Abstract: A control circuit is disclosed for synchronizing a prime mover or electrical generator with an electrical grid or power line such that when the prime mover is connected to the grid;(a) the instantaneous phase and slip are within allowable limits;(b) the phase and slip expected at breaker closure, as contrasted to the instantaneous measured phase and slip, will be within given limits;(c) the phase and slip anticipated at the breaker closure instant are optimal for the given trajectory, i.e. when they are the closest to their most desired values during the given transition. The circuit includes a clock for measuring the phase difference between the power line voltage and the generator voltage and means for calculating the rate of change of the phase difference which is a measure of difference of the respective frequencies known as the slip.