Alternator -- New or Rebuild?

[Cadence]

Originally Posted by jimbunyard

Supposedly the main thing is that they're ignition protected so they don't cause your boat to explode if there is gas leak if you have a gas engine or a propane/butane/LNG leak if you use those. Starters too.

Thanks. I wondered. No sparks on the wipers I guess not brushes since it ac till the diodes. I may not buy that but I have been wrong before.

[model 10]

Originally Posted by Cadence

Thanks. I wondered. No sparks on the wipers I guess not brushes since it ac till the diodes. I may not buy that but I have been wrong before.

The only difference with my Delco that is spark protected, is a screen over the rear openings. Inside it's the same.

[hellosailor]

" I wonder if there are bootleg Balmar alternators out there?"
Why not? Everything from spark plugs to aviation bolts is counterfeited. I go to Shenzhen, order a thousand alternators "make blue! Must make blue!" and some shiny boxes to go with them, and voila, I've got a thousand "Balmars" I can peddle at a $500 profit each. Sell 'em out of my trunk, sell 'em on eBay (and close the account after six months before trouble reports come back) and I'm in business.


The screening that provides ignition protection is supposed to work by literally absorbing the HEAT of a flame front hitting it, quenching the flame. Of course it also obstructs airflow quite a bit, so I'd expect that invisibly, the other components have been selected to operate under higher heat, or with some better cooling.


You go to Outback, you go to Ottomanelli (prime meats), you grill one steak from each and pour A1 sauce over it. Yeah, they look identical. They sure won't be.

[GrahamHO]

Originally Posted by Guy

The only difference with my Delco that is spark protected, is a screen over the rear openings. Inside it's the same.

Is that screen a wire mesh as in a Faraday Cage?

[St. Elsewhere]

Originally Posted by GrahamHO

Is that screen a wire mesh as in a Faraday Cage?

A Faraday Cage protects things inside it because the charge is conducted over the external surface of the cage. Here I think that the process is to provide an arc tight environment to contain an ignition source to inside the case. What is unclear is, if explosive vapors are present they could pass through a screen to where an arc may be present inside the case.

[model 10]

Originally Posted by St. Elsewhere

A Faraday Cage protects things inside it because the charge is conducted over the external surface of the cage. Here I think that the process is to provide an arc tight environment to contain an ignition source to inside the case. What is unclear is, if explosive vapors are present they could pass through a screen to where an arc may be present inside the case.

I thought that even if that happened, the flame would not get past the screen. Some sort of diffuser.

[GrahamHO]

Originally Posted by St. Elsewhere

A Faraday Cage protects things inside it because the charge is conducted over the external surface of the cage. Here I think that the process is to provide an arc tight environment to contain an ignition source to inside the case. What is unclear is, if explosive vapors are present they could pass through a screen to where an arc may be present inside the case.

I remember school science experiments where a wire mesh was held over a Bunsen burner and the flame would not go up past. Likewise if the gas was lit on top of the mesh the flame would not travel down to the Bunsen.
That was to demonstrate how a Davis safety lantern for miners worked.

The Faraday cage principal is similar but different as it involves electricity, very roughly like the way a bird can sit on a 100,000 volt line.

I wouldn't think there is an absolute guarantee the vapours wouldn't ignite. It's just a bit safer. Somebody was killed and others badly injured about 4 years ago in NZ when they were inspecting a water supply tunnel. One of them took a photo with a point and shoot camera and that set off an unknown pocket of gas with a violent explosion. It doesn't take much sometimes.

[hellosailor]

Graham-
The mesh DOES NOT prevent vapors from igniting inside the alternator, which is where the ignition source (the sparks) are. As I said before, the mesh absorbs so much heat form the flame front, that it quenches the fire before it can pass through to ignite additional vapors surrounding the alternator.


The mesh does make the metal body of the alternator, which already functions as a Faraday Cage, more functional by closing off some of the holes in the cage. A Faraday Cage can be made of mesh, or bars, and the size of the holes in only relevant in relation to the wavelength of any electrical charge hitting it, i.e. whether the charge is something like radio waves, which penetrate differently depending on frequency, or something like a mass charge from a lightning strike. A "perfect" Faraday Cage will be made of metal sheeting, with welded, soldered, or lapped seams, and a layer of mumetal or other material to absorb ay magnetic eddy currents that would penetrate the skin and cause additional damage.


The publicly accessible military papers discussing how to secure critical buildings against the EMP from nuclear blasts go into a great deal of detail as to just how cpmplex it can be unless you have been secured in a solid tin can. (Like a submarine undersea.)

[a64pilot]

Originally Posted by DeepFrz

However one heavy duty alternator dialed back to about 20% under maximum current output should last a long time.

They are kind of like batteries. They often don't die a natural death they are murdered by running them to hot for to long.

Some regulators also monitor alternator temp and dial the power back to keep from burning up an alternator, but also as alternators are fan cooled, driving them at higher RPM will also result in a cooler running alternator, plus I believe an alternator making the same power but running at a lower RPM will generate more heat, so it's a double whammy.
In other words, don't run just above idle to charge your batteries, and or change your pulleys to achieve a higher drive ratio.

[DeepFrz]

IMO all marine alternators should have thermal sensors that modify the external regulator output. But they are sort of a last resort. An alternator should not run at that high of a temperature that they will be damaged if not shut down. An alternator running at low speed wont have the fan power but it also wont be generating the high power either so it may be a wash. That is just a thought without any personal knowledge. I think that a high power alternator is served well by having an external fan helping to keep it cool. Steve Dashew even went to the trouble of having special alternators built with the rectifiers and heat sinks external to the alternator frames.

[model 10]

Sometimes the fans don't even blow the right direction.