How to shutdown the alternator ?

[john61ct]

Because you're likely to blow your diodes.

A cheap starter batt then high-amp B2B is another solution.

[john61ct]

If you need the alt to kick in, means house bank is low. Lead bank right? Won't get back to full for 4-6 hours anyway, so why turn the alt off?

When SoC rises, bank resistance drops acceptance rate, alt stops using fuel, no problem.

[a64pilot]

I believe but am not sure that the reason the diodes will blow is because when there is a sudden interruption of power the electromagnetic field collapses and there is a voltage surge in the output wire, enough of a surge to possibly blow the diodes.
I believe a similar field collapse is how the coil works in gasoline ignition systems, field collapses when points close, sudden surge goes through coil, I think.
Purpose of the diodes is of course to turn the AC power that an alternator makes into pulsating DC power that you can charge batteries with and run a DC system.
I am not certain that is what blows a diode pack, I am trying to put 2 and 2 together and may of course be wrong.
However I have seen diode packs blown by sudden disconnection in person and can attest to something does sometimes blow them

[transmitterdan]

There are probably 10,000 hits on google search about why you should not run an alternator without a battery attached. 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.

If there is nothing at the output then there is no resistance and the output current will find a home. The diodes provide that home. The voltage will build up until the diodes reach their avalanche voltage and then the current will be able to flow backwards through the diodes. The voltage will be in the range of a few tens of volts depending on the diodes. There will be enough power to damage the diode junctions. There may even be enough voltage to arc inside the alternator.

Do not try this at home. I worry that your lack of knowledge on such a well known D.C. systems topic bodes ill for your "charging upgrade". You have gotten good advice from people with decades of experience. You would be well served to listen and heed their advice.

[stormalong]

Originally Posted by rom

Edit: still, I would like to understand why I cannot switch off the ouput of the alt, if someone really knows.

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.

[Wotname]

Originally Posted by a64pilot

I believe but am not sure that the reason the diodes will blow is because when there is a sudden interruption of power the electromagnetic field collapses and there is a voltage surge in the output wire, enough of a surge to possibly blow the diodes.
I believe a similar field collapse is how the coil works in gasoline ignition systems, field collapses when points close, sudden surge goes through coil, I think.
Purpose of the diodes is of course to turn the AC power that an alternator makes into pulsating DC power that you can charge batteries with and run a DC system.
I am not certain that is what blows a diode pack, I am trying to put 2 and 2 together and may of course be wrong.
However I have seen diode packs blown by sudden disconnection in person and can attest to something does sometimes blow them

For a mechanical guy this is as good as any description of why a diode fails when the output of the alternator is disconnected while under load. To expand a little more from an electrical viewpoint, EMF (i.e. voltage) is created when a magnetic field moves though (or cuts across) a conductor. The stronger the magnetic or the faster it cuts, the greater the voltage produced. When the an alternator is under load and especially when it's output is high (lots of current), the resulting magnetic field around the stator windings is large. Disconnect the output of the output windings (the stator) results in sudden drop of the output current. That is from it's high value to zero nearly instantly. Thus the magnetic field around the stator windings must collapse and the rate of collapse is extremely high.

Now remembering the size of voltage produced is directly proportional the both the size of the magnetic field and the rate it passes though (or cuts) the conductor thus a large field moving extremely fast will produce a very high voltage. This can be in the order of tens of thousands of volts for milliseconds. More than enough to exceed the peak inverse voltage rating of the diodes (which is typically only hundreds to a thousands or so volts).

Note, this only occurs when you disconnect the output while the alternator is running and producing some output current.

See below for what might occur if the output is disconnected when the alternator is stationary...

[rom]

Thank you guys for your concern, please no need to be rude, I am getting contradictory information in the 10000 google hits on that matter and even got contradictory information in this thread. It takes a little time to sort it all out...

Ok so I thought disconnecting the alt would leave an open circuit, but forgot that the rectifier could close it shall the voltage rise above the diodes rating, and destroy them. I am still not sure whether this is a "likely", a "may", or a "will", but that's enough for now. If someone cares to point me to a well documented analysis of how it all works I may well spend some time on that latter on.

So a switch on the field it is. Simple and elegant solution. fine.

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...

[Dockhead]

Originally Posted by Wotname

For a mechanical guy this is as good as any description of why a diode fails when the output of the alternator is disconnected while under load. To expand a little more from an electrical viewpoint, EMF (i.e. voltage) is created when a magnetic field moves though (or cuts across) a conductor. The stronger the magnetic or the faster it cuts, the greater the voltage produced. When the an alternator is under load and especially when it's output is high (lots of current), the resulting magnetic field around the stator windings is large. Disconnect the output of the output windings (the stator) results in sudden drop of the output current. That is from it's high value to zero nearly instantly. Thus the magnetic field around the stator windings must collapse and the rate of collapse is extremely high.

Now remembering the size of voltage produced is directly proportional the both the size of the magnetic field and the rate it passes though (or cuts) the conductor thus a large field moving extremely fast will produce a very high voltage. This can be in the order of tens of thousands of volts for milliseconds. More than enough to exceed the peak inverse voltage rating of the diodes (which is typically only hundreds to a thousands or so volts).

Note, this only occurs when you disconnect the output while the alternator is running and producing some output current.

See below for what might occur if the output is disconnected when the alternator is stationary...

Thanks to both you and A64 for all of these great explanations!

[Wotname]

Originally Posted by transmitterdan

There are probably 10,000 hits on google search about why you should not run an alternator without a battery attached. 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.

If there is nothing at the output then there is no resistance and the output current will find a home. ..............

TxDan, I'm not sure why you posted this, I'm sure you realise it isn't true

If the output is disconnected, then the resistance is extremely high, at least a couple of hundred megohms (essentially an open circuit). Further more, current does not need find "a home", ohms law tells us quite simply what the current will be. In essence, the current will be zero when the output is open circuit. Regardless of the voltage of the output (within reason), when the output is open circuit, the current will be zero.

For instance, let's say the output voltage of the open circuit alternator spinning at normal engine speed might reach say 100 volts and the open circuit resistance is 100 megohms, then Ohm's law states the current flow will be 1 microamp.

But this isn't the whole story...

[transmitterdan]

Wotname,

The field winding current creates a magnetic field within which the rotor turns. A wire (the rotor) moving through a magnetic field will produce a voltage proportional to the magnetic field and the length of the wire. The wires in an alternator are very long such that the voltage attainable is much higher than 12V. So without a load the diodes avalanche and current flows backwards through them thus they are destroyed.

In normal operation an alternator is always operated in such a way that the external circuit never allows it to produce its maximum voltage. The battery acts as both a capacitor and a resistance so the current has a path, The regulator adjusts the current in the field winding which in turn adjusts the magnetic field through which the rotor turns. This in turn controls how much current the alternator can produce. If you put a dead short on the alternator output the current does not become infinite as ohms law would predict. That's because the amount of current the rotor can produce is limited by the magnetic field through which the rotor turns. it becomes a current controlled current source.

But if the alternator is terminated in an open circuit the voltage it can produce is much higher than the diodes can withstand even with a very small field current. The only way to prevent diode failure is to make the field current zero.

Put this search string into google: disconnect battery with engine running

[Wotname]

Originally Posted by transmitterdan

.......The voltage will build up until the diodes reach their avalanche voltage and then the current will be able to flow backwards through the diodes. The voltage will be in the range of a few tens of volts depending on the diodes. There will be enough power to damage the diode junctions. There may even be enough voltage to arc inside the alternator.

...............

I dispute the claim of few tens of volts but I agree with you general thrust of this argument.

So what does happen when the stationary alternator has an open circuit output and it is spun up on the engine. Depending on how the regulator senses the output, at least two different things might occur.

If the an internal regulator senses the output voltage inside the alternator , then when the output voltage reaches ~14 VDC, it limits the field current to maintain this voltage. Clearly this isn't going to blow any diode

However if the regulator sense is external, then the regulator will keep pumping amps into the field windings in order to get the sensed voltage up to ~14VDC. As the output is disconnected, the sensed volts isn't ever going to reach ~14VDC so the regulator will deliver it's maximum current into the field windings.

Now I don't know what the maximum voltage an alternator can reach but I know it is dependant on only three things.

1. The size of the magnetic field produced by the field windings. In the case we are examining, if the field current is at maximum, the magnetic will be at maximum.

2. The rate of change of the magnetic field though the stator coils. This is essentially related to the speed of rotation of the alternator thus it doesn't change when the output is open circuit.

3. The number of coils in the stator winding. Clearly this doesn't change when the output is open circuit.

Therefore the maximum voltage the alternator will reach with an open circuit output is defined mainly by the size of the field current.

Alas, I don't know what that is but I suspect it is only in the order of a few hundred volts (AC or DC). Is that enough to blow up diodes - maybe. The peak reverse voltage of power diodes can range from a few hundred volts to 1 or 2 thousand volts, depending of the manufacturer and the purpose of the diode.

I don't know what the "average alternator diode" is rated at but I suspect it varies between manufacturers and is directly proportional to the price of the diode pack.

So given my best guess of the max output voltage is say 300VDC which means the AC voltage will say 500VAC and the cheapest diode pack peak reverse voltage might be also 500 Volts, then yes, the pack might cook. Bt then again, if my guess is a little off, it might be OK.

[Wotname]

Originally Posted by transmitterdan

Wotname,

The field winding current creates a magnetic field within which the rotor turns. A wire (the rotor) moving through a magnetic field will produce a voltage proportional to the magnetic field and the length of the wire. The wires in an alternator are very long such that the voltage attainable is much higher than 12V. So without a load the diodes avalanche and current flows backwards through them thus they are destroyed.

In normal operation an alternator is always operated in such a way that the external circuit never allows it to produce its maximum voltage. The battery acts as both a capacitor and a resistance so the current has a path, The regulator adjusts the current in the field winding which in turn adjusts the magnetic field through which the rotor turns. This in turn controls how much current the alternator can produce. If you put a dead short on the alternator output the current does not become infinite as ohms law would predict. That's because the amount of current the rotor can produce is limited by the magnetic field through which the rotor turns. it becomes a current controlled current source.

But if the alternator is terminated in an open circuit the voltage it can produce is much higher than the diodes can withstand even with a very small field current. The only way to prevent diode failure is to make the field current zero.

Put this search string into google: disconnect battery with engine running

We are posting at almost the same time

We or at least I am not disputing what happens when you disconnect a battery with the engine running.. The diodes fry

And Ohm' law is never wrong; just sometimes we don't account for all the factors.

Really the only things we are unsure of is:
1. The maximum voltage of the stator windings when the field current is at max.
and
2. The Vrrm of the diode pack.

You seem sure the diodes will cook.
I'm not so sure but will accept it might be that way.

Let the OP do the field testing for us

[a64pilot]

Originally Posted by rom

I am still not sure whether this is a "likely", a "may", or a "will", but that's enough for now. If someone cares to point me to a well documented analysis of how it all works I may well spend some time on that latter on.

...

It's a likely or a may, I believe how much current the alternator is putting out is the variable. That is to some extent the problem, you can often get lucky and no damage is done so it leads you to think just disconnecting is viable, then one day not too much later your alternator is dead leaving you to wonder why. It's been a while since the mod so you don't connect the cause to the failure and you replace the alternator, then later it of course dies again.

[rom]

Originally Posted by Wotname

Let the OP do the field testing for us

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

[transmitterdan]

Originally Posted by rom

I am still not sure whether this is a "likely", a "may", or a "will", but that's enough for now. If someone cares to point me to a well documented analysis of how it all works I may well spend some time on that latter on.

So a switch on the field it is. Simple and elegant solution. fine.

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