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Damper resistances and reactances at sub-transient frequencies in large generators are computed rapidly from standard design data in a program written for repeated use in optimal design. Two non-linear, two-dimensional field problems in complex algebra, one for each of the direct and quadrature axes, are set up and solved in five to eight seconds on an IBM 4341 digital computer. Local negative sequence losses can be computed in a further three to five seconds. A diakoptic treatment, the modelling of singularities and the use of surface impedances give the method its economy. Comparisons with another method and with measurements suggest an accuracy within ten per cent.