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Old 11-13-2012, 06:05 PM   #21
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42MT Starter motor specifications :: Delco Remy

The specs may read as follows:

5. DATA
Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 v dc
Torque (lock min) . . . . . . . . . . . . . . . . . . 26 1b-ft (at 500 amps, 3.5 volts)
Pinion speed (no load) . . . . . . . . . . . . . . . . . . . . . . . . . . 7000-10700 rpm
Pinion rotation (facing drive-end) . . . . . . . . . . . . . . . . . . . . . . . . clockwise
Number of teeth on clutch assembly (pinion) (one tooth blank) . . . . . . . . . . . . . . . .12
Mounting data:
Number of mounting holes... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Diameter of mounting holes . . . . . . . . . . . . . . . . . . . . . . . . . 0.6592 in.
Mounting hole circle diameter. . . . . . . . . . . . . . . . . . . . . . . . . 5.75 in.
Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.96 0.12 in.
Diameter (field frame) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.56 0. 04 in.
Weight (approx) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 lbs
4
Sorry, no "Locked Rotor Current" values given here. I suppose you could disable the starting on your specific engine and then use a clamp on DC amp meter around the + battery cable for a reading X seconds it was spun overvv
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Old 11-13-2012, 06:12 PM   #22
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Sorry, no "Locked Rotor Current" values given here. I suppose you could disable the starting on your specific engine and then use a clamp on DC amp meter around the + battery cable for a reading X seconds it was spun overvv
What part of "torque (lock min) with amps are you having trouble with?

If you lock the rotor the spec say you will get a minimum torque of 26 lb-ft while drawing 500A with a voltage across the terminals.

A motor will produce the highest torque at 0 rpm.
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Old 11-13-2012, 06:46 PM   #23
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You can determine the theoretical locked rotor current of a motor by measuring the DC resistance of the motor and using that figure, the supply voltage and ohm's law to calculate the current.

In reality, the current will be less because of the voltage drop due to the resistance of the connecting cables and the internal resistance of the battery. Also, in reality, the cables will probably melt or the battery will explode pretty quickly.
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Old 11-13-2012, 06:49 PM   #24
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Rick, I'm being nice here, but " lock min" does not equal an acceptable amperage draw for "Locked Rotor Current". Besides, a starter has a armature. Not a rotor��. I just don't to have guys tooling for a number out there that doesn't exist.
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Old 11-13-2012, 06:52 PM   #25
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............ You don't even need a shunt, figure out what length of your battery cable equates to a resistance of .001 ohms and measure the shunt voltage at those points.
As long as you account for the increased resistance of the cable as it heats up from being overloaded.
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Old 11-13-2012, 06:55 PM   #26
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Old 11-13-2012, 08:03 PM   #27
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.......... If one wishes to work with electricity and wiring on the DC system of one's boat, knowing X-sectional area of the conductors used is far from BS. It is the basis of wire sizing and current capacity...............
The crossectional area of a conductor is important and something that's discussed in classrooms and textbooks, but in real life, we either follow the manufacturer's installation instructions as to wire or cable size or we use tables based on wire or cable size and allowable voltage drop.

In real life, I've never seen a tech strip a conductor and then use callipers and a calculator to determine the current carrying capacity.
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Old 11-13-2012, 08:37 PM   #28
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Ford Lehman manual for the 120 specifies locked rotor draw of 1070 A, running draw of 650 A.
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Old 11-14-2012, 06:26 AM   #29
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In real life, I've never seen a tech strip a conductor and then use callipers and a calculator to determine the current carrying capacity.
Now why is that not a surprise? I bet there is a lot of real life stuff you have never seen a marine technician do.

Since you seem to be more interested in trolling for an argument than anything else, and don't appear to know about stuff like this, (not that that has stopped you from being insulting about it) one reason most folks don't need to measure the wire cross section is because the wire size printed on the sheath reflects that figure.

Now, read very very carefully ... since the OP probably knows the cross sectional area (you can call it wire size if it helps you) of his starter cable, he doesn't need a tech to strip it and measure it with calipers. He most likely does not know the cross sectional area of the shunt but he can determine that by measuring the width and thickness of the conductors and then compare that figure with the starter cable.

If that is a bit much for you to take in all at once, ask someone to help interpret.

I see that another reader has also provided you with information you have never been exposed to in the form of the Lehman locked rotor current. Just in case you need more information or don't believe his post either, have a read through the link that follows and if you can read the charts (the figures might be a bit small print) you will learn a lot about the current draw, voltage, and rpm relationships in a starter motor.

Starter Motor
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Old 11-14-2012, 06:37 AM   #30
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"I want to be able to measure the total usage of the battery bank. "

Wit an engine in modest condition that starts in 2 to 45 seconds of cranking the juice used is to minor to worry about.

And not worth the danger of putting a fuse in the circuit. At high loads a shunt becomes a fuse.

A SOC meter for the HOUSE batt setrequires a shunt, is a good idea and probably should be the first up grade on a boat being upgraded from dock condo to cruiser.

This will have its own ground so the shunt would be matched to the highest loads.

Probably an air cond or water maker powered by an inverter.

If you wish to monitor the starter system , the Volt meter should be powered by the starter terminals.
Watch the voltage drop during cranking .
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Old 11-14-2012, 06:53 AM   #31
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The OP wrote "I want to be able to measure the total usage of the battery bank. " and went on to ask if there would be any problems.

So why does everyone have to try so hard to convince him to do something else? It is none of our business why he wants to do something or if it is even practical or has any use other than personal amusement. Why do any of us have a boat? Probably very few of us need one for transportation or safety and owning one isn't necessarily the best idea in the world. How would you feel if people starting posting here about what a bad idea it is to have a boat?

Why not just answer the question and if you can't or don't know what the question means then keep out of it?

And why is a shunt any more likely to become a fuse than any other part of the circuit? Geez, the wiring inside a starter or alternator is microscopic compared to the battery cables or a shunt, how come that doesn't upset anyone?

Either help the guy with his project or not, it is HIS project and HIS choice to do it, he didn't ask what you thought about the idea, just if it can be done and if the shunt would present a problem.
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Old 11-14-2012, 07:21 AM   #32
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and if the shunt would present a problem.

YES, it poses a danger.
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Old 11-14-2012, 07:40 AM   #33
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Either help the guy with his project or not, it is HIS project and HIS choice to do it, he didn't ask what you thought about the idea, just if it can be done and if the shunt would present a problem.
It always happens when people feel there is a better way they state their opinion
This is good, its how forums work
Even a manufacturers tech support person may question the op's desires and give alternate ways
This may give the OP pause for thought , but as you say Rick its his decision in the end
Hopefully he will come to a informed conclusion based on the information here
He just has to filter out all the bickering comments in this thread
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Old 11-14-2012, 07:52 AM   #34
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This is good, its how forums work

Thread expansion is probably the most useful part of most responses as frequently the OP has no idea what he is actually asking .

Folks that have a desire sometimes need help in defining their Desirements.
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Old 11-14-2012, 09:18 AM   #35
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... frequently the OP has no idea what he is actually asking .
It seems a bit arrogant to assume that when someone asks what time it is he really wants to know what readers think is the best watch is for themselves.

It is noon somewhere, why do you ask?
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Old 11-14-2012, 09:21 AM   #36
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... YES, it poses a danger.
Really, what danger is that? Has there been a rash of shunt induced boat fires recently?
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Old 11-14-2012, 09:27 AM   #37
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ask a simple question!
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Old 11-14-2012, 09:43 AM   #38
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ask a simple question!
You should know better
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Old 11-14-2012, 11:25 AM   #39
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Ford Lehman manual for the 120 specifies locked rotor draw of 1070 A, running draw of 650 A.
I stand corrected. And informed!
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Old 11-14-2012, 11:28 AM   #40
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As long as you account for the increased resistance of the cable as it heats up from being overloaded.
True. Or the increased resistance of a cold engine.
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