Alternator clutch

[halcyon]

Re: That\'s not a solution...

You can fit a tempreature sensor on the engine, this stops the VSR cutting in until the engine has warmed up.

Brian

 

[DaveS]

Re: That\'s not a solution...

My point on alternators' response of current to speed, linear rise then flat, was recalled from my University research project on alternator cooling. This was carried out some 30 years ago, but the technology hasn't changed much.

I am aware that the alternator turns faster than the engine. I inserted (engine) when quoting rpms to prevent misunderstanding of this.

I think my statement on the VSR's function is correct. It is perfectly true that with a well charged cranking battery the VSR will close quickly, but that in no way invalidates my statement. Incidentally, with a low domestic battery, after the VSR closes, current will initially flow into it from both the alternator and the cranking battery. Only later, once the voltage has risen, will the two batteries share the charging current.

 

[pvb]

Take a look...

Take a look at the output curves in this brochure on the Prestolite alternators which Adverc (amongst others) recommend and sell.

Talking of Adverc, they have an interesting article on VSRs.

 

[DaveS]

Re: Take a look...

Point 1.
These output curves are certainly quite a bit different from those I used to work with. Some elements are the same: after the "knee point" the output is not really flat (I previously over-simplified) but actually rises gently: this is down to the increased cooling at higher speeds.

What is surprising is how different the various models are from each other. One shows a linear rise to beyond half output, one appears to have no linear region at all. The critical speed, below which there is no output, varies quite a bit. All, however, show a far more rounded "knee" than I would have expected.

I wonder, and this is pure speculation, if the clue lies in the statement "adjustable voltage regulation, as well as pre-set". The graphs are titled "stabilised output @ 24C": I'm not sure what "stabilised" means in this context. Are we in fact seeing the output curve for an alternator whose regulation is being varied? This could well give an output shape above and to the left of the linear rise line expected of a passively regulated alternator. (Similar to my point in an earlier post re. adding a smart controller upsetting the classic cooling theory.)

Point 2.
I must say I was not too impressed by the Ardverc article on VSRs v. diode blockers. The latter's advantages were praised while the very real downsides were glossed over. It then highlighted one potential problem with VSRs (the possibility of chatter) while playing down their benefits. To balance this picture it is only fair to point out that chatter will only occur in a situation of relatively low alternator output combined with a relatively large capacity, deeply discharged domestic battery. The manufacturer of the VSR used to illustrate the article (with whom I have no connection, etc... ) makes this point, and gives clear and sensible advice on maximum recommended domestic bank capacities for particular alternator outputs.

 

[William_H]

Re: That\'s not a solution...

Hi PVB I think your last line is a little misleading at least under some circumstances. "Adding the domestic battery will share the available charge current"
That would be the case only if the batteries were very flat. In normal circumstances and after a few minutes each battery will take from the laternaor what they can absorb at the regulated voltage. My point being you will not be any where near the alternator limit so total charge into 2 batteries (or a larger one) will be greater than just one battery connected. Assuming a normal 14 volt regulated charging system. Not a sharing but rather taking more total. Sorry perhaps your words were not carefully chosen. olewill

 

[pvb]

Don\'t misquote me...

Please don't misquote me! My words were carefully chosen. I wrote "Once connected, the batteries will share the available charging current according to their needs." This statement is true. The "available charging current" will obviously vary according to the state of charge of the batteries.

 

[pvb]

Re: Take a look...

Output curves are at constant voltage, so it's unlikely Prestolite were tinkering with the adjustable regulator!

If you disagree with Prestolite and Adverc, maybe you could take it up with them direct.

 

[halcyon]

Re: Take a look...

Alternator were always rated in watts, at a voltage, into a resistive load. This is more relative to the atomotive industry that they were designed for. A battery is not a true resistive load, it has a capacative element, thus the alternator curve may / can be differant. Added to this is how much domestic load the boat is using, on a power boat this can be the output of a alternator.
VSR chatter as stated only occures through bad design, or if you have a very flat large service bank compared to engine battery / alternator capacity. Normally with a cut in voltage of 13.8, and a dropout voltage of 13.0 volt, the chances of chatter are low. But if the service battery is very low ie 11.0 volt, the cut in / drop out will be very slow and not chatter, as the engine battery acts as a capacitor spplying the initial surge. It can also be said that this reduces the high surge / running loads on the alternator from a large flat bank, on initial engagement.

Brian