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Slip test on salient pole machine

  1. Sep 8, 2015 #1

    cnh1995

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    In slip test, we provide a constant 3 phase voltage across the stator of the machine. Then why does that voltage vary? Its like a variable inductor circuit due to different air gap length but basically if the voltage impressed is kept constant, why does the voltmeter show variation?
    Is it like changing the core of the inductor at its steady state. What will happen if I inserted iron core in the inductor when it reaches the steady state in a dc circuit?
     
  2. jcsd
  3. Sep 11, 2015 #2
    In my opinion, during the slip test the synchronous motor did not locked in that means it is an induction motor still.

    The excited rotated poles induce an a.c. EMF in the stator of a frequency of (1-s)*netfrequency.s=[(60*p*f)-rpmrotor]/(60*p*f) [the slip]

    p=number of pole pairs.f=network frequency.

    Both voltages -network voltage and EMF- will present a common standing wave voltage [nodes and antinodes] that means the voltage has a minimum and a maximum with s*f frequency.
     
  4. Sep 12, 2015 #3

    cnh1995

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    The machine is an alternator, not a motor. For the test purpose, it is run as an IM. And the rotor is not at all excited in this test.
     
  5. Sep 12, 2015 #4

    jim hardy

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    Can you clear up this apparent contradiction ?

    So is voltage constant or did it vary?

    What voltage is the voltmeter indicating ? Must be different from the constant one that's impressed.


    U nder constant impressed voltage a current meter would vary with pole position.
    Go back to that simple one turn machine in an earlier thread.
    Magnetic path for D & Q axes is not the same , air gap is different.
     
  6. Sep 12, 2015 #5

    cnh1995

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    The voltage impressed across stator is constant but the meter reads fluctuatiting voltage. I don't know how that happens. The current too fluctuates which I can understand. I don't understand the voltage part.
     
  7. Sep 13, 2015 #6

    jim hardy

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    Hmmm you still insist that voltage is both constant and not-constant.

    Perhaps this has to do with your question in another thread about 'regulation'.

    Regulation in a voltage source, be it a power supply or an alternator or transformer or even a battery , is the change in output voltage over its full range of load current.
    A regulated supply with nominal rating of ten volts & one amp, that puts out 10.0 volts unloaded and 9.9 volts when loaded to 1 amp,
    has regulation Δvolts/nominal volts = 0.1 volts/10 volts = 1% .
    Sooooo
    Whatever power source you used to impress "constant volts " across the stator has some regulation.. That is, its output voltage drops by some percentage as you draw current from it.
    If the volts it impressed across your stator were actually constant your voltmeter would have reported that.
    Did your stator source have a dial or switch with numbers that you set ? That just sets what it tries to make ,

    I assume you used a low voltage AC source for this test because applying full stator volts to a synchronous machine with its field open circuited should destroy the field winding.

    You apply low AC volts to the machine then observe impedance as the rotor poles move under the stator poles. Impedance changes because the air gap of a salient rotor machine is not constant like a round rotor one. Try using Z = observed volts/observed amps

    You use the same technique to find open rotor bars in a squirrel cage machine. Apply low voltage (to just one phase if a 3ph machine) and slowly rotate the shaft by hand while watching the ammeter.
    Because the squirrel cage is a short circuit, a fair amount of stator current will flow so that's why you use low voltage. A broken rotor bar passing under the stator poles will make the ammeter dip because it's not a shorted turn like the good bars are.
    You may notice a broken rotor bar in a running motor that's loaded,
    it'll make the ammeter 'dip' sharply at slip frequency
     
  8. Jan 26, 2017 #7
    actually ,voltage across armature is varied but supply voltage remains constant .
    constant applied voltage -minimum impedance drop = armature terminal voltage ..
    as in slip test current varies with respect to rotor position thus impedance drops are also not constant .
    as a result armature terminal voltages are not fixed but these swings in voltage is so less because of low impedance drop in leads
     
  9. Jan 26, 2017 #8

    cnh1995

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    Right.
    Back then, I was not aware of this impedance drop in the leads because it was not shown in the circuit diagram and our professor also didn't discuss it in the lecture.
    I read a detailed explanation later in a reference book.
    Thanks for your reply though!
    And welcome to PF!:smile:
     
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