What are the similarities and differences between variable-frequency motors and ordinary motors + frequency converters?

There are cases where the variable frequency motor and the inverter are installed separately, and there are cases where the inverter and the motor are installed together. For the former, it can be connected directly by linking the three phases U, V, W at the output of the inverter’s main circuit to the motor’s u, v, w terminals; for the latter, it simply needs to be connected to the power supply. The following is the content from Baidu Baike: Characteristics of variable-frequency motors 1. Electromagnetic design For conventional asynchronous motors, the performance parameters that are primarily considered during design are overload capacity, starting performance, efficiency, and power factor. In contrast, since the critical slip rate of an induction motor is inversely proportional to the supply frequency, it can start directly when the critical slip rate is close to 1. Therefore, overload capacity and starting performance do not need to be given excessive attention; the key issue to address is how to improve the motor’s ability to adapt to non-sinusoidal power supplies. The methods are generally as follows: 1) Minimize the stator and rotor resistances as much as possible. 　　Reducing the stator resistance can lower the fundamental wave copper loss, thereby compensating for the increase in copper loss caused by higher harmonics. 2) To suppress the higher harmonics in the current, it is necessary to appropriately increase the inductance of the motor. However, due to the large leakage reactance in the rotor slots, the skin effect is also significant, resulting in increased copper loss at higher harmonics. Therefore, the magnitude of the motor leakage reactance must take into account the rationality of impedance matching across the entire speed control range. 　　3) The main magnetic circuit of variable-frequency motors is generally designed to be in a non-saturated state; this is done first to prevent high-order harmonics from exacerbating magnetic circuit saturation, and secondly to increase the output voltage of the inverter appropriately at low frequencies in order to boost the output torque. 　　2. Structural Design 　During the restructuring design, attention is primarily paid to the impact of the characteristics of non-sinusoidal power supplies on aspects such as the insulation structure, vibration, noise, and cooling methods of variable-frequency motors. The following points should generally be taken into consideration: 　1) Insulation class, which is usually F grade or higher; it is necessary to enhance the insulation strength against ground as well as between windings, with special attention being given to the insulation’s ability to withstand shock voltages. 　　2) Regarding the vibration and noise issues of the motor, it is necessary to fully consider the rigidity of the motor’s components as well as the overall structure, and strive to increase its natural frequency in order to avoid resonance with various force waves. 　　3) Cooling method: Forced ventilation cooling is generally used, that is, the cooling fan of the main motor is driven by an independent motor. 　　4) Measures to prevent shaft current: Bearing insulation should be employed for motors with a capacity exceeding 160 KW. Primarily, it leads to magnetic circuit asymmetry, as well as the generation of shaft current. When the currents generated by other high-frequency components act together, the shaft current increases significantly, which can result in bearing damage; therefore, insulation measures are generally required. 　　5) For constant-power variable-frequency motors, when the speed exceeds 3000/min, a special high-temperature-resistant lubricant should be used to compensate for the increase in bearing temperature. Key features of variable-frequency motors Variable-frequency dedicated motors have the following characteristics: B-class temperature rise design, F-class insulation construction. The use of high-polymer insulating materials and a vacuum pressure impregnation process, along with a special insulating structure, significantly enhances the insulation voltage resistance and mechanical strength of the electrical windings, ensuring they can handle high-speed operation of the motor as well as withstand the high-frequency current surges from inverters and the damage to insulation caused by voltage. It features a high balance mass and a vibration level of class R (vibration-reduced level). The mechanical components are manufactured with high precision, and specialized high-precision imported bearings are used, enabling high-speed operation. The forced ventilation cooling system uses imported axial flow fans, which are ultra-quiet, have a long service life, and deliver strong airflow. It ensures effective heat dissipation of the motor at any speed, enabling long-term operation at high or low speeds. The YP series motors designed using AMCAD software offer a wider speed control range and higher design quality compared to traditional variable-frequency motors. Thanks to a special magnetic field design, higher-order harmonic fields are further suppressed, thereby meeting the requirements for wide frequency operation, energy efficiency, and low noise. It features a wide range of constant torque and power speed control capabilities, with smooth speed regulation and no torque fluctuations. It provides good parameter compatibility with various types of frequency converters; in combination with vector control, it enables zero-speed full-torque operation, high torque at low frequencies, as well as high-precision speed and position control along with fast dynamic response. The YP series of variable-frequency dedicated motors can be equipped with brakes, and encoders are available, thereby enabling precise stopping as well as high-precision speed control through speed closed-loop control. A \"micro motor + dedicated variable-frequency motor + encoder + inverter\" setup is used to achieve precise control of ultra-low speed with stepless regulation. The YP series of frequency conversion dedicated motors feature good versatility; their installation dimensions comply with IEC standards, allowing them to be interchanged with standard motors. Construction principle of variable-frequency motors Speed control and regulation of motors are among the fundamental technologies for various mechanical devices in industry and agriculture, as well as for office and household electrical equipment. With the remarkable advancements in power electronics and microelectronics technology, the AC speed control method that uses \"specialized variable-frequency induction motors + frequency converters\" is, thanks to its excellent performance and cost-effectiveness, driving a transformation in the field of speed control by replacing traditional methods. The benefits it brings to various industries include a significant increase in the degree of mechanical automation and production efficiency, energy savings, improved product qualification rates and quality, an increased capacity of power supply systems, smaller equipment sizes, and enhanced comfort. It is currently replacing traditional mechanical speed control and DC speed control solutions at a rapid pace. 　　Due to the special nature of variable-frequency power supplies, as well as the system’s requirements for high or low speeds of operation and rapid response to changes in speed, stringent demands are placed on the motors that serve as the power source. This presents new challenges for motors in terms of electromagnetism, structure, and insulation. Applications of variable-frequency motors Variable-frequency speed control has now become the mainstream solution for speed regulation, and it can be widely used in various industries for stepless speed adjustment. 　　Especially with the increasing use of frequency converters in the field of industrial control, the use of frequency-driven motors has also become more widespread. It can be said that due to the advantages of frequency-driven motors over conventional motors in terms of frequency control, it is not difficult to find such motors wherever frequency converters are used.

Variable-frequency motors are used in conjunction with variable-frequency controllers. They represent a new type of motor that adjusts its speed by changing the frequency; in fact, their functional capabilities are not very different from those of ordinary motors combined with frequency converters.

It’s mainly due to the different cooling methods for the motors. Reason: Industrial variable-frequency motors and ordinary motors can both operate properly within the 0-50HZ frequency range; the V/F ratio for variable-frequency speed control remains constant, which ensures that the power output by the inverter is steady. If the output frequency of the inverter is reduced while the motor load remains unchanged, since the fan of a conventional motor is mounted on the motor shaft, a decrease in motor speed leads to a reduction in the volume of cooling air. This results in poor heat dissipation by the motor; over time, this can cause the motor’s lubricant and insulation to age, ultimately leading to damage to the motor’s mechanical components and windings. Since the frequency conversion motor housing is equipped with a dedicated cooling fan powered independently, the amount of air used for cooling the motor remains constant, regardless of the speed of the main motor.

What was said upstairs is correct; it’s mainly the difference in heat dissipation. We have used the combination of a regular motor and an inverter on site. With this setup, the motor should not operate at low frequencies for extended periods, as excessive heating can lead to malfunctions, especially in larger motors. We have a 30KW motor that uses this configuration; the electrical engineers advised us that it’s best to operate it at frequencies above 25Hz. Smaller motors fare better under such conditions. We also have a 3KW motor that is used in the same way as an inverter-driven motor, but it operates mostly at frequencies below 25Hz. However, the electrical engineers said that it’s better to use an inverter-driven motor when operating at frequencies below 25Hz for long periods. We asked the electrical engineers about the price difference between the two types of motors, and it’s around 1/3 to 1/2. This post was last edited by Pseudo Xiaobao on 2009-2-21 at 19:57

For those using variable-frequency motors, it is designed during the design phase； For those using a regular motor + inverter, the modification is carried out during renovation.

Relatively speaking, motors generate more heat at low frequencies. In the case of ordinary motors combined with frequency converters, the fan speed slows down at low frequencies, resulting in poor heat dissipation. Additionally, I disagree a bit with what the brother on the 4th floor said: in actual use, frequency conversion motors cannot operate across the entire 0–50Hz range; some motors tend to experience high torque and overload issues when operating below 5Hz. People from engineering fields may not always be correct. This post was last edited by Pseudo Xiaobao on 2009-2-21 at 20:05.]

In simple terms, a variable-frequency motor is just a regular motor with added insulation; there’s no other difference. Since the harmonic components in high-voltage, high-power products were not well controlled in the early days, this could lead to insulation breakdown in motors; that is why variable-frequency motors came into existence.