soft starting of a induction motor


Thread Starter

Madhukar Parikh

I understand that there is a method of reducing the applied voltage to a 3 phase induction motor by passing the current through a chemical solution whose resistance reduces as the motor speeds up. this allows lower voltage at starting and provides a soft start to th motor. Can any one provide me with more information about this specially about reliability, harmonics, availibility etc.
It a great deal less hassle to just use a solid state soft starter. These are reliable and made by hundreds of companies worldwide.
The above method is usually used only for rotor resistance starting. The starter has three tanks for each phase filled with coductive water (added with baking soda) and three electrods that gradually dip into the water.
The moving electrodes are shoted to each other The rotor wires are connected to the three individually insulated tanks.

During starting the electrodes are positioned at the minimum contact level and slowly advanced by a mechanism towards the maximum depth. At the end of the electrode travel the three electrodes plug into three sets of conntacts on the tanks that short the tanks and electrodes giving the motor the full speed. The amount of soda into the water determines the minimum starting speed.

This method is not used on the primary side of a motor as it involves water,mains electricity and risk of electric shock as the tanks are continously live.

Best regards,

Shishir Gupta

i understand you are speaking about HT induction motor. To reduce the starting current you have to have GRS or LRS.

For more info you can contact me at [email protected]

Shishir Gupta
Responding to Madhukar Parikh's query:

Liquid rheostats as variable resistors were often used in the 'g'olden days for motor-starting and even for speed-control. Today they are mostly used to test generators.

Essentially they consist of a set of movable and fixed electrodes immersed in an electrolyte solution, the simplest of which is salt water. If used in the motor supply circuit three isolated "tanks", one per phase, must be used. If used in the rotor circuit of a wound-rotor motor then there are three movable electrodes immersed into one tank.

Resistance change is a accomplished by varying electrode separation, or immersion depth. An alternative method is to use isolated metal plates
as the movable electrodes. The surface area is shaped to provide increasing contact as it is lowered into the electrolyte solution.

If additional info is required, contact me at "[email protected]", mailto:[email protected] .


Phil Corso, PE
(Boca Raton, FL)
Responding to J. Raefield's Sat, Dec 21, 11:38am, reply:

Liquid rheostats for reduced-voltage starting have also been used in the primary supply to ordinary induction motors. My Sat, Nov 20, 4:32pm,
response on subject can be found in List Archive.

Phil Corso, PE
(Boca Raton, FL)
[[email protected]]