AC THEORY

[Therealcrt]

Years ago, we were called to figure out which conductors in a 4-wire cable went to which ceiling speaker in the den of a home. The cable had one black wire, one red wire, and two white wires. The lady of the house told us that two previous electricians couldn't figure it out.

She went next door for a minute, and we were finished and sitting on her front porch when she came back. She couldn't believe we did it so fast until I showed her the two pairs, identified by twisting each pair together. I simply used a AA battery and listened for the clicks.

I’m also confused on what you did there

 

[LarryFine]

I’m also confused on what you did there

I held one white wire on one end of the battery, and touched the black or red wire to the other end of the battery, until I heard a click from one of the speakers. I double-checked by trying the two remaining wires.

 

[Sea Nile]

I held one white wire on one end of the battery, and touched the black or red wire to the other end of the battery, until I heard a click from one of the speakers. I double-checked by trying the two remaining wires.

I've done that to figure out which wires went to which speaker on a car stereo, nice trick.

 

[junkhound]

Am primarily aerospace an power electronics.

When ya gotta figure out why a satellite on orbit is displaying abnormal power supply characteristic, you DO need to know as much theory as you can.

 

[__dan]

Serious question. How often or when really would you ever need to know most things taught in AC theory levels 1 through 4 out in the field? To me it all seems unnecessary

Honestly the op reads like troll bait and it's working pretty good getting a lot of replies. You should know loop and node analysis minimum and if you don't, take a course.

My opinion is you don't need to know anything to do the job. I have seen plenty of that with more or less a range of success.

You can sell the job and never touch a screwdriver, or you can just build muscle memory working, or fall into niches where they won't let you do anything even if you want to. The field is so broad no one can know everything even if you try.

You do have to know the limit of what you don't know, which can get extremely dangerous for people who don't know what they don't know. Know *where* the limit of what you don't know is. It is too easy to find it the hard way.

What you have to know is enough to satisfy your piece of mind. So if you want to know something and it bothers you to not know, you may want to chase after that. So it's not for job security, it's for your own personal feeling, feeling secure you know what you're doing or feeling insecure constantly not know if you're doing it right, might get caught, have to tear it out and do over. Some places the do overs are where the profit is made, some places do overs just kill you quicker.

If you know how to or are accidentally profiting from do overs, avoiding learning more is working for you. I have seen that more than once.

In the old days it was understood that if you did bad work everyone would know or find out and you would be dead from it. No one would hire you. Generally but that's not true everywhere.

So ask yourself about what you need for your own peace of mind and about the do overs. If you profit from them keep doing what you're doing, many do. If you worry about do overs and it bothers you things you don't know, that's probably most people and it is a lifetime of learning to get there. Just keep taking bites at it and try to enjoy the journey. That's one of the things they try to take away from you, your peace of mind, and your enjoyment of the journey.

 

[Carultch]

And you can tell your guys not to use an extension cord while it is coiled up because of inductive reactance will cause it to heat up.

Since the extension cord has two wires carrying equal and opposite currents, is there really any net inductance caused by it being coiled up?
I would expect that the magnetic field of the black wire and the magnetic field of the neutral, would both add up to close enough to zero, whether it is coiled or straight, and nullify any inductive reactance you'd get.

There is a reason to uncoil the extension cord before using it, but it has nothing to do with inductive reactance or magnetism. It's entirely a thermal reason, since it has to get rid of its Ohmic heating through its surface area, and the extension cord is built for use in free air.

 

[LarryFine]

There is a reason to uncoil the extension cord before using it, but it has nothing to do with inductive reactance or magnetism. It's entirely a thermal reason, since it has to get rid of its heat out of its surface area, and the extension cord is built for use in free air.

I agree, due to the concentration of normal IR losses, not unlike conduit-fill adjustments

 

[Sea Nile]

nullify any inductive reactance you'd get.

It's mitigated, but not nullified.

 

[Strathead]

Sure helps you understand why it hurts so much when you touch the high leg of a 120,240 delta

It helps when someone tells you that you get a bigger shock from the neutral than the hots, which I have

First half-answer: It's absolutely impossible to predict when & where it will become essential. And when it does become essential, you will never know it became essential if you haven't learned it -- you'll just fail to understand what you're looking at.
Second half-answer: You will absolutely never be disadvantaged by learning it.

I confess, I'm the total opposite. I'm a card-carrying nerd with a penchant for learning anything & everything. But there have been some surprising opportunities when learning something that was apparently unrelated, unnecessary, and/or irrelevant became useful.

An installer might ask why we need to learn about corner-grounded delta; nobody installs that any more.
In three minutes, I diagnosed why a newly-installed VFD was refusing to start, after three people had spent a week looking only at the VFD itself.

Almost everybody asks whether we "need" to learn that much math.
My driving improved significantly after I mastered calculus.
(you'll just have to take my word for it; there's no way to explain what happened unless you understand calculus, at which point I won't need to explain what happened)

What does escape velocity (satellites, orbital mechanics) have to do with paint rollers? Quite a lot, as it turns out.

This is well said. I am the same way, and I try to convey it often. It usually doesn't come out as well. That would be an example of how studying English (reading and writing) would come in handy. Learn music theory and you learn something about electricity.

 

[drcampbell]

... Learn music theory and you learn something about electricity.

Math theory, too. Yet another example of the interconnectedness of seemingly-unrelated things.

A very-geeky friend was experimenting with digital audio synthesis, back when an hour of CPU time was needed to produce a minute of digital audio. One of his discoveries was that superposing two sine waves whose frequencies were related by a ratio of √2 would produce a bell, chime or gong. Low-frequency sinusoidal amplitude variations produced tremelo, exponentials produced realistic decay, and he even synthesized a vibraphone. (sorry, don't remember the math that achieved that)

 

[Joethemechanic]

I can think of a lot of times knowing theory help me recognize a dangerous situation. When you go out on a service call you never know what you're going to walk into

 

[zbang]

It's mitigated, but not nullified.

mitigated to the point of negligible.

 

[Strathead]

Math theory, too. Yet another example of the interconnectedness of seemingly-unrelated things.

A very-geeky friend was experimenting with digital audio synthesis, back when an hour of CPU time was needed to produce a minute of digital audio. One of his discoveries was that superposing two sine waves whose frequencies were related by a ratio of √2 would produce a bell, chime or gong. Low-frequency sinusoidal amplitude variations produced tremelo, exponentials produced realistic decay, and he even synthesized a vibraphone. (sorry, don't remember the math that achieved that)

And of course, Physics even more than math.

 

[Joethemechanic]

And of course, Physics even more than math.

Some basic structural too. Know what moment, shear, compression, torsion, tension are, And be able to recognize some basic structures like a cantilever, or a bridge or a column. You don't know how many times I see stuff falling down because the guy that installed it didn't know any basic structural concepts

 

[retirede]

 

[Carultch]

Math theory, too. Yet another example of the interconnectedness of seemingly-unrelated things.

A very-geeky friend was experimenting with digital audio synthesis, back when an hour of CPU time was needed to produce a minute of digital audio. One of his discoveries was that superposing two sine waves whose frequencies were related by a ratio of √2 would produce a bell, chime or gong. Low-frequency sinusoidal amplitude variations produced tremelo, exponentials produced realistic decay, and he even synthesized a vibraphone. (sorry, don't remember the math that achieved that)

It's called the Fourier Series.

The way you can tell the difference between two different instruments both playing the same note, is that they all have a different composition of overtones for the waveform shape. The pattern of amplitudes and frequencies of all the overtones relative to the fundamental frequency (the lowest and usually the loudest), defines the type of tone you get, and Fourier series is the math behind how to calculate it from knowing the wave shape. That, and the envelope of the wave shape, which is the exponential decay terms you described. The sine wave is the pure tone, and multiples of its frequencies form the overtones.

This has applications for electrical power, due to the use of non-linear loads tend to produce harmonic currents. The harmonics at 180 Hz and multiples of it, which are very common, are the ones that accumulate, rather than cancel on the neutral conductor. This explains why some applications call for a 200% neutral, and why there are some applications where you have to treat the neutral as a CCC for ampacity calculations.

 

[jaggedben]

... Learn music theory and you learn something about electricity.

Harmonics.

 

[LarryFine]

Harmonics.

 

[jaggedben]

View attachment 2565472

<Facepalm> No. Not that.

This.

 

[rambojoe]

Harmonics.

Music theory has more to do with division of the major scale and oscillation... And harmony.
If you asked mozart about harmonics you might get that look... If you ask jim root he would smile. There is alot of history behind dividing a octave into 11 half steps.