How many AC volts can coils have

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Model T Ford Forum: Forum 2008: How many AC volts can coils have
Top of pagePrevious messageNext messageBottom of page Link to this message  By A.Boer on Saturday, December 20, 2008 - 10:21 am:

A friend of our Dutch Model T Register made an
electrical eguipment to charge the magnets in the
car [engine] He used it with greet sucses and can start his engine with the first pull.
I used that equipment on an engine that had 7 vac
at maximum speed. After charching that same engine, has 20 vac at low speed and 35-40 volt ac
at high speed !!!!!!
My guistion now is how much acv can ignition
coils have
MERRY CHRISTMAS and a HAPPY MODEL T YEAR
Greetings from Holland
Toon


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Saturday, December 20, 2008 - 09:02 pm:

Toon:

The answer is "it depends". The amount of AC voltage that is applied to the coil is directly proportional to the engine speed. When the AC voltage is at 60 cycles (450 RPM) then the magneto is putting out about 10 volts. If you double the RPM you also double the frequency and the voltage output should also double if all is working OK. Thus at 900 RPM (120 cycles) the mag voltage is putting out 20V nominal. The coil operation is basically the same since its primary is an inductor and thus you haven't really changed the power to the coil since you have doubled BOTH frequency AND voltage but because of the inductive reactance of the coil primary - its AC resistance also then doubled. In fact for the coil to work properly with your magneto - the voltage HAS to go up with increasing RPM (doubling the RPM should double the voltage) or the thing won't run. From a practical point of view you cannot keep increasing the voltage and frequency forever and expect the coil not to care since there is a FINITE amount of time necessary for the points to operate and if you go too fast that won't happen and then the coil points cannot fire on each pulse to thus limit the primary current. I suspect that upwards of 3000 RPM, the coil points couldn't keep up but that is a WAG (wild a-- guess). Somebody with some time, a well balanced HCCT driven by variable speed motor and a properly adjusted coil might perform an experiment and see when the coil quits due to high RPM limitation of points but be careful since spinning the large flywheel at 3000 RPM is going to have a lot of energy.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Noel Denis Chicoine, MD on Saturday, December 20, 2008 - 09:57 pm:

John, doesn't the HCCT have 16 points for 1 coil to fire in 1 revolution, whereas the T ignition system only fires the coil once every 2 revolutions. (OK, there may be 2 or 3 as the timer still may have contact at the next 2 sine wave peaks but the first one ignites the charge) I would guess that a HCCT at 3000rpm would be like a Model T at 96000rpm. I must be missing something.
Noel


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Monday, December 22, 2008 - 01:42 am:

Noel:

The coil fires on either peak of the AC waveform so 8 cyles are generated each revolution and that produces 16 sparks if there is no timer and the coil is connected directly to the mag as with an HCCT. Disregarding the fact that the coil takes a bit longer to rise and fire for the FIRST spark than it does for all the rest of the sparks in the train while the timer dwell is present, the fact is that the points take a FINITE amount of time to move from their AT REST position to the point where they then OPEN. Normally the determining factor as to how long this takes is governed by the signal driving the coil namely the magneto voltage. The higher the applied voltage, the faster the coil rises to the firing point. That rise time gets shorter and shorter as the RPM increases because both the frequency and voltage are increasing from the magneto so the points fire faster and faster until finally they cannot follow any speed increase of the applied voltage so then this finite amount of time becomes the determining factor as to how fast the engine can go before the coil will MISS a beat. That is what I was guessing at. Assuming that once the coil cannot follow the AC waveform connected to it then the ignition has reached its highest RPM and that is true whether the timer is connected as needed in an engine or whether there is no timer as with an HCCT. I frankly have never tested it so I just guessed. I could be way off. I could probably calculate a better educated guess based upon magneto pulse width in REAL TIME at a given RPM and assume that the coil would have to have at least 1 or 2 milliseconds to ramp up and fire but I just haven't done any such testing. If a coil needs at least 1 millisecond to fire then that means it would be coming from a flywheel spinning 1 Revolution in 16 milliseconds which is 3750 RPM. I kinda doubt the T coil would fire that fast but I am not going to swear it can't.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Barker on Monday, December 22, 2008 - 06:45 am:

I'm puzzled. If you connect a coil to a DC supply, it fires at 150 to 200 cycles per second. This seems to imply that it takes about 6ms to go from rest-to-spark-to-rest. Let's guess that rest-to-spark takes 4 or 5ms.
At 2000rpm, the approximate maximum for a standard T engine, the magneto produces 533 voltage peaks (+ve and -ve) per second, so each pulse takes just under 2ms.
So how does the coil manage to fire at this speed?

A related question; if it does fire in these conditions, does it also fire at each of the 7 succeeding voltage peaks available during the half-crank revolution (~90 timer degrees) that a roller timer is in contact? Subjectively, I would say 'no'. If you listen to a coil box with an engine running (slowly), on battery you hear 'buzzes', whereas on magneto you hear 'clicks'.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ron Patterson on Monday, December 22, 2008 - 11:46 am:

Chris
The coil operates differently on DC than on AC. When operating on DC the coil points do not go from REST to SPARK to REST, the points operate in a very narrow range with the points very close together. On magneto (depending upon engine speed) the points open wider because the coil primary winding must fully reverse polarity of the next pulse.
In the last part of John Regan's message above you can see the guesstimate theoretical maximum limit of the points to follow the magneto current pulses at higher engine speeds.
John and I have discussed this subject at length. As he points out more testing needs to be done in this area and I hope to do that when we finish the variable speed motor driven flywheel magneto/coil/timer dyno we are building.
On your last point, the camshaft (timer) rotates at one half the speed of the crankshaft and the timer dwell case segment dwell is 90 camshaft degrees. i.e. For every 90 crankshaft degrees the timer roller moves 45 degrees. It is theoretical possible that two magneto pulses will occur during this interval. Depending upon engine speed and spark lever position the coil will likely only respond to the first of these current pulses and only the first current pulse that produces a coil spark output fires the mixture charge in the cylinder. The other coil operations will occur at lower engine speed, but as speed increases the coil will start to miss every other pulse. I have looked at this with an oscilloscope connected to a running engine, but is it very hard get good traces because of the noise created by the operating coils.
Ron Patterson


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Monday, December 22, 2008 - 11:58 am:

Chris:

The speed at which any magnetic field builds up when controlled by an electromagnet is directly proportional to the amount of voltage applied. Thus your statement of "If you connect a coil to a DC supply..." is not a defined set of circumstances unless you state what the DC voltage is. The higher the voltage the faster the coil will rise to the firing point thus the "delay" to fire it shorter. Eventually applying further higher voltages will NOT result in any further increase in the speed of the coil. This is the RPM limitation I was referring to in my previous posts. You need to read Ron Patterson's articles on the Model T ignition and how it works. Those articles are excellent. You can find them at my web site for free at www.funprojects.com - look in the document library.

For design reasons known only to Ford, the timer closes for about 87 crankshaft degrees (not 90) so that encompasses at least 3 and usually 4 pulses from the magneto during that timer closure. The FIRST pulse does the job and its time to fire is just a wee bit longer typically than the next pulses that immediately follow because the coil is typically fully AT REST before that first firing. Depending on engine speed, subsequent firing on adjacent pulses may be faster since the points may NOT have had a chance to return to the AT REST position.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Barker on Monday, December 22, 2008 - 02:25 pm:

Ron, John,
thanks for taking the trouble to de-mystify me.
As Ron knows, I have read his excellent articles and Forum postings - including the phrase that the T ignition system is a trap for logical people (or something like that).
I didn't mention a voltage because I don't sense much effect - maybe one musical tone higher at 12V DC than 6V DC, which would be 12%. That's why I was surprised at the 'guestimate' of a minimum of 1 - 2 ms to fire a coil as it implies an operating frequency above 500Hz.
Ron seems to confirm that at higher speeds, there won't be a spark for every 1/16 crank rev. and I understand the point about the coil not going back to rest, especially when fed DC.
I look forward to hearing about the experiments yet to be performed.

Thanks


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Monday, December 22, 2008 - 08:32 pm:

Chris:

The formula and bench testing confirms that a T coil is exactly twice as fast using 12V battery versus 6V battery. The rate of rise of current in an inductor is directly proportional to the voltage applied unless the inductor is near saturation. It turns out this is exactly why a T coil needs more than 6V to run well at higher RPM. NOT for more spark energy but just to get the coil charged up quicker to firing point so the timing doesn't seem sluggish. This is all shown graphically in Ron's article. I did the bench testing of the coil and the graphs are in Ron's fine article.


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