Balanced Neutral

[K8MHZ]

080915-2123 EST

K8MHZ:

Put a load on a voltage source that is not a pure resistance and you can separate the current into two components, resistive and reactive. The resistive component times the source voltage is the power being consumed by the load. This in phase current component and the applied voltage allow you to calculate the equivalent shunt resistance of the load.

An inductor in series with a resistance can be replaced by a pure inductor in parallel with a shunt resistance. Here it is easy to see what the resistive current component is and the inductive component.

Now suppose you have two impedances with equal equivalent shunt resistances, but different reactances, then in the center tapped transformer circuit the neutral current contains no resistive component, but has a reactive component.

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I am quite familiar with reactance. That was not what the OP was referring to, however, and somehow phases started getting discussed and it threw me.

I am quite familiar with the OP's question and it's answer as it was drilled into our heads in the second year of our apprenticeship. Granted, the correct way to outline the problem would have been to use the phrase 'of equal impedance' but real issue was the true purpose of using a common neutral in a center tapped application.

Thanks for taking the time to offer an explanation.

So...how's the weather on your side of the state, anyway???

 

[Strahan]

Why not? With an unbalanced load, the common neutral will just carry the imbalanced amperage. Typical MWBC.

Ok maybe I'm missing something here but if we were to have to loads of different impedences that were sharing a neutral and we were to lift the neutral of course one of the loads would see a higher voltage drop than the other.

 

[K8MHZ]

Ok maybe I'm missing something here but if we were to have to loads of different impedences that were sharing a neutral and we were to lift the neutral of course one of the loads would see a higher voltage drop than the other.

Yes, that is how it works. If not, there would be no need for a common neutral. Without the neutral, current is the reference as removing the neutral changes a parallel circuit to a series circuit.

 

[gar]

080916-1829 EST

The OP did not specify the type of load, but it is reasonable to assume a resistive load which makes the discussion simpler. However, for others reading the thread it makes some sense to discuss impedance.

Obvious the word phase becomes important in an AC circuit. For a center tapped transformer where the center tap is the voltage reference point, and with equal voltage magnitudes on each side of the center tap, then one outer voltage will be 180 degrees out of phase with the other.

I am not yet sure that grantcool has an intuitive idea of the circuit and can visualize its operation. Larry's DC circuit should provide the best starting point.

K8MHZ:

Weather has been a little cool but very nice. Do you actually have those call letters?

.

 

[K8MHZ]

080916-1829 EST

The OP did not specify the type of load, but it is reasonable to assume a resistive load which makes the discussion simpler. However, for others reading the thread it makes some sense to discuss impedance.

Obvious the word phase becomes important in an AC circuit. For a center tapped transformer where the center tap is the voltage reference point, and with equal voltage magnitudes on each side of the center tap, then one outer voltage will be 180 degrees out of phase with the other.

I am not yet sure that grantcool has an intuitive idea of the circuit and can visualize its operation. Larry's DC circuit should provide the best starting point.

K8MHZ:

Weather has been a little cool but very nice. Do you actually have those call letters?

.

Hi Gar,

Yes, those are my call letters. I am an Extra Class Amateur Radio Operator and teach the subject at a local school here.

It's supposed to be sunny and 70 tomorrow. Yipee!!

I don't agree with the 180 degree assertion. In any single phase system with a resistive load, center tapped or not, all voltages will be of the same phase. If they were 180 off, they would cancel each other out. Another way to think about it, if one phase were 0 and another were 180 out, that would make a total of two and we only have one.

I have had this discussion with many over the years and am surprised at how few electricians really understand a single phase center tapped supply it's phase singularity.

If I remember correctly, there is a small town called Saline near Ann Arbor. We did some work for Verizon there installing a transfer switch and a meter socket. I think it was a DMUX site.

 

[gar]

080916-2100 EST

K8MHZ:

Saline is about 5 miles southsouthwest.

Label the transformer secondary terminals L1, CT, and L2.

Connect an oscilloscope common to CT. Synchronize the scope with the sync selector switch set to line. Connect one probe to L1. Adjust the sweep frequency to display 1 full sine wave across the screen with the left half cycle positive. Now move the one probe to L2, no change in the sync reference. The waveform will be inverted and that is a 180 degree phase shift of L2 from L1 relative to the center tap.

You could not build a full wave center tapped rectifier if there was not the 180 degree phase shift between the two ends of the secondary relative to the center tap.

If you have two secondaries of equal voltage and phase them to have the same phase angle relative to a point labeled CT (but it is not actually a center tap) and measure the voltage between them the result will be approximately 0 because the instantaneous voltages are equal and in phase. Use the oscilloscope test and you will see they are in phase. Wire this the same way you would for a full wave center tapped rectifier and you will only have a half wave rectifier. Essentially two diodes in parallel.

.

 

[grantcool]

yeah i looked at Larry's DC diagram with the switches, but if that was a 120/240 volt circuit the bulbs would burn out when you opened the center (neutral) switch. right? isn't that the whole point of making sure you don't lose the neutral?
but if the neutral really doesn't carry anything, you wouldn't need it in the circuit......i'm still confused.

 

[K8MHZ]

yeah i looked at Larry's DC diagram with the switches, but if that was a 120/240 volt circuit the bulbs would burn out when you opened the center (neutral) switch. right? isn't that the whole point of making sure you don't lose the neutral?
but if the neutral really doesn't carry anything, you wouldn't need it in the circuit......i'm still confused.

In the rare instance that the loads are and remain equal, it is true the neutral is not needed.

The other 99.997 percent of the time the neutral is needed lest the voltage imbalance (rise) due to current reference increases to the point of equipment damage.

The potential of 240 volts with an open neutral exists as a maximum. Low voltages can also be damaging to motor loads due to reduced REMF and resultant heating of the conductors.

 

[gar]

080916-2222 EST

grantcool:

There is something, and it is not clear what the something is, that is causing your problem analyzing the information that has been provided.

Lets go back to your original question.

If you have a nearly zero resistance wire and you apply exactly zero volts across that wire, then how much current flows? I stated near zero resistance because I can not divide by zero and get a meaningful answer unless I know how an equation reacts as I approach 0 for the divisor.

Ohm's law states that I = V/R. Since V is zero and R is non-zero, then I = exactly 0.

Now if I have two voltages of exactly V volts and put this low resistance wire between them, then how much current flows in the wire?

Answer that and we can continue. But not tonight.

.

 

[LarryFine]

yeah i looked at Larry's DC diagram with the switches, but if that was a 120/240 volt circuit the bulbs would burn out when you opened the center (neutral) switch. right?

It wasn't my drawing; the battery narrative is mine. That aside, two equal 120v loads in series is perfectly happy on 240v. Operating the switch changes nothing.

If you placed a voltmeter between the supply neutral and the center point between the loads, you'd read 0v (in theory). 0 volts across any resistance results in 0 amps.

 

[roger]

yeah i looked at Larry's DC diagram with the switches, but if that was a 120/240 volt circuit the bulbs would burn out when you opened the center (neutral) switch. right?

It is Ronald Colemans diagram, and being 120/240 AC would not change anything, the loads will operate perfectly with the center switch open.

If the loads were different sizes you would have a problem.

Do you have access to any study materials covering series and parallel circuits?


Roger

 

[grantcool]

ok. i'm finally starting to get it. among the ton of stuff i'm studying to pass the masters is series & parallel circuits, and after doing some examples i now see what you guys have been trying to tell me.
thanks for all the help & wish me luck for the test next month.

 

[roger]

Good Luck.

Roger

 

[Dez LMS]

protected/unprotected neutral...

protected/unprotected neutral...

I'm a first year apprentice electrician and my boss was trying to show me what protected and unprotected neutrals were on a mains switchboard. I would like someone to post a more detailed run down of this so I was a little more prepared when he shows me again next week. Remembering I know very little already. Cheers, Dez.