A water rheostat using untreated water

In power machinery and other heavy equipment laboratories there is a universal need for a compact, rugged, yet flexible and continuously variable rheostat suitable for use on low voltage. The requirements for this rheostat are: dc motor starting, dc and ac single and 3-phase loading, wound-rotor induction motor speed control, and many other applications. These requirements were met by the design of a balanced 3-phase equivalent delta circuit water rheostat which also may be used in dc and single-phase circuits. Each 3-phase rheostat consists of a 7-electrode assembly held stationary within a single tank, and the resistance is changed by varying the water level with respect to the electrodes. The electrolyte used in the rheostat is untreated city tap water that has an average resistivity of 4,600 ohms per inch cube at 20° Centigrade. Corrosion tests on nine different materials that are economically feasible for rheostat use showed that stainless steel was the most resistant to electrolytic corrosion. The water level is changed by a positive displacement type pump driven by a 115-volt 3-phase induction motor. Potentials ranging from zero to 300 V can be satisfactorily handled by the rheostat. With 230 V 3-phase applied, a single rheostat unit will dissipate 75 kw; although the measurements of the tank are only 14 by 27 in x 36 in high. With 77 V 3-phase applied to the rheostat, the maximum power dissipated in a single rheostat is 10 kw.