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Old 08-12-2018, 03:45 PM   #41
woofa.express
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Default Re: question. simple starter motor conversion to 12v

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Originally Posted by harryc View Post
The problem I had when running a 6 volt starter on 12 volts was that it engaged with such a jolt that it started chewing up the teeth on the ring gear. I finally switched to a modern starter drive which engages very gently. I love it.

Yes, mine is going to chew up the ring gear teeth if I don't get it modified.
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Old 08-12-2018, 07:56 PM   #42
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Default Re: question. simple starter motor conversion to 12v

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Steve, torque would only apply to a gear tooth engagement load if the rotational inertia / moment of inertia of the starter armature and Bendix assembly was zero. Since this assembly has mass, and in a specific three-dimensional physical arrangement, it gains rotational inertia as it accelerates. That becomes the "relevant quantity" as the gear teeth crash (moving object meets stationary with a huge inertia component). I should have differentiated as to whether, in this starter motor problem, it was average power, instantaneous power, or mechanical power that I made reference to. All three have a time component, but differ in how that is applied.

In the case of this starter motor problem you are looking at near-instantaneous power with the time factor approaching zero, something in the millisecond range as the rotational inertia of the starter and Bendix is overcome and the angular velocity of the huge, massive flywheel and crank assembly increases. The difference between how many milliseconds to gear tooth engagement (slam!) for 6V vs12V is a function of force, in this case the amperage creating the magnetic field. 4X more amperage will result in faster armature/Bendix acceleration. You will have 4X more rotational inertia (function of RPM) at the point of engagement if voltage doubles.

OK, lets look at this slightly differently, as mechanical power. That is a product of force and movement. Movement will always contain a time element (exception- some particle physics). For a running motor, that's the product of torque on the shaft and angular velocity (RPM). Now, for this "slam" problem, movement instantaneously becomes near zero. The kinetic energy in the rotational mass as the gear teeth crash (moving object meets stationary with a huge inertia component) becomes the primary factor in the mechanical force applied to the gear teeth.

That said, I believe I am correct. Power will be a product of the amperage squared times the motor's resistance, you will have increased kinetic energy in the rotational inertia of the starter, and thus 4X the instantaneous 'crash'.

If you want to look at this a bit deeper, reverse EMF in a motor from a moving armature will decrease the amperage as the motor speed increases from starting (zero RPM). This reduces the power output. This is clearly evident in the MTFCA graph, amperage goes up/down as the starter motor slows (compression stroke) and increases (compressed mix passes TDC). At near zero elapsed time on that graph (X axis) the amperage is purely a product of Ohm's law. At about 25ms you see a strong amperage decline as the armature accelerates and creates an increasing reverse EMF. Then you see the up/down/up/down amperage as the starter runs fast/slow/fast/slow across the compression strokes.

Also on that graph you see an interesting huge 'down' spike in the amperage between T=0 and the few milliseconds to the initial start of motor rotation. That is the reverse EMF created by the buildup of the magnetic field in the iron cored motor armature and field. When the field stops building (max, but motor still not moving) the amperage spikes back up. That graph would be extremely interesting and revealing if a third component, angular velocity of the armature, were shown along the same time line.
Mike ,
I feel like we're speaking two different but related languages: you, engineering and me, physics (although perhaps somewhat out of date). Perhaps you could answer a question to see if we have some common ground.
Do we agree that Ohm's Law, V=IR, cannot reliably used for circuits with inductance or capacitance elements?
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Old 08-13-2018, 12:30 AM   #43
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Default Re: question. simple starter motor conversion to 12v

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Mike ,
I feel like we're speaking two different but related languages: you, engineering and me, physics (although perhaps somewhat out of date). Perhaps you could answer a question to see if we have some common ground.
Do we agree that Ohm's Law, V=IR, cannot reliably used for circuits with inductance or capacitance elements?
Steve, Good grief, kind of a loaded question. Even what appears to be a simple purely resistive circuit, like a battery, two wires, and a light bulb will have some, albeit near immeasurable elements of reactance/inductance and capacitance. That said, Ohms law is never an absolute.

Anything that functions on the relationship between electricity and magnetism, like any motor, will be highly inductive and, while running will not simply follow Ohms law. Case in point: The starter motor in this discussion. If it draws 125A @ 6V while running at cranking speed it WILL NOT draw twice that @12 while running. For most parallel dual field DC series motors that number will be about 1.4X the amperage if you double the voltage. Now, does this mean you will not get 4X the power? Yes and no. While running, no. Stalled, yes. At that point it will draw near 2X the amperage. The motor will transition from 4X power (actually ~3.8X if you want to set up a dyno on a test bench for the starter in question) to about 2X as the inductive reactance changes in a non-linear relationship with armature rotational velocity. The near-stall speed 4X (OK, ~3.8X) is what imparts the increased momentum energy in the armature/Bendix that creates the nasty crash engagement.

On anything Ive posted on Model A forums Ive always tried to keep things in perspective of the audience. Not too many engineers here, so I have rarely delved into application of the work of Henry, Farad, Steinmetz, etc. . . for electrical problems. Even if I did, this forum lacks a math editor/font, and if I started posting anything with calc or functions derived from the work of anyone beyond Volta, Ampere, and Ohm Id be talking to an audience of one (You!). I spent too many years scribbling that kind of stuff on blackboards in front of wannabe EEs and MEs with blank looks.
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Old 08-13-2018, 11:02 AM   #44
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Default Re: question. simple starter motor conversion to 12v

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... I spent too many years scribbling that kind of stuff on blackboards in front of wannabe EEs and MEs with blank looks.
Mike, Okay, that little flashback is a good place to wrap this up. Now I'm picturing years of blank looks staring back at me in P. Chem. classses. I'm supposed to be doing this for fun.
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Old 08-13-2018, 09:22 PM   #45
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Default Re: question. simple starter motor conversion to 12v

I was a blank look once but I am feeling much better now.
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Old 09-25-2018, 09:23 PM   #46
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Default Re: question. simple starter motor conversion to 12v

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good day.
I went and put a starter bendix spring in line. it now has slightly less of an impact. Only slightly less but not very significant. I have ordered a new field coil. gary
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Old 09-26-2018, 07:54 AM   #47
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Default Re: question. simple starter motor conversion to 12v

I'm surprised nobody mentioned this:

http://www.qualitypowerauto.com/item...-Flatheads.htm

Not the cheapest, nor original, but probably the simplest solution to convert from 6V to 12V.
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Old 09-26-2018, 10:00 AM   #48
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Default Re: question. simple starter motor conversion to 12v

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on anything ive posted on model a forums ive always tried to keep things in perspective of the audience.

Not too many engineers here.

Id be talking to an audience of one (you!).
Mike I always enjoy your posts. Well thought out and although too technical for some not so for others. I understand them.

Not too many engineers but I'm sure more than 1.

So an audience of perhaps 2.
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Old 09-26-2018, 01:47 PM   #49
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Default Re: question. simple starter motor conversion to 12v

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Originally Posted by MALAK View Post
I'm surprised nobody mentioned this:

http://www.qualitypowerauto.com/item...-Flatheads.htm

Not the cheapest, nor original, but probably the simplest solution to convert from 6V to 12V.
I have mentioned those many times.
I have used those since they came out many years ago.
They can be had for around $100 on eBay if you watch for them.
At one time I even showed how to use a floor type headlight dimmer switch mounted in the original start pedal hole to activate it.

Last edited by Pete; 09-26-2018 at 06:15 PM.
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Old 09-26-2018, 03:54 PM   #50
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Default Re: question. simple starter motor conversion to 12v

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Originally Posted by MALAK View Post
I'm surprised nobody mentioned this:

http://www.qualitypowerauto.com/item...-Flatheads.htm

Not the cheapest, nor original, but probably the simplest solution to convert from 6V to 12V.
I wished I'd purchased that new starter. I did purchase a new field coil and airfreight it hopefully in time for our national meet. the coil cost, in AUD 70 and the freight AUD 99. To airfreight the new starter would have cost me my left you know.
Mostly I get my sons who are frequently in the US to pick up but time didn't permit.
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Old 09-26-2018, 04:15 PM   #51
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Default Re: question. simple starter motor conversion to 12v

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Originally Posted by Pete View Post
I have mentioned those many times.
I have used those since they came out many years ago.
They can be had for around $100 on eBay if you watch for them.
At one time I even showed how to use a floor type headlight dimmer switch mounted in the original start pedal hole to activate the it.
Pete,

That's interesting. Their contact information for Quality Power indicates they are located in Yucaipa, CA, about 15 miles from my house!

I might stop by and talk to them.

David Serrano
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Old 09-26-2018, 05:50 PM   #52
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Default Re: question. simple starter motor conversion to 12v

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Pete,

That's interesting. Their contact information for Quality Power indicates they are located in Yucaipa, CA, about 15 miles from my house!

I might stop by and talk to them.

David Serrano
Wow! I didn't notice that when I posted their link. I'm just a few miles myself.
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Old 09-27-2018, 08:24 AM   #53
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Default Re: question. simple starter motor conversion to 12v

I once took a Pontiac starter apart that someone had cut some of the pole pieces off, trimmed both ends of each pole pieces a bit, and stuffed some plastic in to keep the windings centered. I imagine they were attempting to reduce the efficiency of the starter, by reducing the magnetic field. This would be wastful of battery amps I suppose, but would reduce the slam effect.

In thinking about it, I suppose you could also reduce a starters power output by rotating the end plate a bit one way or the other, so the brushes moved the armatures magnetic force to a less than optimal position, again, this would reduce power output, slow the speed, but not reduce the current draw, so this is not the "best" solution.

However, if I was in Australia, I might be tempted to try one of those "outside the box" ideas rather than pony up for fast shipping if I was in a pinch. Just a thought...
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Old 09-28-2018, 12:17 PM   #54
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Default Re: question. simple starter motor conversion to 12v

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Originally Posted by MALAK View Post
I'm surprised nobody mentioned this:

http://www.qualitypowerauto.com/item...-Flatheads.htm

Not the cheapest, nor original, but probably the simplest solution to convert from 6V to 12V.
This looks very much like the one I have on my pickup, that came from Snyder's.
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Old 09-28-2018, 03:47 PM   #55
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Default Re: question. simple starter motor conversion to 12v

Thankyou all for your help. I value it.


I have a new field coil coming but regret not buying a whole new starter motor. But it's done now. thanks again, gary

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Old 10-14-2018, 06:46 PM   #56
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Default Re: question. simple starter motor conversion to 12v

I took the starter to an old an versatile fellow who is jack of all trades and master of most. he was uncertain and endeavoured to seek help. however he came back without success, perhaps in fear of damaging it.
I ordered a 12V field coil from Snyders which I now have in my possession and will convert with that.
I got various opinions on reducing slam by running power through bendix coil and amp meter showed no reduction nor resistance and auto electrician said it was bull shit. However I did try it and yes it did reduce somewhat but insufficient.
Since all of that an A owner demonstrated his new type, made in UK which engaged prior to rotation. This was most impressive and I wish I had known about it prior to purchasing field coil.
I thank you Corley for your input which has been received with gratitude and all others who likewise gave suggestion. gary.
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Old 10-14-2018, 06:48 PM   #57
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Default Re: question. simple starter motor conversion to 12v

The starter I spoke about is what is illustrated by CarlG number 54 below.
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