Question about the neutral wire in an electrical system

  • #1

Main Question or Discussion Point

IF a phase touches another phase in a 3 phase AC supply it goes bang due to them being 120 degrees apart and therefore being at different voltages when they touch (this, i understand), so WHY, after passing through an electrical component, can all three phases be joined onto a 0 potential wire (the neutral)? Surely they are still out of phase and should still go bang?

all text books seem to be really poor at explaining this and im yet to meet anyone that has got anything other than a half arsed answer that strays from the point... any help appreciated.
 

Answers and Replies

  • #2
MATLABdude
Science Advisor
1,655
4
IF a phase touches another phase in a 3 phase AC supply it goes bang due to them being 120 degrees apart and therefore being at different voltages when they touch (this, i understand), so WHY, after passing through an electrical component, can all three phases be joined onto a 0 potential wire (the neutral)? Surely they are still out of phase and should still go bang?

all text books seem to be really poor at explaining this and im yet to meet anyone that has got anything other than a half arsed answer that strays from the point... any help appreciated.
The three phases "drain" into the neutral to force one end of the device to ground potential, and have the entire voltage drop to be across the device. They're no longer out of phase, because it's 0 volts (more or less) and so nothing goes 'bang' (unless the load happens to be a short circuit--putting hundreds of volts through a short circuit gives you an impressive 'bang').
 
  • #3
4,662
5
In three phase 240 volt power, all of the current in the load (like a three phase motor) should be only between the three phases. I think the phase-to-phase voltage is 208 volts. The fourth conductor is a ground, connected to the center of the wye in the power transformer. The ground is connected to the case of the motor, and should not carry any current, unless there is a ground fault in the motor. In 120 volts AC, there is a hot, a neutral (for the return current), and a ground. Often, the neutral and ground are connected together where the power enters the building. But the ground should not carry any current.
 
  • #4
140
1
IF a phase touches another phase in a 3 phase AC supply it goes bang due to them being 120 degrees apart and therefore being at different voltages when they touch (this, i understand), so WHY, after passing through an electrical component, can all three phases be joined onto a 0 potential wire (the neutral)? Surely they are still out of phase and should still go bang?

all text books seem to be really poor at explaining this and im yet to meet anyone that has got anything other than a half arsed answer that strays from the point... any help appreciated.
For a delta-wye wound transformer supplying both three phase and single phase power, the secondary windings are spatially configured like the letter "Y". The potential developed between each of the three endpoints of the Y relative to each other (E, the "phase voltage") is determined by the primary to secondary turns ratio for the three windings. The voltage from each endpoint to the common center point is governed by the spatial orientation of the seconday windings and for the wye type becomes [tex]\frac{E}{{\sqrt 3 }}[/tex]. Before being connected to anything external to the transformer, the potential of the common center point is basically "floating" (or unreferenced) relative to anything other than the three end points at the opposite end of the three windings. The induced voltages into the secondary are spatially coupled to the primary so even though the center point is electrically floating, the "overall region" of secondary potentials (and therefore the common center point) are somewhat determined by the "overall region" of the primary potentials relative to earth. By "overall region" I don't mean spatial region - I mean range of potential values. The primary phase voltages, even though equally balanced relative to each other, aren't arbitrary relative to earth. Somewhere along the power distribution they are in some manor referenced to earth and held within a desired working range relative to earth ground. So the secondary voltages, even though only spatially coupled to the primary, and precisely determined relative to each other by the turns ratio, aren't entirely arbitrary relative to earth either. The ungrounded center point is "floating" but floating within a reasonable range of potential values relative to earth determined by that of the primary. Since the secondary windings are coupled by induction to the primary rather than physically connected, the common center point of the wye can be tied to earth since its already within an acceptable range of values. I wouldn't try connecting the center point to some other system of power that's being firmly referenced to say 10,000 volts relative to earth. Nor would I try connecting it back to one of the phases of the primary. It may be "floating" and unreferenced before connecting it to earth but only in a loose sense. There is some acceptable range of potentials about the earth reference that won't produce damaging effects but we aren't totally free to connect the common center point to just any external reference potential either.
 
  • #5
754
1
WHY, after passing through an electrical component, can all three phases be joined onto a 0 potential wire (the neutral)? Surely they are still out of phase and should still go bang?
This is no different than a 120V single phase scenario: If you connect a 120V line directly to neutral, it will go "bang." But, if instead you connect the 120V line to a load and connect the "other side" of the load to neutral (i.e. pass the phase through an electrical component to a zero potential wire), all is okay.

The reason there is a "bang" (in either situation) really has nothing to do with phase angles, rather it has to do with voltage potentials. If there exists a potential between any 2 wires of more than 0 volts, connecting the wires directly to each other creates a short-circuit (i.e. a no-load situation). This means that the resistance in the circuit is 0 ohms and, by Ohm's Law, this causes an extremely high current, thus the "bang."
 

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