This paper endeavors to model the effects of governor- turbine dynamics on the average system frequency behavior of a multi- generator system after a major generation loss or load change when the system remains in synchronism. The maximum frequency deviation and the time at which the maximum occurs are the main quantities of interest. Two independent concepts are presented to convert a closed-loop, high dimensional, nonlinear model into approximate, open-loop, low dimensional, linear ones (the delay and the canonical models). The delay model uses pure time delay to model "fast time constants" and valve motion and converts the closed loop model into an open loop one. The canonical model expresses the turbine reheats respo'nse as a linear combination of a set of "basis" functions and provides a basis for combining many machines into one simplified, low dimensional model. The two models can be implemented concurrently, to give a very large reduction in complexity when a multi-machine system is analyzed.