What is the major difference between soft starter and liquid starter?

A soft starter is an electronic device The liquid  starter has movable plates

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A soft starter is an electronic device used to help protect motors. Not all motors are initially equipped with soft starters, but they have become common, especially with delicate motors that can be easily damaged by sudden influxes of power. Soft starters are composed of a series of rectifiers that work together to manage voltage

  The liquid  starter has movable plates rather than relieing on heating to change the resistance.  When starting the motor the operator monitors the stator current and manually controls the starter, keeping the stator current just below the motor protection pick up point.

Generaly soft starter is an electrical device make a Smooth starting process without any adverse effect on mechanical system, motor and power systems

Electronic Soft Starter It is used for the soft start and protection of the low-voltage squirrel cage motors with capacity ranging from7.5kw to500kw.

Liquid Resistor Starter which is widely used in protecting high voltage squirrel-cage motor or any other electrical products installed in an enterprise and for systems or motors above500KW

Compared with primary resistance starters, soft starters are more flexible and reliable.Primary resistance starters offer limited performance because:

• start torque cannot be fine-tuned to match motor and load characteristics.
• current and torque transients occur at each voltage step.
• they are large and expensive.
• liquid resistance versions require frequent maintenance.
• start performance changes as the resistance heats up. So multiple or restart situation are not well controlled. 
• unable to accommodate changing load conditions. e.g. loaded or unloaded starts.
• they cannot provide soft stop.

http://www.indiamart.com/worldwideelectricals/soft-starter.html

soft starter means you will use a programable switch that will operate according to software being dumped on it. you can control it by single click.

soft starter is used in VSD and VFD 

 The liquid  starter has movable plates changing resistance

ALL the answer are right but U know what here is a big difference, (correct me if i am wrong). Liquid starter are used for slip ring motors ranging to5MW while soft starter are used even in place of any star delta application.