Dual Alternators help please

iangrant

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My Volvo lump has a vacant fitting and wiring on the port side for the standard Alternator.
On the Starboard side I have a Prestolite Alternator rated at 90 amps working perfectly to two battery banks.

Both sets of wiring go to two sterling alternator controllers.
Currently the Prestolite alternator (a new one last year, I smoked to old one) is not wired through to the controller, just working as nature intended. I couldn’t face taking the covers off and soldering a wire onto the lump that cost a b;**dy fortune. Add to that the man in the shop said that sterling controllers cook alternators with small pulleys at low revs.

The wiring is in place for the second of the dual alternator setup and it should be a simple matter of mechanicals (ha) to bolt in the new port one.

Both sets of output wiring seems to be connected to a “1 Both 2” switch, I’ll check that though!!

The added advantage of a second alternator will be the second belt driving the water pump and separate charging for domestic and engine battery banks.

My question is will it pop both alternators if they are inadvertently connected together?
It would be good if one alternator carried on if the other one failed etc…


Ian
 

boatmike

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Ian,
I am not entirely sure of your setup but its fairly certain that you need a blocking diode in the system. You should not connect the output from two alternators directly. Battery sensed diodes are available and you need one specifically designed for twin alternators (input) and however many batteries banks you have (output)
 

tome

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Perhaps someone can explain to me why blocking diodes are required when connecting 2 alternators together, given that each of the 3 windings on both already go through diode rectifiers?
 

kandoma

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re cooking volvo alternator: my old volvo alternator cooked twice with an external regulator. I would add an external temperatur sensor and switch off the field in case the alternator temperature goes higher then 75 °C.

Sitting at anchor for one week with an anchor firmly set, last day bevor departure charging the battery just margenial, next morning starting the engine in idle, trying to lift with the anchorwindlass the anchor, nothing goes, increassing rpms of the engine to get a higher voltage at the windlass, finally getting up the anchor and leaving under engine, motoring to the harbour to check out: result: alternator winding burnt, diodes soldered out and both batteries damaged beyond repair. That is how it happend twice to me with an external regulator without temperatur controll of the alternator. I wouldn't be surprised, if the build in regulator has some temperatur contoll included. Never had a problem with the inbuild regulator.
After the second incidence, I replaced the Volvo alternator with a "marine grade" alternator. I believe. the only difference is a thermostat controlled field in the marine alternator.

connecting two alternators together: there should be no problems with connecting two alternators to the same battery. I have run with this setup for quite some time with a shaftalternator and the ordinary alternator going to the same battery. When I forgott to switch off the field of the shaftalternator, both charged the same battery.

Peter
 

halcyon

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There should be no problem running two alternators in parallel, for charging.
The problem is switching to isolate the battery banks when not in use. With a rotory switch that you have, as long as it maintains feed back to the alternator, when switching, you should be ok.
If you want auto isolation go for a bi-directional relay, it will isolate batteries when off charge, but allow for aternator redundancy. Should either alternator fail, it will allow the good one to charge the bank off charge. It can also provide for link starting if you have a low engine battery.

Brian
 

boatmike

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The diode rectifiers on the alternator are built as a 4 way bridge to convert the AC produced by the field windings to DC. They do not stop current passing from the charge line back to the field windings. In theory, if two alternators are running together on the same engine witrh similar outputs they will both charge the batteries but if one were to stop (lets say the drive belt were to break) The one that was running would send DC current back to the field windings of the other which is highly undesirable. The diode bridge built to seperate the charging currents of two alternators are primarily used for twin engine installations and ensure that current from one cannot flow to the other but both can charge the batteries. Essential when one engine is running and the other is not. I suggest that even with two alternators running off the same engine the safe thing to do is wire through one of these which will ensure a malfunction with one does not affect the other. I hope this is clear.
 

tome

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I'm aware that each of the 3 stator coils has a diode rectifier (it's 2 way, not a 4 way bridge). The field winding is fed via the regulator, not direct from the output. Therefore I don't understand why a blocking diode is necessary.

In the situation you describe, the regulator of the faulty alternator will limit the field current to safe limits and no damage should result. The output current will drop to zero on the faulty unit, but Kirchhoff's 2nd law still applies and the remaining alternator will happily feed it's load.

So I still don't understand why it's necessary - hope this is clear!
 

halcyon

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It's normally a 3 x 3 diode packs, a pos and neg output, and 3rd positive diode pack that feeds the rotor, the regulator then switches on the neg side.
As you say loose output on 1 alternator, the output diode packs block any current, as you have no back feed to the stator, there is no output from field diode pack. All you have is a feed to the rotor via the warning light, so the max current is 2.2 / 5 watt from the bulb.
In practice, during the last 23 years we must have supplied 3,000 - 4,000 systems to boat builders were we have had two alternators running in parallel, and to date we have had not a problem, well no one has shouted, so it must be fairly reliable.

Brian
 

tome

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Thanks for this Brian. To be clear, are you saying that you parallel the outputs without any further blocking diodes?

PS I'm aware of the extra smaller diodes on some alternators for a dedicated regulator/rotor feed.
 

iangrant

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Right you B*ggers - now I'm all confuzzed.

I'll just sum up.
1: EXPENSIVE alternator connected and working on starboard side of engine.
2: Two mouldy alternators in garage
3: Wiring and mechanics in place on port side of engine ready for mouldy refurbished alternator.

What I don't want to do is end up with total meltdown with the "1 both 2" switch that the "system" seems to go through connecting them in parallel (maybe).

I can trace the wires and sort it out but mr Adverc is saying he needs alternator types to spec the diodes.

Chuck in another poster that has melted an alternator with a sterling controller -

What to do -- Doh --

Ian
 

halcyon

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Yes, they were mainly three battery banks with two engines, the engine would charge it's battery, then link to charge the service battery. So you had both alternators both charging the service battery. This gave two advantages, by reducing the load on the alternator we got a higher charge voltage, and if you lost an alternator, the good one charged the faulty engine battery. We have done the same with twin engine / twin battery bank.


Brian
 

tome

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Dinnae fret lad, we'll sort it. I was puzzled by the diagram you sent me from Adverc hence asked the question and Halcyon has confirmed my own understanding.

Speccing the diode splitter is easy- just go for the highest output and add a generous safety factor.

To prevent meltdown, we have to make sure that any battery switching doesn't disconnect the regulator sense wires with the engine running.

Perhaps Brian will confirm.
 

halcyon

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To a point yes, my original comment was use the rotory switch, as long as it's a modern one, and is not break before make, ie does not run alternator open circuit.
As for diodes I'm the oppsite, spent the last 30 years designing systems to provide an alternative to blocking diodes, and avoid there problems. Think my views on diodes are about the same as yours on relays.
Might be an intersting topic one quiet winter evening?

Brian
 

tome

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Fair enough Brian, there's always an alternative view and I'd welcome a discussion. My main interest at the moment is to help get Ian away on a pain-free ARC trip and extended cruise afterwards. If he has any alternator problems, I'll be hearing from his missus!
 

halcyon

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First thought is to :-

Run a primary battery bank from the alternator with the Adverc via position 1.

Fit a second alternator ( standard regulator around 14.2 / 14.4 volt ), and power a second smaller bank for starting and providing back-up power to primary bank.

Then fit the rotory switch terminal 2 to this alternator.

Fit Hella type isolator switch between primary alternator and second bank.

Thus normal use Adverc charges primary bank, if this alternator fails, switch to position 2, after charging turn switch to off, or position 1 to isolate banks. Alternator 2 charges your start / reserve power bank, if you loose this alternator, close Hella switch and charge from number 1 alternator.

Thus you have two independant battery banks, each with an alternative charge source, minimum parts, and no volt drop.

Brian
 

tome

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I'll sketch out your suggested schema and ponder it over the weekend whilst afloat. Thanks- I can see where you're coming from and like the options.

Cheers
Tom
 

William_H

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If two alternators are charging any one battery or series of batteries in paralell (even via diodes) then there is the potential for one alternator to bring the voltage up to charge voltage , the other alternator sensing this charge voltage will not charge at all. Until such time as the current limit of the alternator maxed out and the voltage fell from winding resistance. This assuming that each alternator has a standard regulator. The outcome would be a shorter life for that alternator. On twin engined aircraft you always see a balancing regulator fitted to ensure equal load to both alternators. Mr Adverc obviously doesn't see the need and simply paralells the feild wire to both alternators so that current from each alternator will be similar providing they are running at the same speed and have the same field and stator resistance characteristics. Any difference in alternators will give different current load. Perhaps it doesn't really matter in terms of cost for result. olewill
 

iangrant

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Tome, be sure you'll get the blame if it does go pop!

Anyway - reading through this lot I think the wiring and switch is all in place. I'm off to get the new alternator this morning and have a poke about with some wires tracing them through and draw it up!

Ian
 
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