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Two 3-phase power systems of differing frequencies are to be connected by closing the breakers at the moment the voltages across them are minimum. The phase voltages va, vb, vc of the abc system  and the voltages vA, vB, vC of the ABC system are represented in the left figure by phasors of constant amplitude rotating at the speed associated with the corresponding system frequencies. The voltages vaA, vbB, vcC across the breakers are seen in the right figure as amplitude modulated phasors rotating at the difference frequency (slip). This arrangement forms the basis of the operation of the synchroscope.



                                                                 Opposite sequences

© M. Riaz