Generator repair.

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Model T Ford Forum: Forum 2017: Generator repair.
Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Sims--Reed City,Mich. on Tuesday, June 06, 2017 - 12:16 pm:

What should the resistance be in the generator field coils? I have rebuilt my generator after testing everything and it still will not charge. I have rebuilt many generators with good results. Set the null point correctly and the generator motors when hooked to a battery.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Rolf Oehman Oslo Norway on Tuesday, June 06, 2017 - 12:26 pm:

Hi . Don't know the resistance, but you say set the null point and the generator motors when hooked up to the battery.. The correct position of the brush plate is where it not motors I think.But you probably know this. :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Tuesday, June 06, 2017 - 03:48 pm:

It shouldn't motor with the third brush lifted, but should when it's down in contact again.

I've had luck with mixing parts from several generators and following the MTFCA electrical handbook.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Sims--Reed City,Mich. on Tuesday, June 06, 2017 - 06:23 pm:

I should been more clear. No motoring with brush lifted but does with brush in place. New brush plate insulator and new brushes.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Scott Conger on Tuesday, June 06, 2017 - 07:54 pm:

Generators are relatively simple devices, but the process to rebuild one is a somewhat lengthy process if done thoroughly. There are a number of reasons why you're not getting what you expect and since none of us saw you rebuild it, this will be a nearly infinite game of "did you do this?", or "did you do that and if so what did it do then...?". Please invest in the MTFCA electrical book, read it, and test your generator step by step according to the "generator" section. It will work when you're done.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Stephen, South Texas on Tuesday, June 06, 2017 - 09:42 pm:

I think the field coils total something like 2 ohms but it's been a while since I've metered them. If you have the pigtails loose, it's good idea to check for a short to ground. If they're old, they can short to the pole shoes or the yoke.

You can also check the armature for shorts to ground (shaft). It doesn't do much good to meter the armature coils since there's two coils attached to each commutator pole. Just make sure none of the contacts show a short to ground.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Brad Marble on Wednesday, June 07, 2017 - 03:21 pm:

Jim; It should motor with the third brush lifted and you should be able to reverse the rotation direction by slightly rotating the brush rigging, you need to loosen the four screws at bell end. A good brush neutral is when it just wants to rotate clockwise when looking at the shaft end. If you have done this and you still have low or no output, try to find someone with a growler, some of the older motor shops still have one. If you turned the commutator and didn't undercut the mica, you might have caused a short between two commutator bars, that is all that it would take reduce the output to almost nothing. Good Luck. Brad.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ted Dumas on Wednesday, June 07, 2017 - 04:44 pm:

Did you cross the field leads?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Sims--Reed City,Mich. on Wednesday, June 07, 2017 - 07:18 pm:

Just so you understand. I have rebuild at least 10 generators with good results. Armature has been tested 3 different test with a growler. Null has been set by lifting the brush and then rotating the brush plate both ways and setting in the middle when the armature does not rotate. Checked the field coils for shorts to the case and also for continuity. Have tried crossing wire as needed and also not crossing them. New brush plate insulators and new brushes. This is why I am confused. Will not pay some one else to fix it.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ted Dumas on Wednesday, June 07, 2017 - 09:11 pm:

Maybe you need to polarize the generator.

If all else fails let it sit for a day or two and go through it from a fresh start.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Thursday, June 08, 2017 - 03:29 am:

That's why it's good to have several generators to scrounge parts from.. Once a field coil seemed OK when testing with my crude instrument, but would still not charge properly.. (It's nice they're available new too)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Brad Marble on Friday, June 09, 2017 - 07:55 am:

Jim; I to have rebuilt dozens of model T generators, but you have something going on in this one that I don't understand. Please keep us informed on what you find. Brad.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Friday, June 09, 2017 - 09:13 am:

A short between turns in any inductive device be it transformer, choke, or field coil can make the thing test good with an ohm meter test yet not function at all when installed. Essentially such a short then bypasses the field current through the short where it creates no magnetism and thus the generator doesn't work. The best way to test very small values of resistance is to use a constant current source such as a DC power supply and a series resistance that controls the amount of current then you connect the field ends to this and measure the DC voltage drop across the windings using a DIGITAL voltmeter so you can see small differences in the Voltage Drop. Then use that setup to compare your field winding readings with known good field winding made by the same company. Measuring the inductance of the field windings on a properly made bridge will usually show a substantial difference in inductance between a good field and a shorted one even if the DC resistance between a good and bad one is rather similar. If/when field windings are not expensive and are available it might just be easier to put in new fields. Growlers are rather inexpensive on ebay. I picked up mine for around $25 but they are heavy to ship. Almost all motor shops have one and will usually test an armature for you on a walk in basis and often will do it for free if you drive up in a model T.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ron Patterson-Nicholasville, Kentucky on Friday, June 09, 2017 - 04:33 pm:

Jim
I have one question, did you install a new field winding in this generator?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Sims--Reed City,Mich. on Friday, June 09, 2017 - 04:47 pm:

I did not install new field windings. I believe in repair instead of replace if possible. That is why I initially asked about the resistance in the windings. I have many other generator and will find good windings to use. I have already been told to buy new windings, new armature or send it out to spend much money to have it rebuilt. That is not my way of doing things.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ron Patterson-Nicholasville, Kentucky on Friday, June 09, 2017 - 05:42 pm:

Jim
The DC resistance for a Model T Ford generator field winding is 3 Ohms, but for the reasons cited by John Regan above, that number means nothing.


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