alternator fires

[Rotten Ricky]

Originally Posted by team karst

I use remote sensing. It removes the diode V drop from affecting the charge voltage

Karst, thanks for your response. Please elaborate on remote sensing.

My system was a simple one where the sensing wire went straight from alt to battery terminal. Does your system involve a smart regulator that allows you to set it to take into account the additional voltage drop?

Is there any particular reliable smart regulator of good value that you recommend, or is it just a case of the more money you spend, the better value the unit?

Many thanks,

RR.

[team karst]

Originally Posted by Rotten Ricky

Karst, thanks for your response. Please elaborate on remote sensing.

My system was a simple one where the sensing wire went straight from alt to battery terminal. Does your system involve a smart regulator that allows you to set it to take into account the additional voltage drop?

Is there any particular reliable smart regulator of good value that you recommend, or is it just a case of the more money you spend, the better value the unit?

Many thanks,

RR.

My alternator has terminals brought out the back for various functions.
But, its not "smart", it doesn't have remote battery termperature monitoring, or dual phase charging. But, it does have a DF terminal, that could either go to the B+ (big red wire) terminal for local V sense, or, you can run a small gauge wire to the BAttery + terminal, or at least downstream of an isolator and the wire V drop. That's what I did. Other terminals include a Tach drive (W marked), and a D+ output. That one typically drives the warning light.
So, back to the isolator. Mine has inputs from each engine, and outputs for both starting batteries, and the house battery. So, actually, 2x charging systems that are quite independant from each other. In fact, our last cruise happened without the stb alt putting out any power. This is all in addition to 2x solar chargers.

THis is all not possible with an automotive type "one wire" alternator".

[Jammer]

Originally Posted by Rotten Ricky

While the concept of battery separation has merit, be aware that the use of battery isolators can screw up the sensing of voltage by the regulator, due to the 0.7 volt drop across the isolator diodes. I had trouble with charging and in the end I removed the isolator, and charging was fine after that.

Good, modern isolators use MOSFETs instead of diodes, and there is no voltage drop.

Could someone advise what size and type of fuse should be used? I assume it is on the main output cable? My alt is the legendary Delco Remy CS-144 rated at 100 amps continuous, though typically only needs to output half that at most.

Use the smallest fuse that won't blow when there isn't a fault. If the fuse blows for no reason the diodes in the alternator will probably fail, an expensive repair. I would use 200 amps. You can still have a fire without blowing the fuse, which is one of the reasons it is uncommon to see fuses on the alternator output.

[Rotten Ricky]

Originally Posted by team karst

My alternator has terminals brought out the back for various functions.
But, its not "smart", it doesn't have remote battery temperature monitoring, or dual phase charging. But, it does have a DF terminal, that could either go to the B+ (big red wire) terminal for local V sense, or, you can run a small gauge wire to the BAttery + terminal, or at least downstream of an isolator and the wire V drop. That's what I did. Other terminals include a Tach drive (W marked), and a D+ output. That one typically drives the warning light.
So, back to the isolator. Mine has inputs from each engine, and outputs for both starting batteries, and the house battery. So, actually, 2x charging systems that are quite independent from each other. In fact, our last cruise happened without the stb alt putting out any power. This is all in addition to 2x solar chargers.
THis is all not possible with an automotive type "one wire" alternator".

My boat should be simpler than yours as I have one engine and alt only. Yes, the one wire systems are quite inadequate for charging efficiently or promptly.

I understood that connecting the alt DF wire to the alt output stud was not a good idea as it would have the same effect as connecting to the battery positives as it is the same wire, and not even allow for any voltage drop in the main charging cable, not that there should be much.

As has been kindly pointed out, mine might have been the older form of isolator that used diodes and not a more modern one that uses MOSFETS that have virtually no voltage drop. My batteries were never charged up enough.

I am a fan of connecting the D warning wire to an audio device as well as visual indicator, as not all of us can watch the warning light at all times and a loud buzzer will get our attention immediately. Before you notice the light, damage could have been done.

On my previous boat I was the "smart regulator" (though my wife might not always agree!) as I made up a simple set of resistors so that I manually adjusted the field current to obtain maximum charging current - as the voltage rose I just stepped the field current down. It worked very well. In case I was distracted, I had intended to install an inexpensive audio 14V over-voltage alarm but never got around to it, but I never cooked any batteries anyway. I might do the sale with my present installation. An inexpensive form of smart regulator.

Thanks again, RR.

[Rotten Ricky]

Originally Posted by Jammer

Good, modern isolators use MOSFETs instead of diodes, and there is no voltage drop.

Use the smallest fuse that won't blow when there isn't a fault. If the fuse blows for no reason the diodes in the alternator will probably fail, an expensive repair. I would use 200 amps. You can still have a fire without blowing the fuse, which is one of the reasons it is uncommon to see fuses on the alternator output.

Thanks for that info, Jammer. I nearly had an electrical fire in my boat recently so I am keen to prevent another incident!

Like you, I seldom have heard of fuses in the main charging cable. However, if diodes could be destroyed by a charging current fuse blowing, if one placed a high-value (ohms), high-capacity (watts) resistor in parallel with the fuses, would this allow a small current to continue flowing such that the diodes would not blow from a sudden open circuit?

I think there is a device you can buy to save diodes but I wondered if the use of a resistor would be a cheap but effective alternative. I would think that there would be a small amount of heat continuously emitted from the resistor when in use, but I think that would be a small drain on the engine efficiency.

Incidentally, as with the initial purchase price of the rugged Delco-Remy CS-144 alternator (built for ambulance and fire truck use), all the parts, including replacement diodes sets, are very inexpensive. I read that sometimes people will install diode sets that are much higher capacity (like 300 amps) to help safeguard against failure from a possible open circuit.

Cheers, RR.

[team karst]

Originally Posted by Jammer

…You can still have a fire without blowing the fuse, which is one of the reasons it is uncommon to see fuses on the alternator output.

Cars consistently fuse alternators, for many decades now. In the old days, fusible links hidden in silicone insulation. Then, large fuse in the fusebox. I replaced my Traverse alt last w/e and a 200 A fuse appeared in the wire harness. Alt rating is 170 A, btw. The difference with boats is that car el systems are designed by engineers.

[CharlieJ]

Fuse is required at conductor attachment to battery B+ or its bus.

[Jammer]

Originally Posted by Rotten Ricky

Thanks for that info, Jammer. I nearly had an electrical fire in my boat recently so I am keen to prevent another incident!

Like you, I seldom have heard of fuses in the main charging cable. However, if diodes could be destroyed by a charging current fuse blowing, if one placed a high-value (ohms), high-capacity (watts) resistor in parallel with the fuses, would this allow a small current to continue flowing such that the diodes would not blow from a sudden open circuit?

Alternator diodes fail on an open circuit because the stator windings are inductive. I don't know your background but if you have studied calculus and AC circuit theory you will know that the voltage across an inductor is proportional to the derivative of the current through it. Abruptly reducing the current from, say, 100 amps to 0 amps, produces a large voltage spike, and the diodes don't stand a chance. A parallel resistor that can dissipate even as much as an amp, would reduce the voltage by only a small amount, 1% or so.

I think there is a device you can buy to save diodes but I wondered if the use of a resistor would be a cheap but effective alternative. I would think that there would be a small amount of heat continuously emitted from the resistor when in use, but I think that would be a small drain on the engine efficiency.

The devices you refer to are MOVs. There are two facts to consider. The first is that MOVs can fail short and burn up, and so if you put them on the alternator side of your fuse, there's a hazard there. The second is that I believe most of the MOVs being sold to protect alternators are probably undersized and will, at best, absorb one or a small handful of "hits" before they fail.


If I were going to install a fuse I would choose a large, high-quality one that isn't going to blow absent a serious fault. In reality the most important thing it's going to prevent is a fire from a short in the B+ lead to the alternator rather than a fire from a failure in the alternator itself.

Incidentally, as with the initial purchase price of the rugged Delco-Remy CS-144 alternator (built for ambulance and fire truck use), all the parts, including replacement diodes sets, are very inexpensive. I read that sometimes people will install diode sets that are much higher capacity (like 300 amps) to help safeguard against failure from a possible open circuit.

The CS-144 and AD-244 are high quality machines with a long history.

[Rotten Ricky]

Originally Posted by CharlieJ

Fuse is required at conductor attachment to battery B+ or its bus.

Charlie, any recommendations about ampacity, fast or slow blow, holder and type of fuse, such as for 100 amp alt? Would a breaker be better?

I know - the smaller the better.

Thanks, RR.

[team karst]

Not a hard and fast law, but semis usually fail shorted. BUT, if that causes a massive current, then wire bonds fuse them open.

[goboatingnow]

Originally Posted by Rotten Ricky

While the concept of battery separation has merit, be aware that the use of battery isolators can screw up the sensing of voltage by the regulator, due to the 0.7 volt drop across the isolator diodes. I had trouble with charging and in the end I removed the isolator, and charging was fine after that.

Could someone advise what size and type of fuse should be used? I assume it is on the main output cable? My alt is the legendary Delco Remy CS-144 rated at 100 amps continuous, though typically only needs to output half that at most.


Cheers, RR.

Both low drop and no-drop splitters are available also

[CharlieJ]

@RottenRicky #24:

Charlie, any recommendations about ampacity, fast or slow blow, holder and type of fuse, such as for 100 amp alt? Would a breaker be better?

Assuming that the conductor has been sized properly to handle the full output from the alternator, the fuse should be installed at the battery connection and sized to the maximum ampacity of the conductor. For the 100A alternator that you mentioned, barring voltage drop considerations, AWG 2 Boat Cable (not SAE) has an ampacity of 125A inside the engine room. For this example I would specify a 125A fuse.

There is no need to fuse at the alternator end as modern alternators are considered to be "self limiting"; i.e., they can only output what they were designed to produce whereas a chafe short in the conductor between the alternator and the battery B+, or a short internal to the alternator puts the full short circuit capacity of the battery onto the B+ conductor.

Slow or fast acting doesn't really apply in this application as the potential fault that is being protected against is not a quick pulse but rather a fast ramp up to max and then a decay as the battery discharges into the short circuit.

Be mindful of the requirement for ignition protected fuses or fuse holders in a gasoline engine compartment.

A breaker could be used but would be more difficult to spec out to ensure the proper trip characteristics. Breakers also age and change those characteristics so they are not my choice.

I use the following fuses in order of preference: MRBF (only one crimp required); MEGA; ANL.

[Nicholson58]

Add a new high capacity, high charge rate battery bank and you will overload any alternator not designed for continuous full load. Use an external charge controller and set the max output to 80% or less. Ask me how I know.

[Dieseldude]

Originally Posted by Journeyman

Does anyone have any anecdotes about alternators literally burning up?

I have heard of one recently and was unaware of the potential danger, particularly with high output alternators.

Also, I have thought of alternator faults as most likely self limiting, but a short to ground in a running alternator I guess is basically an arc welder capable of starting a hard to put out Metal Fire:

https://www.wtamu.edu/~cbaird/sq/mob...t-metals-burn/

A heat or flame sensing alarm in the engine compartment would give early indication of a problem. A smoke alarm would be good to but might cause false alarms during cold starts before thermal expansion seats exhaust system joints.

Periodic inspection of alternator bearings, perhaps annually, would find bad bearings before they can overheat and cause fire. To inspect, slack the belt. Rotate alternator by hand. If it feels gritty, does not rotate freely, or has noticeable side play, bearings are worn, and need replacement. If spun rapidly, the alternator should continue turning a few revolutions after letting go. If it stops immediately, bearings are suspect. At same time, all belt driven accessories can be given the same inspection.

If the system has a a belt tensioner, this should be checked at same time. They can develop side play and put side strain on accessory bearings, damaging them. If side play is extreme, a flat serpentine type belt will be seen to run over the edge of pulleys. This is a good visual indication that can be seen easily without effort. With v belts, visual indication will not be so apparent. You might see uneven wear on the belt or fine rubber dust over surroundings. I had a water pump fail on a Cummins engine. After replacing the pump, I noticed the belt riding over pulley edges. Replacing the tensioner solved the problem. I replaced the alternator as a precaution. It was quite old, so likely due for trouble.

A fuse in the alternator output of same amp rating of alternator would help avoid fire. No guarantee, as overheating could possibly develop below max current rating.

Commercial generators, motors, and transformers are tested annually for insulation weakness. This is commonly called "megger testing". It requires a special resistance meter with internal high voltage source to test for high resistance that a regular ohm meter will not reliably measure. The test is looking for any high resistance partial shorts between windings, and between each winding and case ground. This would require disconnection of the diodes and regulator to avoid destroying them with high voltage. After testing, windings must be shorted momentarily to bleed off any stored charge due to capacitance effect. The test instrument is not cheap, but it can be used for testing other devices.

The procedure for insulation testing must be known before attempting. The applied voltage of the insulation tester is selectable. Usually they have 50, 100, 150, 250, 500, and 1000, volt settings. Care must be taken not to apply excessive voltage that would damage insulation under test. This test is not intended to over stress insulation, but to apply enough voltage to measure high resistance that a regular ohm meter cannot reliably measure. Rule of thumb for applied voltage is a setting that is just above the normal operating voltage. For 12 or 24 volt systems, 50 volts applied should be OK. As stated, all alternator electronic components must be disconnected (diodes, regulator, and any other device). The brushes must be lifted away from the commutator. Alternator output to the electrical system and exciter coil must be disconnected to avoid high voltage destruction throughout system. The allowable insulation resistance of the alternator windings must be known. If test reading is below this figure, the alternator should be repaired or replaced. Of course meter probes and device terminals should not be touched while testing to avoid high voltage shock.

Note that high atmospheric humidity can cause false results. You might measure resistance of humid air rather than your alternator or other device. May need to choose a time and place of low humidity.

See https://us.megger.com/blog/december-...megger-a-motor
for more info. The page contains other links for more details.

[Dave9111]

quote:

Commercial generators, motors, and transformers are tested annually for insulation weakness. This is commonly called "megger testing". It requires a special resistance meter with internal high voltage source to test for high resistance that a regular ohm meter will not reliably measure. The test is looking for any high resistance partial shorts between windings, and between each winding and case ground. This would require disconnection of the diodes and regulator to avoid destroying them with high voltage. After testing, windings must be shorted momentarily to bleed off any stored charge due to capacitance effect. The test instrument is not cheap, but it can be used for testing other devices.

end quote:


Where does this happen annually ?? Certainly not in the USA!!