feasibility of voltage surge when switcvhing of ignition firld supply of alternator

[catmandoo]

Got two alternators running off same engine . each alternator has ignition light , switch and diode in field supply .

as I dont want to create a voltage surge when field suppy shut off i would like experienced corroberation of following reasoning

1 Before start up one alternator field shut off - at start up alternator does not produce current other alternator charges
2 after start up and when both alternator fields are on, one alternator keeps charging as the voltage from the D+ provides the field current and open circuit and diode prevent ignition light from glowing
3 if switched ed on again likewise nothing happens as alternator is still charging however if engine stopped and restarted after field switched off alternator will not charge

You might wonder why I would want to switch one alternator off however out put is 80 amps and 70 amps respectiveley but only at 6000 rpm . pulley ratio means engine would have to go at 3000 rpm to get that but max continuous rating of engine is at 1800 rpmand max speed is 2200rpm so need two alternators to charge batteries quickly however when motoring with charged batteries one alternator off would add perhaps another half knot to boat speed


Any comments

 

[pvb]

however when motoring with charged batteries one alternator off would add perhaps another half knot to boat speed

If the batteries are charged, the alternator will be producing minimal current, so will not be loading the engine. You wouldn't get any noticeable speed improvement.

 

[catmandoo]

If the batteries are charged, the alternator will be producing minimal current, so will not be loading the engine. You wouldn't get any noticeable speed improvement.

Yes on reflection that would seem the case .picked it up from Sterlings blurb on regulator justifying switch on white wire etc

 

[pampas]

Generally once an alternator is excited in cannot be switch off charge except by stopping the engine.

However I have seen only Once where the drive had a home made clutch fitted to the driving pulley and achieved what you want.

 

[pvb]

However I have seen only Once where the drive had a home made clutch fitted to the driving pulley and achieved what you want.

But it wouldn't achieve what he wants - once the batteries are charged, the power needed just to spin an alternator is negligible.

 

[VicS]

Generally once an alternator is excited in cannot be switch off charge except by stopping the engine.

It can if its modified so that the field current can be switched off. Would need a switch in the brush wiring

 

[William_H]

There is no reason to think that there will be a voltage surge as you switch off the field current. That is assuming that you are switching off the field current not just the start up field supply that comes through the ign light. As Vic says it is not common to see the field current being able to be switched off although common on light aircraft alternators.
if the 2 alternators are charging one battery then if one alternator is charging its voltage will tend to discourage the next alternator turned on from charging seeing an apparently
fully charged battery. You need a ballancing regulator. of course charging separate batteries you have no problem. olewill

 

[neil1967]

Why not change the pulley sizes so that the engine and alternators are better matched?

Neil

 

[Birdseye]

But it wouldn't achieve what he wants - once the batteries are charged, the power needed just to spin an alternator is negligible.

I suspect the situation is a bit more complicated. For a start has the op actually measured and compared the current flow with one or two alternators running. He would get a shock if he thinks the output is a combined 150 amps. Nothing like.

The current flow is governed by the voltage state of the batteries and the resistance of the charging circuit in the internal resistance of the batteries thecmselves. Once the output voltage gets up to the regulated 14.5 or so, it wouldnt matter if he had 5 alternators in circuit, the current flow would be the same. So would the drag on the engine, friction apart.

The other issue is the time taken to get the batteries fully charged. Its likely that from half charged to full it would take several hours at 14.5 volts assuming the op has overkill on battery capacity

 

[catmandoo]

Thanks for all the comments. I don't fancy fiddling with the internal field wiring . Got enough issues with standard Lectrica marine( Volvo penta use so manufacturers say )regulator setting of 14.6 volts . Sterling and others state should not run batteries with that loading for more than 5 hours . this makes me wonder why alternator manufacturers push out new machines with that setting . They say they are in step with engine manufacturers in this respect so are all new engine and battery combinations so designed ??

I think changing pulley ratios means work out of proportion to its value . One alternator is directly on engine utilising engine belt driving circulating pump etc . alternator pulley currently at smallest size available and don't want to change pump speed etc . External alternator maybe but difficult to access and at my age I often get cramp in difficult positions.

 

[catmandoo]

I suspect the situation is a bit more complicated. For a start has the op actually measured and compared the current flow with one or two alternators running. He would get a shock if he thinks the output is a combined 150 amps. Nothing like.

The current flow is governed by the voltage state of the batteries and the resistance of the charging circuit in the internal resistance of the batteries thecmselves. Once the output voltage gets up to the regulated 14.5 or so, it wouldnt matter if he had 5 alternators in circuit, the current flow would be the same. So would the drag on the engine, friction apart.



The other issue is the time taken to get the batteries fully charged. Its likely that from half charged to full it would take several hours at 14.5 volts assuming the op has overkill on battery capacity

In actual fact I do have shunts on each alternator outlet . combined output is about 60 to 70 amps reducing to less as the batteries charge up after providing power at anchor for say two days . I have 3 125 Ah service batteries and one 90 Ah starter battery. None of the batteries are allowed to down below 12 volts