Single Phase Sine Wave

Status
Not open for further replies.
F

fred-d-c

Member
Location
Tennessee USA
Occupation
Maintenance Tech
I have a question. Why does the following happen? When you take a scope meter; Fluke 190-204, Putting the Channel 1 test lead on L1 and the ground clip on Neutral you will see the typical -170... +170 volt sine wave at the same time use Channel 2 put the ground clip on L1 and the Channel 2 test probe on Neutral you will see the same sine wave but 180˚ out of phase from Channel 1 measurement.
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Same reason that if you take a multimeter and put the red lead on the + terminal of a batter, and the black lead on the - terminal you will get one reading, and when you put the black lead on the + terminal and the red lead on the - terminal you will get the - of that reading.

Voltage (or potential difference) is always measured _between_ two points, and the polarity of that measurement depends on the polarity of the sensor.

With your specific example, you got very lucky, because on most scopes the ground leads of the different channels are common. With most scopes if you try to repeat the experiment you will measure the available short circuit current of your 120V supply, and likely the fecal containment capacity of your pants.

-Jon
 
F

fred-d-c

Member
Location
Tennessee USA
Occupation
Maintenance Tech
winnie said:
Same reason that if you take a multimeter and put the red lead on the + terminal of a batter, and the black lead on the - terminal you will get one reading, and when you put the black lead on the + terminal and the red lead on the - terminal you will get the - of that reading.

Voltage (or potential difference) is always measured _between_ two points, and the polarity of that measurement depends on the polarity of the sensor.

With your specific example, you got very lucky, because on most scopes the ground leads of the different channels are common. With most scopes if you try to repeat the experiment you will measure the available short circuit current of your 120V supply, and likely the fecal containment capacity of your pants.

-Jon
Click to expand...


Yes I was aware not to attempt that with a typical scope. Does that mean that the Neutral is alternating too?
 
J

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
(oh no, not this again)

fred-d-c said:
... Does that mean that the Neutral is alternating too?
Click to expand...

That question has no real meaning. As Jon said a voltage (alternating or not) is always between two points. Therefore a single point cannot 'alternate.'

(Also, technically you don't have a neutral unless you also have an L2, which you've not mentioned yet.)
 
hbiss

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
fred-d-c said:
I have a question. Why does the following happen? When you take a scope meter; Fluke 190-204, Putting the Channel 1 test lead on L1 and the ground clip on Neutral you will see the typical -170... +170 volt sine wave at the same time use Channel 2 put the ground clip on L1 and the Channel 2 test probe on Neutral you will see the same sine wave but 180˚ out of phase from Channel 1 measurement.
Click to expand...

If you connect a DC volt meter to a battery, red to positive and black to negative you see the voltage displayed. If you swap the leads, black to positive and red to negative you see the voltage displayed but with a (-) sign in front of it.

If you can understand why that happens you should have no trouble understanding your waveforms with swapped leads.

-Hal
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
fred-d-c said:
Yes I was aware not to attempt that with a typical scope. Does that mean that the Neutral is alternating too?
Click to expand...

As I noted, voltage can only be measured between two points. It is impossible to say that one point or the other is alternating, only the voltage between them.

However: it is very common to pick a reference point and _define_ it as being a fixed '0V', and then the voltage difference between this reference and any other point can be associated with those points. Generally 'ground' is _defined_ as 0V, and since the neutral gets 'grounded' then in this scheme the neutral is also at a fixed '0V'. But this is a definition, not physics.

-Jon
 
J

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Dennis Alwon said:
I believe most people use the term neutral interchangeably with the grounded conductor. Some even call it the white wire....LOL
Click to expand...

I wouldn't always bring it up just to nitpick, but in this case it has some relevance to the subject, since a neutral is defined by the vector sum of waveforms.
 
F

fred-d-c

Member
Location
Tennessee USA
Occupation
Maintenance Tech
I understand a lot what everyone is saying, and thanks for all the responses, I have learned some things. I do understand why swapping the leads from + to - causes the meter to go from + to -. I am still not sure I totally get why it is 180˚ out of phase, if I keep thinking about it surely I will have an aha moment. Keep in mind, I am not an electrician and certainly not an EE or Physicist. I do tinker with things both mechanical and electrical and have for years. when I run into something I don't fully understand, I start digging in.
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
fred-d-c said:
I am still not sure I totally get why it is 180˚ out of phase, if I keep thinking about it surely I will have an aha moment.
Click to expand...

Danger Will Robins!

You have walked into one of the never ending discussion triggers. Lots of different folk have very strong opinions on the topic.

When you swap the leads you invert the measurement. An inverted sine wave looks _exactly_ like the same sine wave offset by 180 degrees, and in situations that require phase offset (like the stator of a 3 phase motor) an inverted sine wave can be _used_ like a sine wave offset by 180 degrees.

Weather it is actually a 180 degree phase offset or 'merely' an inversion is a philosophical discussion that will stretch for north of 1000 posts :)

-Jon
 
G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
fred-d-c said:
I understand a lot what everyone is saying, and thanks for all the responses, I have learned some things. I do understand why swapping the leads from + to - causes the meter to go from + to -. I am still not sure I totally get why it is 180˚ out of phase, if I keep thinking about it surely I will have an aha moment. Keep in mind, I am not an electrician and certainly not an EE or Physicist. I do tinker with things both mechanical and electrical and have for years. when I run into something I don't fully understand, I start digging in.
Click to expand...
This is an issue that always causes problems with discussion of whether or not 120-0-120 is single phase or two phase.
The 180 degree phase shift is different from all other phase shifts when looking at a sine wave:
A 180 degree phase shift is exactly the same as reversing the polarity of the sine wave.
(A trigonometric identity tells us that sine(180+theta) == -sine(theta) )
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
We had quite the debate about this a few years ago. We concluded that it is a polarity issue that, because it involves AC, merely resembles a 180-degree phase shift. If it involved DC, there would have been no polarity-vs-shift debate.
 
Status
Not open for further replies.
Top