Have I fired my alternator diodes?

[tinkicker0]

SWMBO once knocked the isolator switch off with a bag of bread intended to feed the ducks, brand new alternator too.

Only found out after we had returned to dock, the alt was still providing current to run the motor open circuit and the voltmeter was showing 15v instead of the usual 14v. Odd, but it was a brand new alt, so did not think much of it apart from a frown in the direction of the voltmeter.

Borrowed the Picoscope from work and did a ripple test on it. It was fine, the cascade electrickery had done its thing quick enough and protected the output diodes from the spike. Dunno about if it was recharging a flat battery though, bigger surge and bigger spike.

I do know a voltmeter will not detect a failed diode set. You need to do a ripple test or a diode continuity test on all three phases.

 

[bryantee]

12OFF switch

a slight drift. have the above switch with the field wire contacts, so that if you go to OFF inadvertantly it cuts excitation to the alternator. is this worth wiring in ,and which wire is put into this circuit. have a new vetus engine 3.28with a loom wiring system.have located the ignition/warning light to alternator,so that when lower nav lights are on ,this feeds a relay to put steaming light on when engine is started.

head down waiting for enlightenment please.

 

[VicS]

You need to do a ripple test or a diode continuity test on all three phases.

All very well if you have access to the equipment and the skills and knowledge to use it!

Best advice I think to anyone else would be to take the alternator to an auto electrician and get it tested if there is any doubt about its performance after any mishap with the isolator, but if it appears to be performing normally it probably is!

 

[Stu Jackson]

So many of these "brother-in-law" and "my crew turned the switch" stories.

I know it's a bit off topic, but can somebody please help me understand why skippers can't simply tell crew: "Please, do NOT touch this switch. EVER!"???

It would save a lot of alternator repair bills.

Oh, and if you take the alternator output off the switch, it would also save a lot of grief. Maine Sail asked about that in his reply #14 above. I don't understand why some of you keep "fighting" that concept.

 

[cimo]

Or modify the switch to limit its travel. plenty of ways to skin a cat.

 

[Iain C]

I know it's a bit off topic, but can somebody please help me understand why skippers can't simply tell crew: "Please, do NOT touch this switch. EVER!"???

It would save a lot of alternator repair bills.

Erm, I'm the skipper. I made the warning label. I turned the switch. I stupid.

(in my defence you can't actually see the damned switch unless you bend double or lie in the quarterberth...it's on the side of the engine box. Still stupid though...)

 

[VicS]

I know it's a bit off topic, but can somebody please help me understand why skippers can't simply tell crew: "Please, do NOT touch this switch. EVER

One of the troubles is that people do like these big impressive looking 1,2,0ff,both switches mounted in prominent positions!

The boat I crewed for many years had separate isolator switches for the two batteries mounted directly on the terminals. The owner did not allow anyone else to operate them!

Their location was a bit of a pain because they were under my feet when in my bunk ... but you get used to someone lifting the cushion you are sleeping on to switch batteries and they were not in a tempting location.

I think people do not like the alternator connection going directly to the battery because they want everything isolated when the switch is off.

 

[Stu Jackson]

One of the troubles is that people do like these big impressive looking 1,2,0ff,both switches mounted in prominent positions!

It's important to understand how it is wired. Continuing the use of the 1-2-B switch is easily possible. You just have to understand why factories wired boats originally and how a simple change can save your alternator diodes.

Try Reply #11, here; http://c34.org/bbs/index.php/topic,4949.msg30101.html#msg30101

Everything is still OFF when the switch is OFF.

 

[cimo]

No.

They will remain low until the load is reduced by the battery charging load reducing, from recharged battery, or the voltage coming up to regulation volts.

The regulator will only limit max volts, the current will be limited by alternator windings, or voltage available at battery,

Generating amps equals horse power, increased horse power equals more load on engine and less revs.

Brian

Hello Brian
what you have said is really "first approximation" alternator theorey. (though I wouldn't agree with what you say re: current being limited by alternator windings - rather its the voltage regulator that controls stator current).

My previous comments refer to that finite time interval between switching the alt. output to a relatively heavy load and, just before your alternator theorey begins to apply.

In practise, under the OPs load conditions, its a characteristic of the alt.s closed loop control system to over-shoot its target before reaching a steady state point (you may be familiar with the term non-critically dampened?).

The greater the difference between the loads the more evident this overshoot becomes to the ear (i'd be very suprised if it was just my ear!) i.e. initial drop in revs followed by a relative rise in revs. In my own setup this time interval can sometimes be between 1 to 2 seconds.

I doubt if you will have ever seen it mentioned in any battery charging texts. It's more the subject matter of closed loop control systems. Perhaps advancements in modern alt. regulation have moved to a more criticaly damped response? Although I haven't come across any that I know of to date.

of couse, you are correct, more current = more hp.

rgds
c

 

[VicS]

Try Reply #11, ............ Everything is still OFF when the switch is OFF.

But the alternator output terminal and wiring is still connected to the battery!

In your suggested scheme the isolator switch only isolates the domestic panel and the starter circuit (and presumably the engine control panel and all the engine electrics connected thereto.). You are still left with a live 12v positive connection in the engine compartment on the back of the alternator Touch it with a spanner and the sparks will fly (in the case of some Volvo Penta engines its the oil filter that will touch it while you are changing it .... happily doing so believing that with the main battery isolator is off it is all isolated.

If I have a main isolator I want it to isolate everything not just most things.

Your scheme may have its merits but the fact that the main isolator leaves one circuit still connected is my objection to it.

BUT

please tell me I have missed something .. your diagram #3 shows the alternator output connected only to "PDP" with no explanation as to what "PDP" is If PDP is another isolating switch to isolate the alternator from the battery then it overcomes my objection to some extent, although I'd prefer just one isolator to isolate everything

 

[cimo]

a slight drift. have the above switch with the field wire contacts, so that if you go to OFF inadvertantly it cuts excitation to the alternator. is this worth wiring in ,and which wire is put into this circuit. have a new vetus engine 3.28with a loom wiring system.have located the ignition/warning light to alternator,so that when lower nav lights are on ,this feeds a relay to put steaming light on when engine is started.

head down waiting for enlightenment please.

hello bryan
yes id say its worth wiring. i suspect you need to connect the field excitation circuit directly. How easy this will be I can't say right now. Your alt. may have the field wire accessible or you may have to dismantle the alt. to bring the field circuit to the outside world. you alt.s excitation mode (P or N) will also have a signifiance. I would speak to vetus to get their recommendations in the first instance. perhaps you can come back and enlighten us!

 

[Stu Jackson]

.... your diagram #3 shows the alternator output connected only to "PDP" with no explanation as to what "PDP" is...

A PDP is a "positive distribution post" like a bus bar or a Power Post, simply gathers multiple "incoming" lugs to the house bank (+) post to avoid landing them on the battery post; one wire goes from the PDP to the battery; the multiple leads can be: alternator output, solar panel, charging relays, stereo memory, bilge pump, etc. It is not a switch. To offset your concerns about a live alternator, one could make it a switch. In 12 years with the arrangement I have never sparked an engine. Nor have I heard of anyone who has done so with any issues. It would appear that fi the alternator case was grounded, every time anyone used this method installing the alternator would zap his power system. That just doesn't happen.

 

[VicS]

A PDP is a "positive distribution post" like a bus bar

OK got it from that.

It would appear that fi the alternator case was grounded, every time anyone used this method installing the alternator would zap his power system. That just doesn't happen.

Of course it does not happen why would it?

BUT with no isolator it would be very advisable to disconnect the alternator wire from the PDP before attempting to remove the alternator or it could short to the engine block ... then there would be some sparks!

.

 

[Stu Jackson]

it would be very advisable to disconnect the alternator wire from the PDP before attempting to remove the alternator or it could short to the engine block ... then there would be some sparks!.

Thanks, Vic. I have installed a fuse on the alternator output within seven inches of the house bank PDP. It is removed whenever servicing the alternator.

 

[VicS]

I have installed a fuse on the alternator output within seven inches of the house bank PDP

That sounds like a sensible thing to do .. hope it never blows while running

 

[halcyon]

BUT with no isolator it would be very advisable to disconnect the alternator wire from the PDP before attempting to remove the alternator or it could short to the engine block ... then there would be some sparks!

.

Vic, l'm still trying to work a major advantage with the system.

It worries me as well, the alternator regulator circuit is normally powered from the positive output post on machine sensed units, so is also live. A positive diode failure can cause a short via the stator winding, at the best draining the battery flat.

Brian

 

[VicS]

It worries me as well

I'm overplaying the issue a bit. You don't have any isolation between the battery and the alternator in a car do you?
What is the likely hood of a diode failing to a short circuit ... I've had them fail open circuit ... once... but not the other way.

I think my main objection is having an isolating switch which does not totally isolate everything.

I might be tempted to re-label the 1,2, off, both switch "Battery Selector"
and have a full "Isolation Switch" in the common battery negative

 

[halcyon]

I'm overplaying the issue a bit. You don't have any isolation between the battery and the alternator in a car do you?
What is the likely hood of a diode failing to a short circuit ... I've had them fail open circuit ... once... but not the other way.

I think it is looking at the problem from different sides of the fence. In the late 70's we ran an alternator exhange/rebuild firm with a mate, so saw many alternator failures. Since the early 80's we were building mains battery chargers, and still service them, so again have two power diodes in each. Gives one a fair over view of diode failures with around 5000 in service.

Once spent a day with a fire investgator on a motor cruiser with the back end burnt away. Fire occured while tied up in the marina, it had been there for sometime, but powered up. Fire was caused by a flywheel diode, fitted in the auto helm motor control failing short circuit. This resulted in a overheated cable, causing secondary failures in other circuits, till there was a dead short in retro fitted power cables.

The problem is not with current owner. it's the next, or the chap who is not that with it, leaves a fuse out, or fits an over size replacement.

Have not ruled it out, it is just that there are many more negatives than posiives with the system.

Brian

 

[cimo]

couldn't agree more.

If I'm following all the posted links and understanding them, the main driver for this arrangment appears to be an attempt to address the potiential for acciedential isolation of the alt. o/p when charging through the Off-1-B-2 switch. (this appears to have been discussed to death, even in the short time since I joined the forum!).

Although it addresses this potiental problem very well, it is my own opinion that the solution creates a new probelm, potientially more disatrous.

The problem of accidential atl. isolation can otherwise be easily resolved via meachanical means, involving a simply modifucation to the off-1-b-2 switch itself. My own has had a threaded pin inserted that will limit the lever travel. OFF cannot be selected without removing the pin first. Its unlikely that this will be done in error. Taking the alt o/p off the common post could be considerded more elegant perhaps, but definitely less effective IMO.

 

[Iain C]

Well, I think I got away with it. The battery I'd flattened a few weeks ago was still flat (Forgen not enough to recover it on it's own I guess!) but a few hours charge on a mains battery charger was enough to bring it to life, and more importantly with reasonably heavy use all weekend (still dark early!) it still stayed nicely charged and started the engine fine by teh end of the weekend.

Another thought, the alternator light stays on until you give the engine some revs after starting (and always has done)...if it was not charging would this light still go out when you bring the revs up if the alternator was damaged?