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So let’s think about the thermal fuse, what it does and how it works. This is a one-time fuse that opens in response to excessive heat for a prolonged period of time. In a dryer, I can think of a few things that can cause excessive heat:
– restricted vent. Not just clogged with lint, but too long, too many turns, partial collapse, stuck vent hood on the outside of the house, clogged lint filter, etc.
– the cycling thermostat may be stuck closed causing the heating the heating element to cycle off and on via the hi-limit thermostat which has a much higher cutoff temperature than the cycling thermostat and will eventually cause the thermal fuse to open. For this reason, whenever the thermal fuse opens, I replace the hi-limit and cycling thermostats in addition to the thermal fuse. And then I inspect the vent.
– Grounded heating element. If the heating element breaks in such a way that part of the heating element coil touches the casing, this will create a grounded element that will stay hot whenever the dryer is one, even when the thermostat is open. You can test for this easily by removing one of the heating element wires and then checking continuity from each heating element terminal to the casing. Should read open. If you read some low resistance, then you have a grounded element.
How many times have you replaced the thermal fuse?
Hi Philip,
I think you meant to say this is a dryer, right?
You may not have reached the Troubleshoootng module yet, but there we teach a troubleshooting procedure called the Ten Step Tango. Let’s use this topic as a preview.
The first step in the Ten Step Tango is to make a succinct problem statement that answers one or both of these questions in a simple sentence:
1) what is the appliance doing that it should NOT be doing?
2) what is the appliance NOT doing that it should be doing?
Go ahead and answer that and we’ll continue through the other steps of the Tango.
Thanks for letting us know you’re enjoying the course, Kevin! The Student forums here are a great resource and way to get interactive help with the lessons. Keep up the good work in your studies and you will reap the rewards in your troubleshooting.
the load should drop all the voltage, so there wouldn’t be any left to kill me.
Is that right?99%! You are correct that the load is dropping all the voltage. Now, on the Neutral side, the voltage DIFFERENCE between Neutral and Ground is ZERO. Electrons can’t move without a difference in voltage potential between two points. Therefore, electrons will NOT be pushed through your body.
Keep in mind, too, that there is still CURRENT (movement of electrons) in the Neutral line. In fact, it will be 100% of the current on the Line side of the load.
To paraphrase that old Bengals song: “Walk like an electron…”
Myles, just forget that the problem even said it’s a dryer. Analyze the circuit as given. That’s the intention here. It’s not about guessing what the problem may be based on the type of Appliance. At this point in the course, you are analyzing circuits, nothing more. So, take dryer off the table, strike that from your memory, and analyze the circuit.
Glad to help, Dave. Sleep tight! ? ? ?
Hi Dave,
Good question!
There are three different temperature and vapor/liquid zones in a condenser:
1. first couple of rungs: superheated vapor
2. middle rungs comprising most of the condenser coils: 100% saturated liquid
3. last rung: subcooled liquid
When we’re talking about condenser temperature, we’re usually referring to the temperature of the saturated liquid zone, same as with the evaporator.
In a situation with low refrigerant, the compressor will be taking in very hot vapors from the high superheat. The gases entering the compressor cylinder will expand due to the heat resulting in lower vapor densities. The compression ratio will be higher than normal causing low volumetric efficiencies. As a result, the compressor will not pump as much vapor (also seen in the lower amp draw) and all components in the system will be starved of refrigerant. Since the heat energy is carried by the refrigerant, less refrigerant moving through the system means less heat is being carried through all components, including the condenser.
The 100% saturated liquid zone in the condenser will shift toward the exit of the condenser resulting in very little (if any) subcooling because the condenser is not receiving enough refrigerant vapor to condense it to a liquid. So the middle of the condenser, which is normally the saturated liquid zone, becomes vapor (either saturated or with some small superheat). But again, since there’s less refrigerant in the system, there’s less heat energy being carried which means that temperature effects will be smaller.
Make sense?
I go into these thermodynamics in detail in Module 3. Be sure to watch those videos and let me know if you have any other questions.
A load is something that does work. Period. Full stop. That’s your bottom line answer. Now let’s unpack that.
Work can be thermal (BTUH), mechanical (horsepower) or electrical (watts).
Electrical work is called power.
All four parameters of Ohms Law come together in loads: voltage, current, resistance, and power:
The voltage source (or supply) pushes electrons (current) through the load to make the load do work. The amount of work the load can do is the product of the supply voltage and the current through the load, P=I*E
The amount of current through the load is directly proportional to the supply voltage and inversely proportional to the resistance of the load, I=E/R
Just to tweak what Sam said— in the single evap, mechanical cold control scenario that the question envisions, the cold control controls the power supply to two components: the compressor and the evap fan. The cold control itself is part of the control system but the evap fan is part of the air flow system. What we see in this case is how these systems are actually all inter-related and the distinctions I offer troubleshooting aids to get us to think in terms of systems and their interrelatedness rather than a various discrete components.
In refrigerators with thermistors, the thermistor does not control anything! NTC thermistors are used for temperature measurement and always work with a control board. It is the control board that activates the loads based on sensory input from devices like NTC thermistors.
As far as checking the condenser fan, you can do this from the front on almost all of these models. You’ll need to get on the floor and remove the toe grill. Doing this you can almost always tell whether or not the condenser fan is running by either sound or feel.
Is this just to prevent it being used as a heat source?
Yes because, believe it or not, manufacturers have been sued because idiots did this, despite the warnings in the user’s guide (that they either didn’t or couldn’t read) and died.
Is it just using it for a prolonged time is bad?
Essentially, yes. It’s duration of exposure and accumulation.
When you’re cooking, the oven is cycling off and on a set temperature. The oven produces CO while the burner is lit but it the amount produced gets diluted in the air volume in the kitchen. When the burner turns off, the CO production stops and the CO in the room gets further diluted by normal air changes that occur in every house.
When the oven is used to heat a house, the door is open so it never reaches the set temperature and the burner stays on, cranking out CO the entire time. Over time, (we’re talking hours here) this will produce much more CO than will be produced during normal use. The volume of the kitchen and construction of the house or building will determine how quickly the concentration of CO will exceed the 8-hour time weighted average.
Make sense?
Hi Jason,
This is a good question because it shows you’re thinking about things!
All of the examples you’ve seen in the course (and that we do) are all in purely resistive components: heating elements, resistors, etc. These kinds of components are called passive because they are not what’s called reactive.
Reactive components are things like capacitors and inductors. In addition to pure resistance that you’re familiar with, they also present another kind of resistance to current that’s called reactance; capacitors have capacitive reactance and inductors have inductive reactance.
With inductive components, we leave Kansas! Voltage and current are no longer in phase with each other and different things become possible.
Motor windings are electrically just big inductors. So they will have inductive reactance in addition to pure resistance. But your meter only measures the resistive component of the inductor’s opposition to current flow.
The inductive reactance varies with the frequency of the AC power supply. This video explains further:
You’re over-thinking the question. The question, and this whole course, is in the context of repairing appliances. We are not teaching the National Electrical Code.
The purpose of this unit in particular is to familiarize students with circuit breaker basics, outlets, and GFCIs.
This particular question was simply asking about the concept or principle of a ground fault. That concept is the same regardless of the circuit breaker panel and sub-panel arrangement.
As to your other question,
I really think a different set of answers would be better for question #12.
I’m open to suggestions. What do you suggest would be more suitable answers? And, more importantly, what do you think the correct answer should be (if not the actual correct answer)?
As to your other comment:
It’s the same explanation given in question #11.
Good point! Although it does not change the correct answer, that explanation doesn’t fit very well with the answer. Somehow, during an edit, that explanation got copied from the previous question. I have corrected that. Thanks for pointing that out!
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