What Are Liquid Rheostats?

I would be interested to know the answer to this as well!!

John.

Liquid rheostats are variable resistors comprised of copper plates with an electrolyte between them. The resistance is varied by changing the area of the plates that are covered with electrolyte, either by immersing the plates further into the electrolyte solution, or by varying the level of the electrolyte. The electrolyte needs to be electrically conductive. One article I read suggested sodium carbonate ( "soda ash"), about six percent by volume, in distilled water for the electrolyte. As I recall in "motors lab" in college, we used salt water.

Liquid rheostats are used to control the speed and/or starting torque of wound rotor induction motors by varying the rotor resistance and hence the slip characteristic. Increasing the resistance decreases the slope of the torque speed curve and hence increases the slip that the motor runs at for a given load, lowering its speed. The cost for this is higher losses in the form of heating of the electrolyte in the rheostat (I remember watching it boil in college!)

Sir Thanx for the response.It is really useful. I needed this info w.r.t one of my projects where BFP's with LR control panels are to be erected and commissioned. regards Raghavendra

Liquid rheostats are a simple and smooth way of resistive control for load testing AC/DC generators,welders and other power equipment from low to thousands of amps.

Steel blades much like an air capacitor with an alkaline soda solution tank to avoid corrosion.

Induction motors are now best speed controlled by solid state frequency shifters.

Another application of this technology is the resistive tilt sensor where the angle of the base of the liquid container determines the depth of the electrode(s) in the electrolyte, thus changing the resistance proportional to angle.

Liquid Rheostats are used for torque and amperage control of wound rotor motors. They are connected to the rotor of the motor and control how much current is generated by inductance within the rotor of the motor.

They are used as starters for high inertia loads, like large fans. Typically in this application, once the load is up to speed the rheostat is "shorted" so the motor then performs like a squirrel cage motor.

In applications with rapidly changing loads liquid rheostats are kept "on line". While this is inefficient it helps to provide rapid response to radical changes in load for the purpose of "backing down" the break down torque to a lower rpm and for the purposes of controlling peak motor amperages. A typical example of this application is for automobile shredders in the 2000 to 10000hp range.

For very large motors they provide a very reliable method, though inefficient method to do the work. They do not produce the harmonics nor entail the complexity of more efficient modern silicon alternatives so for applications where near bullet proof reliability and sticks and stones simplicity count it is still the method of choice.

Power factor for a large wound rotor motor with a liquid rheostat may be as poor at 0.73 when fully loaded. This can be radically improved with static VAR compensation capacitors, and even more with switching VAR compensation capacitors.

Another method of accomodating for poor power factor is putting a large syncronous motor on the same line and varying the excitation, a method commonly used in steel mills.