Effects of finite length in solid-rotor induction machines

The author presents an analytical solution of Maxwell´s equations for the fields in a finite-length, solid-rotor induction motor in which the rotor is not fitted with conducting endrings. It has been assumed as a boundary condition that zero flux enters the ends of the rotor; this is borne out by tests on a typical machine. The solutions for the field distributions are approximate only, but the approximations are good over a wide range of slip. A major advantage of the solutions is that they are expressed in closed, rather than series, form. It is shown that the field distributions and the equivalent-circuit impedances may be considerably different from those obtained by ignoring finite-length effects. Some simple criteria are derived to assess the importance of finite-length effects on machine impedances. The analysis is carried out ignoring rotor saturation and slotting.