Analysis of MW-level offshore wind turbine generators with dual star–delta fractional-slot windings

This paper investigates the use of fractional slot concentrated windings (FSCW) in large-scale (MW level) offshore wind generators. It focuses specifically on a power rating of 3MW and uses an ex-isting direct-drive synchronous PM machine (DD-SPM) with 480s/160p and dual 3-phase inte-ger-slot winding (ISW) as a baseline. A multiple of the common 12s/10p FSCW machine is used that matches the electrical frequency of the ISW machine, yielding a 192s/160p dual 3-phase machine. The hybrid star-delta connection has grown increasingly popular owing to its unique harmonic cancellation properties that can help reduce rotor and PM eddy current losses in FSCW machines. In this paper, two dual 3-phase star-delta wound machines are scaled to 3MW and included in the investigation. Specifically, a 384s/160p dual 3-phase and dual star-delta winding machine which are a multiplication of the 24s/10p machine, and a 192s/176p dual 3-phase and dual star-delta winding machine which are a multiplication of the 24s/22p machine. These machines are investigated using finite element analysis (FEA) and compared on their air-gap flux density harmonics, open-circuit electro-motive force (EMF), torque performance, and losses and power. It is found that the pro-posed 384s/160p dual star-delta winding machine has the best electromagnetic performance of all, with a stator power that is 1.2% greater than the baseline ISW machine. However, this machine has a coil pitch of 2 and so loses the manufacturing and fault tolerant advantage of having concentrated windings. If concentrated windings are desired, then the proposed 192s/176p dual star-delta winding machine is the best choice with a stator power only 2.6% less than the baseline ISW machine, but unfortunately still has significant rotor and PM eddy current loss.