三相電的問題

3-phase system refers to that 3 alternating voltages are generated, transmitted and distributed simultaneously because
1.3-phase 3-wire system required lesser volume of conductor for transmitting
bulk energy. It is most economical of all the A.C. system.
2.3-phase machine have a more even torque, higher efficiency, higher power factor
and self-starting.
3.The ripple content is smaller during rectification to obtain direct curren
All 3-phase electrical equipments must connect to 3-phase power supply.

Most 3-phase systems operate at either 208 Volts or 240 Volts, but sometimes at 480 Volts phase-to-phase. In any case, Using a single isolation transformer with sufficient capacity to operate the entire facility is the optimum approach to designing a technical power system. If this is not possible, 3 or more single-phase transformers loaded evenly across the 3-phase source will also work. This application could include use of rack-type systems plugged into 120- or 208/240-volt single-phase outlets on a 3-phase distribution grid.

3-phase power operates most cleanly and efficiently when the load current is balanced across all three power legs of the system. If current is too unbalanced, current distortion will be the result.

The major drawback to using one single phase transformer on a 3-phase grid is a tendency to unbalance the 3-phase system's current if particular care is not taken in evening out the load with careful electrical distribution planning. For this reason, a single transformer with a 3-phase primary and a balanced single phase secondary is recommended. Conventional 3-phase to single-phase isolation transformers help to some extent, but they fail to evenly balance the load current across the 3-phase system's primary to the desired degree.

The best distribution method for a 3-phase system involves the use of a 3-phase to 6-phase "wye" isolation transformer. The 6-phase "wye" secondary has three single-phase outputs -- all are balanced 120-volt outputs with 60 Volts to ground. Each output circuit is rotated 120 degrees from the adjacent output circuits. All three single-phase outputs share a common grounding reference. This configuration resembles a 6-point starburst.

The advantages to this design are simple. There is but a single grounding reference point for all three output circuits. This insures that there will be no variation in grounding potentials across the system. Secondly, all three outputs are coupled to the same magnetic flux field insuring a more stable current alignment -- this stabilizes the power factor in a 6-phase system.

In either case, whether using a 3-phase to 6-phase system or just a simple single-phase set up, balancing all 120 volt circuits will dramatically reduce noise and harmonic power distortion.

(Note: Some equipment requiring a 208-volt single phase circuit for power supply power requires a 208-volt balanced circuit -- 104 Volts to ground. 3-phase transformers can be engineered to output multiple voltages from a single secondary coil array when required under these conditions.)