Autonomous power plants have been given a boost by the development of electric vehicles. The presented power plant based on a switch reluctance machine with a new principle of phase winding excitation is a new technical solution. The new principle of exciting phase windings is based on the features of such electrical machines’ operation. This feature is the trapezoidal form of the phase voltages. This allows switching and excitation of phase windings in the natural mode. That is, this technical solution eliminates the need for tracking and switching phase currents depending on the position of the rotor. The winding is excited by switching capacitors of the idle phase. This can greatly simplify and reduce the cost of electronic control equipment of the switch reluctance generator. Due to the design novelty, the considered technical solution has not previously been investigated by either foreign or domestic scientists. This article provides a mathematical description of the work and developed a mathematical model of the switch reluctance generator with capacitor excitation. This mathematical model allows computing the current and EMF in the electrical machine, and in the external electric circuit, and the value of the torque. The presented results of calculations and experiments confirm the adequacy of the mathematical model. The developed model allows one to visually examine the electromagnetic processes, the performance and functioning of the system in various modes under various external influences, to develop control algorithms and evaluate the energy performance of the generator. The model can also be the basis for a technique of designing and construction of new facilities.