To reliably determine a state variable (G) of a polyphase synchronous machine (1) that is supplied with electric current by a polyphase power network (2) with n phases (U, . . . , N), where n≥3, the current values of at least n−1 phase currents (Iu, . . . , In-1) from at least n−1 phases (U, . . . , N-1) of the polyphase power network (2) are detected. The state variable (G) is determined by means of the at least n−1 instantaneous current values (Iu, . . . , In-1), the current values of all n phase currents (Iu, . . . , In) from the n-phase polyphase power network (2) being detected and the n phase currents (Iu, . . . , In) being correlated with one another. The result of the correlation of all n phase currents (Iu, . . . , In) is used for a plausibility check of the n−1 phase currents (Iu, . . . , In-1) used to determine the state variable (G).