Sam Brown

[Sam Brown]

Why wouldn’t we read 120 if we didn’t have a valid neutral?

All voltage measurements are a comparison between two points. You put one lead on the place where you want to measure the voltage, and the other lead on a reference point that you know has 0 volts of potential — that’s Neutral. If your reference point is not connected to Neutral, then your measurement is undefined.

An undefined voltage measurement with a loading meter will read 0 volts. That’s because, when you’re missing Neutral, there is not a valid circuit, and so there’s no voltage drop across your meter.

If you’re using a non-loading meter, like the VAC function of your multimeter, things can be a bit more fuzzy. You might still read 0 volts, but you could also read some ghost voltage. This is why we recommend that you always use a loading meter for AC voltage measurements.

Either way, you would not read a solid 120 VAC unless you have a valid Neutral.

[Sam Brown]

Would this mean that the door switch is for sure good? You would still measure 0 across the push to start even if neutral was open because of the door switch, right?

The 0 VAC reading by itself would be ambiguous, but we did a reading before that with the switch open and got 120 VAC. We would not have gotten this reading if there were not a valid neutral. That’s how we know that we are getting a valid neutral through the door switch.

[Sam Brown]

Service manual clearly stated all voltages supplied from main board. This model has 3 different Voltages, supply, feedback and signal. Was getting supply voltage and signal but no feedback.

The feedback signal is definitely not an output from the board. It is a signal generated by the motor’s Hall sensor when the rotor spins, which is then sent back to the board. So it is an input to the board, not an output.

If the manual says or implies that it is an output, then that is an error in the manual. Errors like that are not uncommon, especially in manuals that have been translated, like LG’s.

Hopefully that clears it up! This was a great question, and in the future, questions about problems you encounter on the job are usually best posted at Appliantology. That way, you can get help not just from us, but from a bunch of other experienced techs, too.

[Sam Brown]

I guess with a compressor, the line voltage is supplied separately to the inverter. It is using the line voltage (AC) to put out the appropriate DC power, which the switches are commutating into effectively AC power, right? So it goes AC input from line, changed to DC output that board is actually creating but that output is effectively changed back to a 3 phase AC voltage via the switches?

Great summary! This is all correct.

For example, this BLDC compressor motor shown in the second video of 1.13 at about 15:12. The return circuit for the BLDC motor connects to the inverter. This portion is an AC circuit? That would require my meter to be in AC and requires the neutral that is connected to the inverter to be a complete circuit?

You can’t really make sense of the output of the inverter with a standard meter because it’s 3-phase power and the frequency varies widely from your standard 60 Hz power. But yes, the inverter takes a 120 VAC power supply (which consists of both Line and Neutral — you need two legs to make a power supply), rectifies that into high voltage DC power, and then commutates that into what is effectively 3-phase AC power with varying frequency.

And yes, separately from all that, the inverter also converts its 120 VAC input to a low voltage DC power supply for its internal logic.

[Sam Brown]

1. In a slave system, how is the freezer compartment evaporator coil supplied with liquid refrigerant (rather than vapor) after the refrigerant passes through fresh food evap coil? Won’t the refrigerant turn from liquid to gas in the first evap coil and therefore only deliver vapor refrigerant to the freezer evap coil? Generally, it goes evap coil – compressor – condenser in order to return the refrigerant to a liquid so that it can change back to vapor in the evap coil.

Only a portion of the refrigerant passing through the first coil turns to vapor. So a mix of liquid and vapor is then fed into the second evaporator, which turns into 100% vapor by the end of the coil. This way, you get cooling in both evaporator coils.

2. The last video states that the lines leaving the evaporator coils in a dual evap (with 3 way) merge together to form the suction line (3:50), which leads to the compressor. It later says when the fresh food compartment needs cooled, the compressor sucks refrigerant from the RC evap coil (5:39). Does it have the ability to suck refrigerant from only the RC coil, is that not what the video meant, or is that irrelevant?

The compressor sucks refrigerant from both evaporators, since the suction tubes merge before the compressor. However, if the 3-way vale (called the rotary valve in this video) is in a position such that one evaporator is closed, then no refrigerant will move through that coil.

[Sam Brown]

4. I assume the reed valve is not fixable – requires new compressor?

Correct, that’s an internal component of the compressor, so the compressor must be replaced if the reed valve is damaged.

[Sam Brown]

In the second video of 1.13 at 15:19, the evaporator fan is compared to the condenser fan. The evaporator fan is a BLDC fan with an inverter in the fan. I do not see where this fan has its own Line supply to convert into the 3 phase AC voltage that would power this BLDC. I believe I am confused

Little BLDC motors, like the ones used for evaporator fans, run on much lower voltage. This is why they usually take something like a 12 VDC power supply to their internal inverters. You just don’t need a lot of juice to run these guys. But they’re still doing the same thing inverters always do — making 3-phase power.

A compressor, on the other hands, needs a much higher voltage. Hence why compressor inverters take the full 120 VAC line supply.

I believe I understand that all motors must run on AC, and are named a DC motor if that is the power supply. The brushes commutate, etc. But for the compressor, why is it a BLDC motor if the AC input into the inverter is being used to power the compressor?

The inverter does output DC power, but this is then commutated by the switches into what is effectively AC power. Basically, the electronic switches in the inverter are doing the same thing the brushes do on a DC brushed motor. It’s simply being accomplished electronically rather than with physical brushes.

[Sam Brown]

The reading mentioned internal thermal protectors (that require the compressor to be replaced), but the videos did not. Are they not prevalent in residential refrigerators?

Residential refrigerator compressors almost always have resettable thermal protectors. They are also usually external, especially on split-phase compressors. So the overload can be replaced separately from the compressor itself.

The reading mentioned internal thermal protectors (that require the compressor to be replaced), but the videos did not. Are they not prevalent in residential refrigerators?

PTC start devices are by far the most common start device for split-phase compressors.

I understand that I might test a compressor for continuity with ground. I understand that a compressor rotor will naturally develop leaks to ground, and that this is not an issue with the compressor (normal). If I measure and find continuity with ground, is there a way to know which it is? Or is is just one runs and one does not?

When paths to ground develop in a compressor, they are very high resistance paths. The continuity function on your meter would not pick them up. The path to ground would be something very high resistance, in the high k-ohms or mega ohms.

[Sam Brown]

1. You said, “Or am I just assuming that based on appearance (rime ice buildup) that the defrost is not being initiated (if the peripheral components are proven good?” You’re on the money here. If defrost is being initiated properly outside of test mode and all of your peripheral components are good, then you should not have excessive frost buildup. In cases like this, putting the board into forced defrost is more of a confirming test than anything else.

2. If you jump L1 to DF, you’re just using a jumper wire to physically connect those two pins, bypassing the ADC. When the jumper connects those two pins, you’re simulating what the ADC does when it closes its internal switch and supplies voltage to the defrost circuit. So there’s nothing fancy going on here — you’re just manually bypassing the ADC.

3. If it’s the first trip, then you’re correct: the customer should leave the unit as-is, plugged in with the food in there and the doors closed. If necessary for disassembly, you’ll often pull the food out of the freezer yourself and put it wherever the customer wants it. One thing you could do is ask the customer to have a cooler on hand (if they have one) before you go out so that there’s someplace to put the food if you need to remove it.

4. Condensate drains can get clogged for a number of reasons. It’s a common side-effect of a defrost failure. Sometimes it’s just a fluke — like some kind of crud gets caught in the drain, and ice forms around it until the drain is completely plugged. Other times, there’s a design flaw in the drain, and there’s a replacement drain kit that needs to be installed (this was the case with an older Whirlpool refrigerator, not so common these days).

Either way, once you melt all the ice you can get with the steamer, that’s when you pour the salt water down the drain — if the drain hasn’t already been cleared. Often, the steamer is all you need to do the trick. You can tell that the drain has been successfully cleared when all of the water you’ve made by clearing it empties into the drain pan at the back of the refrigerator. As long as you’ve addressed any underlying issues that caused the drain to clog, you can reassure the customer that it won’t happen again.

And yes, the tubes referred to in the video are the evaporator coils.

[Sam Brown]

You got it! Your reading of the schematic is absolutely spot-on.

[Sam Brown]

Remember that, in order for the reignition system to detect that a flame is established, current has to return to the board through the chassis of the appliance. If there is a buildup of grease, char, or other crud, that can act as an insulator and interrupt the return path. When we’re talking about a burner, think about where you might find built-up debris that could cause this issue.

Hopefully that makes sense. If you’re still uncertain, you may want to review the video in module 7, unit 3.

[Sam Brown]

Apologies for the late reply — looks like my response from a couple days ago didn’t post.

To understand what the correct answers are (there are two of them), you need to understand the difference between an early-stage leak and a fully progressed leak. While a fully progressed leak will manifest differently depending on if it’s a low-side or high-side leak, an early stage leak manifests the exact same way as an undercharge.

This makes intuitive sense if you think about it — in the early stages, refrigerant has been lost through the leak, and therefore you do have an undercharge of refrigerant in the system. It’s this undercharge that causes the ice ball, since the below-spec charge of refrigerant in the system all flashes to vapor toward the beginning of the evaporator coil, creating the ice ball.

Does that all make sense? Do you know what the two correct answers are now?

[Sam Brown]

If you look at the diagram at around 7:00 in the first video, you’ll see that when both evaporators are being run in series, a lot of the refrigerant has already turned to vapor by the time it gets to the freezer evaporator. Less refrigerant to boil means less cooling. So while you might get some cooling at the freezer evaporator in this configuration, it won’t be the “normal” amount.

This is why, in the other 3-way valve position, it just sends refrigerant to the freezer evaporator. That is necessary to keep the freezer properly cold.

[Sam Brown]

it was a very low temp so I figured “subcooled” but how do I find the answer. Sub cooled wasn’t it.

What’s tripping you up is that you’re thinking of subcooling and superheat in terms of “really cold” and “really hot”. That’s not what they mean.

A substance is subcooled if is cooler than the saturation temperature for its current pressure. Similarly, it’s superheated if it’s hotter than the saturation temperature.

So all you have to do is look at that temperature you’re measuring, compare it to the saturation temperature for your given pressure, and see if it’s higher or lower. That tells you if you’re superheated or subcooled.

Make sense?

[Sam Brown]

When checking some harness connections or even on the board / invertor board. I can’t find a place to test the dc voltage or hertz. Some harness connections, have the wire going into the connector where the wire is not exposed or I’m not able to get a probe in there .

You do sometimes have wires completely encased in a plastic connector, but you can still test those. You just need to have thin micro-leads for your meter. Something like these.

Using very thin leads like that, you can do what’s called “backprobing”, where you push your meter probes into the back of the connector and make contact with the wires that way.

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