How to shutdown the alternator ?

[john61ct]

So wrong

[rom]

Originally Posted by Wotname

I am certain that if the regulator senses the output voltage inside the alternator, then no harm will occur. The output terminal will sit happily at ~ 14 VDC and as it is an open circuit, no current will flow and the field voltage/current will be near zero.

That is somewhat what I expect but still need to clarify a few things before testing...

[rom]

Originally Posted by transmitterdan

Most alternators have sufficient residual magnetism in the core to produce enough voltage to run the regulator. It will try to excite the field and the voltage will most likely run away.

There is one thing you can try that should work. You can connect a short battery cable from the + output of the alternator back to the alternator frame. That will protect the diodes and "probably" will not draw significant power from the belt. Measure the current in the battery wire with a DC clamp type ammeter. If the current is nearly zero then it is drawing no power from the belt. By shorting the output there will be no voltage to excite the field winding and the alternator will be forced to shut down.

But only do it at your own risk.

The more I think about the more I think you're right, in theory the short circuit seems like better option than open circuit if I were to use either option to shutdown the alt *before starting the engine*. But I've got those two other cables R&L entering the alt and I have no idea if that could trigger the field or not ...

[rom]

Originally Posted by rom

This handbook, though interesting, says nothing about that.

I missed it the first time, actually the book page 12 says "AC voltage at open terminals, no load connected, can easily approach 100V"

The quicktifier external rectifier is rated 300V as an example. I have no idea about the rectifier in my hitachi though. This debate seems to go nowhere unfortunately.

Talking about Hitachi, I made a few mistakes and need to be corrected. This morning I disconnected the R&L cables from the alternator which would then not produce any current. I thought it was a self-exciting alt, which it obviously isn't .. right ?
If that is correct, then I would not need to modify the alternator to shut it down, a simple switch on R and/or L (?) next to the control panel is enough.

[Wotname]

Originally Posted by rom

I missed it the first time, actually the book page 12 says "AC voltage at open terminals, no load connected, can easily approach 100V"

The quicktifier external rectifier is rated 300V as an example. I have no idea about the rectifier in my hitachi though. This debate seems to go nowhere unfortunately.

Talking about Hitachi, I made a few mistakes and need to be corrected. This morning I disconnected the R&L cables from the alternator which would then not produce any current. I thought it was a self-exciting alt, which it obviously isn't .. right ?
If that is correct, then I would not need to modify the alternator to shut it down, a simple switch on R and/or L (?) next to the control panel is enough.

On R, it goes to the +12V when the key is on.
L just goes to the indicator light.
IMHO.

[Wotname]

R = regulator
L = light.

[john61ct]

Originally Posted by rom

I missed it the first time, actually the book page 12 says "AC voltage at open terminals, no load connected, can easily approach 100V"

The quicktifier external rectifier is rated 300V as an example. I have no idea about the rectifier in my hitachi though. This debate seems to go nowhere unfortunately.

Talking about Hitachi, I made a few mistakes and need to be corrected. This morning I disconnected the R&L cables from the alternator which would then not produce any current. I thought it was a self-exciting alt, which it obviously isn't .. right ?
If that is correct, then I would not need to modify the alternator to shut it down, a simple switch on R and/or L (?) next to the control panel is enough.

There you go!

Overkill, but Blue Sea's ​DPST Dual Circuit will switch both at once if you don't want to figure out which.

[john61ct]

I believe on some the "just a light" plays a role in soft starting, like others use an oil pressure switch.

[stormalong]

Originally Posted by rom

The more I think about the more I think you're right, in theory the short circuit seems like better option than open circuit if I were to use either option to shutdown the alt *before starting the engine*. But I've got those two other cables R&L entering the alt and I have no idea if that could trigger the field or not ...

Shorting the alternator output is lunacy. Either you will burn up the alternator or start a boat fire. DON'T DO IT.

[rom]

Originally Posted by stormalong

Shorting the alternator output is lunacy. Either you will burn up the alternator or start a boat fire. DON'T DO IT.

Don't worry I would not do it unless I am 100% sure the field is null. Shorting an alt is not lunacy though, it is a good way to test its amp output. (provided you have a proper control on the field once again).
see those pages for example, you will find plenty other:
Synchronous Machine Open and Short Circuit Tests
Short Circuit Test on Three Phase Alternator (Theory) : Electrical Machines : Electrical Engineering : COE PUNE Virtual Lab

[lateral]

What was that about a horse and water?

[stormalong]

Originally Posted by rom

Don't worry I would not do it unless I am 100% sure the field is null. Shorting an alt is not lunacy though, it is a good way to test its amp output. (provided you have a proper control on the field once again).
see those pages for example, you will find plenty other:
Synchronous Machine Open and Short Circuit Tests
Short Circuit Test on Three Phase Alternator (Theory) : Electrical Machines : Electrical Engineering : COE PUNE Virtual Lab

You are literally playing with fire. Your own supplied reference states that this should only be done with minimum field current and you you are advocating such an action on a public discussion board without spelling that out. As resistance goes to zero (short circuit) current goes to infinity (actually maximum that can be produced). A voltage regulator looking at a zero voltage output would max field the alternator into a dead short. The current would rise to the point where the windings would melt.

To advocate something like this on an open forum where the audience may not realize this is reckless. Please retract you suggestion.

[transmitterdan]

There will be no field current with the output shorted and disconnected from a battery. An alternator needs a source of power to excite the field. With the output shorted there can be no field thus no output.

And even if there was some current there would be no fire.

[stormalong]

Originally Posted by transmitterdan

There will be no field current with the output shorted and disconnected from a battery. An alternator needs a source of power to excite the field. With the output shorted there can be no field thus no output.

And even if there was some current there would be no fire.

That is only true with an internal regulator. An external regulator would still energize the field. It there is no alternator output what do you hope to accomplish by shorting to ground?

[Wotname]

Originally Posted by stormalong

You are literally playing with fire. Your own supplied reference states that this should only be done with minimum field current and you you are advocating such an action on a public discussion board without spelling that out. As resistance goes to zero (short circuit) current goes to infinity (actually maximum that can be produced). A voltage regulator looking at a zero voltage output would max field the alternator into a dead short. The current would rise to the point where the windings would melt.

To advocate something like this on an open forum where the audience may not realize this is reckless. Please retract you suggestion.

Originally Posted by transmitterdan

There will be no field current with the output shorted and disconnected from a battery. An alternator needs a source of power to excite the field. With the output shorted there can be no field thus no output.

And even if there was some current there would be no fire.

@ Stormalong.
Furthermore an alternator by it's very construction is current limited. Even with the regulator in circuit and working properly a 55 amp alternator will never deliver more than ~55 amps even into a short circuit. It isn't really any different than say charging a large flat AGM battery.

As for the OP, he was only considering using a short on the output with the regulator inactive as a preventive measure to protect the diode pack rather than risking potential issues with the alternator output open circuit.