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Why do existing inverters need to use soft starters?

Dec 23, 2020

Why do existing inverters need to use soft starters?

If you have a car, you still have to ride a motorcycle, an electric car or a bicycle; if you have an air conditioner, you still have to blow a fan, and occasionally find a fan to fan yourself twice; you can eat wagyu every day at home, and many people also eat vegetables and vegetables. . There are countless examples of this kind, and one product may not be able to meet all application requirements. Although the inverter is powerful, the soft starter also has its advantages. Even in many occasions, the traditional star-delta start is still used.

Use everything

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 adjustment, so that the motor can work in different speed ranges to meet the working requirements of the equipment. The factory buys the frequency converter, mainly for this.

Because the inverter needs to adjust the speed, this process requires frequency and voltage adjustment, so it needs to use expensive power devices such as IGBTs to achieve it, and the control of the chopper inverter output is more complicated, and the requirements for components are very high, and it also involves Complex calculation algorithms, etc. These factors determine that the price of the inverter will definitely be much more expensive than other products.

In addition to the speed regulation function, the frequency converter can of course also realize the so-called 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, and the focus of the process is reduced voltage start , Because the current will be much larger than the normal current when the motor starts. If a very high voltage is applied all at once, the current will be too large, which will not only impact the power grid and affect the transformer, but also may have adverse effects on motor bearings and other components. Therefore, when the voltage is lowered when starting, and it can be increased slowly, these problems can be avoided.

The soft starter, even the star-delta start realized by the contactor, is only designed with the function of step-down start. They do not have the so-called speed control function, so the structure is simple and the requirements for components are not high. The soft starter is just one Thyristor voltage regulators have no frequency modulation function, but sometimes simplicity is also a force.

Many devices only require a small motor starting current, and after the start is completed, it switches to the power frequency. In this case, the use of a frequency converter will cause waste. It is completely unnecessary. Use a cheap soft starter, even if it is broken. It is easy to repair and the overall maintenance cost is low. Apart from the local tyrants, who doesn't care about the "money". For example, a 160KW soft starter is about 3,000 yuan, while the same 160KW inverter costs about 13,000, which is four times the price. Of course, the price of a low-power inverter and a soft starter is not much different. The fundamental reason why low-power soft starters are rarely seen.

In some cases, it is not suitable to use the inverter

The frequency converter does not output a true sine wave, but a series of square waves output by PWM chopping and inverter, which has a relatively large interference to the power grid and equipment. The use of a soft starter with 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 only to start, and there are many expensive and delicate instruments and meters on site, the use of frequency converters may affect the operation of these meters, so it is not thankful to spend money.

The inverter is generally not suitable for switching on the output contactor and other devices. The main reason is that the inverter control is too complicated. When the motor is switched, the phase of the motor idling and the phase of the inverter output may be very different. A relatively large impact will affect the life of the IGBT of the inverter.

The design of the soft starter itself is to consider the switching, pay attention to these details, use some phase-locked loop technology to track, etc., but it does not have these functions, because the thyristor is cheap and reliable, and can be controlled with high voltage resistance. Silicon is used to make up, and IGBTs are too expensive, and inverters generally do not consider switching to industrial frequency applications. Therefore, soft starters in these places may be more ideal than inverters.