Forum Replies Created
-
AuthorPosts
-
Hi Kevin,
Sorry for the looong delay in replying to your post. I just happened to see yours when I was answering another one. For some reason, the forum software never notified me of your post. Even though my reply is late, I hope it’s still useful.
The relationship between resistance power is one of the several relationships defined by Ohm’s Law. Here’s the Ohm’s Law pie chart from Unit 3 in the Basic Electricity module:
You’ll see in the P quadrant that two equations describe the relationship between power and resistance. Let’s talk about each.
P = I2 × R
This equation is actually used to calculate power loss in conductors. It is commonly referred to as “I squared R” losses. Power companies use this to calculate the power losses over their transmissions lines. It says that the power loss in a conductor increases as the square of the current flow through that conductor and is directly proportional to the resistance of that conductor. In circuits, we ignore I squared R losses because 1) current flow rates are relatively low (compared to power lines) and 2) the lengths of the conductors are very short (again, compared to power lines).
P = E2 / R
This is another equation for power in terms of voltage and resistance. It says that the power available in a circuit increases as the square of the voltage in that circuit and is inversely proportional to resistance.
Please let me know if you have any other questions on this.
I have a thought question for you to ponder until you get to Unit 6: An AC circuit is properly grounded and supplied with One Million Volts. You are standing barefoot on the wet ground and grab a hold of the bare neutral wire. What happens to you?
You have to start with the understanding that current, the flow of electrons, is the RESULT or EFFECT of a voltage difference between two points. It is that difference in potential or voltage between two points that makes the electrons seek the positive (or get away from the negative).
Once you understand that, then you should immediately see that the driving mechanism for current, whether AC or DC, is voltage. In an AC circuit, the voltage is switching polarity, positive to negative, 60 times a second. Current, the flow of electrons, simply goes where the voltage drives it. That’s the principle of electrostatic repulsion and attraction at work.
I encourage you to re-watch my screencasts in both Unit 1 and Unit 2 because I explain this in some detail in both. In fact, as stressed in the Orientation, you should re-watch ALL the technical screencasts at least twice AND be making notes. This is called active viewing. You just won’t get everything out of them with a cursory, passive viewing. Yes, it takes time and work but so does anything worth doing or learning.
Let me know if this still isn’t clear to you.
In normal circumstances, the chassis is NOT used as a conductor. It’s only when something goes wrong. An example would if a hot wire carrying 120 vac contacts the appliance cabinet or frame (chassis). In that situation, if the cabinet and house wiring are properly grounded, the cabinet (chassis) shorts the current to ground and causes the breaker to trip, preventing a deadly electrical shock.
Many techs make voltage measurements using the chassis (cabinet or frame) as their neutral reference point. This is poor technique and assumes an awful lot about the wiring of both the appliance and the house. Most of the time, they get lucky because things are wired correctly.
The correct reference of AC voltage measurement is neutral, NOT chassis. It is true that if everything is wired correctly, the chassis and neutral should be at the same potential and the voltage measured would be the same whether using chassis or neutral as the reference. And it’s okay to do that as long as you 1) have verified that chassis and neutral are at the same potential 2) the house wiring is properly grounded and 3) that you do so with the understanding that you are ASSUMING the wiring is all correct.
Personally, I always use neutral as my reference, never chassis. Open neutrals are a ubiquitous problem in appliance troubleshooting and using the correct reference will help catch this.
You’re asking two distinctly different questions here: AC and DC in circuits and AC vs. DC in power transmission. I’ll answer them separately.
1. AC and DC in circuits.
When we’re talking about measuring AC or DC voltage in circuits, we need to know what our reference is for that measurement.
Voltage is always measured with respect to or relative to some reference point. There is no absolute 120 vac or 10 vdc “out there” somewhere.
Voltage is always measured as a difference in voltage between two points: the point of interest and the reference point. In an AC circuit, that reference is neutral.
DC circuits are used in electronics and are their own separate electrical system, including ground, that is separate and distinct from the neutral reference used in AC circuits. The engineers don’t want the sensitive, low voltage electronics circuits affected by switching transients and noise on the AC system because it screws up the electronics and makes it difficult, unreliable, or impossible to distinguish low voltage DC pulses, like digital data pulses representing logical 1’s and 0’s, from the background noise on the AC system.
DC circuits for electronics are like someone with a breathing problem who needs specially filtered air to breathe correctly because they aren’t robust enough to breath the outside air with all the pollution and pollen in it.
2. AC vs. DC in power transmission
Power transmission lines are used, like the name says, to transmit power. Power, P is equal to the voltage, E, times the current, I.
P = I * E.
AC is used because it can be stepped up to very high voltages using transformers. Looking at the equation above, if you jack the voltage way up, you can drop the current way down and transmit the same amount of power.
It is desirable to reduce current flow in wires because this reduces the losses due to resistance (“friction”) in the power conductors.
No conductor allows electric current flow without some small amount of losses. In circuits, the losses are small enough (because the currents are low enough and the physical lengths of the conductors are so short) that we ignore them. You can’t do that with power lines because the currents are much higher so the losses are real, especially over long distances. Reducing the amount of current needed to supply the same amount of power is the answer and this is done by raising or stepping up the transmission voltage.
Correct! Good job, Al!
Hi Al,
Although A to C does have the highest resistance of the pairs, think about what you’re actually measuring there– it is the SUM of the resistances of the two windings, main and start. Re-watch the screencast and study the diagram again and then see if you can tell me which pair of terminals is connected to the start winding.
Scott
Hi Chuck,
You have to begin with the appreciation that warranty work is not a money maker. In fact, quite the opposite: warranty work is a money loser.
I do warranty work for Daco and AGA Marvell precisely because I don’t actually end up fixing many of these appliances while they’re still in warranty. Instead, I get the COD referral which is profitable.
Keep in mind that if you do warranty work for Whirlpool and GE, you will be running lots of service calls on these appliances while they’re still under warranty and it will pay next to nothing. In fact, you could get so many warranty calls for these brands that you will have a hard time running profitable COD calls.
The carrot that manufacturers use to lure servicers into running their warranty calls is that they give access to tech line and their service or website. But, in my opinion, this is not a sufficient payoff for the real cost of running lots of warranty calls. Especially when you have a resource like Appliantology where you can get tech help, service manuals, and tech sheets for appliances you’re working on.
The best way to get the phone to ring is to be smart about how you promote and advertise your business. Make sure you’re making full use of all of the free and low-cost online venues for advertising your business. Having a Google plus page is a great start and Google Adwords can be very cost-effective once you get the system set up and dialed in.
If the 4 and 6 you’re referring to have circles around them, they are just cams, not contact points.
The video in this forum topic is an addendum to the longer video in the Using Schematics to Troubleshoot Appliances, Part 2 lesson in the Troubleshooting module. Both the original, longer video and this addendum are posted in the original lesson. Watch the longer video and then the addendum again. Once you understand how to read the diagram and timing chart, you should be able to trace out the motor circuit yourself. But if you’re still confused, lemme know!
Hi Igor,
Whoa– this one slipped by me. I don’t have such a list. But some things like compressor relays, here are several good topics on this in the Dojo at Appliantology.
My only caution is to avoid using generic parts. Even OEM parts from another manufacturer used in a different brand is far preferable to generic parts.
Intermolecular force is an intrinsic property of a material. The molecules that comprise a particular material have a unique and characteristic intermolecular force that is different for different materials. If you heat any material, it weakens the intermolecular force in that material. If you heat it enough, it will melt or boil. The amount of heat needed to make this happen is different for any given material because of the different intermolecular force of that material.
Intermolecular force is measured in kilocalories/mole in laboratories. Engineers use BTU/pound. So let’s look at an example.
Water has a boiling point of 212F at atmospheric pressure. At its boiling point, it takes 970 BTUs of energy per pound of water to make the water molecules leave the other water molecules as steam. In other words, you will have to add 970 BTUs of energy per pound of water to overcome the intermolecular force of water at atmospheric pressure. This is called steam. This is an intrinsic property of water and it never changes… at atmospheric pressure.
Now, if you change the pressure that the water is in, make it higher or lower, the amount of energy needed to overcome its intermolecular force changes. But this change is predictable and is always the same for water. There are published tables of this data widely available. They’re called steam tables.
It’s exactly the same with refrigerants. It will take differing amounts of energy at varying pressures to overcome their particular intermolecular forces enough to create refrigerant “steam” or vapor. The refrigeration effect all happens in this phase change from liquid refrigerant to a vapor because, by design, the boiling point of refrigerants is very low at operating pressures. So when the pressure of the liquid refrigerant is suddenly lowered, this lowers the boiling point of the refrigerant which also means that the intermolecular force holding the molecules together is reduced. When this liquid is exposed to outside heat energy, like liquid refrigerant in an evaporator coil, the external heat energy is greater than the intermolecular force holding the molecules together and, as a result, the molecules leave the liquid and boil off as refrigerant vapor.
If you look at a refrigerant table, like this one, you’ll see that the vapor pressures for each refrigerant is different at the same temperature. The reason for this variation is exactly the variation in the intermolecular forces among the refrigerants.
Did that help? If not, keep asking questions!
Hi Dan,
I just replied to your email telling you that the problem is fixed. I’ll go ahead and post here what I sent you by email.
Hi Dan,
Thank you so much for sending this to me! I was able to track down the questions that needed editing and make the necessary changes. I really appreciate your feedback and letting me know about this.
It was a little glitch from when we added the module exam questions. They’re pulled from a pool of questions and the exam is built on-the-fly by the LMS software when the student requests the exam. The questions weren’t originally worded as stand-alone questions but as a series with the assumption that they would always appear together and in sequence. The original quiz was put together way before the module exam was so we had forgotten that little nuance about the original quiz question construction.
Thanks again for your help in tracking this down!
Hi Dan,
Yes, please, that would be great if you would email that to me. That’ll help me find the right question so I can edit it and add the additional needed info so it can be answered.
Thanks again for reporting this!
Scott
Hi Dan,
Thanks for your comments. The questions on the module exam are pulled from a pool of questions so if you could paste in the question here or tell me enough about the question so I can find it, I’ll check it out for you.
Also, based on feedback from you and other students, I’ve increased the exam time from 60 to 90 minutes. *wild cheering* 🙂
Hi Matthew,
You got it! His example was maybe a little misleading but I used that quiz question as a teaching moment precisely to bring out the fact that current in parallel circuits is not necessarily the same– it depends on the resistance of the load in that circuit. As you pointed out in his example, the resistance in each branch was the same so, since each parallel circuit “feels” the same voltage and since the resistance in each branch is the same, then it so happens that the current will be the same. But this is a special case and only works out that way because of the equal resistance in each branch.
E = I * R ==> I = E / R
We see from this simple Ohm’s Law relationship, I=E/R, that if the resistance goes up, the current must go down (the denominator gets bigger). The simple math will keep your thinking straight.
I’ll reset you so you can take the quiz again.
-
AuthorPosts
