auxillary/field/excitation wire, alternator.

[baffle]

hello people. Does the field wire need to be live all the time an alternator runs, or just when it starts? my starter switch is just a push to close, release to open circuit, and I don't know if this is suitable to include the field wire in. thanks

[nicknorman]

It isn't necessary to power the alternator field wire once the alternator is charging, although not doing so means you can't have an alternator warning light. If the field wire is effectively connected to +ve only whilst the starter is operating! it may well be that the alternator hasn't started charging when you release the button and so may never charge. If your button only has 1 contact it means that the alternator field will be connected to the starter permanently, thus if/when the alternator is charging it will try to power the starter.

 

So for those various reasons it is a really bad idea! Install a toggle switch or similar, to power the alternator field current via a bulb, just like everyone else does.

[by'eck]

Just to add, the field windings are supplied with current via the regulator, from their own set of diodes once the alternator is energised. Although some are self exciting most aren't so require a battery feed connected to the D+ terminal (usually via a warning light) to kickstart them into life.

 

Using the start button feed, as Nick suggests will very likely be inadequate to excite it, due to low battery volts whilst starter is operating, short time and the fact that the alternator has hardly had chance to start spinning.

[baffle]

OK thanks for that

[davidg]

 

will very likely be inadequate to excite it, due to low battery volts whilst starter is operating

 

There are reasons why it isn't a good idea as already outlined, but this isn't one of them. The field only sees about 2V normally as it is supplied via the warning light; if the battery voltage was dipping to 2V when the starter was operating then the starter motor might be having issues.

[by'eck]

 

There are reasons why it isn't a good idea as already outlined, but this isn't one of them. The field only sees about 2V normally as it is supplied via the warning light; if the battery voltage was dipping to 2V when the starter was operating then the starter motor might be having issues.

 

Fair comment but whatever voltage the field does see whilst starter motor is running will be less given that volts are dragged down substantially even with a fully charged battery. Any further reduction will be pro-rata.

[Sir Nibble]

Once upon a time I supplied an alternator to a garage, it came back. Autopsy revealed blown field diodes. After a while the replacement came back too! Same reason. So I asked to look at the car and found an intermittent short between warning light and start solenoid cables. Somehow frowning garage proprietor and car owner managed to get me to fit ANOTHER replacement alternator and fix the short for free. Still trying to work that one out.

[Tony Brooks]

That illustrates what Nicknormal warned about rather nicely.

 

For others who are probably baffled, my take:-

 

The field diodes normally only supply the rotor current which is typically 3 to 4 amps based on it resistance. The current will probably be lower than this because of the rotor's inductance when operating. That is all the field diodes are specified to supply so if you connect the warning lamp cable to the starter after the warning lamp bulb the field diodes would be trying to run the starter and the high current drawn would blow them.

 

However I am not sure how the starter did not pull the voltage to the rotor down fast enough to stop the charging, but it obviously did not.

 

I am not so clear as to why the OPs setup should not work as long as the link to the starter was on the push button side of the warning lamp. I think all that would happen is the starter would whirr as it came off load,as the alternator started to charge and as soon as the button was released the waning lamp would come back on (limiting the current flowing to the starter) and the warning lamp would come back on (limiting the current flow from the diodes to the starter) even though the alternator was charging.

 

Discuss?

[ditchcrawler]

I would suggest that maybe feeding from the start button, even through a warning light the supply will be removed from the field before the alternator is turning fast enough to charge.

I have just fitted a new 24 volt domestic alternator and I had to wire in a warning light to get that to charge. I always start my engine on minimum throttle setting and the engine will run like this without the alternator starting to charge, so in my case wired via the start button would not have worked.

[Sir Nibble]

I would suggest the wheel doesn't need reinventing. The fix is to adopt the usual tried and tested circuit.

[Arthur Brown]

Where there is a standard, you can get help easily if you have followed the standard, if you have not then you are on your own. fit the alternator and wire it how it should usually be, then any service exchange of the right type should work. The Alternator warning light is a useful indicator of what's going on with the charge circuit. That with volt and amp meter will tell you enough about batteries and alternator in use. It's rare to find a boat without some electrics these days.