in throttle. This probably indicates that the 24v alternator was not yet producing 24v. Is the control logic for this fault light any more complicated than that?
- Is the exciter circuitry independent of and unrelated to the voltage regulator?
Anyway, I look forward to returning to Aletes in a few weeks to pursue this and would like to be as knowledgeable as possible if you have any thoughts.
There is a relay that uses the start engine 12v circuit to close the other side of the relay which is supplied with 24 volts. That is how the "excite circuit" works for the 24-volt alternator on all model Amels that I am familiar with that have 24-volt banks and alternators. Here are a few photos:
24 V alternator recently requires higher RPM before producing voltage. Any suggestions about where to start troubleshooting? Exciter circuitry? Thanks!
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