Generator Puzzle: Updated To Show Regulator Tests

[Roy 0]

See my most recent post on voltage regulator testing. Have a S/G puzzle. The one on my 725 will not charge. Starter side works fine. Put new brushes and bearings in. Checked the armature and generator field coil for opens and shorts to ground. Everything checks and looks good but still no charge with the field terminal grounded. Even cleaned up the S/G housing and end caps where they contact the housing and ran a separate ground wire to the chassis. Has me baffled. Acts like an open on the generator field coil side. Obviously I have missed something but don't know what. Suggestions/ideas to find the problem will be welcome. As an aside, I have two voltage regulators that bench check good with a DC power supply but won't work on the tractor. May be due the generator but won't know until I get the generator working (which it did originally). Thanks,

[Willy 2]

Hey Roy if you figure it out,please let me know what you found,I have the same problem. I have two that act the same,I've had them apart a dozen times, every thing cks good but they still won't charge. I just charge the batt once in awhile.

[RedbarnRick 5]

Have you looked at the voltage regulator I had several that the points rusted shut inside could it be you are charging it's just not going thru regulator?

[Roy 0]

Willy, "Hey Roy if you figure it out,please let me know what you found,I have the same problem. I have two that act the same,I've had them apart a dozen times, every thing cks good but they still won't charge." Will do. RedbarnRick, Not the regulator. Won't charge with the field grounded and regulator out of the circuit. Update: Looked today and all checks OK again. Suspect the field coil may be internally shorted but don't have the specifications to check the resistance against. Still have a puzzle to solve. Robbed the voltage regulator and S/G off the B & S Model 19 from my B-1. Had to fix a broken wire in the S/G but have it all working to specification on the 725. Latest update: Looked at Al Eden's electrical writeup and he says the field coil should have 7 to 15 ohms resistance. Checked mine and the resistance is 0 (zero). Must be a shorted coil. Al's article is at "http://www.simpletractors.com/service/electrics/starter-generator.htm" More later,

[Roy 0]

Success: Checked the field coil again with a digital VOM instead of my old, cheap analog VOM. This time the coil checked at 8.1 ohms which was within the specification. Cleaned up the brushes and commutator again and reassembled. Made a "quick" test stand with my ShopSmith, a pulley, a clamp, and the belt & S/G bracket from my Model 19 B & S. Running w/o the field grounded had very little output. Grounded the field and the voltage slowly built up to 13 volts, sort of like a capacitor charging. With the field grounded the S/G works fine. Must try next with a battery and regulator to test my spare regulators. Pictures of temporary test stand:

I'm happy to finally get the generator working. :) Suspect the problem was the new brushes not fully seating until they had "worn" in a bit. A bonus to using the ShopSmith is the ability to vary the generator drive rpm while testing.

[MPH 12]

Like your test stand Roy^. Glad you got it working, my understanding of electrial is pretty much it works or it don't. I can run the wires right.

[HubbardRA 19]

Roy, I thought the charging voltage with a grounded coil was more like 18V. 13V seems a little low. The average charging voltage for an automobile is 13.8 after the regulator. If it was 13V with the regulator in place a a 10 amp load, then I would say it is working right. I still think that 13V with no regulator and no load is too low. I hope I am wrong, for your sake.

[Roy 0]

Thanks Marty. Rod, Depends upon the rpm of the generator. Output voltage is a function of armature rpm. After taking the picture I increased the speed and got 14.6 volts. Didn't want to go too fast and smoke the field coil. :D

[tracktor312 0]

I think you may have to polarize the generator after replaceing brushes try a search on polarizing s/g

[Roy 0]

tracktor312, Yes you do. And yes I did. :)

[Al 6]

Roy, One thing you may want to check is if you show a ground from the commutator bars to ground. One other thing that can be happening is if the armauture has shorted turns. The starter may still work, but may have low torque. You are correct that you can burn out the field coils if you have no load on the armature and the field grounded. The most common way to check the armature for shorted turns is with a growler. I believe you could maybe see it if you had a digital ammeter and could monitor the current from a battery to the A term and ground as you slowly turned the armature from commutator bar to bar. If you had a shorted turn, I would expect to se a current increase of about 15% since there only about 6 turns for each winding. Shorting out one turn would decrease the resistance of the coil by 1/6th or roughly 15%. Unless the Commutator has been turned and pollished and the brushes well seated, brush/commutator resistance could also vary giving varing readings as you rotated the armature. Since you are dealing with resistances of fractions of an ohm. Probe contact resistance can be deceiving. This why it needs to be done with about 10 or 15 amps flowing through the windings. Al Eden

[TimJr 206]

A couple years ago the generator and regulator failed on my early 7016. Got a new regulator and had a local starter shop rebuild the generator. Installed everything, and it would charge fine until I hit about 1/3 throttle, then it would quit charging. Rechecked all my wires and outputs etc.. Took the generator back, they tested it and said it was good after turning the commutator figuring it was out of round and the brushes were jumping off at speed. Reinstalled it, acted the same way. Had to be the generator - all else was good. This time I asked to watch them test it - I was getting suspicious of them. Bingo - their bench testing unit was driving the generator at only a couple thousand RPM. These spin at least 9 or 10,000 RPM at full throttle on a Briggs - check your pulley sizes. My armature had a couple broken wires that would expand due to centrifigul force when at higher RPM. The armature bench tested fine, but we eventually spotted the cracked wires. The shop then gave me a new armature and we called it good. Tim

[Roy 0]

Al/TimJr, Good information to have. Thanks,

[Roy 0]

Voltage Regulator Test Update. Set up the test stand to include the voltage regulator today. Had to sand the VR (voltage regulator) points on the first regulator to get it working then adjusted the VR to give an output of 14.6 to 14.8 volts maximum. The second regulator worked "as is" but had to adjust the VR up from around 10.5 volts to a final output of 14.0 to 14.2 volts. The final voltages are with the generator and regulator hot at approximately operating rpm. Had the Shopsmith set to run the drive pulley to simulate the normal engine drive pulley at 3600 rpm. Pictures show the test set up:

Output of first regulator:

Current output was only 1 amp with both regulators since the battery was fully charged. Cut in/out voltage for both regulators was around 11.5 to 12 volts so I did not attempt to adjust either one. The S/G did get hot from the test running. Took a while, the VR points tension adjustment is done by bending a tang and it is difficult to bend just right. That's it,

[HubbardRA 19]

You-Da-Man !!