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Two pm alternators in parallel query

  1. Mar 11, 2009 #1
    Hi Guys, :smile:

    My query is a bit abstract in nature! But i will be very grateful if someone can provide me with the solution to my query. Here goes;

    Consider two Permanent Magnet (PM) Alternators Alt 'A' & Alt 'B' connected in parallel, supplying equal power to a purely 'Resistive' load (unity p.f.). Both the Alternators have equal stator synchronous reactances (stator resistances can be neglected). Please note, they are not connected to the local power grid (infinite bus). Now, if i increase the driving torque of say, Alt 'A', then what changes will occur with respect to the following electrial parameters:-

    1) Will magnitude of induced emf 'Ea' of Alt 'A' increase/decrease ? Please explain.

    2) Will magnitude of induced emf 'Eb' of Alt 'B' increase/decrease ? Please explain

    3) Will magnitude of terminal voltage 'Vt' increase/decrease ? If the magnitude of 'Vt' remains constant or decreases (due to armature reaction), then please explain.

    4) Can someone please provide me with a practical phasor diagram for the above query. I am asking for one, since the phasor diagrams in textbooks have lot of assumed conditions & hence do not relate to practical conditions!

    Any kind of help will be highly appreciated.

    Thanks & Regards,
    Last edited: Mar 11, 2009
  2. jcsd
  3. Mar 11, 2009 #2


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    Staff: Mentor

    Welcome to the PF. What are your thoughts on your questions? What is the context of your looking into this? Are you working on building a project, or is this for school or self study, perhaps?
  4. Mar 11, 2009 #3
    Thanx Berkeman.....How does loading of the 2 PM alternators happen under the conditions, Alt 'A' at 75% loading & Alt 'B' at 25% loading. To increase loading of Alt 'A' to 75%, input torque of Alt 'A' would have to be increased! albiet with a rise in bus frequency.

    I basically want to understand the internal happenings in the 2 alternators with respect to the induced EMFs 'Ea', 'Eb' and terminal voltage 'Vt' for the Alt 'A' (75%) - Alt 'B' (25%) loading conditions under steady state conditions.

    My query is for academic interest & does not reflect practical requirements.

    Thanks & Regards,
  5. Mar 12, 2009 #4
    Someone plz reply to this query too!! :frown:

    Kind Regards,
  6. Mar 12, 2009 #5
    This is a difficult project. You cannot change the RPM of either motor (think of adding 59 Hz and 61 Hz-what do you get?), but you can change the phase of either alternator. The two motors have to be kept in the exact phase, which means you need to use a cogged belt drive (like a timing belt) to both. In addition, if the load on one alternator increases, the phase of the voltage output will be retarded as the torque increases, even if the drive belt doesn't slip. If the drive belts are not cogged, I suspect that the stable relative phase of the two alternators will be 180 degrees out, so no output power is generated.

    It is much easier to put the two alternators in series and use a step down transformer. But you still need to use a cogged belt.
  7. Mar 13, 2009 #6
    :smile: Thanx Bob for your reply, but this is not a project. I put this post to gain a deeper understanding of the physics of two PM alternators when connected in parallel. You can say it is a topic of interest for me.

    I request you to refer post # 1 of this thread and give a suitable reply to my questions so that i can proceed further with queries if any.

    Kind Regards,
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