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Connect two AC sources in parallel unless they have the same voltage

  1. Jun 21, 2009 #1
    basically, you can't connect two AC sources in parallel unless they have the same voltage, frequency and phase.
    OK, know I undestarnd what u guys meant in the above sentence.
    Now, talking about the PHASE, can you give me any advise, links, recommendation, book (anything), on how to get the same phase?
  2. jcsd
  3. Jun 21, 2009 #2
    Re: phase

    I'm not sure in what context you are talking about, but you can connect two AC sources together.

    If they have different frequencies, then you will have two signals superimposed on one another.

    If the frequency is same, then an interference pattern will form depending on their phase, it can be destructive interference where the two signals will cancel each other, or constructive interference where the signals add.
  4. Jun 21, 2009 #3


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    Re: phase

    I'm pretty sure he's talking about power sources. You do NOT want to connect two AC power sources together when they're out of phase.
  5. Jun 21, 2009 #4


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    Re: phase

    This is a follow on from a previous posting thread. It did refer to power sources. You REALLY don't want to see what happens if you get power generators out of phase and in parallel with each other. Actually they have protection against this so it isn't too bad.

    The other condition that is important is that all the generators have to be running on the RIGHT frequency. This is 60 Hz in some countries and 50 Hz in others.
    You could run a mechanical electric clock off one and compare its time with a quartz clock but this is actually done on a cycle by cycle basis to keep exact frequency.

    If you already have one power source on the right frequency (giving the right number of cycles each second) you can have a governor on the engine driving the second one so that it is pulled slightly in frequency until it is in phase, then the frequency is adjusted to get the same frequency again.

    This has to be done in power stations so that many generators can be placed in parallel possibly across a whole State and still be in phase with each other.
    These are large heavy machines so they tend to keep running at constant speed and they are constantly given small adjustments to stay in phase.

    If your power source was an inverter, perhaps from a battery, you could have a circuit that detected when the accurate power source voltage crossed the zero level (ie went from negative to positive, not the other way around) and start a new cycle at that time.
  6. Jun 25, 2009 #5


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    Re: phase

    Just change the speed of the prime mover.

  7. Jun 29, 2009 #6
    Re: phase

    In analysis of three phase AC power systems, usually a set of phasors is defined as the three complex cube roots of unity, graphically represented as unit magnitudes at angles of 0, 120 and 240 degrees.
    The phase vector ("phasor") is a representation of a sine wave whose amplitude (A), phase (θ), and frequency (ω) are time-invariant. In particular similar frequency condition, the frequency factor, which also includes the time-dependence of the sine wave, is common to all the components of a linear combination of sine waves. In this case the sum or difference of multiple phasors produces another phasor which can assume as applied source on parallel impedance circuit. That is because the sum or difference of sine waves with the same frequency is also a sine wave with that frequency. When phase difference of two voltage vectors is big, the vector difference of them or applied source voltage on parallel impedance circuit will be great which can cause severe short circuit.

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  8. Jun 29, 2009 #7
    Re: phase

    There are inverters for solar panels etc. that permit synchronizing to the AC mains. Do you have such an inverter?
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