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Correct! So the type of load is irrelevant. What’s important is the recognition that there must be a load for it to be a valid circuit. And if there’s a load in the circuit with electrons being pushed around by the voltage supply, then we know from the way basic circuits work that all the supply voltage, however much it is, will be dropped across that load.
Many times, this is accomplished by switching the polarization on the motor windings, in other words, by switching Line and Neutral to the motor. The best way to tell exactly what’s going on in any specific case is to look at the schematic for that model and analyzing the power supply circuit for the turntable motor. Do you have a specific model number or schematic?
one of the 3 fuses in L1
Only one of those switches is a fuse. The other two are bimetals. The more general term for all the components in L1 is “switches.” A switch is a broad, general term for anything that can control the power supply to a load by opening. In this case, the load being controlled is the heating element. Any of the switches in either L1 or L2 of the load’s power supply can open and stop electrons from being forced through the load to make it do work (get hot, in this case).
So, any hypothesis we form should include focusing on one of these issues?
Remember your troubleshooting always starts at loads, not at switches. So your hypothesis will always be framed in terms of the power supply to the load of interest.
If you’re not getting power to the load, then one of those switches have opened and you’ll use voltage measurements across the switches in that circuit that you’ve identified on the schematic and see if any of them are open. A closed switch should act like a wire so the voltage drop across a closed switch should be 0 VAC. If you read 240 VAC across any of the controlling switches in that circuit, then you’ve found your open.
A shortcut method for doing this would be to use your jumper wire and amp clamp. Jump out one switch at a time and see if you get normal operating current. Whichever switch you jump out that restores operating current is your bad actor and would be replaced. This webinar recording is a good one to watch for using jumpers and cheaters and for understanding voltage difference vs. voltage drop.
You should also watch this webinar recording which workshops 4 different dryer troubleshooting case studies.
Something else like a loose connection was slowly creating more resistance somewhere in the circuit thereby lowering the overall amp draw?
That’s a valid suspicion. When electrons start moving in circuit, especially LOTS of electrons like in this case, metal tends to heat up from the movement of the electrons. When electrical connections start to heat up, metal can expand and cause the beginnings of a loose connection. As it progresses, that loose connection will generate increasingly more heat, enough to discolor or burn the wire insulation or metal terminals.
In this case, you could take the lowest amp reading after you’ve energized the circuit for a few minutes. Let the amp reading get to its lowest reading where it’s steady and not dropping anymore. You should also be measuring the voltage supply to the circuit while you’re doing this to make sure the voltage isn’t sagging while the circuit is energized. Then take your amp and voltage measurement and calculate the wattage and compare to the wattage spec for the heating element. P=IxE.
You’ll see lots of other circuit configurations as you apply the troubleshooting principles you’re learning here to real-world situations. Some of the other types of circuit configurations you’ll see are voltage dividers, loads in series, sensing lines, triac-controlled Neutrals, etc. But not matter the circuit configuration, ohms law still applies and they will all function according to ohms law. I illustrate and explain many of these in the webinar recordings.
Every valid circuit will have three things: a valid power supply, conductors, and a load. If Line and Neutral are connected to each other without a load, what’s that called?
Current Line should exactly equal current in Neutral line
This will always be true in a properly functioning circuit supplied with Line and Neutral. Instead of saying “current Line” we say, “Line current.” Line is a technical term that refers to the polarized line. “Polarized” simply means that the polarity of that Line is constantly changing over time. The polarity of Neutral does not change because it is bonded to Ground in the circuit breaker box. But Line current will always equal Neutral current. So as the polarity on Line changes 120 times a second, the polarity on Neutral never changes BUT the current on Neutral will always exactly equal Line current.
Now on to voltage.
We don’t talk about voltage at a particular point. Instead, we talk about the voltage difference between two points. This is because voltage is always relative to some reference. So we say “What’s the voltage between these two points?” or “What’s the voltage at this point in the circuit with respect to (wrt) this other point in then circuit?”
So, in this example, what happens to the million volts on Line? Why doesn’t it show up on Neutral even though Neutral still has all the current that’s on Line? Because the voltage gets dropped across the load. We call this voltage drop. Very important concept, and you’ll learn more about it in Unit 8.
Voltage difference between two points is the prime mover in a circuit– it’s what makes electrons move. While voltage difference is the first cause for everything else that happens in a circuit (including current, the movement of electrons), voltage drop is the effect of voltage difference forcing electrons through the resistance of a load. This is given by the ohms law equation: E=IxR.
A thermistor is a load whose resistance changes with temperature. The microprocessor is the control that monitors the changes in thermistor resistance and makes algorithmic decisions based on that such as opening a damper, turning on a compressor, initiating defrost cycle, etc. The microprocessor is the control because it directly controls the power supply to loads. The thermistor is just a sensor for the microprocessor to enable it to control other loads in the system.
What was “electricity” called during the development of “electrical” theory (conventional current theory)?
It was still called electricity but it was also during this era (1700’s) that the unfortunate term “current” came into use because of the way electricity was understood in the pre-atomic era.
The term “current” has led to lots of confusion among techs today about how electricity works because it associates the movement of electrons (current) with the flow of water. As a result, you’ll see techs with fundamental misunderstandings about electricity. For example, parallel circuits and one of the loads goes open. Many techs who don’t know ohms law will assume that the current in the other parallel loads must increase as a result because, well, the magical, mysterious “current” has to go somewhere. Stay away from thinking about electricity in terms of water and you’ll avoid that problem.
Conventional current flow is still used today by electrical engineers. Technicians and physicists use actual (electron) current flow. Either way works as long as you’re consistent. The big thing we need to look out for as technicians is that the electrical symbols on schematics are all created by electrical engineers. For example the diode symbol. In actual current flow, the electrons move out of the back of the arrow. But in conventional current flow, the electrons move in the direction of the arrow. This excerpt from a recent webinar explains:
R=Z
You’re not quite following. R (meaning that resistance you can measure with the ohms function of your meter) will only equal Z when X(L) and X(C) both equal 0. This will only be true in circuits with no reactive components such as motor windings. Z = R + X(L) + X(C)
Yes but isn’t it also true that if you can measure the voltage supply and the current in a circuit, you know the resistance, R=E/I?
If you’re dealing with a load that has no reactance, like a simple resistor, then this is a true statement. If you’re dealing with a load with significant reactance, eg, coiled heating element, motor winding, transformer winding, then what you’re actually calculating is impedance.
Impedance has the symbol, Z, as in “LoZ” meaning low impedance (input impedance in the case of a meter). Impedance, Z, includes both real resistance that you can measure with your meter plus reactive components, inductive or capacitive, which you cannot measure with your meter because they only appear when electrons are moving in the circuit.
Reactance is given the symbol X. Inductive reactance uses the symbol X(L). Capacitive reactance uses the symbol X(C).
The total impedance to electron flow (current) offered by a circuit is:
Z = R + X(L) + X(C)
So if you measure voltage and current in a circuit with an AC motor, then the “resistance” you could calculate in R=E/I is actually impedance, Z, and only a portion of that impedance is comprised of real resistance. In this case, the Ohms Law equation becomes:
Z=E/I
where Z = R + X(L) + X(C)
Why wouldn’t the most common measurements be Voltage, AC current, and Power?
Good question! Most techs these days don’t carry a wattmeter. They used to back in the day but they’re not really needed. If you can measure the voltage supply and the current in a circuit, you know the wattage, P=I*E.
Resistance is a dirty measurement but it’s still one you’ll have to use occasionally because of omissions in manufacturer’s specs (eg., giving heating element resistance but not wattage). There are some measurements that are normally made with resistance. For example, the resistance of the three windings in a BLDC motor (you’ll get into that in the motors module) or the resistance of a thermistor where they don’t give the voltage drop spec (which is most of the time).
This is a good example of how the manufacturers have either contributed to the erosion of electrical know-how in the tech community or they’ve responded to it by dumbing down their tech lit. In this case, the manufacturer has omitted the wattage spec on the heating element. This is not only unconscionable but it contributes to the near-universal reliance on the almighty ohms test as the gold standard be-all end-all test for most techs.
Today, most techs wouldn’t even know what to do with a wattage spec if it was given. This is a big change in the nearly 30 years I’ve been in the trade. If you go to Appliantology and download the service manual for the old-skool Maytag Dependable Care dryer, you’ll see they talk about using watts to test AC loads, using amps as a proxy for watts, using volts as a proxy for continuity. Techs used to understand this stuff. Nowadays, it’s the rare tech that does. I rant more about that in this post at Appliantology: https://appliantology.org/blogs/entry/1138-dont-confuse-old-skool-with-dumb-skool/
So, although we have to work with the specs we’re given, we should also be astute enough to recognize when the manufacturer has omitted specs and are pandering to PCMs with their dumbed-down tech lit.
As to your calculations, that’s the best you can do with the specifications given by the manufacturer.
Just a reminder, too, that you would benefit from watching the webinar recordings at Appliantology. Tons of continuing education training that builds on and applies your MST training to real-world troubleshooting situations. Here’s the webinar recordings index page: https://appliantology.org/announcement/33-webinar-recordings-index-page/
From blogpost:
“ohmic loads. These are things like light bulbs and heating elements”
You caught that– very good! That was a typo in Sam’s post that I’ve corrected. It now reads: “…ohmic loads. These are things like resistors and calrod heating elements.”
Make more sense now?
So a motor is non-ohmic but a heating element is ohmic.
Nope, both are non-ohmic. What makes something non-ohmic? If it has significant inductive or capacitive reactance. For us, inductive reactance is the more common one because this occurs anytime you have electrons moving through a coil of wire. These moving electrons create a moving magnetic that expands and contracts through the voltage supply sine wave cycle. The magnetic field in turn induces a back EMF that opposes the movement of electrons in a wire. This is basic physics called Lenz’ Law and is also the basis of how electric motors work (you’ll get into that later in the course).
If a heating element has lots of tight coils in it, it will act like a motor winding because it will have inductive reactance. Not as much as a motor winding which has lots more loops of wire all closely bound to each other but it will still have some. Your meter can only measure the real resistance of the wire bit cannot measure the inductive reactance that the coil of wire creates as electrons move through it real-time. As a result, the ohms that you measure with your meter is not the full story of opposition to current flow.
All of the foregoing is why we don’t check AC loads with ohms– you’re not getting the full electrical story. You check AC loads with amps, convert to watts and compare that to the wattage spec for that element, motor, whatever. Ohms are merely a screening test: if an AC load tests open on ohms (when it should be closed) then you found the problem; but if an AC load tests “good” (ie., in spec) on ohms, it doesn’t mean a damn thing.
Samurai’s Law for testing AC loads and switches: “Thou shalt declare an AC load or switch “good” ONLY after you have tested it in a LIVE circuit.”
The corollary to this Law: “Ohms lie!”
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