Loose neutral

[OldBroadcastTech]

230707-1925 EDT

OldBroadcastTech:

If you have a center tapped secondary on a transformer with a single primary, then the instantaneous output of an ideal transformer on one side of the secondary center tap is of reverse polarity of the other half of the secondary winding. Move the center tap to some non-center point and you have two waveforms that are both replicas of the primary except that magnitudes may be different, and one is the inverse of the other,or a 180 degree phase shift. Note: phase shift does not mean there is or isn't a time delay between waveforms. Phase shift is simplly a measure of a difference in two or more waveforms.

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"one is the inverse of the other,or a 180 degree phase shift"

one is the inverse of the other, or an APPARENT 180 degree phase shift

as I stated before, in a balanced system, the two are impossible to distinguish, one from the other.

 

[OldBroadcastTech]

230705-0841 EDT

shawelectric:

A single phase system feeding a transformer with a center tapped secondary generates two voltage that are phased 180 degrees apart. Both voltages will be approximately equal in voltage, and each 120 V loop will have about equal impedances for their source as seen at the main panel. This usually means that a typical system will have a neutral impedance equal to its hot line impedance. In my home this is about 1/20 ohm in the neutral, and 1/20 ohm in each of my hot supply lines.

If I place a 10 amp load on one phase to neutral.I get a total voltage change in that loop of about 1 V. There is no current change in the unloaded loop. Because most of my voltage drop is in power wiring rather than in the transformer this means I see about a 1 V drop in the loaded voltage side. This makes my unloaded side increase in voltage by about 1/2 V.

I can also do a 240 V measurement that excludes any neutral change. Here I should see abot a 1/2 V change.in the 240 V.readings.

With this expected information if the neutral voltage changes by more than about the expected 1/2 change, then I expect a high resistance in the neutral

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"Phase shift is simplly a measure of a difference in two or more waveforms"

Phase shift is a measure of a time-based difference in a single waveform, usually as measured at two points as the wave moves thru a complex circuit ( such as music thru an amplifier......input waveform compared to the same waveform after one or more amplifier stages).

Phase difference or 'displacement' is when comparing two or more identical waveforms that are time-shifted in relation to each other ( the three phases off the power grid is a perfect example).

( Ask a designer of high-powered audio amplifiers how 'phase shift' enters into his / her design(s). It is possible to inadvertently 'design-in' a 360-degree phase shift( thru the several stages of the amplifier at the highest or lowest frequencies, which is also known as 'positive feedback'. )

 

[Sberry]

What happens if we tighten the loose screw?

 

[ptonsparky]

What happens if we tighten the loose screw?

Sometimes not much. Newer AL alloys are better than back when and Cu can be affected as well. Depending on the condition of the conductor, removing, cleaning it and the lug, them torquing appropriately works. Cutting back to new is better. A FOP test after is a good idea.

 

[gar]

230708-0958 EDT

OldBroadcastTech:

Isolate your scope common from its EGC connection in the power cord. This is both a safety, and instrumentation issue. In other words go from a 3 wire power connection to a 2 wire.

Connect the scope chassis, probe shield and common lead, to the transformer secondary center tap. This determines the scope reference point as the center tap. Connect the probe for channel A to one hot line, and sync the scope on channel A. A sine wave will display on the scope. Next connect the channel B probe to the transformer's other hot line. Now added to the display is a sine wave that is an inversion of the first sine wave. In other words shifted by 180 degrees from channel A. Quite obviously not the same.

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[don_resqcapt19]

230707-2107 EDT

As soon as you go from a 2 wire source to more than 2 wires, then you have the possibility of multiple phases.

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Only if you have multiple transformer windings. A center tap on a single winding does not create multiple phases.

 

[don_resqcapt19]

230708-0958 EDT

OldBroadcastTech:

Isolate your scope common from its EGC connection in the power cord. This is both a safety, and instrumentation issue. In other words go from a 3 wire power connection to a 2 wire.

Connect the scope chassis, probe shield and common lead, to the transformer secondary center tap. This determines the scope reference point as the center tap. Connect the probe for channel A to one hot line, and sync the scope on channel A. A sine wave will display on the scope. Next connect the channel B probe to the transformer's other hot line. Now added to the display is a sine wave that is an inversion of the first sine wave. In other words shifted by 180 degrees from channel A. Quite obviously not the same.

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You inverted one because you kept on connection of the scope at the center point of the transformer winding. By doing that, your measurements become one half cycle out of phase, but in reality you are just measuring at different points in time...the time displaced by half of the winding or half of a cycle.

 

[LarryFine]

You inverted one because you kept on connection of the scope at the center point of the transformer winding. By doing that, your measurements become one half cycle out of phase, but in reality you are just measuring at different points in time...the time displaced by half of the winding or half of a cycle.

Actually, that isn't what is happening. You're measuring the opposing polarity at the same point in time.

Half windings do not displace time.

 

[don_resqcapt19]

Actually, that isn't what is happening. You're measuring the opposing polarity at the same point in time.

Half windings do not displace time.

Sure they do...When you measure N to B, you are starting you measurement a half cycle later then when you measure from A to N.

 

[LarryFine]

Sure they do...When you measure N to B, you are starting you measurement a half cycle later then when you measure from A to N.

What??? "Lucy, you got some 'splaining to do!"

In my opinion, the only thing that "happens" a half-cycle later is when the second half of the wave-form resembles the first half.

Everything else, i.e., the opposing voltages, is instantaneous. That's why it's a matter of polarity, not timing.

 

[don_resqcapt19]

What??? "Lucy, you got some 'splaining to do!"

In my opinion, the only thing that "happens" a half-cycle later is when the second half of the wave-form resembles the first half.

Everything else, i.e., the opposing voltages, is instantaneous. That's why it's a matter of polarity, not timing.

No matter what you call it, there is no change in polarity....just an appearance of a change in polarity because you changed the reference points, just like in my previous example of batteries in series.

 

[wwhitney]

I would say that as sin(x+180deg) = -sin(x), while the A-N and B-N voltage waveforms may be considered 180 degrees out of phase, and thus two different phases, they are not two independent phases. They are a degenerate pair of phases, since one of them is just a real multiple of the other (-1 in this case).

Cheers, Wayne

 

[GoldDigger]

I would say that as sin(x+180deg) = -sin(x), while the A-N and B-N voltage waveforms may be considered 180 degrees out of phase, and thus two different phases, they are not two independent phases. They are a degenerate pair of phases, since one of them is just a real multiple of the other (-1 in this case).

Cheers, Wayne

Which brings us to the point of what happens to a complex waveform which is not just a pure sine wave. When more than one frequency is involved, you can still say that the polarity has been reversed. But there is no time shift which can produce that waveform, so you would have apply a different time shift to each frequency to get a 180 degree phase shift.
When we talk about phase shifting a complex waveform 180 degrees we are usually talking about time shifting the waveform by 1/2 the period of the fundamental frequency. If some of the component frequencies are not harmonically related, the concept of phase shifting the entire waveform becomes problematic to interpret.
But reversing the polarity still applies.

 

[Sberry]

I say about 1/4 a turn.

 

[ptonsparky]

I say about 1/4 a turn.

Again, maybe not much. Was it properly torqued before and you just decided to check it? Was it overheating?

 

[busman]

You need a little load bank, Maybe something like a hair dryer with alligator clips instead of a plug, If you need to pull a heavier load than that you could make up something with burners from an electric cooktop for load.

PoCo here uses various versions of the Beast (Super-beast or Mega-beast). Tool exists, but not cheap.

Mark

 

[Joethemechanic]

PoCo here uses various versions of the Beast (Super-beast or Mega-beast). Tool exists, but not cheap.

Mark

A friend of mine's father built a 500kw one on a little 2 axle trailer. Used some kind of industrial heating elements he got for free, and a 30 HP motor with the fan from some old industrial engine for cooling. Bunch of large contactors he junk picked.

I'll tell you what, when he load tested a generator with it, it really kicked some heat out.

 

[LarryFine]

Used some kind of industrial heating elements he got for free, . . .

Probably motor-starting or dynamic-braking grids.

 

[gar]

230722-21212 EDT

don_resqcapt22:

Your understanding of multiple phases is incorrect. Your approach would flunk you in an elementary course in a basic college AC circuit class.

Do you have available a scope with at least two vertical channels, and a single shot time based channel. The horizontal channel would be a linear sweep vs time, and the vertical channels would be the signals you want to look at vs time.

The horizontal channel is triggered by a signal. This trigger is usually selected from one of the vertical channels, the AC line, or an external source. Also the horizontal sweep can come from an external signal, an internal sweep generator, and the internal linear time sweep can be repetitive, synced or free running, or single shot. single shot would be triggered from some source.

Best to operate under battery mode, or open the EGC path in the AC power cord.

For 60 Hz tests as a starter it is good to use 2 mS or 5 mS time base per major division. Use 10X probes if you are going to look at 120 signals.

Connect the scope common to the center tap of your source voltage.

Label the transformer output terminals, the hot ends, as A and B, and the center tap as N.

Connect the scope common to N.

Connect scope external sync to A with appropriate attenuation. This makes the scope sync common to Channel A. Connect scope probe 1 to A, and adjust sync phasing so that left most half sine wave, relative to sync point, is positive on the screen. Some scopes allow you to adjust the sync start point to the right of the left side of the display.

If you can do this report back.

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[Joethemechanic]

Probably motor-starting or dynamic-braking grids.

It may have been. Memory of it is a bit faded. It had to have been 40 years ago because he died in the late 80's. To tell you the truth I didn't study it the way I should have. I didn't realize what a useful tool it was.