Power control in delta-connected load

[EC Dan]

Since the phase current within a delta-connected configuration is 1.73 times less than the line current, it seems like there would be significant cost savings to locating large power control equipment in the delta circuit, such as contactors and SCR power controllers (in this case, assume it's a three-phase resistive load). It would be the difference between the top right and bottom left configurations below:



The circuit would need twice the number of conductors from the panel housing the power control equipment to the resistive load, however those conductors could be smaller as long as a circuit breaker rated for the reduced current was used in the delta circuit. Also, based on tap rules, would there need to be a circuit breaker on both sets of conductors in the delta circuit? Are there any other concerns (including regulatory based on NEC or UL-508A) with designing a power control circuit in this manner?

 

[synchro]

I see the bottom left configuration as having three independent single phase loads, and it's no longer a self-contained three phase load like the delta load shown at the top right.

... The circuit would need twice the number of conductors from the panel housing the power control equipment to the resistive load, however those conductors could be smaller as long as a circuit breaker rated for the reduced current was used in the delta circuit.

And so each resistor gets its separate pair of conductors, and it's therefore a single phase load driven from two ungrounded conductors which each supplies it's own line voltage. Therefore 2-pole breakers and contactors would be required.

I don't see an issue with using SCR's on just one side of the resistor load. I believe this is typically done on single-phase loads, and it's analogous to using a 2-phase soft starter that doesn't have SCR's on one of the phases. You are correct that the SCR's on the bottom left would only have to handle 1/√3 ≅ 1/1.73 or 58% of the current in the top right delta configuration. The L-L voltage is split between two different antiparallel SCR's at the upper right however, and this reduces the voltage requirement on them. The required silicon area in the SCR's is proportional to current, and so a lower current will typically allow a lower cost as long as the voltage requirement doesn't get too high.

This is not to discourage exploring and evaluating various options, however.

 

[jim dungar]

You are looking at the wrong part if the circuit. The bottom left is simply the current flowing through any one resistor not at the current through each supply line and the resultant overall 3-phase power. For all intents the Open Delta configuration is not using all three resistors because, per its description, one leg of of the delta is open.

 

[wwhitney]

For all intents the Open Delta configuration is not using all three resistors because, per its description, one leg of of the delta is open.

I think the OP is borrowing the diagram from a different context, and in both cases is planning to power the arrangement with a full delta supply. I.e. ignore all the text, just consider the different component placement between the two diagrams.

Cheers, Wayne

 

[EC Dan]

I see the bottom left configuration as having three independent single phase loads, and it's no longer a self-contained three phase load like the delta load shown at the top right.



And so each resistor gets its separate pair of conductors, and it's therefore a single phase load driven from two ungrounded conductors which each supplies it's own line voltage. Therefore 2-pole breakers and contactors would be required.

I don't see an issue with using SCR's on just one side of the resistor load. I believe this is typically done on single-phase loads, and it's analogous to using a 2-phase soft starter that doesn't have SCR's on one of the phases. You are correct that the SCR's on the bottom left would only have to handle 1/√3 ≅ 1/1.73 or 58% of the current in the top right delta configuration. The L-L voltage is split between two different antiparallel SCR's at the upper right however, and this reduces the voltage requirement on them. The required silicon area in the SCR's is proportional to current, and so a lower current will typically allow a lower cost as long as the voltage requirement doesn't get too high.

This is not to discourage exploring and evaluating various options, however.

This is very helpful, I hadn't been looking at it in the perspective of three single-phase loads. In this case it ends up doubling the amount of contactor poles (single three-pole contactor outside the delta vs three double-pole contactors inside the delta), in addition to the circuit breaker requirements, so it's not as clear that this would have cost savings despite the lower ratings.

 

[EC Dan]

I think the OP is borrowing the diagram from a different context, and in both cases is planning to power the arrangement with a full delta supply. I.e. ignore all the text, just consider the different component placement between the two diagrams.

Cheers, Wayne

The pictures are from the SCR power controller manual, but I'll admit I'm confused why they refer to it as open delta configuration, unless at any given time in the three-phase controller operation one of the SCRs is fully off and thus the delta is open on that side.

 

[EC Dan]

So if treating each leg within the delta as a single-phase circuit, but still using the three-phase SCR power controller, this would be the circuit:

 

[wwhitney]

The pictures are from the SCR power controller manual, but I'll admit I'm confused why they refer to it as open delta configuration, unless at any given time in the three-phase controller operation one of the SCRs is fully off and thus the delta is open on that side.

Ah, I misunderstood. I agree with you on the terminology, "open delta" to me implies some breaking of the 3-fold symmetry, which I don't see in the diagram.

Cheers, Wayne

 

[Jraef]

Where are there any cost savings in any of this?

Power is power. A load requires a certain amount of power. How it gets there is mostly irrelevant.