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Faunsdale, AL | The 240/120 3phase 4 wire "open delta" is the one with 2 transformers and cheapest for the power co to supply, not the 208/120 Y which requires 3 transformers fed by 3 utility phases. The open delta is "open" because it is fed by 2 phases (and the power co neutral) and only uses 2 transformers. So if you think of a triangle (the delta), the two transformers are two of the sides, and the 3rd side is "open". So, from the utility point of view, some money savings for making the installation with 1/3 less equipment.
But, like you mentioned, balance of load can be a problem, the open delta can be distorted and result in some voltage imbalance. Usually results from not having large enough transformers, but could be a problem with heavy loading on one utility phase vs the other. We have equal size transformers on all of ours and have very good balance, within 1 volt usually. I know there are some that were setup for very little 3 phase load that had only a small second transformer when they were installed. If either the 3 phase or the single phase load grows, the bank needs re-evaluating before voltage balance gets out of whack and burns up a motor.
Voltage imbalance in a 3 phase motor causes extra heating in the windings and rotor. I think spec is if more than 10% imbalance, the motor must be derated. The imbalance is calculated by averaging the phase to phase voltages (A-B, B-C, C-A) and then comparing each voltage with the average. So, it's not the highest compared to the lowest. | |
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Three phase in the morning
