Liquid Resistance stater

Watch Full Operational Video Of Liquid Resistance Stater

Before going in detail about LRS we have to first know what a wound or slip ring induction Motor means and how it operates.

Wound/Slip Ring Motor Operation

Slip ring Induction motor is similar to that of conventional Induction motor but with high torque capacity.This torque I achieved by making some design changes in rotor. Its rotor has a resistance connected to its winding externally through a slip ring .this resistance can be anything from simple resistance box to liquid resistance box.

Wound Motor

What Isthe Need of Extra Resistance?

Squirrel cage induction motors draw 500% to over 1000% of full load current (FLC) during starting.While this is not a severe problem for small motors, it is for large (10’s of kW) motors Placing resistance in series with the rotor winding not only decreases start current, locked rotor current (LRC), but also increases the starting torque, locked rotor torque (LRT).

Wound Rotor With Slip Ring

Figure below shows that by increasing the rotor resistance from R0 to R1 to R2, the breakdown torque peak is shifted left to zero speed. Note that this torque peak is much higher than the starting torque available with no rotor resistance (R0) Slip is proportional to rotor resistance, and pullout torque is proportional to slip. Thus, high torque is produced while starting. This high torque is very much beneficial while dealing with heavy loads. The liquid rheostat allows for smooth stepless resistance change.

Torque Vs Slip Characteristics

A liquid rheostat consists of a tank of water mixed with a chemical (Sodium Carbonate) that alters the water’s conductivity in a specific way. It’s important to monitor the water on a regular basis and maintain the desired conductivity by adding water or chemicals as needed. Inserted in the tank are three conductive plates attached to the rotor winding of the induction motor. The motor has these winding brought out to three rings for carbon brush connections.

Liquid Resistance Stater Connection Diagram

The three plates are attached to a mechanism driven by a small reversible motor or manually driven wheel that can lower or raise the plates in the solution. When the plates are most of the way out, the resistance is maximized. When the plates are all they way in, (maximum surface area submerged) the resistance is minimized.