The difference between soft starter and frequency converter

The soft starter is actually a motor control device that has a soft start of the motor, soft stop, light load energy saving and various protection functions. It is mainly a three-phase parallel thyristor connected in series between the power supply and the controlled motor and its electronic control circuit. By controlling the conduction angle of the three opposite parallel thyristors, the input voltage of the controlled motor will be varied according to different requirements to achieve various functions.

Inverter, it is mainly used in places where speed regulation is required. Its output not only changes the voltage but also changes the frequency at the same time. The soft starter is actually a voltage regulator. When the motor starts, the output only changes the voltage and does not change the frequency. The inverter manufacturer emphasizes that the inverter has all the soft starter functions, but its price is more expensive than the soft starter, and the structure is more complicated.
The soft starter is used in the place where the step-down start and stop are required. At this time, the motor speed does not change; however, the inverter is mainly used for speed regulation and constant voltage, and the speed is determined by the frequency. The inverter manufacturer believes that the biggest difference between the two is that the inverter can set the operating frequency arbitrarily, but the soft starter only plays the role of soft start and soft stop.

Soft starter definition and function

The soft starter is connected in series between the power supply and the controlled motor, and the thyristor triggering conduction angle is controlled by the microcomputer to realize AC voltage regulation, so that the input voltage of the motor gradually rises from zero with a preset function relationship until the end of the start, and is given The full voltage of the motor is soft start. During the soft start process, the starting torque of the motor is gradually increased, and the rotational speed is gradually increased until the thyristor is fully turned on. The motor works on the mechanical characteristics of the rated voltage to achieve smooth start and reduce the starting current. . Avoid starting overcurrent trips. When the motor reaches the rated number of revolutions, the starting process ends and the rated voltage is supplied for the normal operation of the motor.

How the soft starter works

The soft starter (soft starter) is a novel motor control device that integrates motor soft start, soft stop, light load energy saving and various protection functions. It is called Soft Starter in foreign countries. The soft starter uses three opposite-parallel thyristors as regulators, which are connected between the power supply and the stator of the motor. Such a circuit is a three-phase fully controlled bridge rectifier circuit. When the motor is started by the soft starter, the output voltage of the thyristor is gradually increased, and the motor is gradually accelerated until the thyristor is fully turned on. The motor operates on the mechanical characteristic of the rated voltage to achieve a smooth start, reduce the starting current, and avoid starting the overcurrent trip. When the motor reaches the rated number of revolutions, the start-up process ends. The soft starter automatically replaces the completed thyristor with a bypass contactor to provide a rated voltage for the normal operation of the motor to reduce the heat loss of the thyristor and prolong the service life of the soft starter. To improve the efficiency of its work, and to avoid harmonic pollution in the power grid. The soft starter also provides a soft stop function. The soft stop is opposite to the soft start process. The voltage gradually decreases and the number of revolutions gradually decreases to zero, avoiding the torque shock caused by free parking.

The main function

Overload protection function: The soft starter introduces a current control loop, so it can track and detect the change of the motor current at any time. By increasing the setting of the overload current and the inverse time control mode, the overload protection function is realized, and when the motor is overloaded, the thyristor is turned off and an alarm signal is issued.
Phase loss protection function: When working, the soft starter detects the change of the three-phase line current at any time. Once the current breaks, the phase loss protection reaction can be made.

Overheat protection function: The temperature of the thyristor heat sink is detected by the internal thermal relay of the soft starter. Once the temperature of the heat sink exceeds the allowable value, the thyristor is automatically turned off and an alarm signal is issued.

Measuring loop parameter function: When the motor is working, the detector in the soft starter always monitors the running state of the motor, and sends the monitored parameters to the CPU for processing. The CPU analyzes, stores and displays the monitoring parameters. Therefore, the motor soft starter also has the function of measuring the loop parameters.

Other functions: Through the combination of electronic circuits, various other interlock protections can be implemented in the system.

Inverter definition and function

An inverter that converts alternating current with constant voltage and frequency into alternating current with variable voltage and frequency is called a frequency converter. Reduce the impact load caused by the motor starting, control the motor speed, lengthen the starting time, smooth the current, achieve the purpose of soft start, and improve the efficiency of the grid and motor. In fact, the frequency converter is mainly used in energy saving, and the output voltage, current and frequency are changed by adjustment. The general speed calculation motor uses a frequency converter.

How the frequency converter works

The inverter is a device that converts a commercial power supply (50 Hz or 60 Hz) into an AC power supply of various frequencies to realize variable speed operation of the motor. The control circuit completes the control of the main circuit, the rectifier circuit converts the alternating current into direct current, the DC intermediate circuit smoothes the output of the rectifier circuit, and the inverter circuit reverses the direct current into alternating current (and the core control circuit realizes: Straight-crossing process). The frequency conversion technology was born in response to the need for AC motor stepless speed regulation.