AC current in a parallel wye common? 1

[heaterguy]

Does AC current flow in a parallel wye common? For instance, you have eighteen wye resistor circuits and the common is connected with buss bar. Does current flow thru the buss bar between the eighteen circuits?

 

[ScottyUK]

Are the impedances equal? Is there any external load? Is it balanced? Is this homework?


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I don't suffer from insanity. I enjoy it...

 

[waross]

hi heaterguy;
I understand that there is a common busbar to which all heaters are connected.
Yes, the current will flow through the bus bar between the heaters.
The bus bar may remain floating but there are issues.
With the bus bar floating;
If there is a loss of one phase, the voltage vectors will straighten out into a single phase and the voltage will drop on the remaining heaters.
For instance, If you are using a 120/208 volt supply with a floating neutral, and you lose one phase, the voltage across the heaters on the other two lines will drop from 120 volts to 208/2=104 volts.
If the heaters are not balanced as to impedance, (resistance for resistors or heaters), the group with the lowest impedance or resistance will receive a higher voltage.
If a heater fails open (burns out), the voltage will increase to the group that it was in. If a second heater fails in the same group, the voltage will increase further.
I have seen heaters connected as you describe, but you may want to consider running a neutral conductor from the common bus back to the neutral connection in the panel. If this is not feasible either for technical reasons or differences of opinion, don't worry about it too much. Just remember that unbalanced voltages on the heaters some time in the future is an indication that a heater may need to be replaced.
respectfully

 

[heaterguy]

For this application the voltage is 480V and the load is balanced. The total length of the wye bus barr is approximately 18 inches. I am considering the current flow through the buss bar between the resistors (heater elements). It appears to me that since the wye buss bar has zero voltage difference from one resistor to the next, the buss bar has zero current. May you please confirm this logic.

 

[ScottyUK]

heaterguy,

Yes, your logic is correct. In the event of one element failing, the behaviour outlined above by waross will occur. If you have a neutral, connect it to the star point busbar in order to prevent the over-voltage condition due to load imbalance. In this state you will have a neutral current flowing.


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I don't suffer from insanity. I enjoy it...

 

[waross]

The current from the heaters must pass through the bus bar to flow from one heater to the next to return.
Your statement that there is zero voltage is in error. There will be a small voltage drop. The fact that the voltage drop is very small does not mean that there is no current, it means that the impedance of the current path through part of the bus bar is also very low. Ohms law still works.
The bus bar will have an impedance between the heater connections. It will be very low, but it will have some impedance, basically resistance.
This impedance times the current flow will give the voltage drop for the connections. We will be considering current flow, voltages and impedances from one point of connection to the next, not the entire bus bar.
An exception may be if heater leads from three phases are connected to the bus bar with a single screw or solderless conector.

 

[heaterguy]

We have each resistor spaced about 1.5" apart and eighteen (18) are connected in a row. Again, this is the wye part of a three phase circuit. The bus bar is limited to 50 amps, but the circuit is pulling 190 amps total. Is there a way to calculate the current flowing thru each bus bar?

 

[stevenal]

If connected A-B-C-A-B-C..., the current between sections A-B and B-C will be that of one heater. C-A will be zero, since all is balanced on either side. If connected A-A-A-A-A-A-B-B-B-B-B-B, current will add as you move down the bus until you see 6* the current of one heater in the section between A-B. This would be the maximum, short of the unbalance conditions Waross described.