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Old 09-18-2015, 10:44 AM
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Coil Tests

Don recently sent me several coils to test on a rebuilt A4 I have in my garage. The test was to measure voltage at the coil + terminal and current through the coil primary at various engine RPM. I first measured the resistance of each coil and then measured the voltage and current at 0, 700, 1500 and 2000 RPM. The 0 RPM measurement is with the ignition switch on and the engine off. I have the Pertronix Ignitor installed. I tested 4 coils total, three that Don sent me and one that I had purchased about ten years ago, an Ametek coil with part number CAS-4012-1. The results are attached in a PDF. The voltage was measured at the coil positive terminal and the current meter was connected between the coil negative terminal and the black wire to the EI module.

Some observations from the tests:

1) The current is highest when the engine is off and the ignition switch is on (0 RPM). In this situation, the current through the coil primary can be calculated by dividing the coil + voltage by the coil internal resistance, I=V/R
2) The current decreased in the primary circuit when the engine was started and continued to decrease as RPM increased. This is due to the expanding and collapsing magnetic field generated in the coil as the EI/distributor switches the current on and off. The faster the current is switched on and off, the more the magnetic field opposes the current flow.
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Old 09-18-2015, 10:59 AM
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THANK YOU - VERY COOL
This does illustrate why the A4 is hard on coils - we spend a lot of time at low RPMs and low RPM = high current flow.
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Old 09-18-2015, 11:43 AM
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If I operated under any of those 4 scenarios on a trip south my guess is I wouldn't get a coil past NYC.
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Old 09-18-2015, 11:55 AM
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Hanley,

Your own instrument panel shows that the current through your coil is similar to my test results. Why would you think that the coil would fail? The coils I tested all came from MMI and Don has stated that he has never heard of one of his coils failing.
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Old 09-18-2015, 12:02 PM
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Quote:
Originally Posted by Tim View Post
Hanley,

Your own instrument panel shows that the current through your coil is similar to my test results. Why would you think that the coil would fail? The coils I tested all came from MMI and Don has stated that he has never heard of one of his coils failing.
Yes, current is similar but voltages and resistance are very different.
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Old 09-18-2015, 12:13 PM
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From Don:

"OK Tim, I’m finally back from my travels.



Thanks again for your great test! A couple points of interest;



1) Although the Blue Standard coil started out with lower static resistance, by the time you took it to 1500 to 2000 RPM, it had almost caught up to the other coils in terms of decreasing current flow. Doesn’t this mean that it builds inductive reactance at a different rate from the other coils? The implication being that you can’t judge a coil’s dynamic performance based on its static primary resistance alone.


2) Your test (pretty much duplicating in-service conditions), confirms that either of the Standard UC-15 coils or the NAPA IC-14 keep current flow well below the Pertronix 4 amp restriction.


3) I know of no current flow restrictions imposed by the coil manufacturers, so I see no reason to suggest that the current flow values shown in your test should create excessive heat build-up. For folks operating in very hot climates, a more direct way to minimize coil heat would be to increase the ventilation in the engine compartment or perhaps even move the coil away from the engine.



Tim, if you have the time, I really think your test results and your conclusions would be very useful and welcomed on the Forum. I’m not aware of any credible discussion regarding the profound effects of inductive reactance and I think these results would go a long way to reduce all the concern over the need for additional external resistance. I’d consider not reporting on the Amtek coil, simply because I don’t think they’re available anywhere and their unusually high primary resistance might create some misunderstanding. I googled the name and couldn’t find any connection to ignition coils.



Thanks again for your great report. I’ll find lots of use for the information in providing tech support for customers who call in with questions; many times having been confused by Forum information on the subject.



Don"
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Old 09-18-2015, 12:14 PM
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Tim, I propose the amperage is the same at different RPMs when the circuit is closed via the EI. I further propose the measurement differences are a result of meter interpolation between on and off and is a function of time within the meter. The time function accounts for the measurement variance.

Great testing. Is anyone suggesting a strategic change in EI application or is this for deeper understanding?
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Old 09-18-2015, 12:42 PM
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The current is definitely decreasing as the engine RPM increases. The tests show that the highest current in the circuit is when the ignition switch is on and the engine is off. It is this situation that puts the coil at the greatest risk. None of the tested coils went over the Pertronix 4 amp limit.
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Old 09-18-2015, 12:45 PM
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Anything under 14 volts, unresisted, at coil+ is not realistic in this time of big alternators and extensive electrical systems.
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Old 09-18-2015, 12:48 PM
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Quote:
Originally Posted by Tim View Post
The current is definitely decreasing as the engine RPM increases. The tests show that the highest current in the circuit is when the ignition switch is on and the engine is off. It is this situation that puts the coil at the greatest risk. None of the tested coils went over the Pertronix 4 amp limit.
It is precisely the Pertronix "4 amp mantra" that needs to be ditched to clarify thinking.
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Old 09-18-2015, 12:54 PM
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Well, I'll disagree about the current changing despite the measurement data. As I said in my previous post, the effect of meter interpolation is not being considered. Because you're reading lower numbers on your meter you're concluding the current is changing. I'm suggesting the meter is displaying an interpolated value trying to deal with on and off inputs.

When the EI is closed the current flows at full value, when the EI is open there is zero current flow. Current is not a function of time (although coil saturation is).

I'll not belabor the point, had my say. You can proceed however you choose.
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Old 09-18-2015, 12:56 PM
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From: "Don Moyer"
Sent: 9/8/2015 7:22:18 AM
To: "'Tim'"
Subject: RE: AC/DC

Looks like our emails are crossing in cyberspace! I think I now have a complete understanding of AC/DC as it relates to our coil project.



I don’t think I mentioned this to you before, but when I talked to Pertronix last Friday, they reminded me that until an engine is started, the alternator voltage isn’t even present, so our use of 14.2 volts to calculate amp flow through the module are flawed since prior to starting, voltage would only be battery voltage. Moreover, as you are now demonstrating, as soon as the engine starts, inductive reactance enters the mix and dynamic measurements of resistance (impedance) become quite different (higher than original static resistance measurements) so concerns of high current flow through the Ignitor module and coil are greatly reduced.



Don
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Old 09-18-2015, 01:58 PM
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Just FYI: The ignitor module burning out wasn't the issue, the coils burning out was the issue. As a data point, my Standard UC-15 coil with no resistor at about 13.9-14.1 volts has been good for years now. I estimate it has at least 500-800 hours on it.
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Old 09-18-2015, 02:22 PM
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I wrote Tim privately in an effort to better understand the concept of inductive reactance. Prior to his postings, my understanding was that we were having coil failures due to exceeding a current flow of 4 amps through the primary circuit (a restriction originally posed by Pertronix) unless additional ballast resistance was added to bring total static primary resistance to around 4 ohms.

It was for this reason I asked Tim to test several coils that were in common use on the Atomic 4 (without external resistance) to see if his measured current flow through the primary system ever exceeded 4 amps at any point within the normal operating range of the Atomic 4. I asked him to start with the ignition switch "ON" and the engine not running, to the recommended cruising power setting by Universal of 2000 RPM.

Tim's test data is what it is and everyone is free to react to it (or not). That's the nature of public forums. I have no professional credentials to bring to the table, but intuitively, Tim's test makes a good case for me that inductive reactance is real and is a credible component of the actual total current flowing through our primary ignition circuits. Don

Last edited by Don Moyer; 09-18-2015 at 02:25 PM.
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Old 09-18-2015, 10:01 PM
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Coil test data clarification

As a Tartan 30 owner who is electrically challenged (the owner, not the boat!) I need a clarification on the test data set provided by Tim. In his note he states that "...the current through the coil primary can be calculated by dividing the coil + voltage by the coil internal resistance, I=V/R". I assumed that the amp values in the table were calculated using this formula, with the resistance value taken from the column "Coil", and the + terminal voltage from the columns related to the various engine RPM values. The amp values in each cell are much lower than a value computed using the formula, e.g for the Black Standard at 1500 rpm, I=V/R> I=13.8/3.7> I=3.73 amps. I am clearly missing something in the discussion, so could someone clarify?

Thanks,

dd

Later edit:

OK, nevermind on this one. RTFQ, I see in Tim's post (finally) that the computed value only applies to the zero RPM case, since no current is being produced yet. The other values come from direct current measurement (not computed) between coil negative and the EI module

Last edited by ddomino; 09-18-2015 at 10:41 PM. Reason: Closer reading of initial post.
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Old 09-18-2015, 10:16 PM
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RE: Coil test data clarification

Couple more questions:

Can coil inductive reactance be directly measured, or failing that, can it be computed from measurable data? What data points would need to be measured? Would we need to know the output frequency of the alternator to perform the computation?

To the extent that inductive reactance is a kind of resistance, is it a contributor to heat build-up in the coil?

Thanks again,

dd

Later edit: So to close out my understanding, the monotonic decrease in amps with increasing RPM is a function of the increasing inductive reactance within the coil due to increasing RPM?.

Would the "computed resistance" (making up a term) be a value computed by dividing the voltage by the measured amps? For example, for the Black Standard coil V/I for 1500 rpm yields 13.8V/1.60a= 8.63 ohms. At 2000 rpm the computed resistance is 9.72 ohms. Would inductive reactance in ohms be equal to the total computed ohms minus the measured static value?

Thanks again for listening to this ramble...I'm retired now, you see (g).

Last edited by ddomino; 09-18-2015 at 10:56 PM. Reason: More closer reading.
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Old 09-18-2015, 10:38 PM
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The alternator - unless badly defective - should have no output frequency. It puts out DC with a bit of ripple from the 3-phase bridge rectifiers that is smoothed out by the battery.
The second part can be fairly complex to model if you want to be a real EE geek and get all the variables correct.
To start with as a pure DC system, once the points start opening and closing (or their electronic equivalent), current is only flowing part of the time instead of all the time. Dwell is usually the way this is expressed for ignition systems and the more dwell the more on time. To add another variable, rapidly switching current into an inductor is no longer just a DC problem. If it were, RPM would have no influence because if the points are on half the time they will be on half the time at 100 RPM or 4000 RPM and the average current would be be same.
The AC part comes in with back EMF and inductance/reactance both as variables that can change with frequency. I actually don't have the energy to dig out all my old EE books and get every detail, but these issues tend to reduce current flow with higher RPMs. Back EMF is the nature of a coil, when it is energized the building magnetic field produces a current that opposes the incoming current. It takes a finite amount of time for the current to build to maximum. See http://hyperphysics.phy-astr.gsu.edu...ic/indtra.html for some detail. The higher the RPM the more this is a factor because the coil spends less time at its maximum possible current. I honestly forget how much of RPM/Current curve is determined by back-EMF factors and how much is modeled on frequency vs. reactance.
As a practical matter: the coil's worst case is being on with the engine not running. The next worst case is running slowly. Using EI makes it worse because the ratio of on to off is more on than with points. In cases where coil current is high enough to shorten the life of the coil we use ballast resistors to reduce it. Back in the day many engines used ballast resistors and had a bypass connection on the solenoid to supply full voltage when starting. I think all the "new" type starters have this connection if anyone wants to recreate this system. Also keep in mind different use scenarios. I have run my engine over 48 hours nonstop adding gas from cans as we went. Some people motor 10 minutes off a mooring to daysail and don't use 5 gallons of gas in a year.


Quote:
Originally Posted by ddomino View Post
Couple more questions:

Can coil inductive reactance be directly measured, or failing that, can it be computed from measurable data? What data points would need to be measured? Would we need to know the output frequency of the alternator to perform the computation?

To the extent that inductive reactance is a kind of resistance, is it a contributor to heat build-up in the coil?

Thanks again,

dd

Last edited by joe_db; 09-18-2015 at 10:42 PM.
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Old 09-18-2015, 11:02 PM
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Thanks Joe. The reason I asked about frequency is that the online sources I checked on inductive reactance all had frequency as a variable in their computations. Not all of it was clear to me in any case, but that was the source of my question. I know that by the time the energy gets to the boat it has been rectified to DC, but I wondered whether the computation would require "pre-rectified" frequency. In any case I think I have it "clear enough for government work". I posted a couple of edits to my earlier queries while you were posting yours, so thanks.

dd
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Old 09-19-2015, 05:32 AM
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Quote:
Originally Posted by Tim View Post
Hanley,

Your own instrument panel shows that the current through your coil is similar to my test results. Why would you think that the coil would fail? The coils I tested all came from MMI and Don has stated that he has never heard of one of his coils failing.
Tim, Thanks for the testing. I've never tried to examine this matter on the level you and Joe, and others, do but maybe I should . I notice that you measure amperage between coil- and the igniter whereas I have mine connected in front of coil+. Maybe I'm missing something here. Since I have extra gauges, next time at the boat I will connect one as you have and report back any new data. Thanks again for your detailed approach to this subject.
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Old 09-19-2015, 07:05 AM
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Hanley,

The circuit is in series. The current should be the same everywhere in the circuit.
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Old 09-19-2015, 08:07 AM
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Quote:
Originally Posted by Tim View Post
Hanley,

The circuit is in series. The current should be the same everywhere in the circuit.
That is the hypothesis I want to test. I wonder if some current is being dissipated in the coil (via heat?) which might explain why both you and Neil might be right.
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Old 09-19-2015, 09:37 AM
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The frequency in those formulas would correspond to the RPM of the engine more or less.
You can run through the calculations here to get some ideas about current rise in coils:
http://hyperphysics.phy-astr.gsu.edu...ic/indtra.html

Quote:
Originally Posted by ddomino View Post
Thanks Joe. The reason I asked about frequency is that the online sources I checked on inductive reactance all had frequency as a variable in their computations. Not all of it was clear to me in any case, but that was the source of my question. I know that by the time the energy gets to the boat it has been rectified to DC, but I wondered whether the computation would require "pre-rectified" frequency. In any case I think I have it "clear enough for government work". I posted a couple of edits to my earlier queries while you were posting yours, so thanks.

dd

Last edited by joe_db; 09-19-2015 at 09:42 AM.
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Old 09-19-2015, 09:51 AM
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Old 09-19-2015, 09:11 PM
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Watching to see where this is headed and what, if any, conclusions result from the testing. Is there a trend afoot to abandon the Rule of 2011? Plan on going back to 3Ω coils with EI?

No worries here, I have a reliable installation proven over several prolonged trips under power and a spare engine that is equally reliable so I'm good.
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Old 09-20-2015, 08:40 AM
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Neil,

A coil with 3 ohms or less of internal resistance will have a high risk of failure and the coil test proves it. In the test, the blue Standard UC-15 coil had an internal resistance of 3.3 ohms and prior to starting the engine there was 3.75 amps with 12.5 volts on the coil +. A coil with only 3 ohms of internal resistance would exceed the 4 amp limit for the EI module.
The coil test results show:
1) The highest current occurs with the ignition switch on and the engine off.
2) Due to the nature of coil operation and magnetic fields, the current in the coil is lower when the engine is running and the current decreases as RPM increases.
3) The highest risk of coil damage is when the ignition switch is on and the engine is off.

A coil is a long thin wire coated in a very thin layer of insulating material and is usually wound around an iron core. Exceeding the current limit, even for brief periods, can damage the insulating material. Once the insulating material is damaged, the coil will fail - sometimes quickly and sometimes more gradually.
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Old 09-20-2015, 10:57 AM
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Thanks for explaining coils and the effect of internal heat to me.
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