The difference between soft start and variable frequency start

Soft starter is a novel motor control device that integrates motor soft start, soft stop, load energy saving and various protection functions. It is called Soft Starter overseas. Its main components are the three-way series brake tube connected in series between the power supply and the motor under test and the electronic control circuit.

Soft starters and frequency converters are two completely different products. Frequency converters are used in areas where speed change is required, and their output not only changes the operating voltage but also changes the frequency; soft starters are actually AC voltage regulators, and when used to start motors, their output only changes the operating voltage without changing the frequency. Frequency converters have all the functions of soft starters, but their price is much more expensive than soft starters, and their structure is much more complicated.

The control principles of soft starters and frequency converters are different

A soft starter is actually an excessive control device, an industrial control product between a star-delta starter and a frequency converter. It has more functions than a star-delta starter but fewer functions than a frequency converter.

From the main circuit of the soft starter, it is a three-way anti-parallel thyristor connected in series between the power supply and the controlled motor, and then the internal digital circuit is used to control the moment when the thyristor is turned on within a complete sinusoidal waveform of the AC power. If the thyristor is turned on at the beginning of an AC cycle, the output voltage of the soft starter will be high. If the thyristor is turned on at a certain position in an AC cycle, the output voltage of the soft starter will be low.

It is in this way that we slowly increase the voltage at the motor end during the starting process, thereby controlling the starting current and torque of the motor so that the motor can start smoothly. It can be seen that the soft starter can only change the power supply voltage but cannot change the power supply frequency.

The principle of the frequency converter is relatively complicated. Its function is to convert a power supply with a voltage of 380V/220V and a frequency of 50HZ into an AC power conversion device with adjustable voltage and frequency. The torque and speed of the AC motor are adjusted by adjusting the power frequency and voltage. We can see from the main circuit that its main circuit is composed of 6 field effect transistors. Under the precise control of the control circuit, the six field effect transistors are turned on in turn. In unit time, the more times the tube is turned on, the higher the output voltage and frequency. Therefore, this main circuit realizes frequency adjustment and voltage adjustment of the output power supply under the control of the digital control circuit.

Soft start and variable frequency start are two common motor starting methods. Their main differences are:

1. Different control methods:

Soft start gradually increases the starting current and torque of the motor by adjusting the voltage, so that the motor gradually accelerates, thereby reducing the impact of high current starting on the power grid. Variable frequency starting starts the motor by changing the power supply voltage and frequency of the motor.

2. Different effects:

Soft start is mainly to gradually increase the motor starting voltage so that the current and impact force generated by the motor during the starting process will not be too large, which can protect the motor and other equipment to a certain extent, but it is not suitable for variable load starting or high-speed starting. Frequency conversion starting can flexibly adjust the motor speed according to the load conditions, thereby achieving the advantages of starting at a low speed and reducing load impact.

3. Different scope of application:

Soft start is suitable for starting various types of equipment. Its main features are simple installation, low cost, convenient maintenance, and can greatly extend the service life of the motor. Frequency conversion starting is convenient for precise control of motor speed and load, and is suitable for inverter drives that need to adjust motor speed.

In summary, soft start and variable frequency start are both a starting method that is a modification of the traditional motor direct starting method. Both have their own advantages and disadvantages, and the appropriate starting method should be selected according to the specific situation.