Why LRS is used to start HT Motor ?

Why LRS is used to start HT Motor?

The full form of LRS is liquid resistance starter. Before LRS the first thing needs to be clear that the impact of resistance in rotor circuit of motor. There are some benefits which are given below –

1.???? Starting Torque Control – By inserting resistors into the rotor circuit during startup, the initial high resistance limits the starting current drawn by the motor. This helps in controlling the starting torque, preventing sudden mechanical stresses, and reducing the likelihood of damage to the motor and connected equipment.

2.???? Speed Regulations – The rotor resistance can be adjusted to control the speed of the motor. By varying the resistance in the rotor circuit, the slip of the motor can be altered, affecting its speed. This allows for precise speed regulation, which is useful in applications where speed control is critical.

3.???? Improving efficiency – In some cases, inserting resistance into the rotor circuit during startup can improve the overall efficiency of the motor. By limiting the starting current and controlling the torque the motor can start smoothy without drawing excessive power from the supply.

4.???? Reducing starting current surge – The use of rotor resistance helps in limiting the inrush current during motor startup. This is particularly beneficial in applications where large motors are started frequently or where there are limitations on the available power supply capacity.

We can see characteristics between starting torque and rotor resistance.

Now we try to understand how this characteristic will work.

R1, R2 and R3 are rotor resistance with different values. At R1 we get maximum starting torque.

FOR EASY UNDERSTANDING WE TAKE AN EXAMPLE-

First, when water is added to the LRS tank, it is raw water, and no type of soda is added to it. Suppose R3 is the resistance of the water. Now, if we look at the diagram, it shows that we cannot achieve maximum starting torque at R3. For this, we need to add soda to the water until the resistance value reaches R1. As soon as the R1 value is reached, we achieve maximum starting torque. How ever keep in mind that if we add too much soda, the resistance value will decrease further and reach R2. In that case, we will also not able to achieve maximum starting torque. So, we need to set the resistance close to R1.??

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R3 > R1 >R2

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We should keep the pH value of LRS water around 10 so that we can achieve a value near R1. Keep in mind that if add too much soda, the resistance value will reach R2, which is very low, and if we start motor at this stage, it will draw very high starting current.

There is not any source to check this resistance value. We can just take pH value and if pH value reaches 10, it is good to start motor. Keep in mind that any HT motor which starts on LRS, starting current should not go beyond 1.5 times FLC.

Suppose if motor FLC is 500 A, starting current (with LRS starter) should be below 750 A. If you start the motor continue at high current, flashing can occur in slip ring and it led to motor failure.

If we go theoretically maximum starting torque can be achieved if

R=XL ,