Output Voltage for Delco Generator

Rob58

Rob58

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I'm troubleshooting a charging problem with a old Delco generator and Delco voltage regulator. My basic question is to understand what the correct voltage should be, presumably to be set by the VR, and how to perform such a test. When an old generator stops doing its job what are the possible reasons, besides bad brushes? Thanks for the help!
 
Rob58 said:
I'm troubleshooting a charging problem with a old Delco generator and Delco voltage regulator. My basic question is to understand what the correct voltage should be, presumably to be set by the VR, and how to perform such a test. When an old generator stops doing its job what are the possible reasons, besides bad brushes? Thanks for the help!
Click to expand...

My experience with the old generators is that, aside from worn brushes as you noted, they are pretty reliable. I'd first look at the brushes, then look at the voltage regulator. Worn/burned contacts, etc.?

-Skip
 
Funny you should ask, we just had a whole thread on this. As luck would have it, I just went through this exercise with my tractor Genny. First, check that the generator is in good repair. You can do this easily a couple of ways; pull off the belt and apply 12v + to the arm and - to the field. If it "motors", that is turns (it'll be slow), the brushes, field and arm are good. Then, the belt back on and jump the arm and field. Put a voltmeter from the arm to ground and start the motor at slow idle. If you have voltage that increases with engine speed the Genny is good. Be careful! Voltage can get pretty high in a short time, don't go much over high idle. Mine cranked up to 40v in the blink of an eye. Survived it though.

You can fart around with trying to adjust regulator voltage and cutout, but it can be tough to get right without an external variable power supply. Start with cleaning and adjusting the contact points with a points file. You don't need to remove the pits, just the oxidation. I took a feeler guage and took measurements before I started to get them reset ball park after cleaning. It's '30's tech, ball park is good enough. After you reinstall it, be sure the cutout ( the bobbin with the big wires) remains open until the Genny starts to produce, then it should close. Don't close it manually, the contacts can short together and you'll not get them open again. Charging output should be 13.5 -14.25 volts, but give the regulator a chance to warm up before you'll get a reliable reading. Put the cover back on, it makes a difference.

You can adjust output voltage by opening and closing the voltage bobbin gap (the one with the littlest wires). Cutout and current can be adjusted as well, but I found resetting to pre-cleaning gaps was fine.

Of course, don't neglect simple wiring and terminal corrosion issues, and check that he regulator itself has a good ground
 
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Disconnect both wires from the generator.
Place a ammeter from the (A) post to ground
Place a wire from the battery (+) to the (F) momentarily
The Ampmeter should show a momentary output while voltage is applied to the (F)
When no amperage out put, you have an open field circuit.
probably bad brushes.

I should add, the engine must be running when you do this test.
 
Another check - Look at the wire wound resistor(s - I think there are normally two) underneath the voltage regulator. If any are open, it's time for a new regulator. These resistor(s) are the 1930 version of a flyback diode. If any resistors are open, the regulator points will quickly fry themselves with the inductive kick if the regulator vibrator operation into the inductance of the field coil.
 
Tom-D said:
Disconnect both wires from the generator.
Place a ammeter from the (A) post to ground
Place a wire from the battery (+) to the (F) momentarily
The Ampmeter should show a momentary output while voltage is applied to the (F)
When no amperage out put, you have an open field circuit.
probably bad brushes.

I should add, the engine must be running when you do this test.
Click to expand...

That could also be a bad armature. So the you replace the brushes and polish the commutator, and it's still dead. So now you disassemble the genny and try and find a shop with a growler. Motoring the genny tests both at the same time. BTDT.
 
Tom-D said:
Disconnect both wires from the generator.
Place a ammeter from the (A) post to ground
Place a wire from the battery (+) to the (F) momentarily
The Ampmeter should show a momentary output while voltage is applied to the (F)
When no amperage out put, you have an open field circuit.
probably bad brushes.

I should add, the engine must be running when you do this test.
Click to expand...

Aren't instructions that includes a step about a field applicable to an alternator? Generators don't have a field.
 
I wonder what that (F) post is?
Chip Sylverne said:
That could also be a bad armature. So the you replace the brushes and polish the commutator, and it's still dead. So now you disassemble the genny and try and find a shop with a growler. Motoring the genny tests both at the same time. BTDT.
Click to expand...
You can test the continuity of the armature by seeking continuity from segments 180 degrees apart. more than any 2 segments and it is a shorted armature.

if thats the case toss it and do the alternator up grade.
 
Rob58 said:
When an old generator stops doing its job what are the possible reasons, besides bad brushes? Thanks for the help!
Click to expand...
open field winding most likely a broken wire between the VR and the generator.
place a inductive ammeter on the wire from the (A) post to the VR and adjust the VR to read 14.2 volts.

Bad VR, open in the voltage regulator coil circuit. or bad. contacts on the vibrator switch which regulates the peak voltage.
 
Tom-D said:
place a inductive ammeter on the wire from the (A) post to the VR and adjust the VR to read 14.2 volts.
Click to expand...

Huh???? If the voltage regulator is being adjusted a VOLTmeter should be used. The inductive ammeter is useful for adjusting the current regulator. In this case a load somewhat greater than the generator's rating should be applied. A carbon pile is the best tool for this. For example:
https://www.northerntool.com/shop/t...MI44Gp847B4QIVTb7ACh06CQMlEAQYASABEgLNvPD_BwE
 
Rob58 said:
My basic question is to understand what the correct voltage should be, presumably to be set by the VR, and how to perform such a test
Click to expand...
With a generator, the simplest test is to run the aircraft above idle and check the voltage at the battery. For a healthy 12v system it should over 13.5v but not more than 14.5v.

Rgbeard said:
Generators don't have a field.
Click to expand...
Not quite. Both alternators and generators have field voltage/current, but use them in different methods to regulate output voltage.
 
[QUOTE="Tom-D, post: 2708680, member]

if thats the case toss it and do the alternator up grade.[/QUOTE]

Unless you happen to have a spare known good armature in the cabinet ;)

Rebuilding a genny is about as easy as falling off a log. All you need is an arbor press to press the armature shaft out of the bearing, but a drill press will do in a pinch.
My advice is if you don't need the extra amps, spend the money on avgas instead.
 
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Rgbeard said:
Generators don't have a field.
Click to expand...

Generators certainly do have a field. It is quite different being stationary whereas the alternator's is rotating. Both fields are used to regulate output voltage by varying the field current. The generator field is perhaps less apparent to the pilot since there is usually no circuit breaker/fuse for it and no split master switch as for an alternator. This is because the alternator field must be supplied with battery voltage to "start" the alternator. The generator "starts" itself due to a) the residual magnetism of the field and b) the absence of rectifier diodes to block the low output voltage at start up.
 
The response to my questions are super-helpful. Major thanks to everyone!
 
Ron, that's an extremely valuable write-up you published! Thank you!!!
 
Just FYI, the cutout relay in a mechanical regulator is not a simple relay. It has 2 windings and is actually a mechanical diode allowing current to flow from the generator to the battery but not from the battery back into the generator. Really amazing what smart guys before solid state devices were invented came up with.
 
RonP said:
Just FYI, the cutout relay in a mechanical regulator is not a simple relay. It has 2 windings and is actually a mechanical diode allowing current to flow from the generator to the battery but not from the battery back into the generator. Really amazing what smart guys before solid state devices were invented came up with.
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Yup. Quite a maze involved with that:

upload_2021-3-7_13-43-38.jpeg

The sort of stuff that the ordinary mechanic can fix. Lightly dress the relay contacts, make sure the connections and grounds are all good, and it will usually go back to work. How many ordinary mechanics will have success fooling with this?

upload_2021-3-7_13-52-24.png
 
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