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According to the description on page 159 of Prof. Lipo's book "Introduction to the AC Machine design":

It is important to mention that the harmonic leakage is important only for wound rotor machines. When the induction machine is equipped with a squirrel cage the harmonics in the air gap produced by the non-fundamental components of MMF will each induce a component of rotor current which will tend to “short” the harmonic inductance. Although belt harmonic fluxes still remain in the gap, they are now very small, being the summation of the harmonic stator fluxes and the opposing harmonic rotor fluxes.

However, I could not understand why such induced higher order harmonics rotor current, which cancel out the effect of higher order stator MMF in the case of squirrel cage rotor, cannot happen for wound rotor. After all, the current in the wound rotor is induced by the stator MMF, just the same as squirrel cage rotor.

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  • \$\begingroup\$ Voltage or emf is induced, not current. Current flows because of the induced voltage and a conductive path. I don't know why people say induced current in the context of magnetics; it seems to miss the point entirely; it's not like there are brand new electrons created. A bit like saying that light is conducted from an LED to our eyes. Dumbing down I guess. Rant over. \$\endgroup\$ – Andy aka Jan 23 at 12:43
  • \$\begingroup\$ and I always thought mutual inductance was Induced ! The current and voltage created are just ratios of the impedance transformer. Right? \$\endgroup\$ – Tony Stewart EE75 Jan 23 at 13:13
  • \$\begingroup\$ I think the induced current Lipo just means the induced rotor EMF divided by the rotor bar resistance. Thank you for pointing it out. But here the problem is focused on the analysis of harmonic leakage inductance. \$\endgroup\$ – 8cold8hot Jan 23 at 13:48
  • \$\begingroup\$ I found another thread about the phase number of wound rotor IM. The answer says that wound rotor IM is actually three phase. Is it the reason that it cannot be induced to generate higher order harmonic EMF/current? \$\endgroup\$ – 8cold8hot Jan 23 at 15:44
  • \$\begingroup\$ The squirrel-cage rotor does not have phased defined by windings. That prevents harmonics due to a difference between the stator and rotor windings that varies with relative angular position. It is the squirrel cage motor that has the advantage, not the wound rotor motor, I believe what you wrote reverses what Limo wrote. Look at my answer. \$\endgroup\$ – Charles Cowie Jan 23 at 17:27
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In your statements, you have reversed what is said by Lipo. "harmonic leakage is important only for wound rotor machines.

Here is some material from two other sources:

Alger, The Nature of Polyphase Induction Machines, Wiley, 1951 states “A squirrel-cage winding has no phase belts and its belt leakage is, therefore, zero."

Puchstein, Lloyd, Conrad, Alternating-Current Machines, Wiley, 1954 states “Belt or Differential Leakage Flux In a wound-rotor motor another element of leakage flux is present by differential distribution of the stator and rotor windings, or by the different positions which one phase belt of the rotor assumes with respect to one phase belt of the stator as the rotor turns.”

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  • \$\begingroup\$ Thank you for the lucid answer, actually this is what I mean but it seems my statement is not clear. But according to the description of that quoted section about belt leakage, this kind of leakage is caused by "the harmonic components of MMF which arise because of the discrete number of stator coils". Then this reason exists for both the wound rotor and the squirrel cage rotor. What is the determinant difference between these two rotors, making one of them possess the belt leakage while the other does not? \$\endgroup\$ – 8cold8hot Jan 24 at 19:46
  • \$\begingroup\$ In the wound rotor, the windings are distributed among three phases and a number of poles. As a result, the wound rotor current flows in "belts" that are defined by the winding distribution. In the squirrel cage rotor, a number of identical rotor bars are distributed evenly around the circumference of the rotor. It seems to me that the additional freedom for the current to choose it's own paths and distribution is what makes the difference. \$\endgroup\$ – Charles Cowie Jan 24 at 20:55

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