In the energy-saving technical renovation of electric drive systems, besides optimizing the design of the drive system devices, it is also necessary to vigorously promote the application of variable frequency speed regulation in electric drive systems. The frequency converter is a non-linear device, and its operation inevitably generates high-order harmonics. When the frequency converter supplies power to the driving motor, it inevitably inputs a considerable amount of high-order harmonic currents into the motor, thereby causing adverse effects on the motor. Therefore, in the technical renovation of electric drive systems with variable frequency speed regulation, corresponding measures should also be taken to eliminate the adverse effects of harmonics on the motor. Realizing variable frequency speed regulation of asynchronous motors has been the long-awaited "world dream" since the invention of asynchronous motors a hundred years ago. Through the unremitting efforts of scientific and technical personnel, its speed regulation performance is by no means inferior, and in some aspects even surpasses DC speed regulation. Since frequency itself is a digital quantity, it is possible to obtain very hard mechanical characteristics without the need for external feedback. It also has the characteristics of high speed regulation accuracy, smoothness, stable performance, simple maintenance, and easy automatic control of the production process. The asynchronous motor drive system can undergo variable frequency speed regulation technical renovation without replacing the original motor, that is, by connecting a corresponding type of frequency converter between the motor and the power supply, the best speed regulation effect can be achieved. The low failure rate of its drive system is due to the simple structure of the asynchronous motor, and the power in the rotor circuit does not need to be externally connected, so the probability of failure is extremely low. If the asynchronous motor adopts full-voltage direct starting, its starting current can reach 5-7 times the rated current, which will inevitably cause adverse effects on the drive system or the power grid. However, with variable frequency speed regulation starting, the starting current generally does not exceed 1.5 times the rated current. At the same time, the starting is smooth, without impact, achieving a true soft start for the asynchronous motor. The use of variable frequency speed regulation technology in fans and pump loads can not only achieve speed regulation according to the load operation requirements, but also produce very little vibration and mechanical noise during the starting process. When variable frequency speed regulation is used in the electric drive of general production machinery, it will not produce vibration and impact under conditions such as starting, stopping, decelerating, and accelerating, thereby extending the service life of the equipment. When variable frequency speed regulation technology is applied to the energy-saving renovation of conveyors, if the load needs to accelerate or decelerate during operation, it has a good soft start effect and achieves smooth torque. Especially when starting under heavy load conditions, it can increase the output torque, which is an effect that ordinary starters cannot achieve. Asynchronous motors are widely used in the electric drive of industrial and mining enterprises due to their simple structure, reliable operation, and convenient maintenance. The variable frequency speed regulation technical renovation of asynchronous motors in electric drive systems can achieve speed regulation without replacing the original motor. However, because the frequency converter is a non-linear device, it will generate high-order harmonics during operation, which will inevitably cause adverse effects on the driving motor, so corresponding measures must be taken to prevent them. Asynchronous motors designed according to conventional design are usually designed to work at rated frequency and rated voltage. Only when operating at rated frequency and voltage can the output torque and power on the motor shaft reach the rated design values. However, for asynchronous motors operating under variable frequency speed regulation conditions, since the supply frequency is a variable, this influencing factor must be fully considered when selecting the capacity of the driving motor under different working conditions. Usually, when asynchronous motors operate under rated power and temperature rise conditions, the operating temperature of the motor will not exceed the design value. However, in variable frequency speed regulation drive systems, because the current input to the motor contains a considerable amount of high-order harmonics, the harmonic currents cause additional losses in the motor. Even when operating at rated frequency for a long time, the influence of harmonic currents can cause abnormal conditions such as reduced output torque, decreased efficiency, and increased temperature rise. During the operation of asynchronous motors, if the temperature rise increases, it will lead to accelerated volatilization and degradation of coil insulation, decreased dielectric strength and volume resistivity, and may also cause carbonization of coil insulation and loss of insulation function. In asynchronous motors in variable frequency speed regulation drive systems, due to the influence of high-order harmonics, the magnetic field generated by harmonic currents rotates at high speed relative to the shaft, and the shaft potential generated is relatively high, which may break down the oil film of the bearing, causing shaft current to flow through the bearing and cause harm to the bearing. There are distributed capacitances between the coils of asynchronous motors. When high-order harmonic voltages are input, the voltage sharing among coils is uneven, which often leads to accelerated aging of the insulation of coils bearing high voltage, making the first-turn coil the point of insulation damage. In variable frequency speed regulation drive systems, the amplitude of the output voltage of the frequency converter is more than 3 times the standard voltage, and coupled with the high voltage change rate (du/dt) of the frequency converter, the oscillation it causes will make the motor stress greater, inevitably causing harm to the coils. Under conditions of high switching frequency, if the connecting cable between the frequency converter and the motor is too long, standing waves will be generated, which will cause the motor terminal voltage to rise, causing the motor coils to withstand a terminal voltage higher than the grid voltage, which will inevitably accelerate the aging of coil insulation and affect the service life of the motor. In order to improve the operating efficiency of asynchronous motors in variable frequency speed regulation drive systems, harmonic technology must be used to eliminate the influence of harmonics. Those connected to the motor input end are called motor terminal filters; those connected to the frequency converter output end are called frequency converter output filters. Motor terminal filters are divided into first-order RC series type and first-order RC parallel type filtering methods. That is, the neutral point of the star-connected RC circuit is connected to the neutral point of the DC bus of the frequency converter. This filter has small size, low loss, low cost, and is worth promoting; 3.1.4 LC and RLC two-stage series frequency converter output filters. Working principle of cable detector Shorten the length of the connecting cable between the frequency converter and the motor to avoid the influence caused by standing waves. Do not coil the excessively long part of the connecting cable and place it inside the frequency converter cabinet; this treatment method is not good and will still cause harmonic interference. Secondly, the metal corrugated pipe at the frequency converter inlet m can be used, and the outer shell of the pipe should be well grounded, which is also a measure to suppress harmonic interference. In addition, the control line of the frequency converter can be shielded, and the shielding layer should be well grounded, which can also prevent harmonic interference. In the technical renovation of variable frequency speed regulation, in order to improve the operating efficiency of the electric drive system, a frequency converter that is not prone to outputting high reflected voltage should be selected. If the driving motor is to be replaced, a motor specially driven by a frequency converter should be selected.
