Rotor Initial Position Detection Schemes in Synchronous Machines

Before the advent of power electronic drives, synchronous machines were started as induction motors and brought close to synchronous speed before pulling into synchronism. The introduction of power electronic drives brought a revolutionary change with variable frequency drives becoming the most preferred. Starting the machines still remained a challenge as wrong alignment of the stator space vector can lead to failed start or even in worst case, reverse rotation. Several techniques have been used to detect the initial position of the rotor for proper alignment of stator space vector. Most of them have been based on position encoders while the current trend favors sensor-less, machine model based methods. An examination of the philosophy of sensor-less methods is carried out in this paper with measured results from the 2500 MVA short circuit generator at the High Power Laboratory at CPRI, Bangalore. The determination scheme based on simple machine equations has been found to be excellent in predicting the rotor position.