T&T: Electricity at Dock and Phase Angle

[Ken Bloomfield]

Hi John,

You are quite correct in terms of electrician terminology.  The correct 
"electrician" term is as you state.  However, it is pretty much a case of 
semantics I think, since I think we will both agree that if you put an 
oscilloscope on the two legs and reference it to the neutral leg, you will 
find that the waveforms are 180 out of phase relative to each other.  One 
leg is at a peak positive swing while the other leg is at a peak negative 
swing, hence the 240 VAC RMS difference.  The secondary of the transformer 
does exactly what you said, it is a "phase splitter" and as such will take 
one phase and split it into two circuits that are 180 deg. out of phase with 
each other, relative to the center tap.

In this description, I am using the term "phase" to describe the point in 
the sinewave that each leg is at, where one is exactly at its sinusoidal 
peak positive value at the exact instant that the other leg is at its 
sinusoidal peak negative value.  I do realize that the term "phase" began to 
take on some assumptions in the electrician lexicon.  Being from an 
electronics background, I use the term phase to simply mean the sinewave 
rotational relationship. I realize that going back in history, that (for one 
example) if one spoke of "two phase power" an assumption of a 90 deg. 
relationship was taken.  For example from Wikipedia: "The generators at 
Niagara Falls installed in 1895 were the largest generators in the world at 
the time and were two-phase machines. Some early two-phase generators had 
two complete rotor and field assemblies, mechanically rotated by 90 degrees 
to provide two-phase power."  However, "two phase" does not really dictate a 
90 degree relationship in the sinewaves, it just happened that that was the 
case to allow self starting motors.

What I was merely trying to say was that trying to take two legs off of a 
three phase system has more than just an RMS voltage issue, unless a Scott-T 
transformer is used (doubtful).  The phase relationship between two legs is 
360/3 = 120 degrees in that case.  That has profound implications on what 
the white wire in the cord would have to carry.

Sorry for the incorrect terminology, I ought to know better on this list.

All the best,
Ken.
----- Original Message ----- 
From: Truelove39 at aol.com
To: khtb at bellsouth.net
Cc: trawlers-and-trawlering at lists.samurai.com
Sent: Tuesday, April 15, 2008 7:25 AM
Subject: Electricity at Dock and Phase Angle


Hi Ken -

I beg to say that there are not 2 phases in a 120/240 transformer-derived 
neutral circuit. It cannot be 2-phase because the primary winding is only 
single-phase. The 3-wire secondary in this instance is called "split-phase."

Regards,

John
"Seahorse"

Using the example of the center tapped 240 volt transformer is a very good
way to supply the 50 amp service, and in fact perhaps the only correct way
in my opinion.  Here is why:

1.) The two 120 volt phases will be 180 degrees out of phase with each
other, as they should be, and this is critically important.

2.)  The reasons this is critically important is that the standard Marinco
shorepower cable provides the two 120 volt antiphase supplies on three
wires.  What this means is that the two phases (black and red) share the
common white return wire.

3.)  When properly 180 degrees antiphase, consider if the two 120 VAC
circuits are each loaded to their max at exactly 50 amps on each, -- then
the white wire will be conducting zero amps since the antiphase cancel.  By
the same token, if one 120 VAC phase is at 50 amps, and the second is at
zero amps, then the white wire will be conducting 50 amps.  In either case,
the white wire is nver required to conduct more than its rated ampacity.
This allows it to be (usually) an AWG #6 stranded wire.

4.)  If you have something other than 180 degrees out of phase, then there
can be portions of the two waveforms that will be additive and the white
wire can be required to carry more than its rated amperage.  You know where
that leads.

To make a long story short, it is important to remember that the phase angle
matters as well as the RMS voltage difference between two phases.








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