Wiring heaters on Y connections from a delta source 5

[Dracula7]

Hi,
Could anyone help me understand why this is a 277V configuration across the heaters instead of 480V? The incoming power is 480V, delta - so no neutral, while the heaters are 277V, single phase. My guess is that you can get 277V from a 480V only between phase and neutral -which is not the case here since there's no neutral. I attached here a hand draw wiring with 2 heaters in parallel per phase and just one heater per phase.
Thanks all in advance!

 

[davidbeach]

It's just a simple geometry problem. Draw your 480V delta; place the heaters inside the delta as straight lines of equal length. What is that length?

 

[Dracula7]

David,
Thanks for the input but I'm confused about the length.
I understand the geometry part but why is the length of any relevance in this case?

 

[jghrist]

The voltages and currents through each element will be unstable because the "neutral" point will float. With exactly balanced phase voltages, there will be 277 V across each element. If one element has lower resistance that the others, it will have lower voltage and higher current. If one element had a very low resistance, for instance because of a short circuit, the voltages across the other elements will approach 480V and may damage the elements.

 

[Dracula7]

Thanks jghrist,
but isn't phase A to phase B(For example) supposed to be 480V? Instead, it show as 277V and this is the part I don't understand.

 

[pwrtran]

Hi Dracula7, you are correct that 277 is the phase to neutral voltage. 480 is the phase-phase voltage and the relationship is root 3, 1.732. So 480/1.732=277VAC.

 

[Dracula7]

Thanks pwrtran
So in this case then, why the drawing shows that the 277V heater (not 480V) can be used in this configuration?

 

[stevenal]

You say there is no neutral, but that common connection on the right hand side of the drawings is a floating neutral as jghrist pointed out. I'm not sure why the designer said 277 heaters could be used, since 277 will only occur under perfectly balanced conditions. 480 V heaters connected line to line would not see the unstable voltage jghrist speaks of. Suggest you also perform the exercise Davidbeach mentioned above.

 

[waross]

With three elements, if one element goes open circuit, the voltage across the other elements will drop to 240 Volts. The voltage across the open element will rise to 416 Volts.
With six elements, parallel pairs, if one element goes open, the voltage across the other two pairs will drop. The voltage across the element paired with the failed element will rise.
If an element becomes shorted, the voltage across the paired element will drop to zero. The voltage across the elements on the other phases will rise to 480 Volts.
I have encountered this floating neutral connection on approved industrial equipment. With elements submerged in thermally conductive liquid, the elements can often withstand over voltage without badly overheating.
Elements heating other solid or gaseous media with lower thermal conductivity may be more prone to failure when subject to over voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[pwrtran]

Dracula7, the heater nameplate rating information voltage and wattage wise, it may provide you the answer you are looking for. For example the heater could be single phase type but the designer wanted to gang them and connect them wye? Or perhaps the designer wanted to indicate heaters are not for delta connection. I would suggest you to take a look at the nameplate.