I am not sure (im still a newbe ), but let's work in senoidal regime:
Voltage v(t) = V . sin(wt+p)
w (omega) is in radians, so you could make w an equivalent to RPM:
1 RPM = 1 radian = 2*PI
Of curse, you will also need current and impedances for a deep analysis, but you can get the idea (P = (V^2)/Z)
I hope this is OK, sorry everyone if this is screwed up.
For alternating current (A.C.), the RPM of the generator determines the frequency of its output. In the U.S., power companies try to keep the frequency at 60Hz. When the load on the systems increases, the frequency tends to lower, so more power (steam flow through the turbin) is needed. The opposite happens when the load decreases.