For high powers and in applications where low inertia in brakes is needed, the hydraulic dynanometers are used. These machines are basically torque hydraulic converters, very similar to the ones used in automatic transmissions. In fact, in low cost applications, hydraulic retarders are used.

They consist of one or several rotors in form of hydraulic turbine, faced to stators of similar geometry, immersed in an hydraulic fluid, generally water. The rotor, shared in common to the engines axis (change in this case) acts as an hydraulic pump and moves the water from the spin axis to the periphery, guiding it with its alades form, so the water particles are impulsed by the rotors movement, go out paralelly projected to the spin axis and against the superior par of the strators blades, that gathers and gives them back to the inferior part of the following rotors blades. A water obstacle in form of toroidal eddy is created so it is the viscous attachment between rotor and stator.

According to the amount of water inside the brake, the "hydraulic surface", faced between the blades, is greater or smaller and, therefore, this attachment between movable and fixed parts is proportional to the volume of water inside the brake. But as all the power is transformed into heat, it is necessary to make circulate this fluid to dissipate the generated heat.

Therefore the water moves with the own rotors impulse and each moment brake fillingis controlled with a valve in the unloading that strangles the exit in order to the resistant torque that is desired to apply. Like in electric brakes, the stator is balanced and the load cell motor torque, the regime of turn is moderate and consequently the power. The hydraulic brakes are less voluminous, have rotors with much smaller inertia and its maintenance is practically null.

But this type of brakes have the peculiarity to restrain based on the square of the speed, with which to low returns its turn torque is very small and is necessary to see the size of the bigger brakes to restrain small powers with low regime.