Soft starters and variable frequency drives (VFDs) have significant differences in function, application scenarios, working principles, and output characteristics, as detailed below:
Function
The main function of a soft starter is to achieve soft starting and soft stopping of electric motors. It controls the starting and stopping process of the motor by gradually changing the output voltage, reducing the impact on the power grid and equipment.
A VFD not only achieves soft starting and soft stopping of electric motors but also adjusts the motor speed by changing the frequency of the output power supply, allowing the motor to operate efficiently under different working conditions.
Application Scenarios
Soft starters are suitable for applications where the starting and stopping process requires a certain degree of control to reduce impact, but precise motor speed control is not required, such as the starting of fans and pumps.
VFDs are widely used in applications requiring precise speed control, such as industrial automation production lines, elevators, and central air conditioning systems, to meet the requirements of different processes for motor speed.
Working Principle
A soft starter controls the conduction angle of the thyristor to change the magnitude of the output voltage, allowing the motor to start at a lower voltage, which gradually increases to the rated value over time.
A VFD first converts the industrial frequency AC power into DC power through a rectifier, and then converts the DC power into adjustable frequency and voltage AC power through an inverter, supplying it to the motor, thereby achieving control of the motor speed and torque.
Output Characteristics
Soft starters provide stepless voltage reduction starting. When the power supply voltage of an asynchronous motor is changed, its critical slip remains unchanged, but the critical torque decreases significantly. Therefore, the starting torque will be greatly reduced during low-voltage starting. Although soft starters can reduce the starting current, they have difficulty controlling the duration of the motor starting time.