Current distribution and skin effect factor in the bar extension and the end rings of large two-pole squirrel cage induction machines

The prediction of starting torque and starting current in a large induction motor is a challenging electromagnetic problem. It is important to have an accurate prediction of starting values to ensure that the motor will be able to accelerate the load during starting without a negative impact on the rest of the power grid. The purpose of this thesis is to describe and quantify the effect that the end leakage reactance and the end ring and bar extension resistance have on the starting characteristics of two pole machines. These effects possibly account for the inconsistencies that exist between tested and calculated values of starting torques and currents in large two-pole squirrel cage induction motors.; Starting torque and starting current for six two-pole machines were first computed using two dimensional finite element simulations. The six machines had ratings ranging from 425kW to 2.24MW. Comparisons among the results of these computations and test results indicate that the prediction of starting values is greatly influenced by the estimate of end leakage reactance and end ring resistance used in the calculations.; Three dimensional finite element simulations were used to model the end region more accurately. Several simplified models lead to the conclusion that the stator end coils have a significant proximity effect on the distribution of current in the end ring and should not be omitted when computing the skin effect factor in the end ring. In large two-pole machines, the current is expelled towards the outside of the ring. Because of the stator coils that extend far, with respect to the ring, from the core of the machine, diametric flux lines typical of two-pole machines cross the ring from top to bottom, and lead to this current distribution. The 3D simulations have also shown that for machines with a bar extension, the skin effect factor drops exponentially in the bar extension along the length of the bar, and linearly for the end rings as the bar extensions are made longer. Two simple empirical equations are proposed to model these effects. These equations correlate well with simulation results, and for all tested machines.