I am not saying information is often needed . It may be of interest to somebody.
This is a somewhat technical discussion. In a well, you may not know the wire resistance of a pump that is in place. With (assumption #1) that the 3 current carrying wires have equal resistance for practical purposes, we can measure both the resistance of the pump windings and the resistance of the wires.
This uses Ohm's law, a multimeter, and a current source of some sort. Note that most of us don't have a calibrated "current source" on the bench. You can make one with a battery and a resistor. In the drawing 1 amp is shown, but that is arbitrary for simple math. 600 milliamps or 2 amps or anything in between is fine. You want something big enough to make the voltages high enough to measure easily, and low enough to not overheat the windings. Feed a battery through a power resistor, and you can know the current for other calculations. Put the resistor into line. Measure the voltage across the resistor. The current (I) is the voltage divided by the resistance. Measure for each hook-up, and use the result. Alternatively measure the current with an ammeter in series. Use the actual measured current wherever 1 amp is referenced below. Also note that (Vc-Vb) means the voltage between Vc and Vb. The resistor will need to be a power resistor, such as a 10 ohm 20 watt resistor. The power consumed in the resistor is the voltage across the resistor times the current in amps.
For each measurement, we pass current through the common (yellow) wire and one other wire. A key is that the other wire is not carrying current (assumption #2). So in the drawing shown, we know V2 = V3 = Vc. Thus the resistance of the yellow wire is (Vc-Vb)/1 amp. Due to assumption #1, we now know the resistance of each of the 3 wires.
We then know that the resistance of
(Rblack+Rrun+Ryellow)= (Va-Vb)/ (1 amp)
So subract 2*Ryellow, from the total, and you have Rrun.
Change things around running the current through Rred instead of Rblack. Then remeasure+compute the new current, and you can compute the Rstart resistance.
This is a somewhat technical discussion. In a well, you may not know the wire resistance of a pump that is in place. With (assumption #1) that the 3 current carrying wires have equal resistance for practical purposes, we can measure both the resistance of the pump windings and the resistance of the wires.
Edit: Note the simpler method at the bottom of reply #7 below just uses ohmmeter and math. A 4-wire ohmmeter with Kelvin clips makes low resistance readings more accurately, but a good 2-lead ohmmeter can do pretty well. The current source method just below would be more accurate, because the probe tip resistance does not matter, the ohmmeter-only method in #7 is simpler.
This uses Ohm's law, a multimeter, and a current source of some sort. Note that most of us don't have a calibrated "current source" on the bench. You can make one with a battery and a resistor. In the drawing 1 amp is shown, but that is arbitrary for simple math. 600 milliamps or 2 amps or anything in between is fine. You want something big enough to make the voltages high enough to measure easily, and low enough to not overheat the windings. Feed a battery through a power resistor, and you can know the current for other calculations. Put the resistor into line. Measure the voltage across the resistor. The current (I) is the voltage divided by the resistance. Measure for each hook-up, and use the result. Alternatively measure the current with an ammeter in series. Use the actual measured current wherever 1 amp is referenced below. Also note that (Vc-Vb) means the voltage between Vc and Vb. The resistor will need to be a power resistor, such as a 10 ohm 20 watt resistor. The power consumed in the resistor is the voltage across the resistor times the current in amps.
For each measurement, we pass current through the common (yellow) wire and one other wire. A key is that the other wire is not carrying current (assumption #2). So in the drawing shown, we know V2 = V3 = Vc. Thus the resistance of the yellow wire is (Vc-Vb)/1 amp. Due to assumption #1, we now know the resistance of each of the 3 wires.
We then know that the resistance of
(Rblack+Rrun+Ryellow)= (Va-Vb)/ (1 amp)
So subract 2*Ryellow, from the total, and you have Rrun.
Change things around running the current through Rred instead of Rblack. Then remeasure+compute the new current, and you can compute the Rstart resistance.
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