[en] The properties of nonlinear phase oscillations in a microtron are investigated systematically by a unified calculation technique. Resonant and large-current instabilities of the phase motion are described. The cause of appearance of distribution with two maxima of electron bunch density is mentioned and the physical nature of the restrictions on the energy and current of the accelerated particles is discussed. A mechanical model of the microtron is described. It is demonstrated in the particular case of the microtron as an example how in nonlinear system a transition (due to increase in nonlinearity) from dynamic to stochastic oscillation condition occurs