In this work, investigation of magnetic couplers used in industrial applications such as bioreactors for mixing of liquids has been performed. Nowadays, the best solution as to contamination problems associated with mixing in stirred tank bioreactors is magnetic coupling. In this investigation, analytical treatment was carried out for the factors affecting the value of critical torque. These are: materials of the main design details, permanent magnet (PM) shape, PM width and height, the number of pole pairs, and the air gap. In the framework of investigation, design improvement and optimization were performed for 1 m 3, 4m 3, 15m 3 magnetic couplings (JSC 'Biotehniskais Centrs' production). The results of numerical modelling were compared with those obtained experimentally.