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Ground Circuits: Part 9

Measuring generator electron current engine RUN
Tuesday, December 19, 2017 - 08:00
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Let’s say for purposes of discussion the reading obtained by Current Clamp #7 in Figure 11 is 48 A. (DMM indicates .048) That represents a total of 48 A to operate all vehicle circuits during engine run. It’s an aggregate number but does not afford the opportunity to isolate electron current levels drawn by individual circuits. The reading from Current Clamp #7 includes the battery’s recharge electron current. But we are unable to determine how much of the 48 A is battery recharge electron current. Neither can we determine how much current is being drawn by each individual circuit in the electrical system? That is the reason for Current Clamps #1 and #3.

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But the question comes up: “What would happen if one of the circuits in the vehicle began to draw too much electron current? Wouldn’t that cause the generator to burn up?” No!

In our schematic diagram notice that each individual leg of the electrical system consisting of Lamp Circuit #1 and Lamp Circuit #2 (which represents the vehicle’s total electrical system) has its own individual circuit fuse. If a circuit were to fail and draw excessive electron current because the load developed a short circuit or the voltage side of the circuit developed a short-to-ground, the generator reading of 48 A would begin to increase. But what happens at that point? The circuit fuse blows and shuts down that electrical circuit to prevent excessive electron current from overheating the generator and melting the wiring insulation.

I have received some comments from techs about measuring total generator electron current flowing through the generator (output/red) wire connected to +GEN.
Here’s an example.

Hi Vince ………….. The reason I check generator output is because many of the buses I maintain have two generators. And to decipher which generator is not contributing fully even though the voltage outputs are within spec. If there is a better option that I'm missing? Thanks again, Glen B.

I have redrawn the circuit below showing two generators on this vehicle. Both generators are connected to the same point on the B+ (voltage) side. There is a connection to the engine block on the B- (negative or ground) side. Trace the wires. Both generators are connected in parallel.


Figure 2


Electrically speaking, since the generators are connected in parallel, the laws of parallel circuits apply.

The law for voltage in parallel states in so many words: “Two voltage sources connected in parallel have the same (charging) voltage between their B+ and B- terminals.”  

The law for electron current in parallel states in so many words: “The electron current produced by both generators combines into one total electron current for vehicle circuits,” as illustrated in Figure 2. Electron current from each generator channels through the engine block and combines as it goes up the Engine Ground cable. Trace the path of electron current.

The only place available to measure the electron current contributed by each generator is on the B+ (positive) cable as shown by Current Clamps #7 and #8. The reading obtained indicates how many amps each generator is providing. The method that Glen uses to test generator output is valid for its intended purpose of finding which generator in a dual generator set-up is not contributing to the electrical system. Ideally, the two readings should be within a few amps of each other indicating both generators are contributing almost equally. If one of the generators is defective the reading on that current clamp will be very low or zero identifying the defective generator.

Next time we begin discussing ground voltage.


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