Two models of ferrielectric liquid crystals are presented. A static model is used if no voltage or a constant voltage is applied to the cell. Six smectic layers would be required to describe the system in full, i.e., to include ferroelectric and antiferroelectric phases as particular cases. A simpler system with three layers has been used for computation restrictions. Switching and hysteresis curves of ferrielectric devices are studied in a second dynamic model in which some static features have been neglected, but viscosity terms have been included.