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The main purpose of the design of the frequency converter is to adjust the speed of the AC asynchronous motor, which is the so-called speed regulation, so that the motor can work at different speeds to meet the working requirements of the equipment.

Because the frequency converter needs to adjust the speed, this process requires both frequency modulation and voltage regulation, so it needs to be realized by using power devices such as IGBT, and the control of chopper inverter output is more complicated. The requirements for components are very high, and complex calculations are also involved. algorithm etc. In addition to the speed regulation function, the frequency converter can of course also realize the soft start function. To put it bluntly, it is to allow the motor to gradually increase the speed according to a certain curve. This time period can be set artificially. When the motor starts, the current will be much larger than the normal current. If a very high voltage is applied all of a sudden, the current will not only impact the power grid and affect the transformer, but also may have adverse effects on the motor bearings and other components, so when starting Let the voltage be lower and be able to rise slowly, and these problems can be avoided.

The soft starter is equivalent to a thyristor voltage regulator without frequency modulation function, but sometimes simplicity is also a kind of strength. Many devices only need a small starting current of the motor, and after the start-up is completed, they are switched to power frequency. In this case, the use of frequency converters will cause waste. For example, the price of a 160KW soft starter is about 4,000 yuan, and the price of the same 160KW inverter is about 18,000, which is 4-5 times the price. Of course, the price of a low-power inverter and a soft starter is not much different.

But not all cases are suitable for the use of frequency converters. Because the inverter does not output a real sine wave, but a series of square waves output through PWM wave inversion, which has relatively large interference to the power grid and equipment. However, the soft starter using thyristor voltage regulation will not cause interference when starting, and the motor will work at power frequency during operation, and will not produce any adverse effects. If the motor is just for starting, and there are many expensive and delicate instruments on site, the use of frequency converters may affect the operation of these instruments.

Inverters are generally not suitable for switching devices such as contactors at the output end. The main reason is that the control of the inverter is too complicated. A relatively large impact will affect the life of the IGBT of the inverter. The design of the soft starter itself is to take switching into consideration, and it will pay attention to these details and use some phase-locked loop technology to track, etc., but it does not have these functions. Because thyristors are cheap and reliable, they can be compensated by thyristors with high withstand voltage, but IGBTs are too expensive, and inverters generally do not consider switching to power frequency applications, so soft starters in these places are more reliable than inverters. Suitable.