Calculation of no-load additional losses in the rotor of StrafloMatrixTM synchronous turbine-generators

A numerical method for no-load additional loss calculation is presented using the finite element software FEMAG. The loss model adopts a current sheet lying on a slotless stator lamination to simulate the no-load "slotting" effect due to slot openings. Based on the nonlinear magnetostatic analysis of slot harmonics with flux density amplitudes of the slot harmonic pairs obtained through FFT, the discretized harmonic current excitation is derived for the current sheet to generate a spatial harmonic current distribution. Applying the current excitation with the corresponding rotor frequency to the numerical model, no-load additional losses due to slot harmonics in each conducting part of the rotor of both the 300 kVA and 700 kVA synchronous permanent magnet generators with copper cylinder and respectively stainless steel cylinder as damper are calculated. From the numerical calculation results, loss in the copper damper cylinder accounts for about 94% of the total rotor losses caused by the first two pairs of slot harmonics, while for the second design with the stainless damper it accounts for about 92%. The no-load additional losses are about 5.5% of the rated power for the copper damper design and only 0.3% for the stainless steel damper design. The 700 kVA generator with the stainless steel damper has therefore a much lower damping performance; the damping coefficient is only 0.45 s^-1 compared with 2.65 s^-1 for the 300 kVA generator with the copper damper