What is a Liquid Resistance Starter (LRS)?

SFAKP LRS - High Speed Servo Drives that are being tested for shredding application in MKS factory

As the name suggests, these starters consist of a tank filled with electrolyte of the appropriate conductivity to provide the required resistance.


Liquid Resistance Starters also known as Electrolytic Starters has two sets of 3 phase submerged electrodes. One set of electrodes are movable and the other set is fixed. Movable electrodes can travel vertically towards or away from the electrodes that are fixed at the bottom of the tank.

The value of the resistance varies with the distance between the moving electrodes and the fixed electrodes. The resistance value is maximum (R max) when the moving electrodes are at the top and minimum when they are at the near bottom of the tank (R min). Resistance value also depends on the electrolyte concentration and temperature.

Position of electrodes in the LRS tank for maximum resistance.

Figure shows position of Top Moving Electrode (R max), moving down towards Bottom Fixed Electrode (R min)

Wound rotor motor rotor voltage also known as secondary voltage is highest when the motor is at a standstill (i.e. slip is at highest value, slip=1). The moving electrodes of the electrolytic starter are kept at the top for the highest resistance at beginning of the starting process to limit the rotor current.


The motor will start producing torque once current starts flowing through the rotor and acceleration begins. As the motor increase the speed, the slip reduces and so does the rotor voltage.  The liquid resistance starter, also known as liquid rheostat (LRH), is programmed to drive the electrode downward as the motor accelerates to reduce the resistance approximately proportional to the reduction of secondary voltage. This will enable the motor to maintain the rotor current relatively constant during the acceleration of the motor. This in turn keeps the motor starting torque constant during the acceleration.

Torque vs Speed Characteristics of Wound Round Induction Motors starting with LRS

Torque vs Speed Characteristics of WRIM starting with LRS

Typical Single Line Diagram for Liquid Starter

Electrical Diagram of a Typical LRS

Electrolyte of the LRS is prepared by mixing a specific type of salt with potable water in appropriate ratios to achieve the required conductance. The concentration of salt (also known as soda ash) may vary and will depend on the conductivity level for any given application.

For a short insight regarding LRS maintenance procedures, you can find out more in our recent project : Preventative Maintenance of Liquid Resistance Starter.