Reduction of slot harmonics in a brushless doubly-fed machine by using the tooth notching

Since air-gap magnetic fields of brushless doubly-fed machine (BDFM) are produced by two stator windings of different pole pairs, the spatial harmonic contents of the BDFM are higher than conventional induction machines and have a considerable effect on the machine performance. To reduce the contents of main slot harmonics, the harmonic components of airgap magnetic field for the BDFM are analyzed and a method of using the tooth notching in the stator and the rotor is introduced in this paper. A two-dimensional finite element method (2-D FEM) is applied to model a 72-slot stator, 84-slot rotor and 2/4 pole pairs prototype BDFM. The simulation results show that the method of the tooth notching reduces the first-order slot harmonic of air-gap flux density and torque ripple of the BDFM effectively.