Exciter voltage

[pophops]

To help me diagnose a charging fault, could someone please clarify for me the following?

In the event of the exciter voltage dropping to zero when the engine is running, does the alternator immediately stop charging?

Or does the alternator only require a voltage at this terminal to start it working?

Thank you for any replies.

[Sir Nibble]

If you mean an externally regulated machine with an EXC terminal or an ordinary internally regulated jobbie with a D+ terminal the answer is the same. No voltage here means no output.

[John V]

3 minutes for a definitive answer......is this a record?

[Tony Brooks]

SNibs is correct but I am not sure that is what the OP meant.

 

Taking a conventional 9 diode alternator.

 

If the the rotor circuit between field diodes and earth via the rotor and brushes goes open circuit there will be no rotor excitation so the output would be zero.

 

If the warning lamp failed for some reason, a terminal fell off the warning lamp circuit or someone turned the ignition switch off while the alternator was charging then the alternator would stay charging because the rotor current would still be supplied from the field diodes.

 

Not sure if that helps or hinders though.

 

Just for clarity - you can NOT wreck a conventional alternator by turning the ignition off while it is charging - before that old chestnut comes up.

[pophops]

If you mean an externally regulated machine with an EXC terminal or an ordinary internally regulated jobbie with a D+ terminal the answer is the same. No voltage here means no output.

Thanks. Not sure what kind of alternator I've got but it's the original one fitted to a Vetus M4.14 engine and still painted that gorgeous shade of yellow if that helps!. Your reply, however would fit in with what I suspect is happening. I have been having charging problems and the intermittent fault seems to be in the multi-connector which is mounted on a bracket at the rear of the engine. Waggling the plug causes a variation in voltage read across the domestic batteries while the engine is running. Thanks for all replies.

[Tony Brooks]

Thanks. Not sure what kind of alternator I've got but it's the original one fitted to a Vetus M4.14 engine and still painted that gorgeous shade of yellow if that helps!. Your reply, however would fit in with what I suspect is happening. I have been having charging problems and the intermittent fault seems to be in the multi-connector which is mounted on a bracket at the rear of the engine. Waggling the plug causes a variation in voltage read across the domestic batteries while the engine is running. Thanks for all replies.

 

I do not think that fault has anything to do with the rotor current. If you have more than (say) 13.5 volts then the alternator has energised so the warning lamp circuit has nothing to do with it and the inbuilt regulator will be controlling to voltage unless you have a high charging current.

 

I think you are seeing the result of voltdrop caused by loose/dirty connections. Probably on the main pos terminal to the ignition switch/voltmeter.

[WotEver]

I've seen all sorts of problems with that horrid multi-connector including the Rev counter stopping.

[pophops]

 

I do not think that fault has anything to do with the rotor current. If you have more than (say) 13.5 volts then the alternator has energised so the warning lamp circuit has nothing to do with it and the inbuilt regulator will be controlling to voltage unless you have a high charging current.

 

I think you are seeing the result of voltdrop caused by loose/dirty connections. Probably on the main pos terminal to the ignition switch/voltmeter.

 

Hi Tony

No ignition switch or voltmeter. Just start, stop and glow plug buttons and a rev counter.

Ray

[WotEver]

 

Hi Tony

No ignition switch or voltmeter. Just start, stop and glow plug buttons and a rev counter.

Ray

Same as WotEver's M3.10

[Sir Nibble]

Probably the battery sense wire making intermittent contact

[Tony Brooks]

 

Hi Tony

No ignition switch or voltmeter. Just start, stop and glow plug buttons and a rev counter.

Ray

 

But you will still have a positive lead supplying your buttons and instruments and this almost certainly runs via the at least one multi-plug.

 

Once the alternator starts to charge the supply to the rotor (call it exciter if you want) is internal. Worn brushes or slip rings could give a varying charging voltage in the early states of such a fault but eventually the alternator would not excite or just shut itself down. Excitation from an external source is in effect switched off once the alternator starts to charge.

 

You seem to imply that your alternator starts to charge but you can vary the voltage by manipulating the multi-plug. It is hard to be sure without a wiring diagram but the alternator's main output wire often runs to starter main positive terminal with the battery cable carrying the charge back to the batteries. A boat with split charging may be different but the main charging positive and negative leads do not usually go via the multi-plug. This means the problem is not with the main charging circuit or exciter circuit other wise the alternator would not have energised.

 

Without an ignition switch the engine probably uses a rising oil pressure switch to sunrise the alternator but however that is wired once the alternator starts to charge it is redundant until the next start.

 

So something is causing the charging voltage to vary between the alternator and voltmeter when you manipulate the multi-plug. I think the most likely candidate is a poor connection on the cable supplying the current to the control panel OR it's negative pair. Slit the multi-plug, use a pair of pin nosed pliers to VERY GENTLY squeeze the female sockets up a little, clean any plugs or sockets that are dirty or corroded, apply a film of silicon grease or similar and reconnect. If that cures the problem tape the multi-plug & socket up to help keep damp out.

.

Probably the battery sense wire making intermittent contact

 

Absolutely - forgot Vetus had battery sensed alternators but not so sure they run via the Multi-plug. That's a better diagnosis to check first but it may still require the multi-plug to be attended to..

[pophops]

Thanks for your time. I've found that even lifting the loom to the multi-plug, which is quite heavy and tends to sag down, increases the voltage across the domestic batteries from about 12.6v to 14.4v. At the same time the engine revs audibly drop as the alternator kicks in.

It seems clear that the fault lies in the plug-socket connection. Or is it possible that a cable crimp is failing? I've already cleaned and de-greased the plug and socket with little improvement but I'll try tightening the female contacts as you suggest.

[Iain_S]

 

(snip)

Absolutely - forgot Vetus had battery sensed alternators but not so sure they run via the Multi-plug. That's a better diagnosis to check first but it may still require the multi-plug to be attended to..

 

I'm pretty sure they do.

 

In my case, a corroded multiplug caused high alternator output. Not sure whether the culprit was the sense wire, or the control panel supply wire, though.