What is the minimum RPM required for a magneto to generate sufficient current to excite the ignition system when hand cranking?
Interesting question. Although I don't know the technical answer, consider the fact that a properly tuned T will start easily on the hand crank, and cranking produces probably less than the equivalent of 100 RPM (pure guess), I'd say that sets a sort of minimum.
Remember if the timer is not in the right position [spark advance] when the power pulse comes from the mag cranking = nothing!!Bud.
There are simply too many variables to answer your question. The width of the magneto to field pole gap, the strength of the magnet charge and the quality of the coil you are trying to excite are all factors to operate a Model T ignition coil.
You are welcome to give me a call and we can discuss the problems you are having and I will try to help.
Ron the Coilman
The technical answer to your question, like Ron said, is going to be complicated. However, the practical answer is "Not many." If everything is right, a quick 90 degree upward pull of the crank through the compression stroke is all it takes.
Let's put it another way: On a "new" T what would the approximate voltage be generated by the mag on that quick yank upward?
Notice Dave M. appropriately asked about "current" not voltage? If everyone accepts all the "properly working" stipulations the best answer I can give is a quick hard upward 1/4 turn pull will create sufficient magneto current to operate a Model T coil adjusted to Ford specifications.
Same stipulations; the answer to your question is approximately 2 volts ac.
You can easily simulate both conditions by manipulating and watching a HCCT carefully.
Ron the Coilman
Wow. I thought it might be at least in the 4/6 area.
In a nice "clean" environment, moving a single magnet regardless of pole direction past an iron bar wrapped with copper wire will create a current. Moving the positive and then the negative post of a bent magnet such as the ones used on a model t magneto will create an alternating current. And that to the best of my knowledge is how current is produced. Now enter the strength of the magnet, the number of windings of copper wire wrapped around the iron core, the size of the magnet, the size of the iron core and the speed the magnet is past by the copper wrapped iron bar become independent variables in the equation with the strength of current being the dependent variables. By measuring the independents against the dependent, the strength of the correlations can be established in a multiple regression analysis. Keep in mind in some cases the interaction between the dependent scan also have an effect and will also have to be understood. Though an experiment of such a design is best handled with a computer the variables in measurement error will not give you an exact output but will give you the opportunity to predict the outcome as a range. Also because of the inherent variability in other independents such as wire size of the windings, temperature of the environment, distance of the magnet from the iron bar and... Well it wouldn't take long to understand the difficulty of running such an experiment. So I guess the best answer to your very legitimate question is I dunno.
Oh wait this question referenced a Magnito; I assumed we were discussing a magne... Never mind.
Mike, Professor Unwin will explain it all.
steve, you spend too much time on the computer.i'm sure the professor, if given time, would agree
I had the same question a couple years ago. How fast do you have to crank a T to start with mag power?
So I videoed several attempted starts with some starting and some not. Then using the known video frame rate of 30/sec and measuring the angle of crank rotation from one frame to the next I calculated the speed of each no start and each start. I got this:
19.5 degrees in one frame = 97.5 RPM NO start
20.5 degrees in one frame = 102.5 RPM NO start
23 degrees in one frame = 115 RPM Start
24 degrees in one frame = 120 RPM Start
25 degrees in one frame = 125 RPM Start
28.5 degrees in one frame = 142.5 RPM Start
So it looks like at least on my T that anything at about 115 RPM and higher works for mag starting. I'm sure that other cars will preform differently.
The photo is just a single frame of the video.
Posted in this past thread:
For the sake only of discussion there is one important point that you guys are kinda missing. RPM needed to generate a spark can be very slow and as Ron and others have stated it has everything to do with current generated and nothing to do with the amount of voltage generated other than at higher RPM you also get higher voltage. What really determines whether you will get a spark or not is the instantaneous speed when the magnets pass the magneto coil cores. You might be very surprised to have watched Miriam crank her old roadster at a MTFCI tour. She was older then than I am now and she would simply go around front and lift up the crank and the thing started. No battery just mag. The reason is that basically if you have the lever in the correct 3 or 4 clicks down FOR MAGNETO STARTING ONLY then you get some help in your cranking. Remembering that the magneto timing is going to produce a current peak AFTER the piston has gone past Top Dead Center then the compression of the motor is going to give the piston a "shove" down the chute and cause the magnets to suddenly increase their speed just at the instant you need it.
You can be cranking rather slow at 60 RPM which is 1 Revolution per second when cranking your HCCT and you will discover that it is actually very fast on the HCCT but you have no compression helping you. At 60 RPM of literally any HCCT you will get the full 16 sparks but it is kinda hard to crank the engine that fast yet Miriam can start her car with just a pull through compression. Remember that you only have to pull hard to get the piston UP against the compression but that same compression will aid you to speed up the instantaneous cranking speed once you get to the top. Now on a brand new motor with everything tight well you don't get much help but once things are loosened up - it is pretty easy to hand crank start any engine on magneto if the magneto is at all healthy. I have amazed a few people who swore their car would never start on magneto. I spied they had their lever all the way up when they tried. I just advanced the lever a few clicks and gave it a yank and away it went. The coils need to be adjusted correctly and the magneto reasonably healthy but without the 3 or 4 click advance of the lever - nothing is going to happen. NEVER advance the lever at all when starting on Battery or you will get a broken arm or bent starter shaft.
Just to confirm what John and Ron said before.
I took my home build HCCT, a good coil, an analog AC Amp-meter and a digital V-meter.
I found during the test, when there were 16 sparks ( you can't see the sparks on the photos), 1.3Amp ac current and 1.69 Vac.
Watch also my assistant.
Just a question for John or Ron.
Before The coil give the sparks the current went to 1.6A than dropped to 1.3A, is this normal??
David Menzies - You asked what the minimum RPM would be for a start on magneto,......I guess the answer could be that if everything's in good shape and you set the spark advance in the correct position, the answer is,..........something less than 1 RPM, right?
......."a quick 90 degree upward pull of the crank" as Hal and several others have suggested, hardly constitutes even one revolution, right?
You know, a guy could win a few bets or a free beer down at McDuff's Tavern with that one if he "worded" it right!
It is generally of no concern that the meter on the HCCT goes up higher before the points begin to make and break at which time the coil if properly adjusted will reduce the current to the nominal 1.3 Amps. What you need to remember is that the HCCT has a magneto connected directly through a short bit of heavy wiring to an AC ammeter with low internal resistance which is all in series with the coil primary. There is no timer and no timer wiring. There is also no motor compression to oppose a smooth slow cranking speed. With the HCCT when cranking somewhat slowly you will get small "blips" of alternating current as each magnet passes a coil core. That blip if not wide enough in time and/or not sufficient in current to created enough magnetism to pull the points open still is measured on the meter. The RMS value of current is what the meter measures and it is totally possible to have an RMS current reading of 1.6 Amps occurring with lots of different shaped wave forms. Since the T starter or even hand cranking generally produces a crank speed that is faster than the slow cranking of the HCCT, there really is no problem here but just an anomaly of the HCCT.
Steve, I can't quit laughing. The tears are running and my hands are shaking from listening to that. It turned out to be so in-line with what I was attempting to do. I've got to go now. I want to listen to more. Oh, and the rest of you guys, spending so much time on such a question the answer is; you only have to crank it fast enough. Any increase in RPM's is waste once the car is running. I wonder, if I took Professor Unwin and hooked a probe to my HCCT and the other end to his...