Forum Replies Created
-
AuthorPosts
-
Very, cool! Thanks for sharing that!
If that link doesn’t work, see this post:
https://my.mastersamuraitech.com/appliance-repair-course-support/student-forums/topic/link-for-the-pdf-textbook/-
This reply was modified 3 years ago by
Susan Brown.
Cool! I’ll go ahead and unblock you so you can take the quiz again. Good luck!
Scott
Hi, David,
If it’s a BLDC motor, as used in almost all inverter compressors and front load washers, the stator will actually have at least 4 poles: 2 norths and 2 souths, but usually many more (in even multiples). But regardless of how many poles, the basic principle is the same: like poles repel. All electric motors, regardless of type, work on this principle of magnetic repulsion. The north poles in the stator will push against the north poles in the stator. This magnetic repulsion is what makes the rotor spin.
The Motors module in the Fundamentals course goes into this in great detail but I’ve given you the “Cliff Notes” version of the answer. Does it make sense?
Scott
Hi Matt, great question!
Both of the examples you listed, the Whirlpool Cabrio washer and all modern front loaders, are examples of variable frequency drive (VFD) systems. In the case of the Cabrio, it’s a VFD in a vertical axis washer, like the Fisher-Paykels and newer Electrolux top loaders. Front loaders, of course, are horizontal axis washers.
One of the characteristics of VFD systems is that both the speed AND direction of the motor (almost always a BLDC motor) is completely controlled by the inverter board (called various things by the manufacturers: MCU, speed control board, etc.). The inverter can vary the frequency and amplitude of the voltage it is supplying to the motor to make it spin faster or slower. But it can also vary the sequence of the phase rotation to make the motor switch directions from CW to CCW rotation. So, in other words, instead of firing in a sequence of first motor pin A then B then C for CW rotation, it can fire in the sequence of pin C then B then A for CCW rotation. VFD systems open up the possibility for lots of other variations in the type of work that motors can do. You can review this unit for more info on VFD motor systems: https://my.mastersamuraitech.com/module-8/variable-frequency-drive-systems/
Split-phase, single-phase motors, on the other hand, are not nearly so elegant. To make them reverse direction, you have to have control schemes that will energize the start winding either before or after the main winding. Or you can have two separate start windings that are 180 degrees out of phase with each other and that can be separately energized. You can review this unit for more info on single-phase motor systems: https://my.mastersamuraitech.com/module-8/single-phase-asynchronous-ac-motors/
Please let me know if you have more questions on this!
Scott
I think what may be confusing you is that we did something a little different on some of the quizzes in the Refrigerators course compared to the Fundamentals course. In the Refrigerators course, we experimented with adding several randomly chosen questions from previous units as a type of review. The software would randomly choose the questions each time the quiz was loaded or taken. This is a good teaching technique but we didn’t realize that it was not including the associated images for the questions that had one. So until the software developer comes out with a fix for that bug, we are removing the random questions portion of the quizzes.
Hope that clears things up!
Scott
You mean one of the randomly selected questions? If that’s the case, then that’s a bug in the software that we’ll track down. Thanks for letting me know!
Scott
Hi Dan,
I see what you’re saying. I had actually set that quiz to randomly select four questions from previous units as a type of review. But I see how that can be confusing to students and is apparently causing blocking issues so I’ve taken away those four randomly chosen questions. Try it again and let me know if that fixes the problem for you.
Thanks!
Scott
Hi Dan,
Bot sure what happened but I reset you to the beginning of Module 2, Unit 4. That should clear the quiz, too, so you can re-take it. Let me know if that doesn’t fix it for you.
Scott
Hi Igor, this is a great question!
The condenser fan motor for the model you posted is an enclosed shaded pole motor, part number 4387244. A picture of it is shown below:
You can’t quite make it out in the photo, but the direction of rotation is stamped on the back plate of the motor. If you look a the one you worked on, it will be be stamped there as either CW or CCW.
The direction of rotation in a shaded pole motor cannot be changed because the direction of rotation will always be from the non-shaded pole on the motor to the shaded pole. The only way to reverse the direction of rotation in a shaded poler motor is to remove the rotor and flip it around 180 degrees in the stator. In fact, this was commonly done “back in the day” on the open-face shaded pole motors that were commonly used for evaporator motors. This isn’t an option in the type of construction of motor that you’re dealing with so you’re left with just having to make sure you get the motor with the correct direction of rotation.
Since the motor installed in this refrigerator is rotating CCW and this is apparently the wrong direction, this makes me think that the motor must have been replaced at some point in the past and the wrong motor was used.
I don’t know if you’ve seen the supplemental videos in Unit 4 of the Motors module, but there’s a good video there (the last one) that explains shaded pole motors in a little more detail.
Please let me know if you have any more questions on this.
Hi Chris,
Great question, thanks for asking! This is probably the trickiest part of that 20 minute screencast I posted in the Troubleshooting module (link). I thought the clearest way to answer that would be to make an addendum video, which I’ve added to the unit. I’m also including it below for your convenience. Just like with the original screencast, you’ll want to watch it in a larger view so you can see the details I’m pointing out.
https://player.vimeo.com/video/124167749
Please let me know if you still have questions!
Scott
Hi Matt,
Great question and one that has befuddled many a fine tech out there. You’ve been through the Motors module, so you have the basic understanding of how these PWM inverter drive systems are supposed to (also called variable frequency drives, variable speed drives, and lots of really bad words that I can’t say here 😉 ).
The communications between the MCU (inverter board) and CCU (microprocessor board) is two-way: the CCU sends out a PWM signal of varying frequency to the MCU. The MCU confirms this. If the CCU doesn’t see the confirmation signal from the MCU, it throws the communication error code. These can be very frustrating the track down but, in you’re looking for two things:
– bad wire harness connections at either the MCU or the CCU or worn connection pads on the either the CCU or the MCU boards themselves
– The DC logic circuit on the MCU board itself is bad and incapable to sending a confirmation signal back to the CCU.
Let’s look at your two example cases to illustrate some things:
Case 1: Machine quits mid-cycle and throws communications error code. MCU replaced – no fix.
The way you can tell that this is most certainly a communications problem between the two boards and not a problem with either board itself is because 1) the machine is is capable of entering diagnostic mode (so the CCU is probably good) and 2) the drum actually rotates (so the MCU is getting speed/direction signals from the CCU and is capable of making good output to the motor. So that leaves us with a bad connection somewhere: the wire harness itself or the pads on one of the boards that the harness connects to. I have a video on this very problem that illustrates this:
Practical troubleshooting tip: Flakey wire harnesses or connections will usually manifest as intermittent failures (work sometimes and then not others, fail in different conditions, etc.) whereas failed components, like electronic boards, will create a steady failed condition. With this in mind, let’s look at your second case:
Case 2: Motor won’t run at all.
Here again, it’s imperative to go into diagnostic mode because this gives you important clues. If the CCU enters diagnostic mode successfully, then the CCU is *probably* good leaving a bad wire harness or a bad MCU. You can also retrieve other error codes, which can tell a fuller story.
Remember, though: a failed MCU can cause the CCU to throw a communications error depending on the specific way the MCU failed. The CCU is just running off a program burned into the memory chips. It’s making a pre-programmed best guess at what the problem is. So the error codes are just a clue to point you in (hopefully) the right direction. But you still need to be the technician on the scene who understands the technology and can combine the error code with your other findings to make an accurate diagnosis.
Another trick for testing these MCU (inverter) outputs is by using a phase rotation tester, like this one: http://amzn.to/1AXyu6V
If you’re telling the MCU (via the CCU) to run and it’s not, you can use that phase rotation tester to see if the MCU is putting out all three phases to the motor. If not, no lights and bad MCU (assuming you have a good PWM signal at the MCU and that the MCU is getting a valid 120vac power supply). You can’t measure the MCU output directly but you can use that instrument to at least if the phase is present. And this is typical failure mode for MCUs: they fail in such a way that they completely stop putting out one or more of the phases to the motor so the motor can’t run (as opposed to putting out voltage that’s too high or a little too low).
If you open up the failed MCU on the washer in this case, you’ll probably find a burned spot on the board.
Loud and clear!
Hi David,
For clarity, I’m re-posting the quiz question from Module 2, Unit 3 here that you are referring to:
Freezing complaints in the fresh food compartment of a dual evaporator refrigerator are always due to air flow problems. Period.
This is a true/false question. The correct answer is False. The explanation given with the quiz question is:
Although it is true that freezing complaints in the fresh food compartment of a single evaporator refrigerator are always caused by airflow problems, specifically, too much cold air, this is not the case in dual evaporator refrigerators. In dual evaporator refrigerators, the situation is a little more nuanced. Because the fresh food compartment has its own evaporator and evaporator fan motor, the problem could also be caused by a thermistor that has gone out of spec. In other words, the thermistor could be reporting a temperature to the control board that is much warmer then it actually is inside the compartment. This question illustrates the importance of always keeping in mind the exact type of refrigerator that you are working on and having a solid technical understanding of how that refrigerator works.
If the evaporator fan was NOT running in the fresh food compartment of a dual evaporator refrigerator, as you mention in your question, freezing of food would certainly NOT be one of the complaints.
The section of the lesson in Module 1, Unit 12 you’re referring to is this:
Suppose the fridge is humming along and the fresh food thermistor tells the control that it’s plenty cold in the compartment. The control then kills power to the fresh food compartment evaporator fan. Here’s where the problem comes in: even though the evaporator fan is stopped, liquid refrigerant is still getting pumped into the evaporator coil through the capillary tube and flashing (expanding or “evaporating”) into a vapor, doing that refrigeration thing as it does so.
So you’re still going to have this -20F frost magnet inside the fresh food compartment. Without the evaporator fan running, it won’t do much cooling but it will still lower the relative humidity inside the compartment.
This is describing normal operation in a slave-system dual evaporator refrigerator.
The quiz question in Module 2, Unit 3 is referring to abnormal operation in a dual evaporator refrigerator where food is actually freezing inside the fresh food compartment.
There is no damper in a dual evaporator system– air flow is controlled by simply turning the evaporator fan motor on and off as needed. So, right off the bat, you can eliminate any suspicions about dampers being stuck open as the cause of the problem.
If food is freezing in the fresh food compartment of a dual evaporator refrigerator, you can bet the evaporator fan is running too much– and this is making the food freeze. So, in this case, to fix the problem, would you just replace the evaporator fan motor? If you do, you’ll get a guaranteed call back because this will not fix the problem!
You need to appreciate that the evaporator fan motor in these types of refrigerators is controlled by the main microprocessor control board. And this control board makes decisions about how long to run the evaporator fan motor based on temperature information it gets from the fresh food compartment thermistor. So garbage in, garbage out– if the thermistor is telling the control board that the compartment is too warm, the control board will dutifully keep running the evaporator fan motor until to gets good temperature information from the thermistor.
Now suppose the thermistor is out of spec in such a way that it’s reporting actual compartment temperature to the control board 30 degrees warmer than it actually is? For example, if it’s actually 38F in the compartment but the thermistor is reporting to the control board that it’s 68F. In this case, the control board will keep running the compressor and evap fan in an effort to get that compartment temperature down to whatever the customer set the temperature to be. In this example case, the control board is trying to make the actual compartment temperature 8F (38 – 30) all the while thinking that it’s making the compartment temperature 38F. The result: freezing in the fresh food compartment.
Is this an air flow problem? On the surface, it may appear to be because the evaporator fan motor is running all the time (or nearly so). But it’s just a dumb motor that does what it’s told by the main control board and the main control board is making decisions about how long to run the evaporator fan based on bad information it’s getting from a defective thermistor in the fresh food compartment. So the root cause of the problem in this case is a bad thermistor.
By the way, this is also a very common scenario with some brands, especially Samsung and GE, and so illustrates the importance of always checking the compartment thermistors as your first troubleshooting step on any temperature complaint in refrigerators that use thermistors and control boards.
Does this make sense?
Hi David,
Answer the question and see the explanation. Don’t be afraid to get a question wrong– you can take the quiz as many times as you need and there is no penalty. If you still don’t understand after answering the quiz question, post back and I’ll gladly ‘splain it to you some mo’!
Hi Russell,
Sorry for the delay in replying– I wasn’t expecting a post in this forum yet because this is for the Refrigeration course which hasn’t been released yet.
As for the questions you’re asking, much of this is covered in Module 8 of the Fundamentals course. These are BLDC motors that work differently from the split phase motors you are probably thinking of.
You won’t see five wires going to a specific motor. You will see at most 4.
There are three different wiring configurations for BLDC motors:
two-wire: single speed supplied with +Vcc and gnd.
three-wire: DC supply (two wires) and the third wire will either be the pulse width modulated speed signal or the tacho feedback signal.
four-wire: DC supply (two wires), PWM speed signal, and tach feedback signal.
We also cover this in more detail in the soon-to-be-released Refrigeration course.
-
This reply was modified 3 years ago by
-
AuthorPosts
