Definition and function of soft starter
The soft starter is connected in series between the power supply and the controlled motor. It controls the thyristor trigger conduction angle inside it by microcomputer to realize AC voltage regulation, so that the input voltage of the motor gradually rises from zero in a preset functional relationship until the end of starting, and the full voltage of the motor is given, that is, the soft start. During the soft start process, the starting torque of the motor gradually increases, and the speed also gradually increases, until the thyristor is fully conductive, The motor works on the mechanical characteristics of rated voltage to realize smooth starting and reduce starting current. Avoid starting overcurrent tripping. When the motor reaches the rated speed, the starting process is completed, and the rated voltage is provided for the normal operation of the motor.
Soft starter is a novel motor control device that integrates soft start, soft stop, soft load energy saving and multiple protection functions. The soft starter uses three-phase anti-parallel thyristor as the voltage regulator, which is connected between the power supply and the motor stator. This kind of appliance is like three-phase full-controlled bridge rectifier circuit. When using the soft starter to start the motor, the output voltage of the thyristor gradually increases, and the motor gradually accelerates until the thyristor is fully connected. The motor works on the mechanical characteristics of the rated voltage to achieve smooth starting, reduce the starting current, and avoid starting overcurrent tripping. When the motor reaches the rated speed and the starting process is over, the soft starter automatically replaces the completed thyristor with the bypass contactor to provide the rated voltage for the normal operation of the motor, so as to reduce the heat loss of the thyristor, extend the service life of the soft starter, improve its working efficiency, and avoid harmonic pollution of the power grid. The soft starter also provides the soft stop function. The soft stop is opposite to the soft start process. The voltage gradually decreases and the speed gradually decreases to 0 to avoid torque impact caused by free stop.

Main functions 
Overload protection function: the soft starter introduces a current control loop, so it can track and detect the change of motor current at any time. By increasing the setting of overload current and inverse time limit control mode, the overload protection function is realized. When the motor is overloaded, the thyristor is turned off and an alarm signal is sent.
Phase loss protection function: during operation, the soft starter detects the change of three-phase line current at any time, and can make a phase loss protection response once the current is cut off.
Overheat protection function: detect the temperature of the thyristor radiator through the internal thermal relay of the soft starter, and automatically turn off the thyristor once the radiator temperature exceeds the allowable value, and send an alarm signal.
Function of measuring circuit parameters: When the motor is working, the detector in the soft starter monitors the running state of the motor and sends the detected parameters to the CPU for processing. The CPU will analyze, store and display the monitored parameters. Therefore, the motor soft starter also has the function of measuring circuit parameters.
Other functions: through the combination of electronic circuits, various other interlocking protection can also be realized in the system.

Definition and function of frequency converter
The converter that converts AC power with fixed voltage and frequency into AC power with variable voltage and frequency is called frequency converter. Reduce the impact unloading caused by the motor starting, control the motor speed, lengthen the starting time, flatten the current, achieve the purpose of soft starting, and also improve the efficiency of the power grid and motor. In fact, the frequency converter is mainly used in energy saving. It can change the output voltage, current and frequency through adjustment. Generally, the motor for speed adjustment and calculation uses frequency converter.

Working principle of frequency converter
The frequency converter is the equipment that converts the power frequency power supply (50 hz or 60 hz) into various frequency AC power supply to realize the variable speed operation of the motor. This control circuit controls the main circuit. The rectifier circuit converts AC into DC, the DC intermediate circuit smooth filters the output of the rectifier circuit, and the inverter circuit converts DC into AC again (and the core control circuit realizes the AC-DC-AC process). The frequency conversion technology was born in response to the need of stepless speed regulation of AC motors.