The Induction Motor has been used extensively for as long as we have used alternating current electricity to power our modern civilization. The working principles are well known and reasonably easy to understand. The 3 phase motor is said to generate a rotating magnetic field which passes through short circuited non magnetic conductors within a predominately laminated steel rotor. Electric eddy currents are produced within the conductor bars, and the magnetic field polarity associated with the eddy currents is always aligned to oppose the polarity of the 3 phase current's fields. This forces the rotor to turn in the opposite direction to the rotating 3 phase currents. This motor is cheap, robust and quite efficient. There are many very good animations of the rotating fields on youtube. All are slightly different.
I hesitate to argue against the operational theory of this motor, but for it to be correct, then my ideas that passing a magnetic field through a non magnetic conductor will create a DGF, may be false.
There are 2 types of induction motor, 3 phase and single phase. The first problem encountered is that while both motors have the same type of rotor construction, their operation seems to described differently. The 3 phase is said to be a rotating field, while the single phase is described as a pulsating field.
The next problem is that some designs of 3 phase motors have 2 sets of conductor bars installed within the rotor. The second set is installed further into the rotor, and all bars are shorted together. This gives better starting torque, and assists some designs to overcome self starting difficulties, and has no detrimental running characteristics.
As I see this arrangement, the second set of bars is somewhat further from the electromagnet's pole, the field of which will interact with the bars, and should result in a decreased magnetic interaction and reduced motor torque. To further complicate the situation, the inner bars are moved closer to the point of rotation, and this would normally also result in less torque.
A DGF appears to produce maximum power, or torque, at a distance, and this may be an exponential function. For example, a doubling of the distance from the electromagnet's pole to the inner bars may quadruple the torque output from the inner bars, and only an exponential function could explain the extra torque gain.
A property of a DGF is that it will interact with any other DGF in the vicinity. It will change its outer shape to reflect the pressures exerted on it by other DGFs. The portion of the bar of non magnetic material where the DGF originates will move to the centre of its DGF if this has been distorted, and this will cause rotation as pressure is relieved.
The sine wave form of the AC electricity means that the magnetic flux is pumping out and in from the pole face constantly, a little like the pistons in an engine at 50 or 60 cycles per second. The DGF does not care about polarity, so it is a little like a 4 stroke engine.
0 volts to 250 volts + >
250 volts+ to 0 volts <
0 volts to 250 volts- >
250 volts- to 0 volts. <
The magnetic flux would be passing through the bars at about 50 KPH.
Meanwhile the other two phases, offset at 120deg and 240 deg would be pumping out and in. All 3 phases would be making small powerful constantly changing DGFs, in a pattern which forces them to act like wedges and turn the rotor.
If this is correct, then some improvements or alterations to the motor could improve efficiency. The first is to install more non magnetic bars right down to the shaft. The second is to make sure the poles are an even number, say 12, and connect the coils so that opposing poles of the same phase are connected in reverse. This would make the flux on the opposite sides of the motor attract towards each other, therefore travel further and faster through more non magnetic metal, creating bigger, more powerful DGFs. The third is to increase the frequency of the 3 phase supply. This would increase the speed that the magnetic flux passes through conductor bars, The fourth may be to remove the outer bar as it may be creating excessive drag. (Slip)