Cycloconverter-fed electrically excited synchronous motor systems are widely used in high-power and low-speed applications.Apparently，flux estimation is the foundation of vector control for the electrically excited synchronous motor.A novel flux estimator is proposed by combining the current model and proportional resonant controller-based voltage model.The discontinuous current problem is solved by piecewise linearization compensation.The basic waves are extracted through fixed period sampling and variable filtering.Different conditions such as the steady state，the dynamic state and the full speed range of operation are simulated.The algorithm is verified by 960 V/1.6 MW field test.Experimental results show that the proposed flux estimator is capable of effectively estimating the flux in vector control systems of the cycloconverter-fed electrically excited synchronous motor.