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Tractor Talk Discussion Forum

Cutout on Case Vac

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Don E. Foster

10-04-2006 20:15:12




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Guess my last posting didn't fly. Several days ago I posted that I had a rebuilt generator put on my VAC and put on a new cutout that I had purchased some time ago from Valu-Bult. It produced well as shown by the ammeter. But when I shut it off the cutout stayed closed and drained the battery. I polished the points and polarized it again. Same thing.

What causes the points to open when not in use? It seems to be spring loaded.

CarQuest sure is proud of their cutouts$$$ They want $90+. Valu-Blt has them for $35....

Any ideas, Thanks

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noncompos

10-04-2006 22:15:15




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 Re: Cutout on Case Vac in reply to Don E. Foster, 10-04-2006 20:15:12  
You mentioned polarising and cleaning/polishing the points; did you try to adjust the air gap?
The cutouts I recall had a spring hooked over a tab on the back of the piece that held the top point, the piece that ran over the top of the little coil and closed downward, meeting the fixed bottom point to close the points. The spring tension could be adjusted by bending the tab the bottom end of the spring is hooked to,to give more tension to pull the points open, etc.
The fixed (bottom) point was usually mounted on a long upside-down U-shaped little "stand", the two legs of the upside-down U generally bent inward about halfway up, like being a little knock-kneed. The bottom point could be raised by straightening the legs a bit, or lowered by bending the legs closer together. With us it was a hit or miss situation; if the cutout wasn"t working, fiddling with the point gap and spring pressure couldn"t make it any worse, and sometimes got it working again.

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Bob

10-04-2006 21:39:29




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 Re: Cutout on Case Vac in reply to Don E. Foster, 10-04-2006 20:15:12  
There are two windings in the cutout, a series winding and a shunt winding.

The magnetic field from the shunt winding causes the cutout to "pull in" when the output of the generator reaches a certain voltage. Current through the series winding helps to hold the cutout "in", once it's pulled in.

When the engine is shut down, and the generator stops charging, the voltage holding the cutout "in" drops, and the spring is trying to pull the points open.

Also, the current through the cutout's series winding reverses 'cause the current is no longer flowing from the generator to the battery, but is now flowing back into the generator, causing the magnetic field it produces to reverse, cancelling out the magnetic field produced by the shunt winding. This gives the cutout a split second to "drop out".

So, the action of a cutout to "pull in" or "drop out" is a balancing act of the spring tension, the "air gap" between the points lever and the core, etc.

The original cutout would have had an adjustable air gap, and adjustable spring tension, and the shop manual would give the "specs" for these, and give a "spec" for the "cut in" and "drop out" voltages.

With today's imported cheapy units, and the lack of a local "mom and pop" electrical shop to set them up, about the only thing to do is hope you get a GOOD new unit!

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Bob

10-04-2006 22:35:57




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 P.S..... in reply to Bob, 10-04-2006 21:39:29  
I ASSUME you've got the "BATT" and "GEN" terminals on the cutout connected correctly



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