magnetic field

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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
If these two legs are on the secondary of a 3 phase source then aren't the phases 120deg apart? How are you saying that they are 180deg apart?
Relative to the neutral, each line reaches its peak 120 degrees from each of its neighbors. But, when you measure line-to-line, you get a normal, symmetrical sine wave of 208v, not 2 x 120v.

What you don't get is a lop-sided or offset wave with one side of the wave, one polarity, that is greater, wider, or 'longer' than the other. The weird voltage is the "anomoly", not the waveform.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I believe that the voltages would be 120 degree apart. The fact that they are 120 degrees apart (or what you called timing) relative to each other is why you measure 208V rather than 240V - as you said.
Again, relative to the neutral, yes, the 120 degree offset shows. But, with a 2-wire circuit, there is only a normal 208v sinewave.

I had not heard that about twisted pairs can you place cite a reference for that?
Not off-hand. It's just stuff that I've known about after years of AV interest. You can try various terms in a web search, such as 'utp' and 'twisted pair'.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I had not heard that about twisted pairs can you place cite a reference for that?
That's what I was wondering, too. I am an old audio guy (avocationally), and I know about common mode rejection and differential amps, but that's the first I've heard about noise rejection frequency being dependent on twists per unit length in a twisted pair.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Think about it:

The higher the frequency (of noise), the shorter the wavelength; the shorter the wavelength, the closer the twists have to be to make sure both conductors are exposed to the same portion of the noise's wave.

Remember that both differential-amp inputs have to have identical characteristics (like impedance) for the greatest signal-to-noise ratio, which in this case depends on the difference signal being all desired signal.

The twist rate is the main difference between UTP cable categories (plus CAT-6's spline). All cables will carry all signals; the high frequency limit depends on the highest noise frequency you can reliably reject.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Think about it:

The higher the frequency (of noise), the shorter the wavelength; the shorter the wavelength, the closer the twists have to be to make sure both conductors are exposed to the same portion of the noise's wave.

Remember that both differential-amp inputs have to have identical characteristics (like impedance) for the greatest signal-to-noise ratio, which in this case depends on the difference signal being all desired signal.

The twist rate is the main difference between UTP cable categories (plus CAT-6's spline). All cables will carry all signals; the high frequency limit depends on the highest noise frequency you can reliably reject.
I am thinking about it. I just don't see the connection between the spatial frequency of the twists and the time domain frequency of the noise. Can you cite?
 

ELA

Senior Member
Occupation
Electrical Test Engineer
Larry,
on the dual phase question ... I did not understand why you stated this:

The two legs are 180 deg, not 120, when you only have two conductors

Where does the 180 degree come in?

With regard to the number of twists on the wiring I believe it is much more significant to the crosstalk issue. Sort of what is often called a self-compliance issue. High frequency in one twisted pair affecting an adjacent pair.

I questioned the external immunity to high frequencies due to twists since you are talking about a very high frequency when using a cable with 3 twists per inch for example.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Larry,
Where does the 180 degree come in?
I used the wrong term, but I was trying to respond to the question in the way it was asked in post #12.

Mull seems to be under the impression that 208v from a Y source will not have a symmetrical sine wave, because the peaks occur at 120 degrees (relative to the neutral, of course.)

I was trying to explain that the 120 degrees manifests itself by the voltage being 208v instead of 240v (i.e., less than 2 x 120v), rather than by having a non-symmetrical waveform.
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
Is the magnetic field around a cable canceled when feeding a single-phase transformer with 2 phases of a wye distribution? I would think that it is since it's the current that is developing the field, but a ground conductor is installed with it and I don't know if there would be anything funny going on with the ground reference not being a center-tapped connection on the supply side (as in a single-phase service).

yes ,
The electric fields associated with underground transmission cables are in effect shielded because of the ground provides an effective shield from electric fields.
Magnetic fields are not shielded by being buried and fields would be present especially directly above the cables and within the cable easement. However, underground cables are laid close to each other so the electromagnetic fields created by each cable, cancels each other out. Magnetic field values in the order of 5 to 20 μT are typical directly above a high voltage cable and reduce rapidly to very small values within 10 metres of the cable
 

richxtlc

Senior Member
Location
Tampa Florida
yes ,
The electric fields associated with underground transmission cables are in effect shielded because of the ground provides an effective shield from electric fields.
Magnetic fields are not shielded by being buried and fields would be present especially directly above the cables and within the cable easement. However, underground cables are laid close to each other so the electromagnetic fields created by each cable, cancels each other out. Magnetic field values in the order of 5 to 20 μT are typical directly above a high voltage cable and reduce rapidly to very small values within 10 metres of the cable

About 25+ years ago I was incharge of a group of techs who were responsible, among other things, for measuring magnetic fields. We were doing this when EMF was the topic on the 6 and 11 PM news. We had a complaint from a customer that wanted the pole transformer mounted near her window moved because of the magnetic fields that were surrounding it. We went to here home to measure the fields and they were that strong, but she had some pull with public officials and we eventually moved the transformer to the end of the street, she was satisfied. We also measured the field after the change and found it to be stronger then when the transformer was there. It seems that the transformer was cancelling the magnetic field from the high voltage feeder feeding the transformer. In her case it was mind over matter. The transformer was moved, she felt safer. This is what happens when a news story gets out of hand with the facts.
 
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