Genrator

[Mr Mayor]

My SA has three brush, cutout and 4 position switch. Just started to act up, no charge when starting it up, however when I dead ground the field to the frame it starts to charge normally. This would indicate a bad ground but, when I remove the jumper from the F terminal it continues to charge normally including the high - low function of the switch which I assume requires a good ground. BTW the battery is new, system polarized etc. Any ideas?

 

[Janicholson]

There could be three issues or a combination. If the light switch ground is not good (switch to box lid, box lid to box, box to post, post to platform, the resistance might be too much for excitation. If the cutout relay pull in voltage is becoming too high, there might not be enough generator residual voltage generated to latch the contacts. When the light switch is in the H position, the wire to F terminal on the generator and the switch, and the grounds mentioned should be equal to (and as effective) as direct grounding the gen F terminal. Jim

 

[560Dennis]

Had trouble with the bolts (3) that hold the the controls cabinet to the differential . These bolts corroded will not ground. Removed bolts cleaned them added washers and 1/2 external lock washers ( serrated) . Made a world difference in the tractors starting also .
If you have a vom meter do a voltage drop test .

 

[Mr. Mayor]

Would jumping bat to gen on the cutout before starting be a reasonable test of the pull-in voltage?

 

[Janicholson]

That is what is done to polarize the gen. The generator is disconnected from the electrical system when shut off because the cutout contacts are open. This cutout relay has a fine wire many turn winding on its core as well as a current coil that keeps the contacts closed during charging. When the generator begins to turn, it is still disconnected from the battery/charging system. The residual (small but stable) magnetism in the iron field poles create generator output in the armature. this is still not connected to the system until this voltage rises enough to be greater than the battery voltage. It gets reinforced because as the voltage rises to meet the battery voltage some of it goes through the field windings to ground through the light switch in this example, this adds to the magnetism in the generator allowing more voltage production. when the voltage is higher than battery voltage, the pull in winding mentioned above closes the contacts allowing the generator to be connected to the system, and charging the battery and running the lights. The current flowing now through the hold in heavy wire wrapped on that same core adds to the total force holding the contacts together.
When the engine is shut off and slows down such that the generator is producing less voltage than the battery, the current in the heavy winding reverses polarity and tries to go toward the generator instead of from it. This reverse current pulse now pushes the contacts open and because the voltage is low, the pull in winding is defeated as well. The generator is now disconnected. Jim

 

[Mr Mayor]

I must apologize Jim, my question was poorly worded. What I was thinking of was not a momentary touch as when polarizing but actually connecting the BAT and GEN and leaving them connected when starting the tractor. The F to frame jumper would not be in place. I'm thinking that if I get charging with the cutout bypassed that would rule out a grounding issue and point toward the cutout itself?? Also, thanks for one of the easiest to understand (for a non electrical guy) descriptions of cutout functionality. Geoff

 

[Janicholson]

Don't do that. It can overheat the generator. The residual magnetism and self generated current is all that is needed. If the cutout is adjusted to a pull in voltage that is too high, it won't pull in. If it is adjusted too low the cutout won't open and it will drain the battery. There are solid state replacement cutouts that just have a nice big diode inside them to totally replace the troublesome Mechanical cutouts. Jim