Alternator on/off switch

[cindersailor]

My Yanmar 1gm is only a little engine and I would like to be able to switch the alternator off when max engine power is needed and at start up to allow the engine to get going before loading it. This is an issue as the alternator is not original but rather large (60A) for this engine. Having thought about where to introduce a switch one possibility would be in the excitation wire. However, although this might work at start up when there is no current from the alternator itself for the field coils, throwing this switch once the alternator is generating output would have no effect. I know that smart controlers have a delay built in so that the engine is only loaded once it has properly started, but how does it work? Any ideas?

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[Chris_Robb]

Don\'t do it

I understand that switching off the field could blow the diodes in the alternator. This is the same as switching the ignition key off with the engone running.

If your battery is full, the alternator will be taking very little energy from the engine, so there will be hardly any drag from it. Only when the battery is low will it want to put out a larger charge.

If the engine is so low in power output there may well be something wrong with it. It certainly should not be worried by the pull of an alternator whilst ginging out 6 or 7 amps.


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[maxxi]

The Stuart Sole diesel engine had just this feature as standard, claimed to gain approx 1.5 horsepower of extra propulsion with the alternator turned off.

There are still Stuart agents about, talk to Messrs Norris @ Maldon who may be able to help you.

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[Guest]

Also ...

1gm ..... no power ???? Seems strange.

Its a well known and recc'd engine with good reputation. The drag of an alternator unless you are heavily discharging batterys etc. shouldn't affect you that badly .... unless there is something seriously wrong.


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I came - cos they said was FREE Guinness !
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[kds]

Re: Don\'t do it

"the same as switching off ignition" - surely not.
If the field has no current through it, there would be no output and so no harm is caused.
An off switch for the field coil is standard on some main/selector battery switches and it would solve the problem, I think.
Would appreciate the opinion of an expert, though, as I am planning the same solution to reduce load on engine on occasions when you really do want all the power to the prop.

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[kds]

Re: Don\'t do it

"the same as switching off ignition" - surely not.
If the field has no current through it, there would be no output and so no harm is caused.
An off switch for the field coil is standard on some main/selector battery switches and it would solve the problem, I think.
Would appreciate the opinion of an expert, though, as I am planning the same solution to reduce load on engine on occasions when you really do want all the power to the prop.

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[claudio]

Re: Don\'t do it

There is no reason why the diodes should blow if you disconnect the field winding. The diodes still have the battery load connected and therefore the output voltage will not rise. Disconnecting the field may not however prevent the alternator from generating power (residual magnetism). It is worth a try, if it solves your problem. I don't see that any damage will be caused, after all when you switch the ignition key off, the field is disconnected and the engine is still running until it eventually stops.



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[TonyBrooks]

Re: Don\'t do it

I hesitate to reply, now the word expert has been used.

The "blowing of diodes" is a problem when the BATTERY is disconnected whilst the engine is running. This causes a sudden voltage surge that can blow any semi-conductor device associated with the alternator. Some alternators have a heafty zenner diode in parrallel to the output to "shunt" this surge to negative - most of those I knew tended to fail closed circuit when they were required to do this, so you still got a failed alternator

This is one of the reasons I do not like the 1, 2, both split charging switches - with or without extra wiring, aledgedly to prevent alternator damage.

The voltage regulator on a modern alternator is likley to be switching fiesld on and off at over 120 time a second, so using a switch to do the same should not cause damage. The problem is getting at the field exitation wire, that is almost certainly inside a modern alternator, but any decent auto-electrician should eb able to find it for you.

The Ignition switch only energises the rotor (field), via the warning lamp, on first starting, from then on the exitation currant is provided via the field diodes.

Switching (or pulling off) this wire will prevent the alternator energising on most instalations (not go-faster petrol jobies - they might energise the alternator by residual magnatism at about valve bounce) as long as it is switched on a stationary engine. It should be ok to turn it on when it is accetable to produce charge.

Please note that when people fit very large engine pullys to drive their alternators, especially higher power ones, it is not unknown for the alternator load to prevent the engine reving up.

Tony Brooks

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[pvb]

Probably not worth the bother...

Although you've got a 60amp alternator, that doesn't mean it will be pumping out 60amps - its output will depend very much on the ability of the batteries to accept the charge. Unless you have a huge battery bank, you're unlikely to see a 60amp charge rate.

If you have a separate engine starting battery, you may well have one of those 1-2-Both switches. If so, you already have the ability to select only the engine start battery (which will take very little current normally) to reduce your engine load.

Smart regulators normally work in parallel with the alternator's standard integral regulator. The delay you mention is before they start boosting the output, not before charging starts. This means that the initial alternator output is just the same as it would be with a standard alternator.


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[sailorman]

on a different tack, i would be interested to know what size prop u have a my son has the same boat + same engine & is "over-propped"

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[andyball]

Re: Don\'t do it

agree with Tony......have examined the operation of some alternator regulators (I used to sell electronic replacements for mechanical ones fitted to BMW/MotoGuzzi mc's.).

they switched the field on/off at a particular voltage....no finesse, just OFF at 14.x Volts, made them very useful as a voltage operated switch (delaying charging of a second battery 'til 14.4V on the main battery).



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[andy_wilson]

To the original poster, I agree, this is a good idea, especially if the same switch had a OFF / STANDARD / HIGH charge setting.

To the respondents who think this isn't a problem, it is. I am always concious of this during a night at anchor with the lights and heater fugging up the cabin.

If a rapid escape was required heading into a rising wind (which, lets face it is the most likely scenario), it doesn't take much discharge to get the alternator excited.

At tick over only a few hp are produced. The alternator and it's associated inefficiencies can easily take 1-1.5 Hp, and engaging the gearbox at tickover will take another 2-3hp.

Result, revs will not rise to produce the thrust required, especially with a cold engine.

Having never been in this position 'in anger', I have been concerned enough when it has happened when no danger was present.

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[gus]

Re: Don\'t do it

A comment to all posters. According to 'the man I spoke to at Lucas' - Switching off is not the problem, it is when you switch back on that you have the surge which damages the diodes. His recomendation was that if the ign switch was inadvertantly switched off, then stop the engine before switching it back on.
I can only presume that he is right 'cause it is part of his job to know these things.
Any more comments?

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[clio]

Re: Also ...

having dyno tested more engines than i care to remember you would be supprised how inefficient some engine add on's are, air conditioner comppreesor 15 hp, one 100 amp not to be named, fully loaded almost 4hp so as you can see with a small engine it does count

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[andyball]

Re: more comments?

Only - what's the difference between the regulator switching the field on/off while running & doing it manually?. & why don't battery charger diodes suffer from surge damage when you connect a large bank of batteries?.

Now maybe car/marine alternator diodes are strangely fragile, but in many years of m/cycle electric fault finding, I must have reconnected the field with motor running loads of times( & on a few boats now) & never had a problem-maybe I'm lucky.

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[cindersailor]

This seems to have generated a bit of interest. I should point out that this is an original early 1980s 1gm which is only 6.5 hp according to the makers plate, although the Americans insist that their 1gms are 7.5 hp which has always confused me. Hence the need to squeeze out as much power as possible to push my Cinder 22 along (2 tons, long keel). I do not have the engine in the boat yet, I bought it second hand and am currently rebuilding it (bent con-rod causing cold starting problems). Bench testing with the replacement alternator (£7 from the breakers yard, from a Renault 19 I am assured) showed that into a semi flattened battery (12.5V) the alternator created sufficient load to slow the engine noticeably and cause it cold start problems. I am therefore considering how to fit the mod of the original post. The consensus seems to be that a switch in the regulator to field coil wire should have the desired effect. If it blows up I have only lost £7!

In response to Sailorman’s question, I do not yet have a prop. The old engine was a YSE12 and had a two blade left hand 13x11. The 1gm needs a right handed prop. Castle marine’s program suggests 10x9 should be about right. I would be interested to know what your son has.


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[Phil]

I agree with Robb....don't do it as the BIG no no with alternators is to disconect the lead, either positive or negative whilst the engine is running, read the instructions for your car.

As for putting a switch in the field coils I don't know about that one, but would be interested to see a wiring diagram.

Standard alternators are usually quite a bit smaller than 60 amps, about the 35 mark which is considered ample for most twin battery installations in normal circumstances. If using a lot of battery power when the engine is not running you would then need a higher output alternator to give as much charge as possible in as short a time as possible, and then a smart controller such as an Adverc comes into its own.

Out of interest, a 75 amp alternator controlled by a smart alternator charging a bank of four flat batteries ( 320Amps total capacity) uses less than 2 HP of engine power.


Phil

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[sailorman]

no idea on the prop size it will have to wait until lift-out. i think the previous owner re-used the orininal prop as fitted to the vire 7.
we arnt getting anywhere the revs after 1/4 throttle on more revs but it "free revs"
ok ie. no load. i guess its at least 2 ins over pitched.
i re-engined my boat last nov. removed a 4108 ( 40hp) with 17inch max prop @ 22deg. replaced with nanni 37.5 hp & had to increace pitch by 2 deg to obtain same revs ( 2800). the nanni is also 200cc smaller.
the nanni is a fantastic engine but there again its some 45 > 50 yrs later in design than the perkins.

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