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Induction motor flux variations

  1. Apr 24, 2009 #1
    Hi, :smile:

    Of late, I’m dabbling with flux variation patterns in an induction motor at ‘no-load’, ‘full-load’ and ‘standstill (locked rotor)’ conditions. The concept I’m following is that an induction motor can be considered as a generalized transformer. The equivalent circuit depicts that the mutual flux which links both the stator and rotor conductors falls appreciably as the load on the motor increases and the fall in mutual flux is very much pronounced at standstill condition (short-circuited transformer secondary).

    Also, at standstill and at full load or near full load conditions, there are appreciable ‘reflected’ rotor currents in the stator windings of the motor analogous to the case of a loaded transformer, wherein the load currents are reflected in appropriate ratio on the primary side. But in case of a transformer, all this happens ‘statically’ due to the demagnetizing/neutralizing effects of the secondary winding flux which ‘dampens’ the mutual flux.

    But an induction motor is a rotating machine!.....hence I want to understand the manner in which the rotor induces reflected currents in the stator winding of the motor. Is it due to the dynamically induced EMFs as is the case in a generator (since rotor as well as rotor flux also keeps rotating around the air-gap as the mutual flux revolves synchronously around the stator)….. or by statically induced EMFs as is the case in a transformer? Is there any online link or article available which depicts the mechanism of flux variations or reflected stator currents for the above mentioned conditions in an induction motor?
    Any kind of help will be greatly appreciated.

    Thanks & Regards,
    Shahvir
     
    Last edited: Apr 24, 2009
  2. jcsd
  3. Apr 24, 2009 #2

    Averagesupernova

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    Gold Member

    I suppose you could say that it is the same as a plain old non-rotating transformer. Everything is relative, so with a rotating magnetic field and a rotor that is turning near sychronous speed there is really very little flux that is cutting through the conductors in the rotor. Imagine it like picking up a permanent magnet generator and simply spinning the whole thing in your hands. Everything is rotating so no flux lines are being cut by any conductors.
     
  4. Apr 24, 2009 #3

    I do not know the answer to your question, but it interests me because I once worked for an ac motor manufacturer as a technician. We did sample testing of motors that were randomly pulled from the production line.

    The thing I have trouble understanding about these motors is there ability to run on single phase current once the rotor has reached a curtain speed. Most induction motors designed to work with single phase current have an auxiliary winding connected to a capacitor which provides a 90 degree phase shift from the main winding. This is what provides the rotating field needed to turn the rotor. But once the rotor reaches a curtain speed, this winding is no longer needed. In fact, in some motors such as general purpose motors of the type you might find in swimming pool pumps, the auxiliary winding is completely switched out by a centrifugal switch (capacitor start motor). How does the rotor continue to deliver mechanical power to the load when there is no rotating field? Or, if there is a rotating field, how is it being produced? Is it possible that this field is being provided by the reflected current you are asking about?
     
    Last edited: Apr 24, 2009
  5. Apr 24, 2009 #4
    Your query is pertaining to 'double revolving field' theory, in that a single phase winding flux is considered theoretically as producing two equal magnetic fluxes which revolve in opposite directions in relation to each other. I still do not understand how this theory relates to the practical case you are pointing to, but it works in real motors nevertheless!

    The reflected current is due to transformer action and is not related to your doubt.
     
  6. Apr 24, 2009 #5
    I require one more clarification! The revolving mutual flux is the resultant of the vector sum of the individual three phase (or two phase) magnetic fluxes. It revolves synchronously around the stator air-gap cutting the rotor conductors. This mutual flux is also indirectly responsible for stator inductance or inductive reactance and stator magnetizing current.

    What I want to understand is that, is the back EMF (responsible for stator winding inductance) in the stator winding, developed due to statically induced EMF (transformer action) or dynamically induced EMF (generator action)? :confused:....since the mutual flux 'revolves' around the stator air-gap synchronously!..... and hence, in the process, might cut the stator conductors too just as it cuts the short-ckted rotor conductors. But this is strictly my assumption though!
    Thanx :smile:
     
  7. Apr 24, 2009 #6
    Thanks for the info. I thought there was a theory for this, but I did not know what it was called.
     
  8. Apr 26, 2009 #7

    Someone plz reply! :frown:
     
  9. Apr 26, 2009 #8

    Averagesupernova

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    Gold Member

    I'm certainly no expert, although I believe I have a pretty good grasp of how induction motors work. I would suppose that the back EMF is due to the inductance of the stator. Since an induction motor that is not loaded is running at or very near synchronous speed the flux is not cutting the conductors in the rotor, or at least cut very little due to slip.
     
  10. Apr 29, 2009 #9

    Can someone plz clarify this for me? :frown: Thanx.
     
  11. Apr 30, 2009 #10
    There are two basic theory on single phase motors. First of all there is the well known double revolving field theory secondly there's the cross field theory. The last one deals with transformator and generator action. To understand the crossfield theory I advise you to read the chapters 31 to 36 in "alternating current machines " from Puchstein and lloyd publisher John wiley and sons (you can buy one on abebooks for less than 10$ , publishing date 1942 or 1951, very good). To understand the double revolving field theory ,for pure single phase motors and for capacitor motors, I advise you "electric machinery and transformers by BHag S. Guru chapter 10 ( the book isnt cheap) although "electric Machines " by DP Kothari and IJ Nagrath (alsqo chap 10) is also very good and less expensive you have one for about 15 to 20 $ if you buy them in india. I wich you good study work
     
  12. Apr 30, 2009 #11

    Thanx for your reply :smile:.......but my query has more to do with the way in which the rotor and stator magnetic fluxes interact with each other, the mechanism in which the rotor MMF results in reflected stator currents and hence a counter-balance MMF to neutralize the rotor MMF. Also, the resultant mutual flux developed by the superimposition of the stator and rotor MMFs!
    Thanx :smile:
     
    Last edited: Apr 30, 2009
  13. Apr 30, 2009 #12
    When the rotor is blocked, the 3ph motor acts as a shorted transformer, the rotor frequency at that moment is maximum. So the rotating airgap field encounters a very strong rotating field in the rotor(of course in the opposite direction). Therefore the mutual flux is decreasing . Because Us=Es = 4.44* N *distribution factor * f *flux the core flux has to be constant (Us is a constant) The decreasing of the mutual flux is increased by an increasing stator current. The strong rotor current results in a starting torque.
    At no load the airgap field encounters a small rotating field in the rotor (hence frotor = fsource-fairgapfield) This small counter rotating field of the rotor decreases the total mutual flux and again the stator current has to make the flux in balance.
    At full load the motor speed is decreasing therefore the frequency of the rotorfield is increasing and the counter reaction on the airgapfield is much larger then in the no load situation. The reaction of the stator to balance the flux is of course larger.
    remark the difference between motor and transformer
    prim and sec frequency are equal in a transformer
    prim freq is much larger than rotor freq (sec) motor
    transformer has no airgap therefore noload current is very small
    motor has an airgap large noload currents even up to 70% Inom, has also a large leakeage flux.
    to make small motors disigners work with saturation of the core
     
  14. May 1, 2009 #13
    Dear Gilver,

    Excellent reply! Thanx very much :smile: Although I've understood your explanation, a small re-verification from my side;

    1) What I've understood is that the reflected (flux balance) stator current appears due to the 'demagnetizing' effect of the rotor amp-turns (transformer action) on the mutual flux and not due to flux cutting (generator action) by the rotating rotor MMF.

    2) Also, is the inductance (or inductive reactance) of the stator winding a result of the synchronously rotating mutual flux due to statically induced self EMF (transformer action) or due to dynamically induced EMF (generator action).....since the mutual flux synchronously rotates around the air-gap and may be cutting the stator conductors to induce back EMF in the stator windings. Plz correct me if I'm wrong!

    Can you provide me a link or an on-line article which would depict the phasor diagrams or relation between the various fluxes involved in the induction motor? I'll be very grateful. :smile:

    Thanks & Kind Regards,
    Shahvir
     
  15. May 1, 2009 #14
    Dear Shavir

    The demagnetisation of the mutual flux is due to the fact that when one applies a load on the motor shaft, the first it does is slowing down. Therefore the emf in the rotor is increasing, because the difference between the speed of the rotating field and the motor speed has increased. So a lot more flux is cutting the rotor bars and the emf in the rotor increases. ==> Irotor increases ==> torque and counter rotating field increases and the mutual flux decreases. (So demagnetisation of the mutual flux is due to generation action rather then pure transformer action ) otherwise both rotating fields generate in the stator coils emfs. But finally there is only one flux and because the mutual flux has decreased the
    difference between U source and induced Emf has become larger  Istator is increasing and the flux balance can be obtained. of course this doesn’t happen in a second. loading a machines takes time to settle a new working point. In this case with the same flux(same reactive power from the source) , a larger stator current , but a little bit lesser rotor speed (slip has increased)

    I will post some pictures of a vectordiagram on monday

    but here is a pretty link of a rotating field

    http://www.esat.kuleuven.ac.be/electa/teaching/maxwell/screenshots/ [Broken]

    best regards

    Gilbert
     
    Last edited by a moderator: May 4, 2017
  16. May 2, 2009 #15
    Dear Shavir

    The demagnetisation of the mutual flux is due to the fact that when one applies a load on the motor shaft, the first it does is slowing down. Therefore the emf in the rotor is increasing, because the difference between the speed of the rotating field and the motor speed has increased. So a lot more flux is cutting the rotor bars and the emf in the rotor increases. ==> Irotor increases ==> torque and counter rotating field increases and the mutual flux decreases. (So demagnetisation of the mutual flux is due to generation action rather then pure transformer action ) otherwise both rotating fields generate in the stator coils emfs. But finally there is only one flux and because the mutual flux has decreased the difference between U source and induced Emf has become larger  Istator is increasing and the flux balance can be obtained. of course this doesn’t happen in a second. loading a machines takes time to settle a new working point. In this case the same flux(same reactive power from the source) , a larger stator current , but the rotor speed will be a little bit less (slip has increased).

    best regards
     
  17. May 2, 2009 #16
    Dear Gilver,

    Thanx for reply. :smile: Although your reply is technically correct, still it does not hit the nail on the head, per se. Generator action means EMF induced in stator winding due to rotating magnetic flux produced by the rotor conductors, which, I presume, cuts the stator winding conductors as in any generator (back EMF). Similarly, if i assume rotor speed = synchronous speed, rotor current = 0, hence rotor flux = 0 (no-load). But synchronously rotating mutual flux is still present. It induces an EMF in stator winding due to self-inductance.

    Is this stator induced EMF (self-inductance) a result of the mutual flux self-cutting the stator winding conductors (generator action, dynamically induced EMF) or due to the self-pulsations of the mutual flux as is property of AC magnetic flux (transformer action, statically induced EMF) ?

    Similarly, is the stator reflected current (balancing amp-turns) a result of the neutralizing/damping effect of the rotor MMF of a loaded motor as in case of transformer action or due to the flux cutting action of the stator conductors by the rotor MMF as in case of generator action ? This is precisely what i intend to understand!

    Thanks & Kind Regards,
    Shahvir
     
  18. May 3, 2009 #17
    Dear Shahvir

    My statement that both rotating fields generate an Emf in the stator coils ins’t quiet correct.
    You must see that the rotating field produced by the stator coils is nothing else than 3 alternating magnetic fields which are shifted in space over 120 electrical degrees due to the typical construction of a three phase winding., but also shifted in time over 120 electrical degrees due to the 3 phase current. This 3 fields result in an symmetrical rotating field. The rotating field in only a rotating field if you stand in the airgap or in the rotor. In that space it can cut rotorbar. But it doesn’t cut the stator coils, there you have alternating fields like in a transformer core . The flux in every coil changes polarity and strength by the velocity of the source frequency , one after the other . So the counter rotating field shall decrease the mutual flux if the load is rising, this is due to the Emf and current in the rotor that is generated by the mutual flux (finally there is only one flux in the motor, so you can’t have a rotating flux in clockwise direction and one ccw, you will have a resultant flux in the direction of the largest rotating flux, but equal to flux diference). This results in a decreasing flux and the decreasing has to be balanced by the stator. So each single phase winding will react , and together they will balance the flux. This action can be seen as an transformer action.

    Best regards

    Gilver
     
  19. May 3, 2009 #18
    Dear Shahvir
    My statement “both rotating fields generate in the stator coils emfs” ins’t quiet correct
    First of all a Three phase winding consists of three single phase windings, shifted 120 electrical degrees in space (120,60,40 mechanical degrees for a 2 pole, 4pole , 6pole …) and also shifted a 120 electrical degrees in time due to the 3 phase current source. Each winding is producing an alternating magnetic field, also shifted in space and time. This 3 fields result in an rotating symmetrical field, with constant velocity depending from the frequency and the number of poles. only objects in the airgap or in the rotor can be cut from the rotating flux. In the statorcoils the flux is pure alternating but shifted in time and space. So the rotating field doesn’t cut his own coils.
    When load is rising nrotor is decreasing -> Er is generated and increasing, so is Ir -< the counter rotating field is decreasing the total flux. Finally there is only one flux and it has decreased. Every coil in the stator will react to balance the flux in the motor. This reaction (pure alternating field is the coils, like in a transformer core) is a transformer action due to the decreasing flux.
    So in the rotor flux can change by loading a motor this happens because of generated Emf. The reaction of the stator is a transformation action on total flux change.
    best regards
    gilbert
     
  20. May 4, 2009 #19
    Dear Gilbert,

    Thanx very much :smile: This time you have driven the nail right thru the wall! :wink:

    So, ultimately, i feel only demagnetizing/neutralizing effects of the rotor flux comes into play just as in transformer action and generator action is not available although the rotor is rotating. Plz correct me if I'm wrong.

    Also, the concept of ccw revolving rotor flux is a bit unclear to me, since even at standstill (locked rotor condition) both the stator and rotor fluxes are rotating in the same direction , albeit with an angular difference between them. I did be grateful if you could throw some light on this topic. :smile:

    Thanks & Best Regards,
    Shahvir
     
    Last edited: May 4, 2009
  21. May 4, 2009 #20
    Dear Shahvir

    That 's correct

    The ccw field is only to explain why the total flux is demagnetizing, as I mentioned before ther's only one flux in the motor.

    I've put an attachement to this reply fields in a induction motor on load

    best regards

    Gilver
     

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