Abstract: A hydrodynamic coupling for driving pumps, fans, and the like at variable shaft speeds includes coaxial primary and secondary impellers. The space defined within these impellers contains variable quantities of working fluid for transmitting the shaft torque from one impeller to the other. One of the impellers is equipped with a co-rotating casing, which encloses the other impeller and may serve to force working fluid into the working chamber. The fluid exits from the working chamber through an aperture in the higher-lying impeller and returns to a storage chamber from which a conduit transports it to the casing. A control valve in the conduit permits external flow control.