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On alternator failure.


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Alternators on boats work hard, and alternators on boats with lithium batteries work harder.

 

I have a couple of failed alternators on board. One failed before we got lithium batteries and one after. They are both 70A  A127s.

 

Since the second failure caused me to have install the brand-new Ebay sourced spare that I carried, I thought I'd look at the two failures with the aim of building one functional unit to take over the role as a spare.

 

In both cases the only fault was a failed diode. One was open circuit and one short circuit. I wondered if cooked stator windings might be a problem, but both stators looked fine and metered out OK. I'd already bought a new diode module on Ebay for £12 and fitting that is pretty trivial, as long as you have a BIG soldering iron. The sort your Grandad had, not a 15W tidler suitable for doing electronics.

 

What was noticeable about both units was that the cooling air path around the diodes was clogged with dust and fluff. I suspect if I'd stuck with my good intentions and dismantled and cleaned them once a year,  they would not have failed.

 

So, lessons learned. 

 

A127s survive maximum output for long periods pretty well, and when they do fail, it's normally the diodes.

When they fail, they are easy and cheap to repair.

Keeping them free from fluff and dust will probably stave off diode failure.

 

I have a good spare now, but I think I'll probably carry a spare diode pack too. There's good access with the alternator installed on the engine, and it's actually easy to remove the back half of the case and stator whilst leaving the front half, rotor and belt in place. That allows a quick change of the diodes with very little dismantling of the installation.  It also saves having to undo the pulley nut to move the pulley to a new alternator, which is the most difficult part of a swap.

 

MP. 

 

 

 

 

 

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Not thought about this before. Each diode will be conducting for roughly half the time and blocking half the time. When conducting, they will drop 0.6V or so. With three phases, each diode will be trying to dissipate 0.6V x 70A / 6 = 7W. Which is a lot for a little bit of silicon. That heat has to be drawn away through the packaging, leads and solder joints, then dissipated by good air flow. Lots of dust inside won't help and the diode overheats.

The life of the diode is likely to half for each ten degree C rise in temperature inside, if it is the silicon itself that fails.

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For me to try to take apart an alternator might not be safe for our species' future- certainly not for the somewhat nondescript subpopulation of our species which is moored near Chester. 

Too many bad things might happen, all of them involving fire and destruction. 

 

I wonder if a non-invasive approach might be more advisable- perhaps one of those 'air duster' spray cans, with a straw to direct the air jet inside the casing of the alternator?

Might such a practice be of any benefit in terms of preventing future failures? 

 

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22 minutes ago, Jen-in-Wellies said:

Not thought about this before. Each diode will be conducting for roughly half the time and blocking half the time. When conducting, they will drop 0.6V or so. With three phases, each diode will be trying to dissipate 0.6V x 70A / 6 = 7W. Which is a lot for a little bit of silicon. That heat has to be drawn away through the packaging, leads and solder joints, then dissipated by good air flow. Lots of dust inside won't help and the diode overheats.

The life of the diode is likely to half for each ten degree C rise in temperature inside, if it is the silicon itself that fails.

They are mounted on reasonably substantial heatsinks made of ~5mm thick metal, probably steel which form one of the diode connections. There are two heatsinks, one bolted the case forming the common for all the negative diodes, and one isolated from the case which is the common for all the positive ones.

 

MP.

 

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Good work! I suppose one curative would be to have remote diodes away from the heat of the engine and windings. Take the 3 phase away to a remote set of diodes dangling in the canal. Then they definitely wouldn't overheat!

Of course I would point out that having an alternator regulator that monitors alternator temperature might be a better solution but everyone will just go "blah blah blah!"

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7 minutes ago, Tony1 said:

I wonder if a non-invasive approach might be more advisable- perhaps one of those 'air duster' spray cans, with a straw to direct the air jet inside the casing of the alternator?

Might such a practice be of any benefit in terms of preventing future failures? 

 

For an A127, certainly. The diodes and heatsinks are at the back, just behind the cooling slots, so an air duster would work quite well. Just don't shove anything conductive in there.

 

MP.

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1 minute ago, nicknorman said:

Good work! I suppose one curative would be to have remote diodes away from the heat of the engine and windings. Take the 3 phase away to a remote set of diodes dangling in the canal. Then they definitely wouldn't overheat!

Of course I would point out that having an alternator regulator that monitors alternator temperature might be a better solution but everyone will just go "blah blah blah!"

A smart regulator is a good idea, as long as it doesn't have to throttle things too much. Remote diodes are also a good idea, which I've considered (not dangling them in the canal!). Implementation is a bit of a pain, not least providing three high current terminals on the back of the alternator, and, in my installation, a fairly long run of three phase to get to a position suitable for diodes, heatsink and a fan.

 

MP.

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9 minutes ago, nicknorman said:

Good work! I suppose one curative would be to have remote diodes away from the heat of the engine and windings. Take the 3 phase away to a remote set of diodes dangling in the canal. Then they definitely wouldn't overheat!

Of course I would point out that having an alternator regulator that monitors alternator temperature might be a better solution but everyone will just go "blah blah blah!"

Having the regulator inside the alternator case does seem a mistake; using the brushes within the regulator pack (on some alternators) to feed an external regulator would not be too difficult.

 

Is there any chance that the regulator is an informal alternator temperature monitor?  And thus protects the more expensive windings?    

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Ive thought about remote diodes. The diodes make a significant fraction of alternator heat so fitting them externally will also make the rest of the alternator run cooler. There is a remote diode box available (for the petrolheads) but it has a fan. There is a lot of steel on a boat that could be used as a heatsink to give a fan-less system. Big "industrial electronics" style diodes are available in modules containing three diodes but they are costly, and many alternators use special zener type diodes so it might be a case of having to build your own module.

For me its currently more trouble than its worth but the balance might shift if Lithiums were involved.

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33 minutes ago, MoominPapa said:

A smart regulator is a good idea, as long as it doesn't have to throttle things too much. Remote diodes are also a good idea, which I've considered (not dangling them in the canal!). Implementation is a bit of a pain, not least providing three high current terminals on the back of the alternator, and, in my installation, a fairly long run of three phase to get to a position suitable for diodes, heatsink and a fan.

 

MP.

The advantage of actually monitoring alternator temperature is that you can extract the maximum power that can safely be provided taking into account such things as ambient temperature, fan rpm etc. As opposed to eg just putting a high resistance in the circuit to limit current.

Edited by nicknorman
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19 minutes ago, Tacet said:

Having the regulator inside the alternator case does seem a mistake; using the brushes within the regulator pack (on some alternators) to feed an external regulator would not be too difficult.

 

Is there any chance that the regulator is an informal alternator temperature monitor?  And thus protects the more expensive windings?    

Don't confuse the regulator and the diode pack; they are physically and electrically distinct. The regulator (and the dedicated diodes that power it) dissipate relatively little power, and the ambient temperature inside the altenator case is not crazy high for silicon transistors and modern resistors. The problem is the diodes that are carrying the full alternator output. As Jen points out the junctions of those are dissipating a lot of power and are likely the highest point temperatures anywhere in the alt by some margin.

 

MP.

 

 

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2 minutes ago, MoominPapa said:

Don't confuse the regulator and the diode pack; they are physically and electrically distinct. The regulator (and the dedicated diodes that power it) dissipate relatively little power, and the ambient temperature inside the altenator case is not crazy high for silicon transistors and modern resistors. The problem is the diodes that are carrying the full alternator output. As Jen points out the junctions of those are dissipating a lot of power and are likely the highest point temperatures anywhere in the alt by some margin.

 

MP.


I wonder if they are Schottky diodes these days (lower forward voltage drop)? Of course one could design an “active rectifier” using MOSFETS  which don’t have an intrinsic voltage drop. Hmm, a project for the back burner I think!

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39 minutes ago, nicknorman said:


I wonder if they are Schottky diodes these days (lower forward voltage drop)? Of course one could design an “active rectifier” using MOSFETS  which don’t have an intrinsic voltage drop. Hmm, a project for the back burner I think!

In an A127 of indeterminate age, I doubt it. Modern designs, very likely.

 

MP.

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A127s are cheap inexpensive alternators and it's a wonder that they suffer boat use! I moved to lorry type units and never looked back...

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5 hours ago, OldGoat said:

A127s are cheap inexpensive alternators and it's a wonder that they suffer boat use! I moved to lorry type units and never looked back...

I’ve run A127’s for about the last 20 years…mines the 70A model…it can get a caning but the only issue I’ve had is noisy bearings after a bit. I like them as they are cheap and readily available from a high st motor factor or rewind place…ideal when you are out boating! 
 

A lot of issues is because they can output high current without spinning fast enough to cool properly especially on boxed in engines….mine has a belt round a JP flywheel so is doing about 2500rpm even with the engine on tickover. 

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14 minutes ago, AndrewIC said:

Is the diode pack for an insulated return A127 different from the conventional automotive type?

 

Don't take this as gospel but I think the difference is a longer mounting boat in the heat sink, an oversized hole in the case end and an insulating bush and washer in the hole/on the bolt. The bolt forms the D- connection.

 

My personal view is that on an inland boat the advantage of an insulated  return system is questionable.

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18 minutes ago, Tony Brooks said:

 

 

 

My personal view is that on an inland boat the advantage of an insulated  return system is questionable.

That is my view as well Tony.

In fact, are there any automotive derived engines used in inland craft with insulated return starter motors, oil pressure and temperature senders etc?

Without such it is a waste of time having insulated return alternators.

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2 hours ago, Tony Brooks said:

 

Don't take this as gospel but I think the difference is a longer mounting boat in the heat sink, an oversized hole in the case end and an insulating bush and washer in the hole/on the bolt. The bolt forms the D- connection.

 

My personal view is that on an inland boat the advantage of an insulated  return system is questionable.


That’s what I thought, so buying a “spare” diode pack would have to be the right type (or alter the wiring to provide a solid negative connection.

 

1 hour ago, Tracy D'arth said:

That is my view as well Tony.

In fact, are there any automotive derived engines used in inland craft with insulated return starter motors, oil pressure and temperature senders etc?

Without such it is a waste of time having insulated return alternators.


I don’t believe my starter is insulated return, although it’s a while since I stuck my head down there. But the glow plugs, senders, etc don’t have any other negative connection, so I doubt that it is as that’s the only negative to the engine.

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7 minutes ago, AndrewIC said:

That’s what I thought, so buying a “spare” diode pack would have to be the right type (or alter the wiring to provide a solid negative connection.

 

I think the fixing screw is a bit like a coach bolt but instead of a square under the head there is a spline affair. During manufacture this is forced into a hole in the diode plate so it is firm and secure. If I am correct then if you had the wrong one you cold punch the screws out of the plate and swap them over. If i did it I would attack the plate around  the hole with a ball peon hammer to close it up a bit before driving the correct screw into place. However I think an insulated return plate will fit an earth return alternator.

 

Anyway the last thing I would be worrying about is the diodes. The brushes, slip rings, and bearings are in my opinion far more likely to cause problems unless people insist in charging very flat batteries on idle or at low speed.

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On 07/02/2022 at 16:12, MoominPapa said:

I'd already bought a new diode module on Ebay for £12 and fitting that is pretty trivial, as long as you have a BIG soldering iron. The sort your Grandad had, not a 15W tidler suitable for doing electronics.

As demonstrated by this chap (caution: extensive screwdriver abuse!) : https://youtu.be/lXskIlxbt-w

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12 hours ago, OldGoat said:

A127s are cheap inexpensive alternators and it's a wonder that they suffer boat use! I moved to lorry type units and never looked back...

Now that's an interesting thought, have you any part numbers?  Aren't moat lorries 24 volt?

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10 hours ago, Quattrodave said:

Now that's an interesting thought, have you any part numbers?  Aren't moat lorries 24 volt?

Yes, (but you can get 12v vesions for adding to Land Rovers etc).

However - in my uncompromising view and experience, a 12v system on todays NBs is completely wrong. 'Twas fine in the days when folks were happy to wear hair shirts. Today people want all the convenience of living in bricks and mortar and IME 12v just doesn't cut it. I take a 12v tap on the battery bank for those items that only run on 12V and lighting / ebersplutter hearing/ inverters are all 24v...

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15 minutes ago, OldGoat said:

Yes, (but you can get 12v vesions for adding to Land Rovers etc).

However - in my uncompromising view and experience, a 12v system on todays NBs is completely wrong. 'Twas fine in the days when folks were happy to wear hair shirts. Today people want all the convenience of living in bricks and mortar and IME 12v just doesn't cut it. I take a 12v tap on the battery bank for those items that only run on 12V and lighting / ebersplutter hearing/ inverters are all 24v...

 

It's not just hair shirts -- 24V wiring is much thinner (a quarter of the CSA for the same power levels) and cheaper than 12V, and pretty much everything needed on a boat is easily available in 24V. Also 24V inverters -- especially bigger ones -- are considerably cheaper than 12V ones of the same power rating. So for building from scratch, a 24V system is likely to be cheaper, as well as being capable of higher power levels if that's what you need. But it's probably not worth ripping out a working 12V system just to upgrade...

 

To go even further, the series hybrid boat I'm having built has the main battery bank at 48V, with a 48V-24V DC-DC converter to power all the DC stuff on the boat, and a small 24V-12V trickle charger to keep the generator starter battery charged. Taking lower-voltage (12V or 24V) taps off the battery bank raises the problem of imbalance between cells, not good especially for bigger batteries... 😞

Edited by IanD
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