Battery Switch: Field Disconnect Protection - SailNet Community

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  #1  
Old 08-06-2008
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Battery Switch: Field Disconnect Protection

Surprisingly: Nobody over on the Moyer Marine forums appeared to know the answers to these, so I guess I'll try here

After getting under way under sail last Saturday, I switched from "BOTH" to "1". The VHF radio beeped and the GPS powered off. So either the switch on our 32-year-old boat is not a make-before-break switch or it's just plain gotten old. Either way: Time for a new switch.

Assuming our A4 has the original Motorola 35A alternator on it: Can I use the field disconnect protection "OFF/1/BOTH/2" switches that are so-equipped offer? I believe the original had an external regulator, and, if I understand correctly, that's what's needed to use this feature.

Secondly: Would a switch rated at 250 amps continuous, 360 amps momentary be sufficient for an A-4? (Moyer recommends a 200A fuse for the starter circuit, if you're so-inclined, so I'm guessing the answer to this one is "Yes.")

Heck while I'm at it: Wrenched on engines a lot in the past, but never had cause to do more than occasionally just out and replace alternators whole-sale. So how do I tell the difference between the original 35A Motorola and an upgrade? Between an alternator with an external regulator (as I understand the original would be?) and one with an internal regulator?

Thanks,
Jim
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Are you sure that external regulation and field disconnect are the same thing? My understanding is that you want to open the field, thereby disabling alternator excitation
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Quote:
Originally Posted by GBurton View Post
Are you sure that external regulation and field disconnect are the same thing?
It's not that they're the same thing, it's that with an internal regulator it's not practical to get to the regulator output.

Quote:
Originally Posted by GBurton View Post
My understanding is that you want to open the field, thereby disabling alternator excitation
Yes.

Jim
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The regulator is what varies the field excitation, so I would say that if you open that circuit with the battery auxilliary field contact you should be fine.
Ie open circuit the field/regulator circuit

This is so that you do not damage the alternator when there is no load attached to it.

Is this what you are trying to achieve?

Last edited by GBurton; 08-06-2008 at 03:46 PM. Reason: Clarify response
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Semijim,

1. Most battery switches these days are make-before-break. You definitely want one of these if you're gonna switch between batteries while the engine is running. If you break the connection, you could blow the diodes in your alternator very quickly.

2. Yes, 200A should be plenty to start an Atomic 4.

3. You can trace the field wire from the alternator and see if it runs to an external regulator. If not (i.e., if it runs directly to the battery or a fuse or the key switch) then the alternator has an internal regulator. You should be able to tell what capacity the alternator is by looking for a model number. Also, if you have an ammeter aboard you can see what it's putting out, though this might not be a good indication of its capacity due to wiring problems or state of battery charge or engine speed.

Bill

Last edited by btrayfors; 08-06-2008 at 08:43 PM.
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I don't know the details of you specific alternator, but usually even alternators with an internal regulator have a field wire coming from the key switch or engine panel. With an external regulator you plug the field wire from the key switch or panel into the regulator, with an internal regulator it goes to the alternator. So it seems to me you could use a field interrupt type battery switch in either case.

The field wire from the panel just turns the alternator field on and off either 0 or 12 (ish) volts. I think this is the section of wire you want going through the battery switch. With an external regulator these is also a field wire with voltage that varies with demand, but this wire should still go directly from the regulator to the alternator.
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Quote:
Originally Posted by GBurton View Post
The regulator is what varies the field excitation, so I would say that if you open that circuit with the battery auxilliary field contact you should be fine.
Ie open circuit the field/regulator circuit

This is so that you do not damage the alternator when there is no load attached to it.

Is this what you are trying to achieve?
In order: Yes, I know. Yes, I know. Yes, I know. Yes, that's what I'm trying to achieve. But with an internal regulator that field exitation wiring is all... well, internal. With one exception--see the URL I'm posting, below.


Quote:
Originally Posted by btrayfors View Post
Semijim,

1. Most battery switches these days are make-before-break. You definitely want one of these if you're gonna switch between batteries while the engine is running. If you break the connection, you could blow the diodes in your alternator very quickly.
Yup. Thus the impetus to do this.

Quote:
Originally Posted by btrayfors View Post
2. Yes, 200A should be plenty to start an Atomic 4.
Ok. Thanks!

Quote:
Originally Posted by btrayfors View Post
3. You can trace the output wire from the alternator and see if it runs to an external regulator. If not (i.e., if it runs directly to the battery or a fuse) then the alternator has an internal regulator.
Ah, yes, if it's an external regulator, the main rectifier output will probably stop on its way to the batteries, as it were, for the voltage sense, right? Good catch. Hadn't thought of that.

Quote:
Originally Posted by SteveInMD View Post
I don't know the details of you specific alternator, but usually even alternators with an internal regulator have a field wire coming from the key switch or engine panel.
Wrong circuit, Steve. That circuit is only used to "prime the pump," as it were. It supplies field current until the alternator gets going, then the alternator supplies its own, whether an internal or external regulator.

Here's a good explanation of How Alter-nators Work. (Sorry for munging the word, but Sailnet's "word-hijacking" stuff was screwing-up the URL.)

The output of the diode trio is what one needs to interrupt with the field disconnect protection. On an internal regulator, that's not accessible, practically speaking.

Thanks for the feedback, guys

Jim

Last edited by SEMIJim; 08-06-2008 at 08:57 PM.
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Take the alternator apart and break the wiring out
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If you end up replacing it, may I suggest a 90 amp Leece-Neville (Prestolite) marine alternator? About a year ago, I got one for about $170....much cheaper than a Balmar. This is a so called small case alternator, so it is the same physical size as your old Motorola. That will make it an easier conversion than if it were a large case. Halekai (Maine Sail) has a nice web site description of the job, though on a diesel, not an A-4. It will take a little more horsepower to drive that higher power alt, but it should be OK. My 21 HP diesel turns it without a problem.

Here is a copy & paste from another site;
I recently upgraded my M25 to a 90 amp marine alternator made by Prestolite, Leece-Neville division. Prestolite is an old, respected name in automotive applications and their Leece-Neville division makes alternators for emergency vehicles, farm & construction equipment and fire trucks, as well as this marine alt. The alt cost me $169 as compared with over $500 for a Balmar plus $100 to $200 for an external regulator. Quite a bit of $avings, IMHO. Please see these URLs for important info on the Prestolite alt;
http://www.prestolite.com/pgs_produc...esh=1185927037
http://www.prestolite.com/literature...128_marine.pdf
http://www.prestolite.com/literature...3_AG_Const.pdf

To buy one in your state;
http://www.prestolite.com/pgs_buy/di...ovince_code=CA
When you buy the alt, donít forget to get a new pulley. The old alt shaft is a smaller diameter, so is not compatible. The new shaft is 17 mm, which is an unusual size. I got a Delco Remy, part # 10495191, for about $5.

Maine Sailing's Photo Galleries at pbase.com
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Thanks for the info, Keith. Tho I've no intention of replacing our boats alternator at this time, I'll make a note of it.

Jim
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