How to shutdown the alternator ?

[rom]

Originally Posted by transmitterdan

It is "none of the above". A better way to state the probability is that "eventually the diodes will fail if you disconnect the battery enough times". Sometimes it only takes one time, sometimes it takes a few more times before they fail. So you get contradictory first hand reports. Some guy did it once and it didn't fail. But if you do it enough times the diodes will eventually fail. And remember there are several diodes in an alternator. So one diode can fail open and the alternator will still seem to work.

This is an old but pretty accurate book on how alternators work:

http://www.melody-in-blues.org/downl...orhandbook.pdf

You do understand that we are talking about starting/running an alternator that is *not connected* to the battery ? As opposed to *disconnecting* the battery from a running alternator. This handbook, though interesting, says nothing about that.

[john61ct]

Please feel free to experiment and report back. Alts are not expensive.

Our time is more valuable.

[transmitterdan]

Originally Posted by rom

You do understand that we are talking about starting/running an alternator that is *not connected* to the battery ? As opposed to *disconnecting* the battery from a running alternator. This handbook, though interesting, says nothing about that.

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.

[fallingeggs]

Originally Posted by rom

Off topic: Though the alternators may represent less than 5% of my electrical needs I would like them to charge faster when needed so the engines don't run for hours. Since I need to modify my hitachi alts to add the switch I might as well go a bit further...

Any ideas on how to make progress there ? Still looking for something simple and cost effective. Can someone tell what kind of improvement I can expect from a quicktifier ?
https://alternatorparts.com/quicktif...rectifier.html
110 USD plus some low cost regulator like the NAPA someone advised here still fits in my budget...

Have you priced out a new, externally regulated alt from either Leece-Neville or Mark Grasser? They might be able to put together a 100 amp alt that meets you need for around $300 to $400. If you sell your current alt, maybe you are in budget? Perhaps email Mark with your wish list (simple regulatory with field cutoff switch).

[a64pilot]

Originally Posted by rom

You do understand that we are talking about starting/running an alternator that is *not connected* to the battery ? As opposed to *disconnecting* the battery from a running alternator. This handbook, though interesting, says nothing about that.

I believe an issue is the damage to the diodes is cumulative. Damage is happening, enough damage accumulates and the component fails.
It's often why things "wear out", just your accelerating the damage is all, and wearing the things out faster, even if you were careful and only disconnected during idle when charging current is very low.

First time I had diodes go out was in an OLD pickup I had, took me awhile to figure out what was going on, I'd turn off the truck and the alternator charge light would illuminate, but the alternator would still charge when the engine was running.
Diodes usually are not that expensive and are not usually all that hard to change, you wouldn't I don't think have to buy a new alternator if you decide to give it a go, just carry a spare set of diodes

[john61ct]

Originally Posted by fallingeggs

your wish list (simple regulatory with field cutoff switch).

This does not require alt replacement nor ext reg.

Just a competent DC electric guy, 20-30 minutes and wiring a switch.

A teenager training at the local vocational school would do it for a couple cases of beer.

Now if you want a proper setup for alt rapid charging of depleted House, hours of rated output without overheating, that's a much bigger project.

But not a switch to zero field current

[mitiempo]

Originally Posted by transmitterdan

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.

That would likely be a dead short. Alternator frame is ground and depending on how the output of the alternator is wired it is probably live whenever the switch is on. Standard wiring on an engine is alternator B+ to starter positive. Alternatively B+ could be wired directly to a battery. Same result either way.

[Wotname]

Originally Posted by rom

What do you mean ? Is there something you would disagree with this method ?

What I mean is:

I don't know for sure what will happen to the alternator if the output is disconnected while it is stationary and then it run up to speed. But I would like to know and so why don't you give it a go and report back.

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.

Where I am uncertain is the case when the regulator is sensing the voltage external to the alternator. In this instance, the regulator will supply it's maximum available current to the field windings and thus the output voltage of the alternator will be at "a maximum" . I don't know what that figure is but I suspect it is most likely to lowish say less than 50 or so VDC. If so, this will not damage the diode pack. But I might be wrong, the voltage might reach say 300VDC. Again, will this damage the diode pack, probably not but possibly yes.

I have never seen any figures for an open circuit voltage of an alternator when it has maximum field current and I have never tried it myself.

If you have an enquiring mind, you could try it yourself and just measure the open circuit output voltage OR you could remove the diode pack, leave the stator windings open circuit and measure the AC volts on any one of the stator coils.

For other readers, please note we are not discussing removing the output wire while the alternator is already under load. When one does that, there will be a voltage spike that will damage the diode pack. For reasons previously explained

[Wotname]

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.
.......

Here is where we differ. I believe the voltage will not run away. I believe it will reach a maximum but I am unsure what the actual figure will be thus I can't say for certain if it is high enough to damage the diodes.

The maximum output voltage when the output is open circuit is given by the RPM of the alternator, the number of turns in the windings (both rotor and stator) and the size of the field current.

As the RPM and windings remain unchanged and the maximum filed current is limited by the regulator deign, then the only unknown is really the size of the field current.

As I understand your argument, you believe this will be very high (run away etc). I not so sure, I think it will be lowish (sub 100V) but to repeat myself, I don't know for certain and I have never seen it written in any alternator data sheet.

[Wotname]

Originally Posted by stormalong

An open circuit on the output of an alternator will cause a voltage spike. The voltage spike will exceed the ratings of the diodes in the alternator and they will fail. Now you have a dead alternator to rebuild or replace.

No, this is incorrect. A voltage spike will only occur if the output is made "open circuit" while the alternator is producing current (i.e. under load).

For reasons explain some posts back.

[Wotname]

Originally Posted by transmitterdan

......They cannot be regulated without a battery or big capacitor and/or shunt resistor. An alternator is a current controlled current source. It is not a constant voltage source. ......

Allow me to correct this...

An alternator is neither a constant voltage source or a constant current source.

Better described as a current controlled, "current limited constant voltage" source.

That is, it's output is controlled by the field current to be a constant voltage (~14VDC) until it reaches it's maximum output current rating.


But perhaps I'm splitting hairs

[Wotname]

Originally Posted by transmitterdan

.......
This is an old but pretty accurate book on how alternators work:

http://www.melody-in-blues.org/downl...orhandbook.pdf

I note the author states the diodes have a typical PIV of 50V. The book was published in 1986 and reflects the state of diode technology of the times (70s to 80s). I accept the PIV would have been around 50 V back then.

If current diode packs still have this very low PIV, then yes, the diodes will fail if the output is left open circuit.

However I would be surprised if modern day alternator diodes still have such a low PIV as most power diodes have PIVs in excess of several hundred volts (as a minimum). I stress I don't know for sure though

Thus question remains open, hope the OP wants to experiment!

[transmitterdan]

Silicon diodes have not changed much since 1986. Today the "high tech" diodes are Schottky and have PIV of about 50V (some a little more). In silicon the higher the PIV the greater is the voltage drop when conducting thus more needless heat is generated. So there is a penalty for high PIV diodes.

[Wotname]

^^ Hmm... the first ones I found on google have Vr of 150V and 200V. I also a few references to the use of Zener diodes in alternators - I hadn't realised they used them in alternator diode packs - that may change the game

[rom]

Originally Posted by Wotname

^^ Hmm... the first ones I found on google have Vr of 150V and 200V. I also a few references to the use of Zener diodes in alternators - I hadn't realised they used them in alternator diode packs - that may change the game

My understanding is that zener diodes are used in the diode trio, not the rectifier.

Originally Posted by Wotname

Better described as a current controlled, "current limited constant voltage" source.

That is, it's output is controlled by the field current to be a constant voltage (~14VDC) until it reaches it's maximum output current rating.

If we admit the regulator is not part of the alternator (it can be external) then I believe "current source" should be enough of a description. It doesn't know about voltage. You can charge a 24 or 48V batt with a 14V alt if you get rid of the regulator or regulate manually (would need higher revs yes).