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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 va1, vb1, vc1 of the a1b1c1 system are represented in the left figure by phasors of constant amplitude rotating at the speed associated with the corresponding system frequencies. The voltages vaa1, vbb1, vcc1 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.

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© M. Riaz