LRS Starter

This post describes the working principle and advantages of the LRS starter used in slip-ring Induction motors.

Introduction:

In Squirrel Cage induction motor, its rotor is a squirrel cage type, there is no external resistance circuit the bars of the rotor are completely slotted. The rotor resistance is very low so that the current in the rotor is high which makes its starting torque poor.

In Slip ring induction motor, it has rotor of wound type, external resistance is connected with a rotor which makes rotor resistance high when starting, so that the current in the rotor is low which makes its starting torque high.

Motor during starting tends to draw 6 to 8 times of full load current depends upon the Motor size and Rating, such high current results in huge voltage dip which may results in stalling of motor instead of starting. During this motor start, the other loads which are connected to this bus may have an undervoltage they may also feel consequence. In order to prevent such high inrush current, a device which limits this high starting current is called a starter.

LRS Starter (Liquid Rheostat starter or Liquid resistance starter):

There are two ways a liquid resistance starter can be used

As a primary resistance starter where the resistance is connected in series with the stator of a standard squirrel cage motor.
As a secondary resistance starter where the resistance is connected in series with the rotor of a wound rotor motor

Working of LRS Starter:

The LRS is a rectangular tank having electrolytes in it and works on the principle of resistance cutting. The electrolyte solution of Sodium Carbonate ( 𝑁𝑎2𝐶𝑜3) acts as a special liquid resistor and is connected in series to the rotor circuit of the slip ring motor. Basically, the liquid resistor consists of metal electrodes in a tank of electrolytes as shown below figure. Usually, the solution used is a few percent of sodium carbonate by weight in clean water. There are two electrodes present fixed and movable electrodes. The fixed electrodes are mounted at the bottom of the tank and adjustable electrodes,

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