ECN Forum Forums Technical Discussion Electrical Theory and Applications Could someone explain phases?

I'm new to the trade, 300 hours for a local commertial company here in Ottawa, Ontario, Canada. I'm interested in learning more about phases, voltages and their advantages.

Would anyone care to explain? 347, 208, 120, 240, 600?

I get confused when talking about peek to peek voltage and RMS.

Only electronics people worry about peak-peak voltages.

In the electrical (power) world we only use RMS voltages. In the US the National Electrical Code has a section that requires us to only use the nominal RMS voltage for supply system calculations.

hmm, alright. i'd still like information about phases and voltage.

Electricity is like water. It has pressure and flow. We call the pressure "volts" and the flow "amps".

When we wire a building, we need to know the pressure (volts), so we can install the proper equipment. The numbers you have listed are some standards. For example, we all know what voltage equipment to install in a house and the stores all sell the right toaster and coffee maker.

We also need to know the flow (amps) so we can install the correct size wire. Too small a wire for the required flow is like a fire truck using a garden hose - the building will burn down.

Can someone explain phases?

Go to google and search "electrical dictionary". And while you are at it, search "basic electrical theory". You will find thousands of pages of enjoyable reading on the topics you are inquiring about.

You may also look into getting enrolled in a good trade school or electrical apprenticeship. It's hard to learn these concepts in the field...

It just "is". There are usually economics involved with selecting voltages, and the standards very from place to place. Generally speaking, the higher the voltage, the smaller the wire you can use to get the same power from it. Here's the equation that governs it:

Volts x Amps = Watts

Double the volts, and you only need half the amps. Half the amps, and you only need have the wire. Of course, that's tempered a bit by safety, as lower voltages are generally safer, and require less electrical insulation. So, we may use a 2000A 600V switchboard to avoid having to use a 6000A 208V switchboard with equally massive cables. In the end, it really is a case of "that's always how we've done it" with different electric companies rolling out different voltages and the world today being a terrible jumble. Good luck reading up, tons of info out there! And don't let 3-phase power confuse you, come back to it later once you've mastered the basics

If you're trying to work out how the different voltages relate to each other, remember that with 3-phase there is a fixed relationship between the phase-to-neutral and the phase-to-phase voltage, based on the square root of 3.

Hence:

120 x Sqrt(3) = 208
277 x Sqrt(3) = 480
347 x Sqrt(3) = 600

and so on. There is nothing "magic" about the specific voltages themselves, they are just standards which emerged over the years. For example, here in the U.K. we have:

240 x Sqrt(3) = 415 volts.

The Sqrt(3) factor comes about by the phase difference between the phases, which is 120 degrees. If you plot a 120V RMS sinewave on a graph for phase A, then plot another 120V RMS sinewave shifted 120 degrees to represent phase B, then obviously the peaks and the zero points will not coincide.

If you wanted to prove the relationship the "long way", you could make multiple calculations every few degrees along the axis of the graph, adding the instantaneous voltage of A and B at each point and plotting the sum. The result would be another sinewave, of 208V RMS.

Peak-to-peak vs. RMS (Root Mean Square), as mentioned, is not generally a concern in power work. For a simple, pure sinewave, however, the peak voltage is 1.414 x the RMS voltage (or conversely, the RMS voltage is 0.7071 x the peak voltage). For a symmetrical sinewave, the peak-to-peak voltage will just be double the peak voltage. In complex electronic waveforms the relationship can be quite different.

So for the common voltage levels of a basic sinewave, the peak voltages work out thus (rounded to nearest volt):

120V RMS = 170V peak
208V RMS = 294V peak
240V RMS = 339V peak
277V RMS = 392V peak
480V RMS = 679V peak, etc.

The reason that RMS values are used with AC is that they produce the same amount of power as the equivalent DC voltage.

For example, if you connect 120V DC across a 12 ohm resistance you get 10 amps of current and 1200 watts of power dissipated.

If you connect 120V RMS AC (170V peak) across the same 12 ohm resistance, you get 10A RMS current (about 14A peak), which results in the same 1200 watts of heat.

Jza

Welcome to ECN!!!

It is great to see that you are interested in learning the "Advanced" things in the Electrical Field.

You would greatly benefit from _Several_ quality Text Manuals, as they will cover the subjects in-depth, and you can reference them anywhere at any time.

Notice I say "Several" quality Text Manuals - by this I am stating to get at least Three (3) different Manuals - or at least review no less than Three (3) different Manuals, and get the better Two (2) Manuals.
This will give you the advantage of explanations and examples which cover as many areas possible.

Along with the books, dropping by here (ECN) and posting questions + "What-Ifs" and "Am I Right" topics will enhance your education drammatically!

BTW, check out the Technical Reference Section for items posted on-line at ECN (the drawings in this message are from the Tech Ref. area).

You can access the "Menu" by clicking on this link:

Menu For Technical Reference Section

Here are a few more things to add in with the information already posted by others:



Well, the term "Phases" can be more than just one thing, but in your example, I believe you are referring to either "Single Phase" or "Three Phase".

Anything other than a "Single Phase" System is known as a "Poly-Phase" System - there are more than just One (1) "Single Phase" in the System.

The typical 208Y/120V and 600Y/347V Systems are Poly-Phase Systems (these are actually 3 Phase 4 wire Wye Systems).

The typical 120/240V 3 Wire Systems used for Single Family Dwellings (Residential Customers) is a Single Phase System.

Single Phase Systems only have a Single "Sine Wave" / Phasor, and only use a Single Transformer Coil (Winding) for their Primary side.
Also, a "Single Phase" Circuit is most of the time only a 2 wire Circuit. (more on this in another discussion!).

Three Phase Systems will have Three separate "Sine Waves" / Phasors, and typically use Three Transformer Coils (Windings) for their Primary side.
3 Phase Circuits may be 3 Wire or 4 Wire (with a system "Neutral").

There are other Poly-Phase Systems - such as the 2 Phase System (older and very rare now),along with 6 and 9 Phase Systems (used mainly for Rectifier Inputs).

Below are some drawings of different AC Systems:



Single Phase 3 Wire System

*** TECHNICAL NOTE ***
Even though this drawing shows a "Split-Coil" - or Two (2) individual Primary Windings, they are considered to be a single unit Winding, and therefore a Single Phase Primary.
The Primary + Secondary Coils are "Split" as to allow for "Dual Voltage" connections, as follows:

1: Primary:

*P-a: Connection of Coils in Parallel for an input Voltage of 6,000 Volts,

*P-b: Connection of Coils in Series (as shown in drawing above) for an input Voltage of 12,000 Volts.

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2: Secondary:

*S-a: Connection of Coils in Parallel for an output Voltage of 120 Volts - 2 Wire,

*S-b: Connection of Coils in Series for an output Voltage of 240 Volts 2 Wire,

*S-c: Connection of Coils in Series - with a "Center Tapped" lead from the "Series Jumper" (as shown in the drawing above) for an output Voltage of 120/240 Volts 3 Wire.

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FPN: Since this Transformer has only one primary winding - with only a 2 wire circuit feeding it, the system is Single Phase - regardless of how many Secondary leads the Transformer may employ.



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3 Phase 3 Wire Delta System

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3 Phase 4 Wire Wye System

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2 Phase 4 Wire System

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6 Phase System

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As you can see, there is a huge difference between a single Phase System and a Poly-Phase System.

The advantages of using Poly-Phase Systems are Equipment Efficiencies, Economics, and the primary reason: Induction Motors.

We can discuss more later if you would like to.
For now I will keep things simple.

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The following drawings should explain how these Voltages are obtained. Why we use thse values is anyone's guess!!!
The Voltage ratings are of normal "Standard", set forth by some group of Engineers, somewhere, at some time for some reason - only IEEE knows the secret of their origin



208Y/120V 3 Phase 4 Wire Wye Transformer Connection Schematic - with Voltages at points.

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"Layout" or "Single-Line" drawing of the 208Y/120V 3 Phase 4 Wire Wye Transformer Connection

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600Y/347V 3 Phase 4 Wire Wye Transformer Connection Schematic - with Voltages at points.

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"Layout" or "Single-Line" drawing of the 600Y/347V 3 Phase 4 Wire Wye Transformer Connection

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As mentioned by others, "Peak-To-Peak" (P-P) Voltage is the "Maximum" Voltage for an Alternating Current - as viewed by its Sine Wave.
Peak Voltages occur when the Sine Wave reaches the "Highest Amplitude" on either side of the Zero Line.
P-P Voltage is the complete wave's peaks on both sides of the Zero Line.

P-P and even Peak Voltage is not too relavent for most Field work - only if you are dealing with Diodes / Rectifiers.
That is why one Member mentioned the Electronics aspect.

The most referenced Voltage level will be the RMS (Root-Mean-Square) value.

Others have covered this area in great detail, so I will leave it as is.

Again, feel free to participate in the discussions here at ECN!

Scott.


06/14/2008 - 16:04:00
edited to fix a few / many spelling air-orrs (errors)

Wun daiye ayee whill bee aybell tew spel betur, butt knot tew-daiye!
(Translated: One day I will be able to spell better, but not today!)

Sincerly:
Scott - the Injun-ear!

Guys,
I'm going to go out on a limb here.
I was under the impression that P-P voltage was used as the maximum voltage that the system cable insulation had to withstand?

BTW, Jza, welcome to the trade and ECN, great to have you along and asking questions, the more you ask, the more you learn.

In a pure sine wave there is a simple mathematical relationship between P-P and RMS, so when you chose one the other can be directly inferred. It just so happens that everything in the power world is done based on RMS voltage not P-P.

Most cable insulation (i.e. THHN) is rated for 600V if this was a P-P value then we could not use it on ungrounded 480V RMS systems and definitely not on 600V ones. Now if you mean that the cable is tested, as opposed to rated, based on P-P you could be correct, after all a megger only puts out P-P (actually DC). But again a P-P voltage can easily be converted to RMS.

In the US the NEC says that all systems are to be referred to by their nominal RMS voltage.