Abstract : Thyristor phase-shifting AC voltage regulation circuits cannot be applied to the starting and running of asynchronous motors; the so-called soft-start principle is theoretically invalid. Keywords : Circular rotating magnetic field, smooth torque; rotating magnetic field with high amplitude vibration, non-smooth impulse torque; low power factor, high reactive power loss; high-order harmonic pollution… The starting current of three-phase asynchronous motors is large, and the harm to the power grid, equipment on the same grid, and the motor itself is self-evident. For many years, people have continuously researched and practiced in solving the problem of asynchronous motor starting, and have obtained many scientific, practical, ingenious, and effective methods and equipment. In recent years, due to the development of power electronics technology, the scientific, simple, effective, and practical frequency conversion equipment that people have long hoped for has become a reality. Variable frequency speed regulation and variable frequency soft-start technology for asynchronous motors are widely used in automation, intelligent control, and automated equipment. "Soft start" has become the most fashionable synonym for modern industrial control technology. Thyristor phase-shifting DC voltage regulation and AC voltage regulation technology have been widely used for many years in DC motor speed regulation, AC and DC welding, and many other fields. In the past, no one thought of using it for starting asynchronous motors because everyone understood that asynchronous motors use sinusoidal alternating current. However, now the industrial control market has thyristor phase-shifting soft starters and related manufacturers, applying thyristor phase-shifting AC voltage regulation technology to starting asynchronous motors and labeling it with the trendy term "soft start." Even more perplexing are the misleading claims in advertisements and manuals, blurring the lines between right and wrong and distorting the truth to deceive consumers. From the working principle of a three-phase asynchronous motor: only three-phase symmetrical sinusoidal alternating current generates a circular rotating magnetic field; the generated electromagnetic torque is continuous and smooth; the main factors affecting the current of an asynchronous motor are: ① slip S; ② power supply voltage Um; ③ impedance Z; ④ power factor COSφ. Frequency converters employing the latest and most advanced power electronics technology can control the motor current to a very small value (IQ ≤ IN) during the soft start of an asynchronous motor. This is because they can smoothly and arbitrarily change and control the frequency f of the three-phase symmetrical sinusoidal AC power supply, thereby smoothly changing the speed n1 of the stator's rotating magnetic field, making it follow the rotor's speed n2, and always maintaining a very small slip (S ≤ SN). Commonly used traditional reduced-voltage starting equipment lowers the power supply voltage Um, allowing the motor to start at a low voltage, reducing the starting current to IQ = (3-4)IN. In the starting method of high-voltage squirrel-cage asynchronous motors, the stator circuit uses a series reactor, the stator circuit uses a series liquid resistor, and the rotor circuit uses a series resistor. All of these methods reduce the starting current by changing the motor impedance Z and increasing the power factor COSφ. In summary, the traditional reduced-voltage starting method does not change the basic properties of a single sinusoidal AC power supply (i.e., the frequency remains unchanged), ensuring that the fundamental attributes of the asynchronous motor—a rotating magnetic field and a continuous, smooth electromagnetic torque—remain unchanged, and that the original mechanical characteristics of the asynchronous motor are not altered during the starting process. Thyristor-based phase-shifting soft starters alter the waveform of sinusoidal AC voltage, transforming it into a non-sinusoidal pulsed AC current. By adjusting the duty cycle (as shown in the figure), the average voltage of the AC current is changed. This average voltage is controllable and smoothly varying. The magnetic field generated on the asynchronous motor windings is a rotating magnetic field with high-order amplitude oscillations; the rotor electromagnetic torque is a non-smoothly varying, impulsive, destructive torque with high-order amplitude oscillations. Mechanically, this generates vibrations (high-frequency mechanical impulses in the rotor, with an impulse frequency f_mechanical = 2f_power), causing mechanical damage to the motor and production machinery. Overvoltages are generated during stator electromagnetic conversion, directly damaging the motor insulation and reducing its lifespan. From an energy efficiency perspective, the motor operates with a low power factor, high reactive power loss, and high active power loss (due to high-frequency mechanical vibrations in the rotor) during startup. These problems are exacerbated, especially as the average voltage decreases (i.e., the control angle α is larger and the conduction angle β is smaller). Some manufacturers claim their soft starter boxes have energy-saving functions. In reality, this non-sinusoidal pulse AC current with constantly changing duty cycle, when applied to an asynchronous motor, results in a lower power factor (COSφ) and greater reactive power loss compared to traditional starting equipment. Furthermore, at low load rates, the duty cycle of the thyristor phase-shifting AC voltage regulator output waveform is even smaller, increasing the proportion of higher harmonic amplitudes, making the aforementioned issues more pronounced. This not only fails to save energy but actually exacerbates losses. A primary purpose of reducing the starting current of an asynchronous motor is to reduce the impact of large currents on the power grid and minimize the influence on other equipment on the same grid. However, the non-sinusoidal pulse AC current with changing duty cycle output from the thyristor phase-shifting AC voltage regulator circuit generates harmful pollution to the power grid with its higher harmonics; it also affects the normal operation of electrical equipment, communication, and television systems on the same grid. It not only fails to provide benefits but actually has detrimental effects. Many units in the Karamay oilfield purchased these so-called soft starter boxes at high prices, only to quickly discard them after a few days of use, replacing them all with genuine soft starter equipment—frequency converters. The thyristor-based phase-shifting soft starter, an immature product that has not undergone scientific verification or practical testing by national authoritative institutions, violates the basic regulations governing the production and sale of electromechanical products. Its widespread adoption will negatively impact normal production in enterprises and result in a significant waste of social resources. Reference : *Electrical Machinery*, published by Machinery Industry Press, edited by Harbin Electrical Machinery Manufacturing School.