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Cams in timers are just the mechanical devices that make or break contacts — so yes, they function as switches. In this particular timer, each cam is capable of making two different connections. For example, cam 10 can either close the switch between contact T17 and contact T16, or between contact T17 and contact T18.
Don’t conflate the cams with the contacts (contacts are the points called out as T16, T17, T18, etc.). The contacts (also called terminals) are just points on the timer that wires from elsewhere in the machine connect to.
Are you able to see now how the Samurai used the timer chart to tell which contacts would be closed during the pump portion of the cycle?
Hi Nate,
As we say in the text of the unit, a microprocessor board that will not enter service mode is bad by definition. This failure indicates that the logic of the board has become corrupted, and it is no longer able to execute programs correctly.
A word of caution: don’t be too quick to pronounce a board as deceased the moment you fail to go into service mode. This is only diagnostically conclusive when you have a) confirmed that the keypad is functioning properly, as the question states, and b) ensured that you are correctly executing the key presses to enter diagnostic mode. Only when you’re sure of those two things can you conclude that the board has failed.
In a split-phase compressor, the start winding’s resistance is expected to be significantly higher than the run winding’s. What you’re seeing with your measurement looks right in line with what you should see.
As for why the first two readings don’t add up to the third: ohms readings are sloppy things, especially when dealing with low resistances like you are. A discrepancy of a few ohms like what you’re seeing is certainly within the margin of error.
The general rule is this: Common to Start will always have more resistance than Common to Run, and Start to Run will always have more resistance than either of the other two readings.
June 7, 2019 at 3:00 pm in reply to: Bas Elec: Circuit Breaker Panels & Power Outlets – Elec Dryer Midterm Exam Video #15999You didn’t miss anything — this is just a place where electricity doesn’t lend itself to being understood intuitively. You have to look at the math to really get it.
Fortunately, the math involved here is just basic arithmetic — subtraction, specifically. The key concept is this: voltage is all about the difference between two points. That’s what your meter is showing when you make a measurement.
When we use a meter to make a voltage measurement between line and neutral, we place one lead on line, the other on neutral, and then our meter measures the difference in voltage between those two points. Line has 120 volts of electrical potential and neutral has 0, so our meter reads 120 volts.
Another important thing to remember: the polarity of the voltage in an AC circuit is constantly reversing from positive to negative, back and forth. So that line voltage will be at one moment positive 120 volts, and then the next it will be negative 120 volts. The difference between line and neutral remains the same, but the polarity of line is changing.
Once you get to 240 volt power supplies that have both L1 and L2, you have to take this concept of voltage difference and reversing polarity a step further. When the Samurai says that L1 and L2 are 180 degrees out of phase with each other, that basically just means that they are electrically complete opposites. Whenever L1 is at positive 120 volts, L2 is at negative 120 volts. Which line is at which polarity swaps 120 times a second, but they always remain opposite. That’s what it means to be 180 degrees out of phase.
Onto your second question: since we’ve established that voltage is all about the difference between two points, what would happen if you placed both leads on the same line? That’s what the Samurai is asking: what would you read if you put one lead on L2 and the other lead also on L2 (in other words, measuring L2 with respect to L2)?
You would only ever want to use a cheater cord on a load that is intended to receive line voltage. If a load is, for example, supposed to run on 12 VDC, then it could definitely be damaged if you connected it to a 120 VAC power supply.
Does that answer your question?
Exactly! Dealing with missing information is a big part of being a technician, so we do our best to prepare you for that.
This is one of those situations where the manufacturer hasn’t explicitly stated some information, so you have to make an educated guess based on other clues.
For example, there are certain conventions concerning the wire colors of line and neutral. Do you remember what those are?
Don’t sweat it! Learning to catch every little detail is part of becoming a top-notch appliance tech. You’re well on your way. 🙂
The abbreviation is given right there in the unit — reread the material closely and you’ll find it.
May 9, 2019 at 3:23 pm in reply to: ADVANCED TROUBLESHOOTING – MOD 5: UNIT 1: EXAM QUESTION #4 #15805Hi JP,
I highly recommend that you rewatch the videos in this unit (especially the second one), because they spend most of their time covering this exact question.
Keep in mind too that, while both legs of the power supply might go through timer contacts, only one of them is really ever switched by the timer during operation.
Let me know if you’re still confused after rewatching the videos.
What type of voltage is applied to BLDC motor windings?
Think of what that inverter is doing. It’s taking DC current and sending it through the windings in one direction, then the other, switching the direction many, many times a second. What kind of voltage do you have when its polarity is constantly reversing like that?
How many windings are polarized at a time to do work in a BLDC motor?
There’s a diagram showing the answer to this about halfway through the video on electronic commutation.
And could this actually be considered 3-phase
Why yes, it is 3-phase power. That’s the kind of power that the inverter board is producing by switching the voltage to each of the three windings like it does. That doesn’t disqualify the motor from being a BLDC motor. In fact, that’s how all inverter-driven BLDC motors work. It’s why they don’t need start devices to get going from a dead stop.
Wouldn’t a longer duty cycle actually contribute to slowing down a motor at high RPMs, despite its voltage increase?
I’m not quite sure what you’re getting at here. Are you thinking about the duty cycle of the PWN signal that gets sent to the inverter from the main control? That’s just a data signal to tell the inverter what to do. The inverter then interprets that data signal and decides whether to run the motor faster or slower.
After reviewing Multisim in a bit more detail, I can already see that it wouldn’t be of much help for your situation.
If I understand correctly, what you’re struggling with is finding on a schematic where you should make your measurements — identifying EEPs, in other words. Multisim doesn’t teach how to find troubleshooting points on a schematic. It’s simply a tool that you can use to create simulations of circuits, without instructional features.
Multisim would only be useful for people who deal with much more complicated circuits than we do — with reactive components, P-N junctions, etc. That’s a whole other level of electricity that we don’t get into as appliance techs. The kinds of circuits we deal with are always, at the end of the day, just loads and switches (yes, even control boards simply function as intelligent switches).
If you were working in electronics or as an electrical engineer, Multisim might be a helpful tool. But for an appliance tech, it’s overkill.
Glad you’re enjoying the courses! Interactive troubleshooting training materials are definitely something we’re looking into right now. We don’t have any personal experience with Multisim, so I can’t give you a recommendation for or against them.
We do have tons of videos that are focused specifically on the area of selecting the correct electrical measurement to troubleshoot a problem. Here are a few that I think you’ll find particularly instructive:
April 15, 2019 at 4:51 pm in reply to: Question about schematic used in quiz question 4 and 5, unit 5 part 2 #15704A check from one of the output terminals to one of the input terminals would not be useful here. As part of the setup for this question, you already have as a given that your power supply up to the noise filter is good. So you know the component has a good input. The only test you need to make is from L2 wrt N2, since those are your two output terminals. That will tell you if the noise filter is capable of putting out a good 120 VAC.
The question is supposed to throw you off your game, so don’t worry — it’s just doing it’s job! 🙂
The point of it is to teach you to always pay attention to text in schematics. If the engineers took the time to write a special note for you, you can bet it’s something important. In this case, they tell you exactly which two wires provide the input, and which two provide the output.
Since they’ve given you this info, you don’t need to know anything about the inner workings of the noise filter. You can just treat it like a black box. Check that it’s getting a good 120 VAC power supply. Then check that it’s putting out a good 120 VAC power supply. That’s it.
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