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Motors, light bulbs, and other loads with high reactance are non-ohmic. This means the resistance you measure with your meter is not the full story on how these loads oppose current flow (the movement of electrons). Sam has a blog post on this that explains more: https://appliantology.org/blogs/entry/1110-when-ohms-law-doesnt-apply/
This is also why we don’t waste time using ohms to check AC loads. It’s not a useful measurement. Instead, use amps and compare to the wattage spec.
What would be a situation where you would want to use VAC instead of LoZ?
Only situations where you don’t want to place a load on the circuit. This is rare in the AC world of appliances repair. For example, if you wanted to look at the voltage gradient between Neutral and Ground. LoZ setting would show 0 VAC; VAC setting may show a couple of volts.
In the DC world, you’re almost always dealing with low amp, limited wattage power supplies so ghost voltage is not an issue. You would always use the VDC function because you do not want to place a load on these circuits.
So many variables to this hypothetical scenario that can affect the results. Not sure what I can offer that would be helpful.
So, you’re saying sweep the leg?
Not sure what you mean by that but I think you got the idea. đź‘Ť
Am I on the right track?
Is this a purely hypothetical scenario or are you recounting an actual service call?
So in an AC circuit, it doesn’t matter the relative position of the load to the actual shunt. So, only in a DC circuit would the load need to be “downstream” of the shunt?
No sure how you could draw that conclusion based on what I said. Let’s look at it again…
“Upstream” our “downstream” doesn’t matter. If it did, alternating current, which goes back and forth 120 times a second, wouldn’t work!
Nothing in there says that AC is different from DC as pertains to shunts and shorts. I was using AC an example TO ILLUSTRATE why the upstream/downstream issue does not apply.
No, AC or DC makes no difference in the load being upstream or downstream of the short or shunt. In fact, it’s best to not think of upstream or downstream in this context. Get away from any water analogies for electricity. Think in terms of electrons bond to each copper atom. That is what current actually is. And those dumb electrons do exactly what voltage tells them to do, always seeking the relatively more positive voltage. They don’t move unless a voltage difference between two points in a complete circuit force them to move.
Yep, just one load connected to the power supply is all it takes to prevent a short condition. “Upstream” our “downstream” doesn’t matter. If it did, alternating current, which goes back and forth 120 times a second, wouldn’t work!
The shunt is only bypassing loads D, E, and F. Load C is still in the circuit
To verify that the ground is valid, I could first set my multimeter to VAC and check the voltage across the receptacle to verify incoming power (120 V).
Meter setting on VAC may give you ghost voltage readings. You almost always want to use LoZ for VAC testing, including this situation. You also didn’t specific which terminals in the receptacle you would place your probes. If you wanted to see if the ground was valid, you could measure from the hot terminal to the ground on LoZ (NOT neutral and ground). Should see 120 VAC if the ground is continuous to the breaker box. If not, you’ll see 0 VAC. If you do this same measurement on VAC, you may see ghost voltage if the ground is not continuous to the breaker box (open ground) which can be deceiving.
We go to plug in our washer and the same thing happens, a bit of copper wire connects the hot prong and the neutral prong. The fact that the ground is not valid has no bearing on the interaction between hot and neutral right?
Correct. If you have a valid Line and Neutral that get shorted, the breaker will trip regardless of what’s going on with Ground.
And so the copper bit falls but it hits hot and ground. Current wouldn’t even flow because, since ground isn’t bonded to neutral at the breaker box, there would be no complete circuit back to the source right?
Correct.
Finally, the copper bit falls and lands on neutral and ground. Again, nothing happens right?
Correct,
However, if we perform the test as shown in figure 6-23, would we not be determining the resistance of the entire circuit since R1 has not been removed or isolated?
Good question! You’ll notice in the drawing on page 94 that they don’t show a power supply connected to the circuit. This means it’s open on both ends– nothing connected to them or connecting them together. Hold that thought…
Do you know why your meter requires a battery to measure ohms? Because when you measure resistance with your meter, your meter is actually pushing electrons through the load being measured and sensing the voltage drop across the load. The electrons are being pushed from one probe, traveling thorough the load, and then “sucked up” on the other probe.
In that circuit as drawn, the electrons have no path to R2. The electrons will leave one probe, travel through the load, and go up the other probe. Since the other end of R2 is open, there is no possibility that the electrons will travel through R2 and confound the reading.
Oddly, it does not affect receptacles 4 or 5 either (We verify this with our Voltmeter).
You may have been seeing ghost voltage depending on how you made this measurement. Were you making this measurement on LoZ or using the regular VAC setting on your meter?
From this, we could determine receptacle 3 is wired in parallel to receptacles 4 and 5. Does this thinking make sense?
Are any of these receptacles wired into a three-way switch?
The most common problems with these washers pertain to the actuator (Whirlpool) or mode shifter (GE). These components in both machines control the mode (spin or agitate) of the basket movement. Problems with these components will result in no spin or no agitate complaints.
In the Whirlpool VM washer, the actuator also functions as the tachometer (speed sensor) for basket rotation, telling the control how fast the basket is rotating. A common fault code you’ll see in this machine is the F7E1 error code. This usually points to a failed actuator but there are other components that can cause this as well. This service bulletin has more info on this. You may also benefit from my training notes on the VM washer.
An open coil on the mode shifter in a GE TL washer can cause the wash basket to spin during agitate. This service bulletin addresses this common problem with these washers.
Here are a couple off webinar recordings that give more info on these BLDC fan motors and the inverters used to control them.
RE: Your question on the video. That section of the video was explaining the PWM signal portion of the inverter’s input. An inverter can be constructed to be able accept a PWM signal that tells it things like varying the frequency or amplitude of the applied voltage to the motor. This changes the motor’s speed and/or direction, depending on how the PWM signal is coded.
Not all inverters are capable of accepting a PWM signal– it’s an “upgrade” feature.
So I am reviewing some videos and I think I found where I got this idea. Is this a typo or did I just misunderstand what you meant?
I attached a screenshot from the video.Which module and unit are you referring to with the video at the 16:40 mark with your question?
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