Application of a model for optimizing steady state and transient control of hydraulic dynamometers

Water brakes are still used in engine test cells, due to their compactness, low inertia and high reliability. Compared to dynamic brakes (i.e., electric motor-generators), water brakes do not require bulky and expensive infrastructures (high-power connection to the grid, inverter cabinets, etc.), while in comparison with eddy current brakes they are less expensive and require lower maintenance. However, water brakes are often considered suited for steady-state tests, due to poor transient capability and steady state speed accuracy. The brake load torque is usually controlled by means of a control valve, that can be positioned both on the water inlet pipe (in-flow control) or on the outlet side (out-flow control): in fact, the braking torque is related to the impeller speed and the amount of water trapped inside the machine. Flow control valves, however, are prone to hysteresis and their dynamic performance is usually not satisfactory. Furthermore, they are expensive, so that they may represent a significant percentage of the total cost of the system. The paper shows how the brake can be modeled, allowing to assess the effect of braking torque regulating systems on transient performance. The approach allows testing in a simulation environment unusual braking torque control technique: at first, the model has been used to assess the torque regulation capabilities guaranteed by the use of a variable speed feeding pump. Results shows that transient performance greatly depends on the pump speed variation capabilities. The model proved to be an efficient tool to evaluate different control solutions, either based on inlet or outlet flow control, and it will be used in the future to design a new control system for water brakes.