In this paper, we present the design of an observer/estimator for the estimation of the power coefficient in a wind energy conversion system (WECS) based on a separately excited dc generator. It is shown that the estimator is capable of supplying accurate estimates of the power coefficient, and that it can handle measurement noise and provide satisfactory convergence qualities. A further advantage of the observer design presented is that it can be easily extended to other WECS where different types of generators are employed.