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9.5: Summary

  • Page ID
    25297
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    Polyphase systems can be thought of as a group of individual sources of the same magnitude that are synced together and where the load is similarly divided into sections or legs. By spreading out the source currents across the waveform's period, a smooth application of power to the load can be achieved. Also, for the same line current, more power can be delivered to the load than that of a single phase system. Loads can be balanced or unbalanced. A balanced load means that all legs of the load exhibit an identical impedance. Thus, the currents coming out of the source will be the same except for the phase shifts spreading them across a single period. While any number of phases is possible, three-phase systems are popular as they deliver the benefits of polyphase while limiting complexity.

    A three-phase source produces currents that are 120 degrees apart. That is, if the first signal is taken as the reference, or 0\(^{\circ}\), then the other two are at 120\(^{\circ}\) and 240\(^{\circ}\). Both the source and the load can be configured in one of two ways: delta or Y. This makes four possible combinations for the source-load connection; namely Y-Y, delta-delta, Y-delta, and delta-Y. In a Y-Y connection the source phase current and load current will be the same. The voltage from one line to another will be \(\sqrt{3}\) times larger than the phase voltage. In a delta-delta connection the phase and line voltages will be the same, but the line current will be \(\sqrt{3}\) times larger than the phase current of the generator or load. In delta-Y and Y-delta connections, the source and load no longer match configuration so neither the phase voltages nor currents are the same. For the portion that is Y-connected, the line current and phase current will be the same. For the portion that is delta-connected, the line voltage and phase voltage will be the same. For those configurations, the other parameter (voltage or current) will be scaled by \(\sqrt{3}\).

    In a system with a balanced load, the system power will simply be three times the power of one leg. If the load has a non-negligible phase angle, power factor correction can be used to reduce the required line current. The compensating items are arranged in a delta configuration, even if the load is Y-connected.

    Review Questions

    1. Describe the advantages and disadvantages of polyphase versus single phase systems.

    2. Define the terms delta-connected and Y-connected.

    3. How are line and load voltages related for Y-connected loads?

    4. How are line and load currents related for Y-connected loads?

    5. How are line and load voltages related for delta-connected loads?

    6. How are line and load currents related for delta-connected loads?

    7. Describe a practical connection for power factor correction of a Y-connected load.


    This page titled 9.5: Summary is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by James M. Fiore via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.