Warm Alternator

F

freedom44

Well-Known Member
Joined
28 Feb 2002
Messages
110
Visit site
My boat is on the hard, conected to shorepower. Yesterday I had the battery charger on, and touched by chance the alternator, which was warm. Does anyone know why and should I be concerned. I do wish I had paid more attention during my physics lessons!

<hr width=100% size=1>
 
Starting with the assumption that you did not make use of your engine at that moment for some time I would say there is something odd in the way your electrical system has been set up. Check if there are any voltages present at the alternator. What might have happened is that one of the rectifier diodes is gone. That would result in a current flow through one of the stator coils. Did you notice bad battery recharging by the alternator itself?
Other thing that could be wrong is that the voltage regulator is not happy letting a current through the rotor coil.
Anyway both causes should also occur when not attached to the shore power, draining your battery very quickly.

If none of the above applies:
- Someone sneakly uses your boat?
- Some very creative wiring in your boat.

Anyway I think you should look into it. Alternators should not heat up by themselves if the engine is not working.

Arno

<hr width=100% size=1>
 
It sounds as though you've got a leaky diode in the alternator.

The way that alternators are designed means that the b+ terminal is connected to the windings (Not the rotor ones!) via a bridge rectifier - except its abit more complex as there are various phases to 'rectify'.

First guess is that one of the diodes is faulty and allowing your battery with charger to discharge through a winding. Other possibilities are an insulation breakdown or a faulty regulator.

Disconnect the b+ (remember its live at 12 volts). You could use an ammeter to check what the current drain is. (If the diodes are ok the drain will be in microamps - small enough to disregard)

<hr width=100% size=1>
 
I think I am right: Since the rectifier is a bridge rectifier, albeit a 3 phase one, for a reverse current to flow there must be two defective diodes in the rectifier, one in the poitive side and one in the negative side. A bit accademic I suppose since a duff rectifier is a duff rectifier regardless of the number of diodes which have failed.

If this is a rectifier fault then there must be a appreciable current flowing backwards through it in order to make the alternator warm. More than a few milliamps I would have thought. This means that when standing, with the isolator closed there will be a constant and significant drain on the battery. On a car, where you have a battery isolator, you would notice it, I think, but perhaps you would not on a boat because when not in use you would open the isolator and you probably have a bigger battery(ies) as well.

Just as a theory in case this proves not to be a rectifier fault. Suppose the alternator has a surge quench diode and the charger in use allows the voltage to rise a bit more than it should could that then start to dump some of the excess getting warm as it does so?

FWIW I put the car battery on charge for a few hours today but there was no sign of the alternator or its (external) regulator getting warm.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>
 
How about a short circuit positive rectifier diode, this will then feed a pos to the stator, this then can feed through the field diode, through the rotor, through the controler to ground. This will give you 2/3 amp flow at 12 volt say 25 / 40 watt may get a little warm.

Brian



<hr width=100% size=1><A target="_blank" HREF=http://link>www.kddpowercentre.com</A>
 
I am hugely impressed by the erudite responses and thank you all for your intelectual input. I think it is time for me to call in the techies as I am out of my depth. No, the boat has not been used sneakily, or if so, they must be an acomplished crane driver. Many thanks

<hr width=100% size=1>
 
Have the same problem with a warm alternator and the batteries run down when not connected to shorepower. 2-3 amps flow the 'wrong way'. Must admit I have just - for the moment - disconnected the alternator when alongside.

Will the charging efficency of the alternator be affected by this fault - it still shoves out 80-90 amps?

<hr width=100% size=1>
 
Good thinking. You've probably sus'd it but it does depend on the exact design though.
It will apply to those with a 3 phase rectifier for the field supply but not those where the field current is taken from the main rectifier with another diode (single)between there and the battery connection. But in the latter case short circuiting of that diode on its own would have the same effect!
I should have taken the trouble to actually look at the circuit diagrams.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>
 
That sounds like current flowing through the field coil. Is your ignition switch stuck on? If so, the charge warning lamp would be lit unless, even worse, someone has shorted it out. The bulb is supposed to allow a small current to flow in the field coil to energize it before the engine starts.



<hr width=100% size=1>
 
You must log in or register to reply here.

Share this page

Top