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Alternator rpm


Jrtm

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Am i correct in thinking that my current alternator rpm speed is far to low?

 

Most cars run around a 4-6 inch crank pully (flywheel efectivly on most boat engines) and most alternators run around a 2-3 inch pully this gives a rough ratio of 2:1 to correctly work out its engine pully devided by alt pully.

 

Now most cars run between 1500rpm and 4000rpm when driving and max of 5-8000rpm

 

Now the maths

To get max alternator rpm you take your max rpm and times it by your ratio so lets say you have a car that does 6000rpm and pully size of 5 inches with an alternator size of 2.5 inchs (this is roughly the averange size)

So 5÷2.5=2 = ratio

2×6000=12000 = max alt speed

 

Nearly all alternators can handle between 12k-16k

 

My current engine pully is 12 inches and alternator is 2 inches.

So 12÷2 = 6

My max rpm is 1000

So 1000x6=6000 max alt speed

But my tickover and general running speed is around 300 tickover to 5-600rpm running speed to be making a charge an alternator needs 1800-2000rpm (at the alternator pully speed)

 

So back to my maths at roughly running speed

Ratio= 6

500x6=3000rpm at alternator (charging but only just when compared to a car at say 2500rpm

So 2500x2=5000rpm

 

At tickover

300x6= 1800rpm (only just turning the alt fast enough any slower and wont be charging)

Compared to a car

800x2=1600 (if any one knows about cars yes they dont properly charge your battery at tickover it will charge but not properly)

 

Now to go back to the boat. I know that rn run a alternator directly from the flywheel. So as my alt pully on the engine isnt very good was thinking doing the same using the above it should be.

Back to the maths

Flywheel= 24 inches

Alt pully 2inches

 

24÷2= ratio 12

1000x12=12000 rpm max alt speed

My more realistic running of 500 worst 700=

700x12=8400rpm max alt speed

This brings my alt speed very close to that of the car

Tickover

300x12=3600rpm max alt speed

This is higher than a car but its not a big issue just means i can charge my batterys fairly well on tickover.

 

With this in mind i should be ok with current alternator but run direct off the flywheel and see good charging at tickover but not over rev the alternator even at max rpm

 

What you guys think?

Edited by billybobbooth
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So its pos a little under but not far off.

 

Sounds like a 3inch pully on the alt with then running off the fly will make a max rpm of 8k running around 5600

And brings tickover down to 2400 ish

 

2 reasons for doing it

1 my pully wobbles all over the place and is wearing the edge of the belt not bad but more than if was strait.

3 will give me around 3inch gap between flywheel and fuel tank rather than the 1/2-3/4 of inch i have now

On a 3rd will tidy it up and look better and will be covered by the flywheel cover im planning to put on alowing me to be able to step over the flywheel. If i need to.

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A atandard old, old mini, just after dynamos would have had an ACR 34amp alternator. Some of the much later ones had the A127.

If your alternator has a big black plastic case on the back and only has LUCAR spade type terminals it is an old ACR, can be from 34amp to about 45amp.

Edited by bizzard
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Had a look all my paper work says is for a mini and is 70amp but by the pully its not a new mini as i work for bmw so know its not a new one

Looks like a lucus a127 but dosnt have the fan bit i dont think but never really studied it im throught it had 70amp on it might have been 75amp

Edited by billybobbooth
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Had a look all my paper work says is for a mini and is 70amp but by the pully its not a new mini as i work for bmw so know its not a new one

TheA127 are either 50 or more usually 70amp. I can't remember the year when the mini's began using the 127, probably early 1980's, similar to some M3 Ford Escorts. Does yours have the big black plastic case on the back ? The A127's have no black plastic case, they have alternative stud and nut and spade terminals on them, also the regulator is fixed on the back, on view.

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Not 100% but its a new alt in 2015 but to fit an old mini if that makes sence.

I thought it had a black cover.

Ah found a few pics

Sorry not vary clear but least know what it looks like

Sorry pics wernt taken for the alt were for the pipe work

post-17927-0-31911300-1479844415_thumb.jpg

post-17927-0-83304700-1479844510_thumb.jpg

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Not 100% but its a new alt in 2015 but to fit an old mini if that makes sence.

I thought it had a black cover.

The old ACR and the A127 are very different to look at. If it has the black plastic case on the back it is an ACR could be 34amp to 45amp. I'm not aware that the ACR's were ever more than 45amp, although perhaps it might be possible to up grade to a higher output with a different regulator, but I doubt it as the more modern A127 is a direct swap anyway.

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Not 100% but its a new alt in 2015 but to fit an old mini if that makes sence.

I thought it had a black cover.

Ah found a few pics

Sorry not vary clear but least know what it looks like

Sorry pics wernt taken for the alt were for the pipe work

That's an ACR, Lucas Prestolite still make new ones, could be 34 or 45amp, doubt if its any more.

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That's an ACR, Lucas Prestolite still make new ones, could be 34 or 45amp, doubt if its any more.

I've got a feeling that Prestolite do make a new 70 Lucas ACR. I sorted a boat here in the summer with a BMC 1.8, not charging. It had a A127 70 amp on it which packed up. The owner, a friend of mine got a new one from Calcutt boats. I told him exactly what to get for BMC 1.8, type A127, output, hand ect. The goons at Calcutt sent him a new Lucas Prestolite ACR, WRONG HAND too!!!. I couldn't believe it, wrong hand, never mind an ACR instead of a A127. However I have a feeling it might be 70amp. The owner got the correct hand A127 alternator from Ebay which I fitted. He did not bother to send the ACR back to Calcutt boats. It is still in its box with its paper work on his boat. He usually comes to his boat on Wednesday, if so I'll pop in and check it out and let you know.

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I thought it said 70 or 75. But after here maybe i read 45

 

Ill check when im on the boat later on.

 

If its a 70/75 is it sutabe to run off the fly or is it a case of new alt to do this?

 

I have to lift and move my engine so wanted to remove pully at the same time

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I thought it said 70 or 75. But after here maybe i read 45

 

Ill check when im on the boat later on.

 

If its a 70/75 is it sutabe to run off the fly or is it a case of new alt to do this?

 

I have to lift and move my engine so wanted to remove pully at the same time

I'd say it will be ok at the speeds you say, from the flywheel, but they are quite happy and should give out more or less their maximum output at about 4000 rpm anyway.

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In my experience an alternator only delivers its rated output for a minute or so after starting the engine.

 

My 40A 24V Leece Neville connected to a flattish set of Trojan clones pumps out 43A for a few seconds, then the current progressively tumbles to about 20A over about five minutes. Then during the following 30 mins it tumbles further to about 10A.

 

If I then fire up the genny and the 30A Sterling Pro Charge Ultra, I then get 29 or 30A charging current consistently for an hour or two before it goes into absorption.

 

Alternators do not behave like decent multistage battery chargers.

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The National we have drives an ancient Lucas ACR , about a 9 in pulley in front of the flywheel, stock 2in on the alternator. Supplies all the amps needed for a wooden ex-working boat with electric start. We have one of those modern vehicle electric hand wash things that draws 20+ amps for about 40 minutes while it warms up, powers that ok too. There is an old large scale ammeter on the alternator which shows a charging current of about 5A at engine idling and a maximum output over 30A

We did have the alternator driven off the flywheel rim at one stage but it's quite difficult to get the belt running true with no belt groove and there was no additional amps available despite the vast increase in alternator revs.

Bill

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That's an ACR, Lucas Prestolite still make new ones, could be 34 or 45amp, doubt if its any more.

Through choice we wouldn't ever buy another Prestolite alternator.

 

Bought a band new one at the beginning of this year and it had a faulty voltage regulator. Sent it back and got another new one which seemed fine, until it shook itself apart 3 miles off Southwold Harbour entrance.

 

The noise it was making had to be heard to be believed and we thought something was seriously amiss with the engine as we limped into harbour (lifting the engine hatch at sea showed no obvious cause for the noise). When we were moored up and could investigate further the bolts holding the casing together had shaken loose. One had fallen out completely and was on the floor under the alternator and two others had shaken loose and were rattling through the cooling fan which is where the noise was coming from.

 

The bolts holding the casing together had no washers or other form of stopping them coming loose. Very poor form. Fortunately the bolts hadn't caught any belts or cooling hoses which could have been much worse.

 

We managed to cobble it back together again, straighten out the fan and made sure that we used some Loctite on the bolts which have since stayed put. We do make sure we check them on a regular basis now though!

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Most alternators will rapidly drop below their maximum output, and don't take that long until they produce as much charge at tickover as they do at higher revs (tickover 850rpm on a modern diesel). You need an ammeter to see what is going on. My Vetus [iskra] alternator (95A) will produce its maximum output at about 2,000 rpm just after starting with a 50 - 60% charged battery. After half an hour 1,600 rpm gives its maximum, i.e. increasing revs doesn't increase current. After an hour or two it will be producing around 20 amps at tickover and nothing more at higher revs.

 

If your alternator pulleys are sized so that the alternator is not producing its maximum output within the engine rev range it just means that first stage takes a bit longer.

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I only have a plug in volt gauge at the mo a proper one will be added into the new system as its only half wired in as we keep adding or changing the design.

 

My main reason for doing it is to get a bit of extra space between yhe fuel tank and pully when i undercoated the fly you cant even get your hand between the the pully and tank.

 

Its prob a very old pully but would like to remove it.

 

The alt is plenty powerfull for what we use as we onlu have a few 8w lights 3 bildge pumps and a diesel heater but later a 12v fridge.

 

Its more a case of tidying things up and making space. I just wanted to check if the fly would cause the alt to be over its max rpm.

 

Think ill just get a 3" pully for the alt this will bring it very close to what it currently runs.

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Note that an ammeter is of much more use to monitor the alternator than a voltmeter. It will allow you to drop revs (when running engine for charging only) and know that you're still getting the maximum that the batteries require.

 

That's not to say you don't need a voltmeter (or SmartGauge or whatever), just that it's not particularly relevant to your alternator RPM questions.

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The one thing that has not been mentioned is belt wrap on the small pulley. As the drive pulley gets bigger this gets smaller. Too small and the belt will slip and squeal.

 

As an example, my A 127 70A is on the limit with a 6 rib poly v belt. The drive pulley is 12 in and the alt pulley about 2.5 in.

A ploy v belt will drive better and run more happily on the rim of the flywheel compared to an ordinary A or Z section v belt.

 

N

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I was going to swap the pully on the alt to a 5-8rib so the belt had a flat running area on the fly. But wanted to check what you guys thought about running the alt off the fly ill then match the alt pully to the correct size so if a 2" pully (same as on now) is ok ill just get a 2" ect ect.

 

I see your point on amp meter.

Edited by billybobbooth
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