Originally Posted By: pabernathy
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First, a ?phase? is a coil of wire in a transformer. When we use the term, we normally mean the voltage (Electrical Pressure) and current (electrons moving) across that coil. Most residences have single phase power as an electrical service to that structure. That means that 240 Volts is supplied to the building. In an attempt to make that safer, they take the center of that coil and ?Ground? it. Making the center of that coil the same electrical potential as the earth or floor we typically stand on, by attaching a conductor (wire) from the electrical and physical center of the winding to the earth, by a rod driven into the earth, hence ?Grounded.
Alternating current is an electrical potential with constantly changing amplitude and a periodically changing direction. That means that the electrical potential across the phase moves in one direction for 1/120th of a second, then stops and returns in the other direction for the next 1/120th of a second. The amplitude in the definition refers to the strength of electrical pressure or Voltage as that potential rises from zero (None) to a positive peak. Then the strength of electrical pressure falls through zero, to a negative peak, then again, through zero to a positive peak and repeats that process.
We call one cycle, the time (Period) it takes to move from one point and direction of amplitude through one peak, say positive, then through the other peak, say negative, then back to the original point (being in the same electrical direction). When the time for 360 electrical degrees, or one complete cycle, takes 16 2/3 thousandths of a second, then we call that 60 Cycles per Second or simply 60 Hertz (Hz).
In any phase the difference of electrical potential between one end of the coil and the other is 180 electrical degrees, much like the dry cell (battery) in your flashlight. If one end is positive, the other end has to be negative. An Alternating Current, unlike the battery is constantly changing.
With Three-Phase, you have three coils of wire in a transformer. Each phase (coil of wire) acts exactly as mentioned earlier. They are however, spaced, relative to time, 120 Electrical Degrees apart. That is likened to three guys all standing around the same tent peg, each taking a turn at striking the peg to drive it into the ground. Each guy is operated at 60 Hz, so the time one guy strikes the peg is every 16 2/3 thousandths of a second. Where relative to time, each guy is 120 Electrical degrees apart and before the first guy hits the tent peg a second time, the other two guys have already struck the peg. Three guys striking the peg can put the tent peg to depth in 1/3rd the time that one guy can put it in.
If it is not important to you to have the peg in faster, you can use smaller guys, or smaller mallets. This opens the door to understand the pros and cons of single-phase and three phase benefits or compromises.
First thing, your Kilowatt-Hour usage is the same, because it measures work. The work (tent peg) does not care if you have single-phase (one big guy with big mallet) or three-phase (three smaller guys with smaller mallets). It is the same work being accomplished. Your electric utility has different rates, where they charge for demand (Big slugs of current needed and for how long), KVAR Hour (Is the current moving when the voltage tells it to, or is it slightly different) and other considerations effecting the cost of those kilowatts.
If your load is under 48,000 Watts or 48KW, then Single-Phase is most likely cheaper, because of how the Electric Utility is likely to bill those kilowatts. If you are starting large motors and your load is above 35KW, then Three-Phase is likely to be cheaper. That being said, let?s investigate other costs.
A larger conductor (wire) allows more electrons to pass, enabling more work to be done. Larger wire is more copper, therefore a greater expense. If work equals Volts times Amps, then a higher voltage will require smaller wire to accomplish the same work.
Single-phase components typically are larger, so equipment utilizing a three-phase source have a smaller foot print and may be cheaper to purchase. Higher voltage equipment, regardless of number of phases, also has a smaller foot print, are lighter and often cheaper to purchase and install.
Switch gear for larger utilization equipment can be very expensive, however in this case, three-phase is typically cheaper to purchase and install.
Load balancing is somewhat more complex for three-phase systems, because most ?electricians? are installers and do not have a comfortable grasp of technical aspects of the first six paragraphs of this post. So there is a legitimate consideration of finding the right firm to provide support or maintenance of your three phase system.
There are two types of three phase systems, being Delta and Wye. In a Delta, the three phases are connected in series. In a Wye, the three phases are connected with one end all tied to Ground.
In heavy industry, Deltas and Wyes are equally common and are normally over 600 Volts, frequently as high as 13,800Volts. In light industry, the Wye is most common and usually 600 Volts or less. In light commercial the Delta is most common and often 480 or 240 Volts.
A very common service used in Strip malls, light manufacturing and rural settings is the Open Delta. This is when the power company uses two phases to make three. As a 4 wire Delta, it has a single-phase service contained within the three-phase installation. Normally a 240 Volt Delta, it has the same service as your house by center tapping one phase, yet can provide 240 Volt three-phase power as well. The electric utility likes this because it holds their costs down by eliminating the need for running all three phases over some great distance and uses only two transformers.
This gives the customer the benefit of a common single phase voltage at any connection to the system and still provide the 120/240 Volt single-phase for convenience outlets. 240 volt motors can be connected across any two lines, or 240 Volt, Three-Phase motors will be connected to all three lines. Office outlets and convenience outlets throughout the facility will be connected on one, of two particular lines to ground. The bad part here is the ?Hot Leg,? or ?Wild Leg? that has a higher Line to Neutral Voltage to ground. Incompetent persons calling themselves electricians can get into big trouble being unaware of this phenomenon
Phase is a term used to indicate the time or distance relationship between two waveforms, such as voltage -to-current or voltage-to-voltage. When two waveforms are in step with each other, they are said to be in phase with each other.
In a purly resistive ac curcuit, the current and voltage are in phase. This means that, at every instant, the current is exactly in step with the applied voltage. They BOTH reach their zero and peak values at the same time.
Phase are often expressed in degrees: one full waveform is equal to 360 degrees. For example, a three phase generator has each of its windings out of phase with each other by 120 degrees.
Ok with that said you have "crudly " lets say each PHASE of a single phase system has its own waveform that we as electricians TRY to balance out so the potential value is zero at the exact same time of the sinusoidal wave form.....other things can effect this like Lag and Lead and well that just goes to much into electrical theory for this site..so I will digress...
Anyway....what you are dealing with is Single Phase and Three Phase....the I THINK I explained the single phase above.....
Paul W. Abernathy- NACHI Certified
Electrical Service Specialists
Licensed Master Electrician
President of NACHI Central Virginia Chapter
Moderator @ Doityourself.com
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