Single phase 240V transformer on pole is fed by one one the hot legs of a 13/7.6 KV overhead line. Using an oscilloscope to see each hot line of the 240 volt, what would be seen? Such as how many degrees are the two legs apart from each other. Also how does the transformer accomplish any degree of separation?
Single Phase Question?
- Thread starter EEC
- Start date
- Status
With single phase there is no degree of separation. If you put each leg on a 'scope referenced to the neutral, assuming there is a neutral, one leg will appear to be 180° out of phase with the other. The appearance of being out of phase is a result of opposite polarities.... yet electrically it is no different than if generated 180° out of phase as to usage.
How does transformer reverse the polarities?
How does transformer reverse the polarities?
How does transformer reverse the polarities?
How does transformer reverse the polarities?
How does transformer reverse the polarities?
It doesn't. If you are referring to my last post, reversed polarity is a result of lead connection to the 'scope. L2 to N is the opposite polarity of L1 to N.
Consider the secondary of the transformer as if it were two C- or D-cell batteries connected in series. Using a DC voltmeter, how would you connect you leads to measure the same polarity on the two batteries.
big john
Senior Member
- Location
- Portland, ME
I was under the impression that transformer polarity actually did reverse between primary and secondary. Where +V on the primary would occur at the same time as -V on the secondary because induced voltage was CEMF.
While that part is true, the polarity primary to secondary depends on which direction the windings are wound with respect to each other, too.
Think of it this way
a single 240 sec, no center tap neut
you have 240 with indeterminate polarity, no reference
now, center tap to ground, establish a reference neut
still have 240 across the coil
take a vom and put one lead on the neut/gnd
Put the other probe at one end and you have 120
as you move towards the center it decreases to 0
now start moving it from the center/neut and move it towards the other end
the voltage increases from 0 to 120
the voltage increases in opposite directions or polarity to the reference/neut/gnd
current will flow outward from the neutral to each load, be combined or commoned and return
a single 240 sec, no center tap neut
you have 240 with indeterminate polarity, no reference
now, center tap to ground, establish a reference neut
still have 240 across the coil
take a vom and put one lead on the neut/gnd
Put the other probe at one end and you have 120
as you move towards the center it decreases to 0
now start moving it from the center/neut and move it towards the other end
the voltage increases from 0 to 120
the voltage increases in opposite directions or polarity to the reference/neut/gnd
current will flow outward from the neutral to each load, be combined or commoned and return
That is an incorrect statement.
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
If you put a two channel oscilloscope between L1-N and L2-N you will see this
An oscilloscope between L1-N and N-L2 or between L1-L2
You can’t use neutral as the base reference with an oscilloscope.
You can... just have to realize how it affects trace.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If the scope probe ground is bonded to chassis and EGC, you will be adding a neutral to EGC bond and possibly drawing a lot of current through the shield of the probe cable.
If you use ground (I.e. EGC) as reference you will have to correct your voltage measurements for any neutral offset.
Scopes with differential inputs allow you to avoid this problem.
No it isn't
the i loops will be opposite and from the neutral to the line end
the will sum/combine at the load end (in a simplified schematic)
Basically speaking, current travels in the same direction (CW or CCW) in full and half-voltage loops.
by convention when doing the schematic current flow is depicted - to +, in this case N to Line
they just happen to be 180 deg out of phase/opposite polarity or sign so if balanced sum (subtract) to 0
A single winding with midpoint tap cannot be positive at both ends.
Even a dual-winding secondary transformer is not depicted with a positive at each end.
Last edited:
- Location
- Massachusetts
What does grounding have to do with polarity?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
You can either show the two halves of the center tapped winding as having opposite polarity and assume identical phase angle or you can show both ectremes as positive and explicitly show 180 degree phase shift. The former is much more common in the US outside of textbooks, but the latter works fine too.
I prefer the latter
when represented by phasors shown at 0 and 180
But as you said works out the same either way
also has both negatives grounded by convention
Last edited:
- Status