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Homework Help: Synchronous Machines - Open Circuit and Short Circuit Tests

  1. Mar 24, 2013 #1
    This was a theoretical / rhetorical question posed by the professor during class. He hinted that this will be on the next quiz. I think I know the answer, but can you guys verify it for me?

    1. The problem statement, all variables and given/known data

    Why do we not perform the Open Circuit Test and Short Circuit Test one after another? i.e. Why must we wait quite some time before performing the SCT after the OCT?

    2. Relevant equations


    3. The attempt at a solution

    For the open circuit test, we must drive the machine into saturation to plot the open circuited voltage characteristics. Once we turn down the field current after getting the different values of voltages, B DOES NOT come down the same curve as it went up due to hysteresis. This means that there will be some residual flux left in the core of the machine right after the open circuit test.

    If we short circuit the terminals right away for the SC test, this residual flux may be very large and cause great amount of current to flow through the short circuit, and this might be very dangerous. Thus, we wait a while for the flux to die down before short circuiting the terminals.


    I think this answer makes most sense to me because core saturation is really the only link between the two tests. However, residual flux is not changing very fast (i.e. [itex]\frac{d}{dt} ∫B dS ≈ 0[/itex]) and therefore not create any emf... so I don't really know if that will really cause lots of current to flow through the short circuit.

    So can someone verify if my thought process is correct? If not, do you guys have any hints regarding this?

    Last edited: Mar 24, 2013
  2. jcsd
  3. Mar 25, 2013 #2
    Residual flux ? I'm not sure about that, fields collapse pretty fast. Usually residual charge is left in the capacitors and it takes time for them to discharge. That's from a practical point of view though when I'm working on circuits, I'm not sure what your equation says.
  4. Mar 26, 2013 #3


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    Staff: Mentor

    Could you say more about that kind of "machine" you are dealing with? Some of your words sound like you are running some sort of test on a transformer, but other parts of your post seem to imply that there is some "mechanism" associated with it.

    Also, could you explain (hopefully with sketches) what the OCT and SCT do?
  5. Mar 27, 2013 #4
    Well, the machine is a cylindrical pole synchronous generator.

    Essentially, the OCT and SCT is used to determine the synchronous reactance of the generator. We do the OCT by measuring the open-circuited terminal voltage while increasing the field current, and same for the SCT where we measure the armature current with increasing field current. The armature current vs field current graph will be a straight line due to the flux of the field and armature reaction cancelling out, so the machine is not driven into saturation.

    The 2 graphs look something like this in theory: http://iete-elan.ac.in/SolQP/S_Final_files/image201.jpg

    Then we find the voltage and current for the rated conditions that we want from the graph (using Ifield to transfer from one plot to the other), and then divide them to find the synchronous reactance.

    I think all of this makes sense in my head in theory, but my prof asked us why in practice, we don't do the short circuit test immediately after the open circuit one... I really have no idea haha...
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