Disabling an alternator

[bluedragon]

A strange request I'm sure, but for those of you who have read my recent posts you'll know why! My question is how to safely disable an alternator whilst having it available for use when needed. I could do it mechanically (take the belt off) but what I'd like to do is to have some electrical means of doing this...that is having the alternator turning but not producing (or consuming) power. I know that if an alternator is suddenly disconnected from a battery whilst producing power, the voltage spike can kill the diodes. But what about if it's disconnected before the engine is started and no exitation has occurred? In principle it won't produce a voltage and therefore no problem...but I have a feeling it's not that simple? What about a switch or relay in the field coil circuit or the warning light feed? As long as this is before engine start, would that present any risk to the unit? Any ideas on how else I might achieve the same effect i.e. taking an alternator electrically off-line?

 

[Stemar]

Many years ago, I had a car with a radio that didn't turn off when I turned the ignition off. This had the predictable result that, on occasions, the next morning, the battery was so flat that the ignition warning light didn't come on at all. No problem. I lived on a hill, and could roll up to 20mph in gear, when the alternator would self energise and off I went.

IMHO, you'd be better off switching the alternator to a load - a couple of 50w bulbs would be plenty - one might be enough, though it might be safer to use more 2 or 4 25w bulbs, in case one blows.

You'll need a "make before break" switch to switch between the bulbs and your "proper" load, or the spike as everything diconnects could well fry the diodes.

 

[Appleyard]

Just a thought...can you fit a magnetic clutch between the belt and alternator...They are used extensively on air-con compressors in cars...cannibalise a scrappy ..it should not be too difficult to fix up and would save wear and tear on the alternator.

 

[ytd]

As you suggest just put a switch or relay in the field connection and turn it off to stop the alternator charging. After all, this is what the regulator does once the battery is charged.

 

[Guest]

My alternator has a switch in the exciter circuit ... so I have accidently not switched on the alteranator before starting engine .... No problem ... The problem is switching it off when engine is running.
So the way to do it - is to find the exciter lead and insert a simple switch in it ... so that you can decide if you want alternator or not.
Don't forget that the alternator may create its own field once engine is running fast ... as the exciter as I understand it provides initial 12v to get field coils energised enough to get alternator producing ... once producing it then self-supports and exciter is reduced to zero - ie ignition light going out. So in fact the alternator could start charging even though exciter is off.
As far as I know - if you switch out the charging from alternator as well as exciter before starting engine - should be ok ... just remember not to switch them back in unless engine is stopped !!

I in fact decided in the end to interconnect the exciter feed t the ignitoin ... so I cannot start engine without first switching on exciter .. that way I do not forget the alternator again.

 

[snowleopard]

Sorry to drag this back to a crude level but all this talk of switches makes me nervous. It's perfectly safe to disconnect an alternator when the engine isn't running. What kills the diodes is that when the current is disconnected suddenly, the rapid collapse of the magnetic field produces a voltage spike. No current to start with - no spike.

So - if you unplug the alternator connector while the engine is stopped: no problem. And you wouldn't want to go fiddling around reaching round a running engine with the risk of getting caught in a belt so there's your safety 'interlock'!

 

[Guest]

Snowleopard ... who would ?

I think it highly unlikely anyone will start reaching around an engine when its running for this ??

My switch is on the start panel ... and interlocked with the start button.

The potential to disconnect an alternatopr exists on most boats in way of the battery main switch ... so risk is already there of blowing the alternator diodes.

This guy is asking if an alternator can be disconnected and still run engine ... I assume he may have a smaller HP engine and needs all the "umph" to the prop .. so wants to reduce load on the engine .... OK he has a problem as the alternator shouldn't be switched out while engine is running ... so he has to make decision before starting engine.

To the post about regulator switching alternator off when batterys are charged ... I may be wrong - but it doesn't actually switch completely to 0v ... if it did then I would assume Ign. light would come back on ?? plus the diodes would be at risk ... I have a feeling an auto-electrician reckoned to me long time ago that a small load is maintained at all times to prevent a 0v situation when charging is completed (actually as we know charging is never completed !!) ...

 

[andy01842]

Re: Snowleopard ... who would ?

I would put a switch in the field/exciter lead with a capacitor across the switch. When the switch is opened the capacitor would charge and then block the DC voltage slowly turning the alternator off and thus advoding the voltage spike.

 

[Guest]

Mmmmm now thats interesting ....

Capacitor across the switch .....

For me - I'm happy enough with enough power alternator on anyway .... 42hp in a 25ft boat ... standard alternator - so really don't notice whether it's on or not !!!!

 

[2Tizwoz]

Re: Mmmmm now thats interesting ....

A car I had developed a charging problem which turned out to be a loose wire on the excitation circuit. It would switch itself on and off at random for long (several minutes) and short periods as vibration affected the loose connection. It took some months before the fault was corrected but once the loose wire was made good it gave no further trouble. The alternator was not damaged in any other way as a result of the constant switching on and off. The ignition light would come on and if the contact was not made for a very extended period the battery gradually became flatter. On one occasion this resulted in the engine stopping as the petrol pump stopped working.

 

[Guest]

Intermittent exciter lead ...

but that is the exciter side and if alternator itself stops charging / starts etc. - that is not same as switching of the main battery switch when alternator is trying to throw out all those amps ....

I sometimes read where they say regulator switches of / on etc. - maybe ... but also read that it is rec'd that Main battery switch is not switched off while engine running ... I also read that many modern alternators have provision to try and prevent diode blows ... but having had my alternator suffer before - I don't trust it.

Better safe than sorry !!

 

[catmandoo]

Not sure if that is strictly true because when you turn off the ignition on a car you kill the field and engine keeps on running for a couple of revs still turning the alternator and diodes OK . Also mechanical regulators consist of bimettallic strips operating a switch which is a crude form of Off / On control . What you are referring to is suddenly putting alternator in a position whee it cannot get rid of current which is like closing the valve suddenly on a pump running at full speed hence the voltage or pressure spike . Its like a kind of electrical water hammer

 

[Guest]

Re: Disabling an alternator ... also

Exciter current is efectively zero when alternator is spinning - as ign light goes out with the alternator supplying its own energising input ...

I would suspect that there probably was more to it than just a dodgy exciter wire ... possibly dodgy connections elsewhere as well - sorted by removal / refit of alternator when fixing exciter wire ?????

 

[snowleopard]

I stand by my original post. If you interrupt the current in the stator (the charging current) the rate of change of magnetic field is massive, the magnetic field collapses and that generates a voltage spike which fries the diodes. cutting off the field coils in the rotor doesn't have that effect.

If you don't believe me, just try removing the charge lead from the +ve terminal of the battery while the engine is running!!

 

[Guest]

Re: Disabling an alternator .... Snowleopard ...

There is an element of confusion creeping in here ....

Your post about faulty lead is as I read it based on the exciter lead ... this can be off / on without - as I understand it - causing blown diodes .....

But actually disconnect batterys - which is NOT what your post mentioned probably would.

I feel that Catmandoo may have jumped a little quickly with his post and got words mixed.

Your later one regarding taking a terminal of the battery - I agree with ... yes blown diodes will result.
Back to original post - you said faulty exciter lead ... this would not cause charge on / off once alternator is up to speed and producing own exciter ??? Or maybe I'm wrong ?? cause exciter lead goes to 0v when alternator starts to work ............

Oh dear ... what happened to simple dynamoes ????

 

[catmandoo]

Re: Disabling an alternator ... also

Well I have two alternators with switches in the field circuits . On many occasions I have switched one or the other off with the engine running with no drastic effects . Also how come mechanical regulators can work without damage . They are in effect mechanical switches going on off as the alternator output voltage decreases and increases respectively . You also mention stator current being cut; but this is when you disconnect the line to the battery . The field current is in fact in the rotating part of the alternator with off/on swtching is part of its method of control .

 

[bluedragon]

As the originator of the post, let me come back in here. I have to confess I lost the track a little somewhere in the middle, but let me summarise what I think I've learned:

Disconnecting the alternator from the battery whilst running is not a good idea as we all know, but a break in the field coil circuit to deactivate the unit would not present any danger to the diodes. If battery disconnection is done before engine start, there would seem to be no problem either. So these are my two options..right?

 

[Stoaty]

An electric clutch on the alternator pulley is the sensible answer to this.

Look here and search site for clutch under engine drives and spares.

 

[bluedragon]

Thanks for the link...I'll look into this. I'm very tight on pulley space so it depends how large they are. But a good thought...

 

[catmandoo]

yup