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Restoration & Repair Tips Board

Re: Allis G cutout relay

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John T

12-26-2012 09:22:03

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Yo Bob, its hard to describe over the net how much is a tad versus say a hair or just a bit isnt it lol lol

I try to tell them to as non abrasively as possible clean/buff/polish the contacts but NOT file and not remove too much of that shiny silvery coating cuz once thats gone they stick fer sure.

Correct me if Im wrong (this is as much mechanical as electrical right) but magnetic force pulls the relay in and its removal would allow it to release HOWEVER IN ADDITION its the short time period of high reverse current that causes the reverse magnetic field which assists in a rapid relay opening????? i.e. it would open in theory buttttttttt its the sudden high reverse discharge current and the reverse magnetic field that really really makes that relay open.

Whatcha think Robert

John T

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Bob M

12-27-2012 09:52:10

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 Re: Allis G cutout relay in reply to John T, 12-26-2012 09:22:03  
A belated "Merry Christmas" to yourself and the lovely Mrs John T!


The cutout on my old Minnie Mo U - which I think is the same as used on the Allis G - has 2 windings. It works like this:

The 1st winding is many turns of relatively fine wire. It is connected between the generator armature terminal and ground. This winding senses generator output voltage. It "pulls in" the relay when gen output reaches 8 - 9 volts, thus connecting the generator to the battery so the battery is now being charged.

The other winding is several turns of heavy wire wound on the same core as the 1st. It is connected in series between the gen output and the cutout "BAT" terminal thru the cutout contacts. When the cutout contacts are closed, full generator current passes through this winding.

Now when the system is charging, the magnetic field of the 2nd winding adds to the field of the 1st winding, helping keep the cutout contacts closed tightly. However when generator output stops (ie. at slow idle, or when the engine is shut down) current thru this winding reverses - ie. battery is now discharging back thru this coil to the generator. The 2nd winding's magnetic field is now OPPOSING the 1st winding's field. The two magnetic field cancel each other and the relay spring causes the cutout contacts open.


Now as a practical matter, a mechanical cutout relay can be replaced with a solid state cutout (no moving parts) for about $35. No more fussing with erratic relays! Agri-Services has them - stock #GDR6030. Or try at any good starter/alternator shop.

...Bob M

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John T

12-27-2012 10:49:39

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 Re: Allis G cutout relay in reply to Bob M, 12-27-2012 09:52:10  
Youre handy to have around Bob, youre a pretty good hand at mechanical/electrical stuff.

Makes sense to me, any cutout relays Ive seen indeed have BOTH a shunt and a series winding. The shunt is higher resistance and small wire while the series is the big fat high current coil.

What you say about how the reverse magnetic field helps unlatch the relay is how I considered it to work also.

John T

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12-26-2012 12:14:39

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 Re: Allis G cutout relay in reply to John T, 12-26-2012 09:22:03  
Quoting Removed, click Modern View to seeJohn, my take is that a high magnetic field in either direction will pull the armature closed. Typically the shunt field is holding it closed and as engine slows/gen output falls & current finally reverses, at some point the shunt & series(current winding) fields become equal & opposite & cancel each other & at that point the spring pulls armature open. Like that?

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John T

12-26-2012 13:17:30

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 Re: Allis G cutout relay in reply to JMOR, 12-26-2012 12:14:39  
Darn if I know fer sure JMOR lol thats why I asked Bobs opinion. I was a thinkin the reverse high discharge current helped facilitate the reverse magnetic field that sorta helped un latch the relay??? Buried deep in the bowells of my computer I have a lot of original Delco data somewhere, this may force me to go to work and study up on this grrrrrr

John T

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