Induction machine model

This model improves the standard model available in SimPowersystem for induction machines

In Simpowersystem, the model of induction machines does not allow the connection of inductances in series to the motor (at least up to my version 2012b). This can be inconvenient when the machine is connected to a real grid, modelled with its equivalent impedance. This model solves the problems by calculating the induced voltages in the stator winding, rather than the stator currents and therefore it is a voltage-source model rather than a current-source model, allowing the connection of series inductances to the stator.