[EN 12792:2003, 155]:speed can be varied either continuously or in steps by a variable speed motor, slipping coupling, gearbox or other meansdamper control[EN 12792:2003, 155]:fan performance is controlled by means of a damper, either on the inlet or on the outlet, creating a variable additional system resistancevane control[EN 12792:2003, 155]:vanes mounted at the fan inlet, which can be adjusted in order to change the fan performance by controlling the swirl at the fan inletvariable blade pitch control (normally only for axial-flow fans)[EN 12792:2003, 155]:the blade angle of the impeller can be varied whilst the impeller is rotating, all blades being simultaneously varied by one operation (i) adjustable pitch if the blade angle of the impeller can be altered only when the impeller is stationary, this method of control is termed ‘adjustable pitch’ (ii) fixed pitch when the blade angle cannot be changed, it is said that the fan has a ‘fixed pitch’