Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

3 Phase Induction Machines

  1. Mar 11, 2006 #1
    This is probably a simple question with a simple explanation... I understand that a 3 phase induction machine generates a rotating magnetic field in the stator which by Lenz’s law induces an electromagnetic force in the rotor which moves the rotor in the same direction as the stator magnetic field. The speed at which the rotor turns is proportional to the relative motion between the stator field and the rotor.. What I don’t understand is the speed of the rotor magnetic field with respect to the stator magnetic field. Will these be the same (i.e 0 relative motion) or different? :uhh:
  2. jcsd
  3. Mar 12, 2006 #2
    Not quite the same. There has to be some 'slip' in order for there to continue to be currents induced in the rotor. So the rotor will always be slightly slower than true synchronous speed.
  4. Mar 12, 2006 #3
    Yes, I understand that there needs to be slip and the speed of the rotor will depend upon the load I guess. But what I don't understand is the speed of the magnetic field set up by the rotor due to the induced voltage? Will it slow down as the rotor spins slower?
  5. Mar 12, 2006 #4
    Ummmm, you don't understand at all. The speed of the rotor is mostly determined by the speed of the rotating magnetic field which is solely determined by the line frequency. There needs to be slip because if the rotor was traveling at exactly synchonous speed then the copper wires/bars inside of the rotor would not be cutting the magnetic field. They would simply be sitting still and no current would be induced in the rotor. There has to be current in the rotor in order for it to turn. I like to think of the rotor as a shorted out secondary winding in a transformer.
  6. Mar 12, 2006 #5
    Hmmm. I would think generally that the speed of the rotor is not only determined by the rotating magnetic field but also by the load. Assuming no losses if there is no load then there rotor will turn at sync speed and therefore no current will be induced.

    But anyway, I guess I need to make my question clearer. What I want to know is, what can we say about the speed of the rotor magnetic field with respect to the rotating magnetic field... not the rotormechanical speed.
  7. Mar 12, 2006 #6
    The rotor speed is to a certain extent determined by the load, but much less than you think. The slower the rotor turns, the faster the rotor is cutting the lines of the magnetic field. So more current is gererated in the rotor. It does depend on how the rotor is designed. How much iron is in the rotor, how deep the copper is buried in the iron and other things that I have forgotten or am not aware of determine how much slip an induction motor has.

    I guess I don't know what you're getting at concerning the rotor magnetic field. The rotating field set up by the stator is simply the magnetic field building up and collapsing back to zero in a synchronous manner between 3 sets of poles. The magnetic field generated in the rotor only exists under load. So in your theororetical example where the rotor is spinning at 100% synchronous speed there is no rotor current. Of course this is not possible in the real world. Off of the top of my head I would say that its frequency is the difference between the line frequency and the would be line frequency of a 100% efficient motor running at the speed that the rotor is actually running at. Jeez that can't make any sense to you. LOL Example: Take a motor that is running at 3500 RPM on a 60 hertz source. There is 1 AC cycle completed in one rotation of the magnetic field in this motor. This is 100 revolutions of slip per minute. That works out to be a circulating current in the rotor of 1 & 2/3 hertz. I don't know this for a fact, but I'm pretty sure that would be the case. One thing I DO KNOW for a fact is that when a motor is designed how far the copper is buried in the iron of the rotor has a hand in determining the starting torque. The deeper the copper is the more inductance the rotor has which limits the current through it which limits the torque the rotor can develop. When a motor is loaded to the point that it is no longer able to maintain synchronous speed it suddenly drops out of regulation and it's speed drops way down. The reason the torque does not increase once past this breakpoint is because the frequency of circulating currents in the rotor increases and the inductance of the rotor causes the current to decrease. This is why an induction motor develops the most torque at near synchronous speed.
  8. Mar 28, 2011 #7
    This probably a dead thread, but I don't want to open a new one since my question is related to this term. A current will be induced in the rotor. There will be a net torque on the rotor. Can you give a picture, or draw something to give me an idea how this thing rotates. I understand the magnet just follows the rotating net magnetic field. But when current is induced and all that give me a hard time.

  9. Mar 12, 2012 #8
    I am given to understand that the Slip in the difference between the speeds of the Stators and the rotors. If this is right then does that mean that over a period of time, the rotor may trail the stator field by one revolution? if yes then is this handled by the construction of the rotor and if no, why not?
  10. Mar 12, 2012 #9


    User Avatar
    Gold Member

    I don't think the magnetic field of the stator cares where the rotor is. All the rotor feels is a magnetic field and acts accordingly.

    Here's my take on these induction AC motors. The starting current induces a rotating magnetic field around the stator. The rotatating magnetic field then induces a current in the rotor. The rotor now has a magnetic field of its own....but in the opposite direction of the stator magnetic field....thus inducing torque.

    How close is this...what am i missing?
  11. Mar 12, 2012 #10

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Were you riding on the surface of the rotor you would experience a magnetic field that changes at slip frequency.

    Were you runing around inside of stator at synchronous speed you would experience a constant field.

    The rotor current must make a field that follows the stator field in lock-step.
    To do that it must precess around the rotor, forward , at slip speed. It's dragged along by stator field.

    Something seems awry. Did i mis-word that, or am i suffering temporary brain-lockout?

    Maybe i addressed wrong question.

    anyhow - large cental station synchronous alternators have a few squirrel-cage windings in their rotors to help stabilize them against sudden changes in power angle when say a steam valve opens a little bit or the voltage regulator changes excitation. Their rotating inertia can get into harmonic motion against other machines on the system so the squirrel cage windings are called "Amortisseur windings" - French for "damper"..
    Last edited: Mar 12, 2012
  12. Mar 12, 2012 #11


    User Avatar
    Gold Member

    Yes, as the rotor spins slower (due to load), the synchronous stator field will cut the rotor more often, dphi/dt, induce even MORE voltage..

    For this reason AC induction motors handle dynamic loading quite well.

    in addition, something i recently learned, i thought it was only with dc machines, but an ac induction motor may be used to generate, for purpose of dynamic braking et cetera..

    If you are running an induction motor normally... then.. under no-load you can decrease stator frequency below that of rotor frequency (i've seen it done with igbts) then the rotor will now induce a larger voltage into the stator... and you can load the motor down with resistor grids and it will actually cause a current to flow into the grids and slow the rotor down because the stator is sourcing current to the grids and is heavily magnetized, making it hard for the rotor to turn. off topic i know but interesting..... I was interested....I thought dynamic braking was inherent to dc machines only, but i now have seen it done with ac induction motors.
  13. Mar 12, 2012 #12


    User Avatar
    Gold Member

    This last item that FOIWATER brings us is the basis for pulsed alternator operation that can be the power source for exotic futuristic weapons such as rail guns, pulsed lasers, beam weapons, etc. In a pulsed alternator, the alternator is spun up to maximum speed with no field, the field is applied to the rotor, creating the rotating magnetic field, and then the armature windings are tapped one by one through a diode bridge to produce a huge pulse of "DC" power. Invariably in current designs the field winding is on the rotor, and there is a four phase armature on the stator with just a very few turns per phase to keep inductance to an absolute minimum so that the phase can be discharged very rapidly. These are truly astounding machines!
  14. Mar 13, 2012 #13
    thanks for the replies, if my understanding is right, 'eventhough the rotor continues to trail the stator, it really makes no difference..it will not lock because of the construction of induction machines".
  15. Mar 13, 2012 #14
    Though the OP might have long gone, I would like to answer this loooong unanswered OP's question. :)
    The frequency of currents induced in the rotor will be s*f . where s is the slip and 'f' the line frequency.
    The magnetic fields created by this current will rotate at 120*s*f/P relative to the rotor.
    But the rotor itself is mechanically rotating at 120*(1-s)*f/P relative to stator.

    So, relative to stator, the magnetic field created by rotor rotates at 120*s*f/P + 120*(1-s)*f/P = 120*f/P = Synchronous Speed, at all times, for all values of slips.

    So, we see that as the rotor spins slower, the frequency of induced currents in rotor increases, and the rotor-magnetic field rotates faster w.r.t rotor. But, since the rotor has slowed, the effect is that the rotor-magnetic field rotates at constant speed relative to the stator.

    Hope I increased the knowledge base of PF. :)
  16. Mar 13, 2012 #15

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    An ordinary induction motor driven above synchronous speed will return power to the mains. Those curves of torque vs slip are symmetric about zero.
    But it must draw its excitation current from the mains, so you can't make a household emergency generator out of a washing machine motor and lawnmower engine.

    Now - regarding "it won't lock"

    That's so for a typical motor with round, smooth rotor.
    Some record player motors have a permanent magnet in the rotor that does lock, ie Garrard Synchro-Lab from 1960's. But I call that a 'line startable synchronous motor' , myself. I've seen them as big as 7.5 horsepower for other applications.
    Tiny electric motors in electric clocks and refrigerator defrost timers have an odd shaped rotor with lobes on it that can lock just because the iron lobe is attracted to the rotating stator field. Those are "reluctance motors"

    so when you run across those two exceptions in your studies, think kindly of us PF'ers.
  17. Mar 13, 2012 #16


    User Avatar
    Gold Member

    It is the amortisseur windings that Jim Hardy brings up which enable a synchronous motor to be started across the line. In that case, the motor is totally out of synch, and produces not torque at all as a synchronous machine. However, the amortisseur windings cause the machine to start moving as an induction motor, enabling it to come up to close to synchronous speed, provided there is little or no load on the machine. At that point, the field voltage can be applied and the machine can be brought into synchronism.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?