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A loose belt/dodgy alternator..... or something worse?


Froggy

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Still don't get it. Doesn't the alternator pulley rotate at the same speed regardless of battery draw. What I mean is; engine revs are 1000 rpm at tickover. Alternator pulley is rotating at 10,000 rpm. Why does a heavy battery draw make the alternator pulley harder to rotate?

 

Because the alternator is harder to rotate if it's generating a high output. It has nothing to do with the speed of the alternator and has everything to do with the load that the alternator presents to the engine.

 

As Biz points out, no energy is free and so generating large amounts of power from an alternator requires the same amount of power (plus a bit extra for inefficiencies) to be input to it from the engine.

 

So alternator giving a high output = hard to turn. Alternator giving little output = easy to turn.

The time has come really where, like most vehicles these days to use multi-V belts arranged in the serpentine method, involving many guide rollers and automatic tensioner, to give maximum belt wrap around pulleys. Probably beyond the skills of most engine marinisers though, or I think they would have already had a go at it.

 

Agreed.

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Still don't get it. Doesn't the alternator pulley rotate at the same speed regardless of battery draw. What I mean is; engine revs are 1000 rpm at tickover. Alternator pulley is rotating at 10,000 rpm. Why does a heavy battery draw make the alternator pulley harder to rotate?

 

We have a TravelPower which is a big alternator.

When we are cruising along and the washing machine heater kicks in the engine exhaust note gets much heavier, the engine slows down just a little under the increased load and a hint of squeal comes from the drive belt. In some cases there is a satisfying whiff of black smoke from the exhaust. The alternator takes mechanical power (torque) from the engine and turns it into electricity (and waste heat).

But this is all still a red herring, the load from an alternator with duff batteries will usually be no greater than the load when the engine starts up with healthy and reasonably discharged batteries.

 

................Dave

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Still don't get it. Doesn't the alternator pulley rotate at the same speed regardless of battery draw. What I mean is; engine revs are 1000 rpm at tickover. Alternator pulley is rotating at 10,000 rpm. Why does a heavy battery draw make the alternator pulley harder to rotate?

 

The output of the alternator will vary from zero and zero revs to either as much as the battery can take (battery wants) or the alternator's maximum output at an unspecified higher revs.

 

When starting with well discharge batteries the limiting factor will be the alternator's designed output. After a wile the battery will be charged sufficiently to demand less that the alternator's maximum output so the battery refuses to accept any more current than it wants. All of this only applies if the alternator is turning fast enough to deliver its maximum output. If it is turning slower then its effective maximum output will be lower so it will deliver the maximum it can or the battery will accept at that lower speed.

 

While all this is going on the the charging voltage will be gradually rising until it is high enough for the regulator to start limiting the output voltage I think this is the point where people can find difficulty in understanding but the same rules apply in that the charging current will either be whatever the alternator can push into the battery at its regulated voltage or if the speed is low the maximum it can produce at that speed, whichever is the lower.

 

The regulator just keeps turning the alternator on and off, altering the mark-space ratio of its output to maintain a near constant voltage. At high revs there will be long spaces and short marks while at low speed long marks and short spaces so at high speed the alternator is effectively turned off for longer than at low speed when it is turned on for longer. The frequency of the ons and offs gets smoothed by the speed of switching and the speed at which the internal magnetic fields build and decay but it can be read with a fast and sensitive enough oscilloscope.

 

The load on the belt depends upon how much time per rev the alternator is actually generating so in effect once you get beyond the initial stages where the load on the belt is directly proportional to speed the belt load will be proportional to the time the alternator's internals are turned on. As with better charged batteries that time will be shorter the load is less.

 

 

 

 

 

 

 

  • Greenie 1
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Still don't get it. Doesn't the alternator pulley rotate at the same speed regardless of battery draw. What I mean is; engine revs are 1000 rpm at tickover. Alternator pulley is rotating at 10,000 rpm. Why does a heavy battery draw make the alternator pulley harder to rotate?

Hmm, try this:

You are riding a bike, and you pedal at the same rpm in the same gear regardless of terrain.

I'm sure that will notice that you have to put more effort in to maintain the same rpm when going up hill.

The hill represents a heavy demand, the flat a low demand and you are the engine.

At some point the hill will become so steep that you are unable to turn the pedals at all; you will then fall off/dismount because the drive is a fixed mechanical arrangement; but the belt relying on friction could slip.

 

edited to say: Ahh I see that Tony has beat me to it with a technical explanation blink.png

Edited by Eeyore
  • Greenie 1
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I thought my "alternator giving a high output = hard to turn. Alternator giving little output = easy to turn" was pretty clear wink.png

Edited; senior moment.

Ah the joys of communicating with non engineers rolleyes.gif

Edited by Eeyore
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ok, if the torque required to turn an alternator was not related to the electrical demand then we could have one tiny diesel engine, or even a hamster in a wheel, connected to a huge alternator and provide electricity for the whole country.

 

................Dave

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Yeah but don't we actually like our boat engines to be as low tech as we can get away with, no electronics, no fancy expensive things to go wrong etc.

 

We have a polyv belt on the travelpower, its not ideal with poor wrap, I have thought about fitting an idler to sort this out but its just not easy to find a suitable place with something to fix to. Also from my previous work stuff I know that an idler in the wrong place is not good at all.

I still reckon several separate belts driving separate alternators and things is the way to go, with an individual polyV for any really big alternators.

Sounds like the op has the worse of both worlds....a single belt driving lots of things....trying to do the job of a serpentine system.

 

...........Dave

The TP on the Beta 43 used to have an idler leaning on the outside of the lower run to improve belt wrap, but it had a bad habit of seizing solid. When it did, it wasn't just the squeal, it was the black smoke filling the engine room until the belt gave up and parted. Had it happen to me once. I damn near half lifted an engine board and let the powder extinguisher go into the hole as I thought the whole shooting match was on fire.

 

Fortunately, I had the sense to stop the engine and take a peek before doing so.

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Which means in the OPs case that the belt has to transmit approx 3 hp to the domestic alternator pulley via a contact area of no more than 2 square inches. This assumes from earlier posts that its a Barrus Shire with one belt passing around four pulleys. Belt life on these engines varies considerably from annual, on a just in case basis; to almost as regular as oil changes. It seems likely that belt life on this particular installation is closely related to how deeply you allow the batteries to discharge between engine starts; and in this case dead batteries appear to the alternator as very discharged.

 

You've obviously been paying attention from earlier threads. ;) Yes, it's a Barrus Shire engine, and both alternators are driven from the same belt.

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Great news folks. Two pairs of hands and a windlass pressed into service as a lever on the alternator allowed us to tighten the belt just a little more, and this has resolved the problem. It's a relief to know that the noise was merely belt squeal and it's obviously something else learned on our new journey into boating. Many thanks for all the great advice provided by everyone, the knowledge i've gained not only helped with this issue but will undoubtedly come in useful in the future. :)

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Great news folks. Two pairs of hands and a windlass pressed into service as a lever on the alternator allowed us to tighten the belt just a little more, and this has resolved the problem. It's a relief to know that the noise was merely belt squeal and it's obviously something else learned on our new journey into boating. Many thanks for all the great advice provided by everyone, the knowledge i've gained not only helped with this issue but will undoubtedly come in useful in the future. smile.png

A quick note of caution.

A piece of wood, typically a hammer shaft, is less likely to cause damage when used to lever the alternator.

There are a lot of aluminium alloy casting used, not just the alternator, often the front covers of the engine are alloy.

Glad you have it sorted.

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A quick note of caution.

A piece of wood, typically a hammer shaft, is less likely to cause damage when used to lever the alternator.

There are a lot of aluminium alloy casting used, not just the alternator, often the front covers of the engine are alloy.

Glad you have it sorted.

 

Many thanks for this advice, I will definitely bear it in mind next time around.

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My way of tightening the belt: Just undo the bolt that stops the alternator moving a little. Trial and error is needed but you want it so it still grips, but can move under pressure. Then take a heavy wooden mallet and hit the bolt so the alternator moves, until it's the right tightness. If you've got the bolt loosened just right the alternator will stay put as you tighten the bolt at your leisure. No blocks of wood, windlasses or extra pairs of hands needed.

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Some of the later replies seem to take no account of the forces that will be applied to the water pump bearings with the implied belt tension. Just pointing this out.

 

 

Yup.

Thats why (in my opinion) the core engine belt, which likely drives the water pump and a smallish alternator, should be left as standard.

The big domestic alternator and possibly TravelPower should then have their own belting, and if you want to be really pedantic these two items could be on opposite sides of the engine so that the side forces on the crankshaft cancel out.

 

I spoke to Beta a few years ago questioning their choice of pulley sizes on the starter alternator and asking their opinion on a bigger alternator and they told me not to make any changes for this very reason.

 

................Dave

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Yup.

Thats why (in my opinion) the core engine belt, which likely drives the water pump and a smallish alternator, should be left as standard.

The big domestic alternator and possibly TravelPower should then have their own belting, and if you want to be really pedantic these two items could be on opposite sides of the engine so that the side forces on the crankshaft cancel out.

 

I spoke to Beta a few years ago questioning their choice of pulley sizes on the starter alternator and asking their opinion on a bigger alternator and they told me not to make any changes for this very reason.

 

................Dave

Have an arrangement almost exactly as you describe

Original 45 AMP. Alternator driven by same belt as Water pump

 

2 75 AMP Alternators driven by added 10 inch twin V pulley on opposite sides of the Engine

 

Belt life is good and adjustment required once or twice annually,average Engine hours 350 plus per annum

 

CT

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Yup.

Thats why (in my opinion) the core engine belt, which likely drives the water pump and a smallish alternator, should be left as standard.

The big domestic alternator and possibly TravelPower should then have their own belting, and if you want to be really pedantic these two items could be on opposite sides of the engine so that the side forces on the crankshaft cancel out.

 

I spoke to Beta a few years ago questioning their choice of pulley sizes on the starter alternator and asking their opinion on a bigger alternator and they told me not to make any changes for this very reason.

 

................Dave

 

Which is why I can not understand reputable marinisers who seem to employ experienced "engineers" designing two alternators on a single V belt.

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