Analysis of directly network connected non-salient pole permanent magnet synchronous machines

The development of Neodium-Iron-Boron and Samarium-Cobalt magnets has made it possible to use permanent magnets even in large size motor and generator applications. Often a permanent magnet synchronous generator (PMSG) is vector controlled and indirectly connected to the network via power electronic converters. Industry, however, has a great interest on applying PMSGs without power electronics in various power generating applications such as generators in small water power plants, ships and submarines. PMSGs have permanent excitation that cannot be regulated. Therefore a properly designed damper winding is required to achieve stable operation without a frequency controller. This paper presents analysis and performance characteristics of non-salient pole three phase permanent magnet ac machines equipped with damper windings in rigid network operation. By analyzing the results given by a two axis vector equivalent circuit based simulation model of the PM-machine, the guidelines for efficient damper winding parameters for network-connected generators are found. It is observed that there exists an optimum value for the damper winding resistance which results in stable operation after transients in the shortest period of time. The optimum value of the damper winding resistance is dependent on the electrical and mechanical parameters of the drive. The effects of the PM-machine electric parameters and the rotor inertia for optimum damper winding resistances are analyzed. The theoretical results are validated by practical experiments carried out on a 5.2-kW prototype permanent magnet machine equipped with damper winding.