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Old 08-11-2018, 07:15 PM   #38
MikeK
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Default Re: question. simple starter motor conversion to 12v

Quote:
Originally Posted by steve s View Post
I don't know what the correct formula is but I'm pretty sure this is not correct. Power is not torque (they have different units). Torque is the relevant quantity here, corresponding to "slam" force.
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.
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