Return Path of Current

[Smart $]

I have actually ran a 3 phase water heater that called for a neutral without it. I realized the elements were tied to a common point, and that the neutral was really only needed to protect the system in the case of a failure. It was new, and we were in the process of installing a delta/wye transformer to accommodate the load; but until then it operated just fine.. I have also seen 3 balanced rows of 277v. light fixtures operate just fine without the neutral connected, (we were trying to determine if the had been wired correctly), they will operate as a single 3 phase load seeing the normal voltage if balanced.

And there you have an example of why the neutral is often required... to maintain line-to-neutral voltage in case of part failure or fault.

 

[GoldDigger]

I have actually ran a 3 phase water heater that called for a neutral without it. I realized the elements were tied to a common point, and that the neutral was really only needed to protect the system in the case of a failure. It was new, and we were in the process of installing a delta/wye transformer to accommodate the load; but until then it operated just fine.. I have also seen 3 balanced rows of 277v. light fixtures operate just fine without the neutral connected, (we were trying to determine if the had been wired correctly), they will operate as a single 3 phase load seeing the normal voltage if balanced.

And if one fixture fails, it will produce higher than normal voltage on the rest of the fixtures on that phase, assuming that you have wired the fixtures to a common point which has no neutral connection rather than just wiring the fixtures line to line. Worst case you apply full line to line voltage to some fixtures instead of just the line to "neutral" voltage. A multi-voltage ballast might survive that.

 

[under8ed]

Another example that may help the OP is how we wire anti-freezing light bulbs in our rural well-water pump houses. We usually take 2 temporary light fixtures & wire them in series. The pump house has only 220 volts, but with 2 identical bulbs wired to it; each will only see the 110 volts and the will operate properly. Unlike the 3 phase, there will be no danger in the failure of one except for loosing the freeze protection completely. The uneducated in electricity will wire one leg to ground with one fixture, it works; but is dangerous. (I tell everyone, your pump house & under your house is the most dangerous place you can be, electrically speaking; everything around you is damp & well grounded!)..

 

[ggunn]

For a 3 phase system ( delta-star) we have three hot wires and 1 neutral. so current flows
through the three hot wires and comes back through neutral.

No. For a balanced load there is no neutral current.

 

[Bugman1400]

I'm surprised no one has mentioned using sequence networks (sequence currents) to help understand balanced vs unbalanced loads on a delta or ungrounded system.

 

[Smart $]

I'm surprised no one has mentioned using sequence networks (sequence currents) to help understand balanced vs unbalanced loads on a delta or ungrounded system.

Do you mean symmetrical component analysis? If not, maybe you're better suited to explain...

If so, judging by the level of understanding exhibited in the OP, I think symmetrical components is a bit advanced for this discussion... but if you can explain it in a really simple manner to begin with, perhaps we can advance it based on response.

 

[Bugman1400]

Do you mean symmetrical component analysis? If not, maybe you're better suited to explain...

If so, judging by the level of understanding exhibited in the OP, I think symmetrical components is a bit advanced for this discussion... but if you can explain it in a really simple manner to begin with, perhaps we can advance it based on response.

Basically, for a 3-phase system with a neutral return, any imbalance that involves a wye-grounded connected load will result in the return current returning through the neutral. If the load is not connected to ground the imbalance is returned as negative sequence current. Similarly, for a 3-phase system without a neutral return, you will have the same result. The negative sequence current rotates in the opposite direction to that of the normal positive sequence current and decreases the amount of power to the load.