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VFD or Soft starter, which one should you choose?

Views: 14     Author: Site Editor     Publish Time: 2022-06-02      Origin: Site

Introduction to Soft starters and Variable frequency drives

Soft Starter

It is connected in series between the power supply and the controlled motor, and the internal thyristor is controlled by a microcomputer to trigger the conduction angle to achieve AC voltage regulation, so that the motor input voltage gradually increases from zero with a preset function relationship, until the end of the start, the motor is full voltage.

During the starting process, the starting torque of the motor gradually increases, and the speed also increases accordingly. Using this starting method can ensure that the motor works under the rated voltage, avoids the problem of tripping due to excessive starting current, and realizes a smooth start. When the motor reaches the rated number of revolutions, the start-up process ends, and the rated voltage is provided for the normal operation of the motor. It can be seen from this that when the motor starts, the soft starter is essentially a voltage regulator, which can protect the motor from overcurrent damage.


  • The frequency of the power supply cannot be adjusted, and zero-voltage start-up and zero-impact start-up cannot be achieved;

  • The speed cannot be adjusted during startup;

  • The soft starter exits the system after starting the motor and loses its protection function.

Frequency Inverter:

The main function of the inverter is to change the frequency. It mainly converts the alternating current with constant voltage and frequency into alternating current with variable voltage and frequency.

The use of frequency inverter can reduce the impact load caused by the motor starting, control the motor speed, prolong the starting time, make the current gradually smooth, achieve the purpose of soft start, and also improve the efficiency of the power grid and the motor. The frequency converter is mainly suitable for energy saving and environmental protection. It can change the starting voltage and frequency of the motor through speed regulation. At the same time, the frequency inverter also has a certain soft start function. It mainly uses frequency regulation to control electrical equipment, such as variable frequency pumps used in industrial production.


  • High cost;

  • At present, the PWM (pulse width modulation) control method used by most frequency converters makes it easy to generate high-order harmonic currents on the power supply side during operation and cause voltage waveform distortion. After the power system is polluted by harmonics, it will affect the operating efficiency of the system. , damage the equipment and even endanger the safe operation of the power system;

  • The overload capacity of the inverter itself is poor.

The difference between soft starter and VFD

①The starting principles are different. The soft starter mainly uses solid-state relays to realize the voltage and speed regulation of the motor through zero-crossing triggering, while the variable frequency starter uses variable frequency speed regulation, and the motor can be arbitrarily rotated forward and reversed during operation. Speed regulation and frequency conversion operation.

②The soft starter is mainly started by reducing the voltage, which will affect the torque when starting, and there is a certain inrush current.

The frequency converter changes the voltage and frequency at the same time, and adjusts the speed on the basis of not reducing the torque, which can realize the start with load without inrush current.

③The soft starter only works during the start-up period, and only regulates the voltage but not the frequency. The frequency converter works in the running and speed regulation stage of the motor, and the frequency regulation also regulates the voltage.

④The basic principle of soft start is to change the resistance connected in series in the circuit to make the current from small to large. This method is suitable for starting high-power motors to reduce the fluctuations caused by the direct start of high-power motors to the power grid, or cause some Unimportant load unloading hassle.

The frequency converter adjusts the speed of the motor when it starts by changing the output frequency of the power supply. In general, the frequency converter is mainly suitable for variable speed equipment.

⑤ The price of the frequency converter is much higher than that of the soft starter. Soft starters have low technical content and stable performance, so the price is much lower than that of frequency converters. Therefore, in the case of no speed regulation, most companies will still choose a soft starter.

To sum up, both the soft starter and the variable frequency drive are the control equipment of the AC motor, and the VFD is basically a soft starter with speed control. In practical applications, if you want to change the motor speed, you should use a frequency converter. If you only want to limit the starting current, you can choose a soft starter.

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