AC motor questions

[forest]

@forest

Your describing an induction motor.  Not all electric machines use induction.  Basically an induction motor has shorted coils in the rotor.  Just like the case above where I was describing the force that you feel when you move an aluminum ring over a magnet, this is similar.  The stator coils produce a changing magnetic field (much like the moving magnet in my earlier example).  The shorted coils in the rotor try to oppose the changing magnetic field of the stator.  As they oppose, it produces movement of the rotor.  The rotor will start to get faster and faster until the speed of the rotor matches the rotating magnetic field of the stator.  When this happens, the rotor coils no longer see a changing magnetic field because they are spinning at the same rate that the stator is energized.  The stator looks like a static field, induction stops and the rotor slows down a little.  Once slowed down a little, the process starts again.  So at no load there is a slight difference between the rotating magnetic field of the stator and the speed of the rotor because of friction.  When you load the rotor, it becomes harder to rotate and the slip gets bigger.  The induced current looks like the stator current but it goes in the opposite direction and is slightly less in magnitude.

Are you sure that this current in stator is not a pure sinewave just going in opposite direction ? Is that really multi phase current inside rotor ? Is there any way to check this ?
Because rotor has a slip and never goes so fast as stator rotating field and direction of rotor movement is the same as rotating magnetic field I assume that the polarity of magnetic field in rotor is opposite to stator field and it works by pulling rotor not by pushing it. Is that correct ?

Did anybody thought about reusing rotor current and put it back to stator ?

[Doug1]

Forest
  Sort of ,I am working on placing tiny caps in the rotor to increase the potential in the rotor to see if will sync better and reduce the slip. I have not found suitable info on the shorted rotor coils so it seems it is time to wing it. I may just jump right to the rebuild of an alternator to fit the concept.

[Charlie_V]

@nueview

lentz law works great for copper inducters or should i say conductors but isn't worth a thing for iron conducters or plastic

Lenz's Law is just to explain that induced currents from an alternating magnetic field are in opposition to the field that induced them.  Plastic is usually insulating and therefore will not have currents induced in it.  Iron is a conductor, typically with higher internal resistance, and the currents induced in it are also in opposition - just like copper.  That's why they use laminate transformer cores so that the eddy currents are reduced - the eddy currents are in opposition just like in copper. 

The magnetic properties of these materials are different though - that's not what I'm talking about here.  When copper is placed in a STATIC magnetic field, the dipoles align to oppose the magnet - this is not the same as the magnetic field induced by a changing magnetic field.  Copper, water, graphite, plastic and many more are diamagnetic - this is a material property.  If any of these materials are conductive, and you place it in an alternating field, the induced currents will overpower the diamagnetic effect (which is very weak in almost all of these materials - pyrolytic graphite has the highest, better than bismuth, and the effect is still very small).

@forest

Are you sure that this current in stator is not a pure sinewave just going in opposite direction ? Is that really multi phase current inside rotor ? Is there any way to check this ?

I guess the current in the stator is a sine wave, if you drive the stator with a sign wave.  You could make it a square, triangle, impulse, anything really I suppose.  Its only multi-phase if you are driving it with more than one phase.  Oscilloscopes can be used to check it. 

Because rotor has a slip and never goes so fast as stator rotating field and direction of rotor movement is the same as rotating magnetic field I assume that the polarity of magnetic field in rotor is opposite to stator field and it works by pulling rotor not by pushing it. Is that correct ?

If your using the device as a motor it never exceeds the driving rotation.  However, if you use an external source (like a wind turbine) and drive the rotor faster than the rotating field of the stator, it becomes a generator and will supply power to the stator - pretty cool huh.  When in motor mode though, it does both pushing and pulling.  The only motor that works off of pure pulling is what is called a reluctance motor.  The rotor in a reluctance motor is laminated steel or iron and usually has groves in it to make it look like a star.  The coils in the stator attract the rotor and then get turned off so the rotor spins past the coil and then they energize again.  I think these are really efficient compared to induction motors - not as efficient as permanent magnet motors though (they are the best efficiency wise).  The problem with reluctance motors is that they cannot act as a generator. 

Did anybody thought about reusing rotor current and put it back to stator ?

The current in the rotor is normally in the opposite direction as the stator.  They would cancel each other and nothing would turn.  BUT, that's an interesting idea.  You might be able to shift the phase or something and utilize it.  I'd have to think about it - good thought!

[Charlie_V]

Did anybody thought about reusing rotor current and put it back to stator ?

Actually, they do - sort of.  Look up universal motors.  These have the rotor coils in series with the stator so they use the same current to rotate.  The cool thing about universal motors is that if they do not have a load, they will spin so fast they destroy themselves!  There might be a way of using that to an advantage.