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To understand soft starters you need to understand the basics of strting induction motors.

An induction motor has a torque/speed characteristic which tells you the torque that the motor will produce at every speed from zero to synchronous speed with rated voltage (400V) applied. The load will also have a torque/speed characteristic. You need to understand these if you are going to play with soft starting. To start the motor must produce sufficient torque at standstill “Locked Rotor Torque” to overcome the load torque of the device it is to drive. Once started the torque produced must stay above that of the load in order to continue to accelerate. Eventually the motor settle at a point where the load torque and motor torque are equal which will usually be around rated speed.

At rated voltage (400V) the motor will produce “Locked Rotor Torque” at standstill, the value of which will be dependent on the design of the motor. The torque will be proportional to the square of the ratio of applied voltage to rated voltage so if you reduce the applied voltage to 200V, the locked rotor torque will fall to 25% of the rated value which may or may not be sufficient to overcome the load torque. If it is not sufficient the motor will simply sit there in a stalled state drawing approx 1.5 to 2.0 times its rated current without any cooling and either trip the protection circuit or burn out.

Electronic soft starters generally control the start by ramping the voltage up slowly and monitoring the starting current so this is not likely to happen. They will detect a constant stalled condition and shut down before you do damage. However, if you are playing with a reduced voltage starter such as star/delta starter, primary resistance or reactor, you need to get the motor and load torque/speed curves and calculate the minimum voltage that will produce the torque you need to ensure that the motor gets away from rest and also passes the minimum torque point often known as the “pull up torque” with the reduced voltage.

Don't forget also that the voltage drop along the cable is increased during starting due to the higher than rated current being drawn and thus the voltage at the motor terminals may be even smaller that the ratio of the applied voltage to the rated voltage.