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A liquid rheostat or water rheostat or salt water rheostat is a type of variable resistor. This may be used as a dummy load or as a starting resistor for large slip ring motors.
In the simplest form it consists of a tank containing brine or other electrolyte solution, in which are submerged to create an electrical load. The electrodes may be raised or lowered into the liquid to respectively increase or decrease the electrical resistance of the load. To stabilize the load, the mixture must not be allowed to boil.
Modern designs use stainless steel electrodes, and sodium carbonate, or other salts, and do not use the container as one electrode. In some designs the electrodes are fixed and the liquid is raised and lowered by an external cylinder or pump. Motor start systems used for frequent and rapid starts and re-starts, thus a high heat load to the rheostats, may include water circulation to external heat exchangers. In such cases anti-freeze and anti-corrosion additives must be carefully chosen to not change the resistance or support the growth of algae or bacteria.
The salt water rheostat operates at unity power factor and presents a resistance with negligible series inductance compared to a wire wound equivalent, and was widely used by generator assemblers, until 20 years ago, as a matter of course. They are still sometimes constructed on-site for the commissioning of large diesel generators in remote places, where discarded oil drums and scaffold tubes may form an improvised tank and electrodes.
Operation was very simple, as adding more salt, more water or varying the height of the centre electrode would vary the load. The load proved to be quite stable, varying only slightly as the water heated up, which never came to boil. Power dissipation was about 1 megawatt, at a potential of about 700 and current of about 1,500 .
Modern designs use stainless steel electrodes, and sodium carbonate, or other salts, and do not use the container as one electrode.
Systems with frequent starting may include water circulation to external heat exchangers. In such cases anti-freeze and anti-corrosion additives must be carefully chosen to not change the resistance or support the growth of algae or bacteria.
Disadvantages include:
Liquid rheostats were sometimes used in large (thousands of kilowatts/horsepower) wound rotor motor drives, to control the rotor circuit resistance and so the speed of the motor. Electrode position could be adjusted with a small electrically operated winch or a pneumatic cylinder. A cooling pump and heat exchanger were provided to allow slip energy to be dissipated into process water or other water system.Igor Karassik et al, (ed), Pump Handbook Fourth Edition, Mc Graw Hill 2008, pages 9-113 -9-115
Massive rheostats were once used for dimming theatrical lighting, but solid-state components have taken their place in most high-wattage applications.
Modern motor starters are totally enclosed and the electrode movement is servo motor controlled. Typically a 1 tonne tank will start a 1 megawatt slip ring type motor, but there is considerable variation in start time depending on application.
They are no more dangerous than , which work on the same principle, but with plain water, or electrical immersion heaters, provided the correct precautions are used. This requires connecting the container to both ground and neutral and breaking all poles with a linked over-current circuit breaker. If in the open, are required.
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