Getting help in understanding 3-phase

[Goph704]

I'm trying to understand more about 3-phase power conversions. I tend to be a pretty holistic thinker and so it's very hard for me to work with just one small part of a system. Currently I have a strong background with single phase, but 3-phase four wire is the next logical jump. I'm confused as to how you get higher power such as 208V by using 2 legs. How do you "balance a Load" How do I know the difference between actual voltage? I'm looking for advice or a basic text to increase my overall electrical understanding. What would be awesome is if someone could post some example math problems to play around with and a little explanation on the side. Ex. I have a 1200 w fixture that runs between 7 and 12 amps would a 100 amp service cover that? or would it be better to use a 200 amp service? how big a how big a distro would I need if the distance is over 200 ft.? I'm reading up on previous posts, but still some basic problems would help alot. The more hypothetical the better, but i am trying to understand enough to fill in a lot of Gaps.

 

[Footer]

Amazon.com: Set Lighting Technician's Handbook, Third Edition: Film Lighting Equipment, Practice, and Electrical Distribution (9780240804958): Harry Box: Books

It is one of the best texts on how three phase power works as it relates to the entertainment electrician.

 

[sk8rsdad]

The answer to your question about being able to draw 7 amps from a 100 amp service seems to answer itself. A 100 amp service can supply 100 amps on each phase at whatever the supply voltage is. How much power this equates to depends on the supply voltage.

Your question about a 1200W unit that draws 7 to 12 amps is answered by Ohm's law (assuming a purely resistive load)

P=VI

Solving for V:

V = P/I

V = 1200W / 7a = 171v
V = 1200W / 12a = 100v

To understand phase-to-phase and phase-to-neutral relationships in a 3-phase system you need to understand trigonometry, alternating current, and it helps to know vector algebra. Determining voltage and current in a polyphase system involves adding sinusoidal waves, fortunately of the same frequency.

In it's simplest form with pure sinusoids shifted 120 degrees from each other carrying a balanced load, the equations are fairly manageable. The relationship between phase-to-phase and phase-to-neutral voltage is the square root of 3 (1.73). The neutral current is zero. That's where 208v/120v comes from. The voltage measured between 2 phases is 208v. The voltage from any phase to neutral is 120v.

Unbalanced resistive loads add more complexity since the neutral current is the actually the vector sum of the currents on the 3 phases. The phase-to-neutral voltage can drift too. Google "Adding Sinusoids" and "3-phase power formula" for some nice graphs and formulae from thousands of sites.

Including reactive loads and harmonics in the calculations make the math much more complex since all the work on adding sinusoids has to be done for each harmonic.

Taking things further, Tesla's work on polyphase power is not limited to 3 phases. Generators can be wound to produce any number of phases. There are reasons why an odd number of phases are desirable (self-starting induction motors come to mind). There are reasons why the world settled on 3 instead of 5 or 7 or some other number.

This site explains the mechanics of polyphase motors. You might find it helpful.

 

[xander]

I have not read the Set Lighting Technician's Handbook that Footer mentioned so I cannot compare, but I think that Automated Lighting: The Art and Science of Moving Light in Theatre, Live Performance, Broadcast, and Entertainment by Richard Cadena and published by Focal Press is the single best book for an introduction to electricity for an entertainment technician. I don't quite understand why it is called Automated Lighting. In reality it is a solid overview of everything an entertainment electrician needs to know from electricity to electronics to optics to networking and protocol. I highly recommend it.

-Tim

 

[masterelectrician2112]

Amazon.com: Set Lighting Technician's Handbook, Third Edition: Film Lighting Equipment, Practice, and Electrical Distribution (9780240804958): Harry Box: Books

It is one of the best texts on how three phase power works as it relates to the entertainment electrician.

I am reading that book right now. I have not gotten to that section yet, but what I have read has been very interesting. I especially enjoy the beginning chapter with the jokes about the different people trying to screw in a light bulb. Hmmm....that gives me an idea for the next off-topic thread...

 

[Chris15]

is answered by Ohm's law (assuming a purely resistive load)

P=VI

NO NO NO NO NO NO NO.
Ohm's Law is that the voltage is proportional to current by a constant known as the resistance. Ohm's Law DOES NOT and HAS NEVER said anything about power.

The phase-to-neutral voltage can drift too.

Err no. The whole reason for the neutral is so that the phase - neutral voltage does not drift. You will find voltage drop occuring as load increases, but that is most definitely not drift.

The size of a distro has nothing to do with the distance involved but everything to do with the currents (and to a small extent voltages) involved...

 

[sk8rsdad]

Ohm's Law DOES NOT and HAS NEVER said anything about power.

OK, so it's Joule's Law which is closely related to Ohm's law. I guess oversimplifying hit a nerve.

You will find voltage drop occuring as load increases, but that is most definitely not drift.

Granted. However I did not think the OP was ready for a 5 year electrical engineering degree from a forum response.

The size of a distro has nothing to do with the distance involved but everything to do with the currents (and to a small extent voltages) involved...

To which I say "huh?".

 

[Chris15]

OK, so it's Joule's Law which is closely related to Ohm's law. I guess oversimplifying hit a nerve.

Oversimplifying is one thing. But it's not Ohm's Law, call it what it is.

We've seen in the discussions surrounding grounded conductors and grounding conductors what happens if we don't adhere properly to established nomenclature.

I'm all for a simple explanation, but no explanation that is inaccurate will ever be acceptable in my books...

The size of a distro has nothing to do with the distance involved but everything to do with the currents (and to a small extent voltages) involved...

To which I say "huh?".

I refer to the OP;

how big a how big a distro would I need if the distance is over 200 ft.?

 

[STEVETERRY]

Oversimplifying is one thing. But it's not Ohm's Law, call it what it is.

We've seen in the discussions surrounding grounded conductors and grounding conductors what happens if we don't adhere properly to established nomenclature.

I'm all for a simple explanation, but no explanation that is inaccurate will ever be acceptable in my books...



I refer to the OP;

Guys--

I have been watching this thread with interest.

Perhaps we need to consider that some questions posed here are beyond the ability of a simple forum reply to fully explain.

For instance, if I asked:

"I have just found a deal on a used Westinghouse nuclear reactor, and I'd like to know how it works before firing it up."

..the reaction from the forum would be predictable.

IMHO, trying to explain three-phase power from scratch, without graphics, falls into much the same category. This is especially true given the wide range of expertise (none to lots) of potential responders.

I suggest that a referral of this type of question to the appropriate text is the best approach.

ST

 

[DuckJordan]

Guys--

I have been watching this thread with interest.

Perhaps we need to consider that some questions posed here are beyond the ability of a simple forum reply to fully explain.

For instance, if I asked:

"I have just found a deal on a used Westinghouse nuclear reactor, and I'd like to know how it works before firing it up."

..the reaction from the forum would be predictable.

IMHO, trying to explain three-phase power from scratch, without graphics, falls into much the same category. This is especially true given the wide range of expertise (none to lots) of potential responders.

I suggest that a referral of this type of question to the appropriate text is the best approach.

ST

+1 as i see every response from here and take it with a grain of salt. As i have no idea what kind of education and experience backgrounds you each come from. As far as literature is involved i would rather have someone refer me to a book than off their basic knowledge as if you have it published you are assuming a small sense of responsibility to any errors made while showing.

 

[derekleffew]

... "I have just found a deal on a used Westinghouse nuclear reactor, and I'd like to know how it works before firing it up."...

Contact the manufacturer at [-] Westinghouse[/-]. EDIT: Westinghouse AP1000 .
CB search results for Westinghouse nuclear reactor.
Use appropriate PPE.
Failure to observe Safety precautions may result in

Mushroom Cloud Triangulations

 

[zmb]

"I have just found a deal on a used Westinghouse nuclear reactor, and I'd like to know how it works before firing it up."

Also talk about a serious way to freak the neighbors when the UPS guy drops it off...

 

[STEVETERRY]

Actually, the link is here:

Westinghouse AP1000

ST

 

[jonliles]

"I have just found a deal on a used Westinghouse nuclear reactor, and I'd like to know how it works before firing it up."

..ST

I bet philhaney or I could give it a good go (from our past lives). The Pressurized Water Reactor is a great way for power generation [though not quite as efficient as a Boiling Water Reactor]. It is an inherently stable design and greatley minimizes the the opportunity for core meltdown - then again we don't use the Chernobyl design, so the likelihood of that ever happening again is almost ZERO. Then again, it may be a little indepth for the 3pahse power question asked by the OP.

 

[sk8rsdad]

I favour the CANDU reactor design with unenriched uranium and a deuterium moderator. Dump the heavy water moderator and the reaction stops. I don't think it's possible for this type of reactor to melt down. One byproduct is tritium which is a key ingredient for high-efficiency lighting, bringing it back on topic, sort of. They suck at producing weapons-grade byproducts so they don't have much of a market in the US.

And yeah, I worked for Atomic Energy Canada Limited once upon a time, in one of the heavy water plants.

 

[LekoBoy]

So my highschool is looking into this to power our 1KLs, Lightronics dimmers, and Innovator.
1. How much do these cost?

2. Would we need to buy three of them for 3phase service?

3. Would zipcord work or do I have to use that thick heavy 4ot stuff?

 

[DuckJordan]

So my highschool is looking into this to power our 1KLs, Lightronics dimmers, and Innovator.
1. How much do these cost?

2. Would we need to buy three of them for 3phase service?

3. Would zipcord work or do I have to use that thick heavy 4ot stuff?

WHAT?! are you talking about the reactors? by gosh man, do you really need that much power?

 

[mc5w]

Actually, a 100 amp service is only allowed to carry 80 amps continuously. However, U.S. National Electrical Code forgets to say that at ambient temperatures above 40 degrees Celsius and circuit breaker, fuse, or fusible switch can only carry 60% to 70% of its rating for standard rated devices. For heavy industrial devices rated to carry 100% of rating continously that only applies at a maximum ambient of 40 Celsius - for higher ambients the heavy industrial devices still need to be derated.

Mike Cole

 

[STEVETERRY]

Actually, a 100 amp service is only allowed to carry 80 amps continuously. However, U.S. National Electrical Code forgets to say that at ambient temperatures above 40 degrees Celsius and circuit breaker, fuse, or fusible switch can only carry 60% to 70% of its rating for standard rated devices. For heavy industrial devices rated to carry 100% of rating continously that only applies at a maximum ambient of 40 Celsius - for higher ambients the heavy industrial devices still need to be derated.

Mike Cole

That is a correct and astute observation. It reads directly on some manufacturers' distributed dimming systems that feed two 2.4kW dimmers from a single 20A thermal/magnetic breaker in a standard breaker panel. That means only 960W continuous power for each 2.4kW dimmer, assuming derating to 80% and an ambient not above 40C.

Our industry has gotten used to dimming systems that are listed for 100% duty cycle, 100% loading using fully magnetic circuit breakers. "Caveat Emptor" applies with systems and distribution schemes that use standard molded case thermal/magnetic breakers.

ST

 

[JD]

Understanding electricity is a "building block" type education. Each new concept learned requires other concepts to be in place in order to understand it. Even in the scientific community, such simple concepts as "does electricity flow from + to - or from - to +" have been under great debate for years. Example- I was brought up in the vacuum tube era. Power flowed from negative to the cathode of the tube, was regulated by the grid, and collected at the plate where it flowed back to B+. Along comes solid state, and everything is drawn backwards! Or is it... The concept of the "flow of holes" was begun. In this concept, atoms generally do not have an excess of electrons, but can have a missing electron. Therefore, the positive charge (atom missing electron) does all the work pulling an electron from its neighbor and cascading down the line until you are back to the power source. In this theory, power does flow from + to -. Is that right? The better question is "Does it matter!?" Does north flow to south, or south flow to north? We know the charge difference is going to cause electrons to move. Are we watching the movement of the electrons, or are we watching the atom ion ratio? Depending on which you watch, both answers are right! It's all perspective.

The takeaway of the concept is one wants the other and it is wise not to be between them! :shock: If even the simplest electrical concept causes confusion, it is no wonder that AC power, and multi phase power are not well understood in the general public. In AC, we think of the "Hot" as being the aggressive wire, and the neutral as being less aggressive. This falls under the category of "Observational Understanding." It lacks science, it is not what's going on, but it gets us through the day!