Question on magneto coils

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Model T Ford Forum: Forum 2012: Question on magneto coils
Top of pagePrevious messageNext messageBottom of page Link to this message  By Dennis Brown on Wednesday, August 15, 2012 - 01:11 am:

If the insulation on a coil is damaged on an edge and 2 or 3 windings in one coil appear to touch together will the mag coil ring still work as long as it is not grounded out at the point of damage. I know most of us have seen coils with this problem, especially if someone tried to remove a starter without first removing the bendix. I have always wondered on this as I am sure there are a lot of engines out there that have that problem and no one knows about it.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Michael Deichmann, Blistrup, Denmark on Wednesday, August 15, 2012 - 01:30 am:

If it works - do not fix it.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kenny Edmondson, Indianapolis on Wednesday, August 15, 2012 - 05:42 am:

That coil will short circuit and slightly reduce performance as long as it's not grounded. But with that said, is the engine apart? If so I would think your labor would be worth more than the cost of a rebuilt Mag ring. If you'd have to pull the engine and replace the mag later, it's not a fun job to do a second time. If the engine is together you can continue running it. If it's apart, replace it and you should have a mag system that will last for another 80 years.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Wednesday, August 15, 2012 - 09:25 am:

Kenny:

If the short is from a winding to the frame it can reduce the performance more than a slight amount. I agree with you that it really is a mistake not to replace even a suspected faulty mag ring while one has the engine apart.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor on Wednesday, August 15, 2012 - 11:07 am:

Having shorted inter-winding turns (not shorted to ground) will lower the cumulative number of coil turns which will lower the magneto output; it may still be functional but not desirable.

I was wondering if anyone had devised a test method capable of determining the health of a magneto coil while still in the car. Resistance measurement alone is not very useful in determining if windings are shorted to ground for example.

The magneto coil inductance might be a good way to check coil health to help judge if a magneto coil needs to be pulled or not. I have searched the forum but did not find any posts on doing this. I contacted a magneto coil rebuilder about this method but they had no knowledge of its use. Does anyone else know if this testing was ever proposed or done?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tim Pearson on Wednesday, August 15, 2012 - 06:36 pm:

bump


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie on Wednesday, August 15, 2012 - 07:18 pm:

If we had some sort of a baseline, you could check it with a Kelvin resistance measurement setup. Google it for more info. It is easy to "trick" a voltage regulator, such as an LM317 into being a current regulator. If you set it up for 1 amp, millivolts of voltage drop equals milliohms resistance.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor on Wednesday, August 15, 2012 - 10:03 pm:

Tom,

I am familiar with the method you referred to using a constant current source to create a milli-Ohm meter. The problem with using resistance as the metric is that the results vary with winding conductor width and thickness as well as defect. It would be difficult to discern problems from normal manufacturing variation. My thought was that coil inductance would be more a reliable test because it would be proportional to the number of cumulative turns provided all magneto coils were made with a specified number of turns otherwise it would suffer from the same issue. I know there were single stack and double stack configurations and two different windings. I would think someone would have information regarding the magneto winding specifications.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie on Thursday, August 16, 2012 - 12:46 am:

My thought was that if we had a baseline, and you knew which coil you had (I think there are only two after the double stack), that the resistances would be within a discernible range. To measure inductance would require an alternating current to be passed through the coil which would tend to demagnetize your magnets. Although with low enough power maybe it wouldn't matter that much.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Thursday, August 16, 2012 - 06:47 am:

Mike,

We measure the health of a magneto in the car by checking voltage output under load.

I call this the Regan - Patterson Memorial Magneto test after its inventors.

To check the magneto voltage one connects a #1156 automotive bulb between the magneto post and ground. This provides a load so the magneto reading is known to be stable.

An analog (NOT digital) AC voltmeter is then connected to the same points. With the engine running at idle you should see maybe 6 volts at idle. With the throttle advanced to high engine speed a good magneto will read 20 - 30 volts. The light bulb will burn out rapidly if this is sustained.

More relevant information is here:
http://www.mtfca.com/discus/messages/80257/96176.html


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor on Thursday, August 16, 2012 - 09:31 am:

Royce,

I am familiar with the magneto voltage test under load with a #1156 bulb. Suppose the measurement is weak, say 3V at 1000RPM (bulb lights very dim) and the mag post contact is good. Now what? In car magnet re-charge would make sense if you knew the magneto coil was good. The process doesn't always go as planned from what I have read and you could wind up de-magnetizing the magnets rather than re-magnetizing them.

The goal is to avoid pulling the engine only to find the magneto coil is good and you should have refined the in car re-charge process. Conversely, it would be nice to know that risking in car magnet re-charge makes absolutely no sense because the magneto coil has been severely damaged.

I know some folks have recently reported using a Gauss meter to access the condition of the magnets to help identify magneto issues. However, I would think there are variables which would make a standard acceptable Gauss specification difficult to achieve since it would be highly dependent upon the position of the probe with respect to the magnet (X,Y, and Z dimensions) probe to probe calibration and surrounding objects in proximity to the measurement.

If each magneto coil segment contains a fixed number of turns then the sum of the total number of turns for all coils would be the same and hence the inductance would also be the same unless turns are shorted together or the coil gets shorted to ground somewhere in between. There would still be some variation due to proximity effects of the coils to the flywheel and coil plate. Hence, I am interested to know if anyone has ever attempted to characterize magneto coil inductance and variation between the various configurations. If not, I would be willing to do so but would need samples of known good magneto coils for the various configurations.

Tom,
I think the resistance differences would be very minimal making interpretation difficult to discern. Probe contact resistance could skew the results if you are not very careful with the way the measurement is applied.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Semprez-Templeton, CA on Thursday, August 16, 2012 - 02:08 pm:

Mike, I would think that Wally Szumowski Or R.V. Anderson should be able to provide the base line values you seek for a new coil. Have you discussed with either of them?


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Semprez-Templeton, CA on Thursday, August 16, 2012 - 02:44 pm:

Mike,
When I was involved in the electric motor repair business the electrical tests we ran on all repaired motors were: Hi-Pot, Resistance (windings to frame, winding to winding, each winding) Inductance of each winding and inductance of all windings connected. Other tests performed were Locked rotor, no load and full load current, but these later tests would not be relevant to a Ford coil ring test. These values were compared to the new motor specs, to determine the results of the rebuild.

Since the Ford stator is internally connected, coil to coil and individual coil tests are, of course, not feasible.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor on Thursday, August 16, 2012 - 05:18 pm:

John,

Thanks for the suggestion and information on electric motor test. Yes, I did inquire with a coil rebuilder but they did not have any information on coil inductance and informed me that they did not provide coils for experimentation purposes. Thanks for the lead on another coil rebuilder, I will follow up.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Vince M on Thursday, August 16, 2012 - 05:30 pm:

Mike,

The way i look at it, If the coil is good, and the magnet recharge is successful, your done! If the coil is bad, and the magnets get de-magnetized, you have to pull the motor anyway to replace the coils, you can send out the magnets for recharge then. I dont think its an expensive deal, and i believe there is a do-it-yourself process for out of the car charging as well.

Vince


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan Hatch on Thursday, August 16, 2012 - 06:14 pm:

A good coil will have about .5 ohm resistance. It takes a real good meter to measure it. The best way to check is out of car, unhook the end that is grounded and check it. It can be done in the car, but again takes a good high grade meter to read it. One or two windings in a coil shorted will be hard to measure. Dan


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Thursday, August 16, 2012 - 06:49 pm:

Vince,

Agreed. If the magneto doesn't have sufficient output an in car charge isn't going to hurt anything and it will tell you everything you need to know.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bede Cordes, New Zealand on Friday, August 17, 2012 - 04:27 am:

John and Mike,

reading this makes me think of VFD's (Variable Frequency Drives) for 3 phase electric motors. Many including most of the ones I work with (Danfoss), will run an "AMA (Automatic Motor Adaption)" or "self tune" to a newly connected motor if you request it. Firstly the basic motor data is entered, then running a basic or advanced AMA will result in the drive causing the motor to sing and grunt, and occasionally the shaft will twitch a bit. A long AMA can take up to 15 minutes. During this time, the drive works out stator reactance, rotor reactance, leakage reactance and god knows what else to improve the motor/drive relationship. I don't really know how the drive does the AMA, but it does apply obviously voltage, and at times quite high current with respect to the nameplate current of the motor. Generally I find motors run sweeter and smoother once an AMA has been done, but then sometimes an AMA will not like a motor, and refuse to complete it (more common in high-efficiency motors, funnily enough). Sometimes on refusal, going back and checking the parameters entered initially into the drive have been incorrect, the the AMA was right- finding an inaccuracy.

I'm wondering if there are boffins out there that can make a VFD do this for a squirrel-cage induction motor, I wonder if similar technology could be invented/used/designed to check the fields of a permanent-magnet alternator (which effectively is what the Model T mag is).
But as mentioned above, we have to have a baseline to work from, which with our VFD's, is the motor data we tell it initially. With a Model T mag, even if we know the characteristics of the field, the differences/conditions of the magnets might make it impossible...

Just putting it out there for discussion, though I think our time might be better spent restoring and driving our T's !

Regards,
Bede


Top of pagePrevious messageNext messageBottom of page Link to this message  By James A. Golden on Friday, August 17, 2012 - 07:57 am:

An easy fix that has worked for me is to clean the bare area with a solvent like alcohol or mineral spirits. Then cover the area with a layer of Devcon's 5 Minute epoxy.

My first T continued to run on Magneto for about 20 years and might still be running, after that repair. I have lost track of it since about 12 years ago.


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