Home>Discussions>ELECTRICAL & LIGHTING>Can someone explain the difference between phases and poles??
90 posts / 0 new
JLMCDANIEL
canuk wrote:

Jack ... I get what you're saying .... I did one mistake there is no phase shift between the primary and the secondary.
the way I figure it we are using two 120 volt supplies in series to produce the 240 volts along with the center tap to drain the difference between the 2 unbalanced loads which creates a split phase.
Which would be aiding ( boosting ) each other resulting in the 240 volts.

This means that one split 120 volt leg would have to be 180 degrees out of phase with the other 120 volt leg. Otherwise they would be opposing ( bucking ) each other resulting in 0 volts.

So mathematically the polar equation :

(120v @ 0degree) minus (- 120v @180 degree) = 240 v @ 0degree.

Just a thought.:)

Ok, look at this way there is no phase in DC voltage, If you take two 1.5 v batteries, connect in series and connect the center connection to ground. It looks like a + and - 1.5 volt power supply. It reads + 1.5v from center to top and -1.5v center to bottom and reads 3 volts top to bottom. They are in phase no phase shift, the lower voltage (just like the 120vac) appears to be shifted but it's not, it's just that we use a refferance point, ground to take the readings.

Jack

JLMCDANIEL
rickpantel wrote:

Well....I started this thread with my question, and I am totally overwhelmed with the depth of knowledge of the members responding. Thanks to all who have responded!

Bsum1 said it well...holy mackerel, you guys are very knowledgable indeed!

So...here is a photo of the power pole and transformer outside my house (see attached). Obviously, the very top wire is the high-voltage line...somebody said it was about 7000 volts or so. This I assume is the single phase distribution from the power grid. The transformer is obvious with the 3 lines for distribution to the houses (2 split single phase hots and a center-tap neutral)....so far so good. I get it this far.

What I can't understand is why there is only one high-voltage line at the top of the pole. :confused: The schematic diagrams drawn earlier show the primary winding of the transformer with two wires on the high voltage side...where is this second wire? Rabbitgun alluded to this by saying that the neutral is not carried on the transmission lines, but I don't understand how this is possible.

Any takers?

It would appear that you have a grounded high voltage system coming to the transformer. In other words it is using ground for the return. I have never seen that around here.
Jack

djohns

The picture is a bit strange in that there is no neutral conductor on the primary . Not something that is done here for sure .

As far as transformer connections go , here is a good source of info . Kinda dated , but still good .

JLMCDANIEL

By the way as a point of referance.
A delta transformer is a three winding transformer with the windings wired so the form a ▲.
A Whye transformer is a three winding transformer with the windings wired so the form a "Y".
Both are for 3 outputs and there would be absolutely no reason to ever use either for residential power.
A step down 2 output transformer is a transformer that steps the voltage down. For instance a residential transformer that has a 7000 volts input and 2 outputs of 120 volts each.
Jack

JLMCDANIEL wrote:

By the way as a point of referance.
A delta transformer is a three winding transformer with the windings wired so the form a ▲.
A Whye transformer is a three winding transformer with the windings wired so the form a "Y".
Both are for 3 outputs and there would be absolutely no reason to ever use either for residential power.
A step down 2 output transformer is a transformer that steps the voltage down. For instance a residential transformer that has a 7000 volts input and 2 outputs of 120 volts each.
Jack

Jack I would be the first to say I do not know utility installations.

A Y transfomer is for instance 120 volts on each of the three phases. since the other end of all three phases are tied together and then grounded phase to phase is 1.73 of each phase. Thus 208 volts is measured from any two phases. the same senerio above is used on 277/480 volt system.

A delta 3 phase is 240 volts measured at each detal connection. To get 120 volts they center tap one of the windings hence the 120 volts.

I seen your drawing in the other post with the pump.Using a single phase tap off of a utility pole feeding a transformer. How do they make the phase shift if it is 180 degress out of phase as you described? A capacitor or inductor? they shift the voltage only 90 degrees? I want to learn something new here

djohns
djohns wrote:

The picture is a bit strange in that there is no neutral conductor on the primary . Not something that is done here for sure .

As far as transformer connections go , here is a good source of info . Kinda dated , but still good .

Start reading at section 15.9 and you will see the schematics . It'll make sense then .

JLMCDANIEL
Ravens53 wrote:

Jack I would be the first to say I do not know utility installations.

A Y transfomer is for instance 120 volts on each of the three phases. since the other end of all three phases are tied together and then grounded phase to phase is 1.73 of each phase. Thus 208 volts is measured from any two phases. the same senerio above is used on 277/480 volt system.

A delta 3 phase is 240 volts measured at each detal connection. To get 120 volts they center tap one of the windings hence the 120 volts.

I seen your drawing in the other post with the pump.Using a single phase tap off of a utility pole feeding a transformer. How do they make the phase shift if it is 180 degress out of phase as you described? A capacitor or inductor? they shift the voltage only 90 degrees? I want to learn something new here

Harry,
As I said there is no phase shift, no 180 degree shift, no 90 degree shift, it is single phase. It appears that there is a phase shift if you connect an O scope reference to ground . If you connect the O scope leads to display the two other legs of the transformer, it appears that as the sign wave goes up for the top leg it is going down equally (180 degrees) on the bottom leg but that is only because you are using the center tap as your reference point. If you use the bottom leg as the reference and hooked the leads to the center and top of the secondary the display would show the top going up to 240 RMS and the center going up th 120 RMS at the same time (in phase). Although you can get the appearance of a phase shift referencing to the center tap there is in fact no shift.
Jack

canuk
JLMCDANIEL wrote:

Harry,
As I said there is no phase shift, no 180 degree shift, no 90 degree shift, it is single phase. It appears that there is a phase shift if you connect an O scope reference to ground . If you connect the O scope leads to display the two other legs of the transformer, it appears that as the sign wave goes up for the top leg it is going down equally (180 degrees) on the bottom leg but that is only because you are using the center tap as your reference point. If you use the bottom leg as the reference and hooked the leads to the center and top of the secondary the display would show the top going up to 240 RMS and the center going up th 120 RMS at the same time (in phase). Although you can get the appearance of a phase shift referencing to the center tap there is in fact no shift.
Jack

Jack .... that's the point .... with reference to ground ( neutral) there is a split in the phases.

If you look at the attached schematic .... this represents the secondary side of the transformer .... also the mains power in the home.

Here is where the (+) and (-) polarity markings really become important ..... so it is clear whether they are aiding (“boosting”) each other or opposing (“bucking”) each other. You can see the split-phase sources in the schematic (each 120 volts @ 0 degrees), with polarity marks (+) to (-) just like series-aiding batteries can be used to represent this.

If we mark the two sources' common connection point (the neutral wire) with the same polarity mark (-), you have to express their relative phase shifts as being 180 degrees apart. Otherwise the two voltage sources would be in direct opposition with each other, which would give 0 volts between the two “hot” conductors.

Power systems in North American households and light industry are most often of the split-phase variety, providing the 120/240 VAC power. The term “split-phase” merely refers to the split-voltage supply in such a system.

. By definition ....
A split-phase power system is one with multiple (in-phase) AC voltage sources connected in series, delivering power to loads at more than one voltage, with more than two wires. They are used primarily to achieve balance between system efficiency (low conductor currents) and safety (low load voltages).

Basically ... this kind of AC power supply is called "single phase" because both voltage waveforms are in phase, or in step, with each other.
Single phase power systems are defined by having an AC source with only one voltage waveform.

Is it any wonder why I used to fall asleep in theory class.;)

JLMCDANIEL

Canuk,
Wake Kent up.:D
There is a significant difference between split phase and phase shift. Because the windings are in the same direction the phase pattern of the two windings are identical. If you ground the bottom line rather than the center tap and you read from the bottom line you would see the center tap to 120 RMS and the top leg go to 240 RMS at exactly the same time, reverse at the same time and go to the same RMS value in the opposite direction. That's because it is single phase. Because you change you reference point does not create a phase shift.
Jack

canuk
JLMCDANIEL wrote:

Canuk,
Wake Kent up.:D
There is a significant difference between split phase and phase shift. Because the windings are in the same direction the phase pattern of the two windings are identical. If you ground the bottom line rather than the center tap and you read from the bottom line you would see the center tap to 120 RMS and the top leg go to 240 RMS at exactly the same time, reverse at the same time and go to the same RMS value in the opposite direction. That's because it is single phase. Because you change you reference point does not create a phase shift.
Jack

Yep ... you're absoluetly correct .... I did say shift and stand corrected using the wrong term.

Darn ... now I have to edit the post .... geez ....all this theory is giving me a head ache .... time for a nap.;)

## Pages

#### TV Listings

Find TV listings for This Old House and Ask This Old House in your area.