Zhejiang Shuyi Electric Co. Ltd

Soft starter is actually a motor control device with motor #softstart, soft stop, light load energy saving and various protection functions. It is mainly a three-phase parallel thyristor and its electronic control circuit connected in series between the power supply and the controlled motor. By controlling the conduction angle of the three-phase parallel thyristor, the input voltage of the controlled motor can be changed according to different requirements, and various functions can be realized.

The #inverter is mainly used 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 is started, the output only changes the voltage and does not change the frequency. The frequency converter manufacturer emphasizes that the frequency converter has all the functions of the soft starter, but its price is more expensive than that of the soft starter, and the structure is more complicated.

The soft starter is used in the place where the voltage reduction is required to start and stop, and the motor speed remains unchanged at this time; but the frequency converter is mainly used for speed regulation, and in the place of constant voltage, the speed is determined by the frequency. Inverter manufacturers believe that the biggest difference between the two is that the #inverter can arbitrarily set the running frequency, but the soft starter only plays the role of soft start and soft stop.

First of all, the purpose of the soft starter and the frequency converter is different. The main problem solved by the soft starter is to reduce the starting current of the large load and reduce the impact on the power grid. The startup of large equipment will generate a very large startup current, which will cause a large voltage drop. If the traditional star-delta and other step-down methods are used, it will not only cause a larger current impact to the power grid, but also cause a larger load to the load. Mechanical shock, in this case, a soft starter is often used to start, to achieve no shock throughout the start, so that the motor starts more smoothly, thereby reducing the capacity of the power supply. The frequency converter is mainly used in places with speed regulation. It can control the speed of the three-phase motor, such as the speed regulation of the spindle motor of CNC machine tools, the transmission control of mechanical conveyor belts, large fans, and heavy-duty machinery.

#Inverters can be used in applications, etc. In general, its functional practicability is far greater than that of soft starters. Secondly, the control function of the inverter of the soft starter is different. The main function of the soft starter is to adjust the starting voltage of the motor to realize the smooth start of the motor and reduce the impact of the motor on the machinery and power grid. However, since it controls the voltage by chopping the conduction angle, the output is an incomplete sine wave, which leads to low starting torque, high noise and high harmonics, which will pollute the power grid. Although the soft starter also has various functions such as the setting of the current limiting function and the setting of the starting time, the function parameters of the soft starter are relatively monotonous compared with the hundreds of function command parameters of the inverter. Generally speaking, the function of soft starter is not as much as that of frequency converter.

Let’s take an example. We have a pump with a very high head. Due to the large power of the pump, it is necessary to implement soft start when starting. And because there will be backwater impact when stopping the machine, which will damage the impeller of the pump, we call it water hammer phenomenon Q, and take corresponding measures. Considering the variation of water supply flow and pressure, we expect to be able to control the pump speed. If you’re just thinking about soft start and soft stop, we just have a soft starter that helps us do that. If we need to consider not only starting and stopping, but also regulating the operation of the pump, we need a frequency converter. It can be seen that the use of soft starter and frequency converter is not the same, the latter has a lot of powerful functions.


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