Time evolution of the magnetic and kinetic energies during the stationary state with imposed background field (negative time) and after the background field has been switched off (positive time) for the three cases (a: 0.0025 G; b: 1.0 G, c: 2.5 G). The induced field in the 2.5 G case is stable when the background field is switched off, showing that it is generated by a subcritical dynamo. A decisive criterion for the onset of dynamo action seems that the toroidal magnetic energy exceeds the toroidal kinetic energy. Shown are the dimensionless kinetic energy, E_kin = 1/2u², and the dimensionless magnetic energy, E_mag = 1/2 (Ek/Pm) B², integration over the simulated shell.