A couple of things that come to mine are less expensive than using 3. But the negative is that with only 2 transformers if one bites the dust your SOL and out of business. With 3, if you loose one you can still function having only 2 but the capacity is reduced to 57% if I can recall correctly.
It's cheaper to install if you're paying for the primary since you only need to bring in two phases. If you're a mile back in the boonies, that can be considerable.
The biggest disadvantage is that its voltage regulation under load is poor compared to a 3-pot bank. This can lead to serious voltage and current imbalances in 3 phases devices, which most of the common heater type protection won't protect against.
I had a water well rep tell me once that he had heard that some pump manufacturers would void their warranties if they found out the pump had been on an open delta.
Three phase is three phase - it always requires 3 conductors. Granted one of these conductors can be grounded. I am not aware of any utility distribution system that uses the earth as the sole return path for a phase conductor.
Also the most accurate comparisons would be between the use of multiple single phase transformers versus a single three phase unit. Advantages may include (but are definitely case dependent): smaller support structure lower purchase costs better utilization of inventory reduced number of spare parts
Open delta three phase can be obtained with only two primary phase conductors. This is done by wiring the primary side in an open wye configuration and secondary in what we know as the open delta. Some one else will have to explain the theory on how the phase shift occurs but this is a common practice in this area
JBD and ptonsparky, I think you are agree without knowing it. You will need three conductors, but only two phase conductors. One can be the neutral or a grounded phase. Either way, you save some money on the insulators.
In theory, you could use the earth for the neutral, but I don't see why you would. Where there is only a single phase, it makes more sense to use the earth because you won't have to worry about conductors that clash => longer spans => cheaper.
[This message has been edited by C-H (edited 11-17-2005).]
Quote: "Three phase is three phase - it always requires 3 conductors. Granted one of these conductors can be grounded. I am not aware of any utility distribution system that uses the earth as the sole return path for a phase conductor."
True. A utility will bring in a neutral regardless of whether it is one, two, or three phases (although I think there are applications in extremely long distribution systems where the earth is the actual return...anyone from Australia confirm this??). I was referring to not bringing in the third hot phase. If you do not have the neutral to corner the bank, you will not get the phase shift. C-H has it right in that regard.
Quote: "Where there is only a single phase, it makes more sense to use the earth because you won't have to worry about conductors that clash => longer spans => cheaper."
The resistivity of the earth varies tremendously. It is not uncommon in our area, which has a lot of sand, to have a 7.2 Kv (or even a 14.4 Kv) line fall on the ground and just lay there hot. The high impedance of the sand prevents enough current from being drawn to trip the line breakers. Extremely good reason to tell your kids to stay away from downed lines.