wye vs delta.

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K2X

Senior Member
Location
Colorado Springs
I just finished 4 chapters of transformers at school and i am still very confused. I'm in a class where the much younger students and the teacher have the common interest of getting out of class 2 hours early which means asking questions in class is pretty much out. So i'm hoping to get some help here.

A month or two ago Larry and others explained the three phase sine waves being 120 degrees out of phase and that these waves were intersecting below the peak thus giving us 208 on the 3 phase wye. I think now I have a good understanding of that concept?

One of my questions would be: Why am I getting 120/240 on the 3 phase delta transformer? Are these sine waves not also 120 degrees out of phase and also intersecting below the peak?? Thanks for any replys.

Also i have a question in regards to using this site. Is there a quick way to find threads that I posted on say 2 months ago?
 
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Volta

Senior Member
Location
Columbus, Ohio
It helps to think of the (high-leg) delta as a single voltage, or three, but not two. The 240 being phase to phase, the others dependant on which phase is measured to the 'neutral' point.

The 120 voltages are 100% in line with one of the 240 windings, so there is no angle to speak of.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Voltage is always a relative measurement. A single point doesn't have a voltage; instead voltage must be measured between two points.

Phase angle is a relative measurement made between two or more voltages.

With a three phase wye system, it is simple to use the neutral of the system as the reference point. Each of the leg voltage is measured relative to the neutral. Measured this way the three leg voltages are 120 degrees apart.

With a delta system, you don't have that nice apparent reference point. From the measurements that you can actually make (from A to B to C) you can mathematically calculate the voltages and phase angles relative to a 'virtual' reference point. So for a delta system you could actually calculate the voltages and phase angles relative to the wye neutral, even though the system doesn't even have a wye neutral. This set of measurements also shows the 120 degree phase angles. You also see the 120 degree phase angles if you compare the A to B voltage measurement with B to C and C to A.

If you select any other reference point other than the wye neutral, then your phase angles will be something different than 120 degrees.

With a high leg delta system, if you use the grounded center tap as your reference point, then your phase angles will not be 120 degrees apart. Looking from this reference point, you get 120V at 0 degrees, 208V at 90 degrees, and 120V at 180 degrees.

-Jon
 
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cpal

Senior Member
Location
MA
When you measure voltage in a 3P wye system your meter is across phase A and B for example it's reading two coils (phases) in series. The voltages in those coils have their reference points 120 deg out of phase and the sum is a vector addition or a multiplier of 1.73. 1.73 X 120 = 208V (sorta)

In a delta system the meter will read across one phase (end to end) and the voltage impressed on that phase (coi) will be indicated by the meter.

In a delta system the current is drawn from more than one phase and the current is a sum of the vector addition (1.73.) The current through one phase X 1.73 = I of the line
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
K2X said:
Why am I getting 120/240 on the 3 phase delta transformer? Are these sine waves not also 120 degrees out of phase and also intersecting below the peak??
Click to expand...
It's easier to start with explaining why the wye's 120 + 120 = 208. We're talking about two lines and the neutral.

The 208v line-to-line voltage is made up of two 120v secondaries sort-of in series, but the two 120v windings do not peak at the same time. Their sine waves are offset.

In a 240v secondary made up of two 120v windings in series, the two windings do peak at the same time. They're identical to a single center-tapped 240v winding.


In a Delta setup, there are three 240v secondaries with the 120 degree offset, but we don't attempt to get line-to-line power from more than one secondary.

Note that the high-leg Delta, which is only so because we opt to ground the center tap of one secondary, can deliver an unusable 208v using 1-1/2 windings.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
New mental image for a wye system:

Let's compare a 120/240v winding like the pedals of a bicycle, with the two pedals at the usual 180 degrees apart, with the axle of the crank as the center tap. Let's say the length of each pedal's arm is 120v long, so the pedal-to-pedal distance is 240v.

Now, let's pretend we have three legs, so there are three pedals, each with a 120v-long arm, equally spaced around the crank axle, and the distance between any two pedals is now 208v long. One day, you lose a leg, but continue to use a 3-pedal bike.


Delta bicycle next, maybe.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Okay, here 'tis:

We still have three legs, but instead of a three-armed pedal crank setup, let's say our bike is a velocipede (and no, it's not a dinosaur) with three pedals bolted to the front wheel.

(Note to engineers: Please, no comment about the physical impossibility of getting a wheel supported in place with three pedals being pushed by a three-legged being. ;))

There is a distance of 240v between each pair of the three pedals in a triangular pattern. (We don't care about the distance from each pedal to the axle (1/2 of 277).)

It happens that a three-legged being (a tripod?) above who lost a leg is as adept at riding either type of three-pedal bikes: a three-armed one or a triangulated one.


If you draw a line between two of the pedals, and make a dot in the center of that line, there happens to be a distance of 208v between that dot and the opposite pedal.

However, there's no pedal at that point, so there's no way to use that point for a two-legged person to be able to pedal symetrically (well, symetrically for a biped.)

However, the two-legged tripod-turned-biped is perfectly happy using any two pedals of either type of three-pedalled bike, as long as he keeps his feet on the pedals.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
K2X said:
...

One of my questions would be: Why am I getting 120/240 on the 3 phase delta transformer? Are these sine waves not also 120 degrees out of phase and also intersecting below the peak?? Thanks for any replys.

...
Click to expand...

winnie said:
...

With a delta system, you don't have that nice apparent reference point. From the measurements that you can actually make (from A to B to C) you can mathematically calculate the voltages and phase angles relative to a 'virtual' reference point. So for a delta system you could actually calculate the voltages and phase angles relative to the wye neutral, even though the system doesn't even have a wye neutral. This set of measurements also shows the 120 degree phase angles. You also see the 120 degree phase angles if you compare the A to B voltage measurement with B to C and C to A.

...
Click to expand...
If you were to connect a 3? wye resistor bank to the delta transformer line terminals, you would establish a system neutral point for measuring voltages from one common point. Measurements taken as such would resemble that of a wye system.

derivedsystemneutral.gif
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Smart $ said:
If you were to connect a 3? wye resistor bank to the delta transformer line terminals, you would establish a system neutral point for measuring voltages from one common point. Measurements taken as such would resemble that of a wye system.

derivedsystemneutral.gif
Click to expand...
Yes, but it should be noted that the Delta-source neutral is not at the same potential as the Wye-load neutral, nor should they be connected.
 
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