Is Variable Frequency Drive Could Be Replaced By Motor Soft Starter?

Is Variable Frequency Drive Could Be Replaced By Motor Soft Starter?

I am meeting more and more customers who ask me a lot of questions and I am very honored to meet them and talk to them about motor start control. Some of the customers alway wonder if frequency drives can be replaced by soft starters. Today I will give you some suggestion:

1. The control principle of soft starter and frequency converter is different

The main circuit of the soft starter is connected in series between the power supply and the motor in three opposite parallel thyristor, through the internal digital circuit to control the thyristor in a complete sinusoidal waveform of alternating current turn-on time, if at the beginning of an AC cycle let the thyristor turn-on, then the soft starter output voltage is high, If the thyristor is switched on at a certain point in a cycle of alternating current, the voltage output of the soft starter is low. In this way, we make the voltage at the end of the motor slowly rise in the process of starting, and then control the starting current and torque of the motor, so that the motor can achieve the purpose of stable starting. It can be seen that the soft starter can only change the voltage level of the power supply, but not the frequency of the power supply.
The principle of the frequency converter is relatively complicated. Its function is to change the voltage of 380V/220V and the frequency of 50HZ power supply into an AC power conversion device with adjustable voltage and frequency. By adjusting the frequency and frequency of the power supply, the torque and speed of the AC motor can be adjusted. Its main circuit is a circuit composed of 6 field effect tubes, under the precise control of the control circuit, so that the six field effect tubes turn on, in unit time, the more the number of tube on, then the output voltage and frequency is higher, so the main circuit is under the control of the digital control circuit to achieve the frequency regulation of the output power supply and voltage regulation.

2. The uses of soft starter and inverter are different

The main problem of soft starter is to reduce the starting current of heavy load and reduce the impact on the power grid. The start-up of large equipment will produce a very large starting current, which will cause a large voltage drop. If the traditional step-down mode such as star triangle is used, it will not only cause a large current impact on the power grid, but also cause a large mechanical impact on the load. In this case, soft starter is often used to start, so as to realize the whole startup without impact and make the motor start relatively smooth. Thus less power capacity.

The use of frequency converter is mainly used in the place with speed regulation, it can control the speed of the three-phase motor, such as in the CNC machine tool spindle motor speed regulation, mechanical conveyor belt transmission control, large fans, heavy mechanical applications can be used in the frequency converter, in general, its function is far more practical than the soft starter.

3. The control function of the frequency converter 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 so as to reduce the impact of the motor on machinery and power grid. However, because it controls the voltage by chopper through controlling the conduction Angle, the output is incomplete sine wave, which leads to low starting torque, loud noise and high harmonics will pollute the power grid. Although the soft starter is limited to the setting of stream function, the setting of start time and other functions, but with the frequency converter, the functional parameters of the soft starter are relatively monotonous. In general, the function of soft starter is not as much as the frequency converter.

4. The price of soft starter is different from that of frequency converter

The two control devices in the same power condition, from the price of the inverter is higher than the soft starter.

In general, soft starter is mostly used for high-power equipment as starting equipment, and frequency converter is mostly used for speed regulation of various power. In most cases, the frequency converter cannot be replaced by a soft starter.