dynobill's pages -->Everything you ever wanted to know about Engine Dynos
What is the difference between a water brake and eddy current absorber? Simply stated, a water brake absorber (also called hydraulic absorber) consists of a rotor and stator in a water-filled cavity. Both rotor and stator have pockets (vanes). As the rotor moves with engine or wheel speed the water will be moved in the direction of rotation and will in turn try to move the stator. The stator is held in position by a strain gauge (torque link) which will measure the force acting on the stator. With proper calibration, this force is a measure for the torque produced by the engine when it moves the rotor in the water. Dynamometer load is varied by varying water level or water pressure in the absorber. The heat generated by the power absorption is rejected in the water.
An Eddy Current (E/C or EC) absorber consists of a rotor made of a high-permeable magnetic material and enclosed by a stator. The cavity in which the rotor turns is usually air-filled (dry gap) although water-filled (wet-gap) models are available. The stator has a series of coils, which can be energized to produce an electric field around the rotor. As the rotor, driven by the engine or wheels, turns in this electric field, small rotating electro-magnetic fields (called eddy currents) are generated, which oppose the rotation of the rotor and try to move the stator in the direction of rotation of the rotor. The stator is also held in position by a strain gauge (torque link) which will measure this force acting on the stator. With proper calibration, this force is a measure for the torque produced by the engine when it moves the rotor in the electromagnetic fields. Dynamometer load is varied by varying the electric current through the coils. The heat generated by the power absorption needs to be evacuated by separate water cooling jackets or by forced air cooling. Water brake absorbers are very efficient and achieve high power-to-size ratios. Because of their compact construction they can easily achieve high speeds. Hydraulic absorbers are typically the most cost-efficient choice. A disadvantage is the gradual build-up of load with increasing speed of rotation, which results in poor loading capabilities at very low speeds. Control of a water brake absorber is also more difficult because water is a less controllable medium than electric current. Eddy current absorbers are relatively inefficient and are bulkier than a water brake absorber for the same power level. They are also more complex and thus more expensive. Because of the greater rotor area and thus size required for high torque levels, rotor tip speed becomes a limiting factor in the design and high-speed versions of eddy current absorbers usually have multiple smaller rotors. This has an impact on cost as well. The eddy current absorber is however efficient at low rpm and is also easier to control accurately than a water brake absorber.
Text courtesy of SuperFlo