alternator relay - why?

ytd

ytd

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Our VP D2-55 has the 120A alternator with a smart regulator fitted. The battery alarm on the VP control panel went off the other day but when I checked the voltmeter the batteries were charging correctly. So I checked the alternator and found a smoking relay clipped to the field wires. Disconnected the relay and the buzzer (to silence it) and went for a sail.

When the relay was opened it was a mass of rust so it hasn't worked for some time. The rust means I can't identify which of the relay input wires went to the coil and which to the contacts but I think brown and red/blue wires from the harness or control panel went to one side of the coil and the D+ wire from the alternator went to the contacts. I assume when the relay is energised by the control panel it connects the control panel wires to D+ since these are the only non-earth connections to the relay.

When I look at generic alternator wiring diagrams the charging light supplies power to the field but in our case the field is supplied by the smart regulator. Searching old posts on the forum I find that D+ is also used for the charging light but connected directly to the control panel. Why have the regulator installers fitted a (non-marinised) relay? The installation instructions for the regulator don't mention a relay but they do warn against running the alternator without a load since the regulator is sensitive to voltage spikes.

I'm all for simplifying the wiring by connecting the control panel directly to D+ and since the relay has been dud for some time this is what we have effectively been operating with. Does anyone have an explanation for the need for the relay
 
Its a puzzle because if the relay switched off the filed current then the alternator would not charge at all.

Is this a secondary alternator? Are you sure its a relay as opposed to an external regulator? Do you have a second regulator like a digi one from Sterling or adverc? If so it could be that your system is wired so that the digi regulator alone is controlling the alternator allowing it to drop the voltage down from the typical 14.5 v of the built in reguilator to a more sensible 13.5 float level. Meantime the relay gives you the ability to switch in the built in regulator if the external one fails.

Does this sound likely?
 
Thanks all for your replies.
BE we do have an external "Next Step' regulator which is connected to the DF terminal. The rusty thing connected to the D+ terminal is a small 15mm cube and definitely a relay and not a regulator.
Len thanks for the Pelican parts link. It clarifies that D+ is for the charge warning light and that when the alternator is not turning or has failed the D+ and DF are connected through the alternator earth. So maybe the relay is to isolate the D+ from the DF when the alternator is not spinning or has failed. Once enough voltage is produced at D+ to energise the relay, D+ is connected to the lamp and it goes out.
So I think I will put another relay in but wire it so that D+ energises the coil and then connects to the panel light and siren.
 
I had a Prestolite alternator with an Adverc controller, and Adverc specified a relay in the wiring. Apparently, without the relay, the alternator would draw a steady small current when not running, and would flatten the batteries.
 
ytd said:
Thanks all for your replies.
BE we do have an external "Next Step' regulator which is connected to the DF terminal. The rusty thing connected to the D+ terminal is a small 15mm cube and definitely a relay and not a regulator.
Len thanks for the Pelican parts link. It clarifies that D+ is for the charge warning light and that when the alternator is not turning or has failed the D+ and DF are connected through the alternator earth. So maybe the relay is to isolate the D+ from the DF when the alternator is not spinning or has failed. Once enough voltage is produced at D+ to energise the relay, D+ is connected to the lamp and it goes out.
So I think I will put another relay in but wire it so that D+ energises the coil and then connects to the panel light and siren.
Click to expand...

The D+ terminal is connected to the battery via the ignition switch and warning light. There are two wires doing this one of which has a resistance in it. The reason that D+ is connected to the battery is that the alternator needs some current through the field coils to generate leccy and this current initially comes from the battery. The DF terminal is the output terminal from the built in regulator. So what it sounds very much like in your case is that you have two regulators, one built in and one external. If you have two regulators working at the same time then all that happens is that the alternator output is controlled by the regulator which gives the greatest voltage across the field coils . The other one is doing nothing.

Built in regulators are crude devices for the most part and cause the alternator to output at 14.5 v. Doesnt matter if the batteries are fully charged or empty, 14.5 is what you get. If your other regulator is a clever multi stage device then it will want to charge the batteries as much as 15v to start, dropping it down gradually and when full keeping the charge at maybe 13.7 volts or thereabouts.

I cannot tell for sure without looking at your wiring but my bet is that the relay is there to allow you to turn off the internal regulator but to have it come back into action if the external one fails. IN which case it is best re-instated. It doesnt need to be a fancy relay - it wont be taking much current.

If you ever find out for sure what the relay is for, let me know.
 
birdseye said:
The D+ terminal is connected to the battery via the ignition switch and warning light. There are two wires doing this one of which has a resistance in it. The reason that D+ is connected to the battery is that the alternator needs some current through the field coils to generate leccy and this current initially comes from the battery. The DF terminal is the output terminal from the built in regulator. So what it sounds very much like in your case is that you have two regulators, one built in and one external. If you have two regulators working at the same time then all that happens is that the alternator output is controlled by the regulator which gives the greatest voltage across the field coils . The other one is doing nothing.

Built in regulators are crude devices for the most part and cause the alternator to output at 14.5 v. Doesnt matter if the batteries are fully charged or empty, 14.5 is what you get. If your other regulator is a clever multi stage device then it will want to charge the batteries as much as 15v to start, dropping it down gradually and when full keeping the charge at maybe 13.7 volts or thereabouts.

I cannot tell for sure without looking at your wiring but my bet is that the relay is there to allow you to turn off the internal regulator but to have it come back into action if the external one fails. IN which case it is best re-instated. It doesnt need to be a fancy relay - it wont be taking much current.

If you ever find out for sure what the relay is for, let me know.
Click to expand...
My Bene has a relay off the alternator, it is powered when it is charging, it switches the anchor winch circuit so that it only works when the engine is on
S
 
skipper_stu said:
My Bene has a relay off the alternator, it is powered when it is charging, it switches the anchor winch circuit so that it only works when the engine is on
S
Click to expand...

While I agree that winching is best done with the engine running, I'd be very concerned if it was enforced...

Engine won't start, being blown onto rocks, can't bear away because anchor down.
 
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