Abstract: With the development of power electronics technology, soft-start technology has developed rapidly, and various new soft-start terms have emerged. In recent years, the concept of variable frequency soft starter has been proposed.
Because its name is very similar to that of a frequency converter, and frequency converters also have a soft-start function, those unfamiliar with the technology often mistakenly believe that a variable frequency soft starter is the same as a frequency converter, confusing the two different products. Here, I will compare these two products to the best of my knowledge so that those who need to distinguish between them can do so.
I. Differences in working principles
1. Working principle of variable frequency soft starter
The full name of a variable frequency soft starter is a graded AC-AC variable frequency soft starter, also known as a discrete variable frequency soft starter. Variable frequency soft starters utilize a three-phase AC voltage regulating circuit. By controlling the triggering sequence of the three-phase thyristors, the thyristors are turned on for one or more half-wave cycles, chopping the grid voltage. This removes a portion of the upper half-wave and a portion of the lower half-wave of the power frequency within several power frequency cycles, achieving the purpose of changing the power supply frequency at the motor end.
Since these frequencies are obtained by dividing the power frequency AC, they are a series of sub-frequencys of the AC power supply. It is impossible to obtain continuous frequencies, so it is called graded frequency conversion or discrete frequency conversion. Because frequency division is achieved by chopping, the resulting waveform is discontinuous.
Considering the phase sequence and symmetry of the three phases after frequency conversion, only the 3n+1 frequency division can meet the requirements among these sub-frequencys. Therefore, the frequency change of the variable frequency soft starter is f/13→f/7→f/4→f/3→f/2→f (50Hz). The 3-frequency division and 2-frequency division are added to avoid excessive impact caused by directly changing from the 4-frequency division to the mains frequency.
As can be seen, the voltage changes when the frequency changes. With proper control, V/f can be proportionally adjusted, thereby increasing the low-frequency torque of the motor and allowing it to start smoothly with high starting torque. Because the frequency is discontinuous, the voltage change is also discontinuous.
After the motor starts, the short-circuit switch is closed, shorting the AC voltage regulating circuit, so that the motor can run directly in parallel with the grid.
Variable frequency soft starters can also achieve soft stop function by using low-speed reverse braking, that is, changing the frequency in the order of f/5→f/8→f/11 to reverse the phase sequence of the motor power supply.
2. Working principle of frequency converter
A frequency converter is a control device that uses the switching action of power semiconductor devices to convert power frequency power into electrical energy of another frequency. The frequency converters we use now mainly adopt the AC-DC-AC method (VVVF frequency conversion or vector control frequency conversion), which first converts the power frequency AC power into DC power through a rectifier, and then converts the DC power into AC power with controllable frequency and voltage to supply the motor.
The output frequency of a frequency converter is derived from a DC inverter, and its frequency can theoretically change continuously. Thanks to advanced PWM modulation technology, its waveform closely approximates a sine wave. The ratio of the inverter's output voltage to its frequency is also proportional, so the voltage change is continuous.
When a frequency converter starts a motor, its frequency changes smoothly, and each frequency change is very small, so it can be considered continuous.
Frequency converters are designed to regulate motor speed and can run motors for extended periods without the need for short-circuit switches.
II. Differences in Main Circuit Structure
1. Main circuit structure of variable frequency soft starter
The main circuit of the variable frequency soft starter is shown in the figure below:
Figure 2. Main circuit diagram of the variable frequency soft starter
As shown in Figure 2, the main circuit of the variable frequency soft starter consists of a three-phase AC voltage regulating circuit composed of six thyristors connected in anti-parallel pairs and a short-circuit switch connected in parallel. When the motor starts, the thyristors work; after starting, the short-circuit switch activates, shorting the thyristors, and the thyristors stop working.
2. Main circuit structure of the frequency converter
The main circuit topology of the frequency converter is shown in Figure 3.
A frequency converter mainly consists of a rectifier circuit, a smoothing circuit, an inverter circuit, and a braking circuit, requiring a total of 12 power electronic devices. Currently, most converters use six power diodes in the rectifier circuit and six IGBTs in the inverter circuit. The rectifier circuit converts the mains frequency AC power into pulsating DC power, the smoothing circuit converts the pulsating DC power into smooth DC power, and the inverter circuit converts the DC power into AC power of the required frequency.
A comparison of the two main circuits shows that the variable frequency soft starter has a simpler structure and uses fewer power electronic devices, resulting in a lower cost than the variable frequency drive.
III. Differences in performance
Although both variable frequency soft starters and frequency converters have large starting torque when starting motors, their different working principles result in many differences in their performance.
The frequency of a variable frequency soft starter is discrete; the stator voltage and current are discontinuous, the torque is also discontinuous and pulsating, and the speed also pulsates, resulting in less smooth operation compared to a frequency converter. Therefore, the motor will experience vibration and noise (low frequency) during operation, unlike a variable frequency soft starter which only produces high-frequency noise from the modulated pulses. Since this vibration and noise only occurs briefly during startup, its impact on the motor is negligible.
Power electronic devices generate harmonics when they operate. The thyristors in a variable frequency soft starter operate for short periods, so they generate fewer harmonics than those in a frequency converter.
While variable frequency soft starters can change the frequency of the motor terminal voltage, their short-time duty design prevents them from operating at variable frequencies for extended periods. More importantly, their frequencies are fixed and cannot be continuously adjusted, and they cannot exceed 25Hz. This means the motor can only operate at half speed or below. Therefore, variable frequency soft starters are unsuitable for motor speed control. Variable frequency drives (VFDs) are designed for motor speed control; using them solely for motor starting results in very low utilization and represents a significant waste of cost and resources.
Conclusion
A comparison reveals that variable frequency soft starters and frequency converters are two distinct products with fundamentally different basic principles and component structures. Consequently, their performance, applications, and costs differ significantly and should not be confused. In practical use, different products should be selected based on the intended purpose. Variable frequency soft starters are used for starting motors, while frequency converters are used for speed regulation of motors, also serving a starting function.
