The influence of the back electromotive force of a motor on its performance

Precautions for motor operation

If the motor stops rotating due to excessive mechanical resistance during operation, there will be no back electromotive force, and the coil with low resistance will be directly connected to both ends of the power supply, causing a large current and easily burning out the motor. This state is encountered in motor testing, such as the stall test, which requires the motor rotor to be in a stationary state. At this time, the motor is very large and can easily burn out. Currently, most motor manufacturers use instantaneous value collection for stall testing, which basically avoids the problem of motor burning out due to long stall time. However, due to various factors such as assembly, the collected values have significant differences and cannot accurately reflect the starting state of the motor.

When the voltage of the power supply connected to the motor is much lower than the normal voltage, the motor coil does not rotate, there is no back electromotive force generated, and the motor is also easily burned out. This problem often occurs in motors used with temporary power lines. For example, temporary power lines use aluminum core wires because they are disposable and to prevent theft. Due to cost control, the voltage drop on the line is large, resulting in insufficient input voltage to the motor. The natural back electromotive force is relatively small, and in severe cases, the motor is difficult to start, even unable to start. Even if the motor starts, it is running at high current in an abnormal state, making it easy for the motor to be burned out.

Reverse electromotive force is generated by the tendency of the current in the resistance winding to change in magnitude. The situation of generating back electromotive force includes: (1) alternating current flowing into the coil; (2) The conductor is placed in an alternating magnetic field; (3) Conductor cutting magnetic field. During the operation of electrical appliances such as relay coils, solenoid valves, contactor coils, and motor windings, there is a phenomenon of induced electromotive force.

The physical meaning of back electromotive force: The electromotive force that opposes the passage or change of current is called back electromotive force. In the energy conversion equation UIt=ε inverse It+I2Rt, UIt is the input electrical energy, such as the input electrical energy to a battery, motor, or transformer; I2Rt is the heat loss energy in each circuit, which is a type of heat loss energy, the smaller the better; The difference between the input electrical energy and the heat loss consumption energy is the portion of useful energy ε that corresponds to the back electromotive force. In other words, the back electromotive force is used to generate useful energy and is inversely correlated with heat loss. The larger the heat loss energy, the smaller the achievable useful energy.

The factors that determine the back electromotive force: For motor products, the number of stator winding turns, rotor angular velocity, magnetic field generated by rotor magnets, and air gap between the stator and rotor are the factors that determine the back electromotive force of the motor. When the motor design is completed, both the rotor magnetic field and the number of turns of the stator winding are determined. Therefore, the only factor that determines the back electromotive force is the rotor angular velocity, also known as the rotor speed. As the rotor speed increases, the back electromotive force also increases. The difference between the inner diameter of the stator and the outer diameter of the rotor will affect the magnetic flux of the winding, and thus also affect the back electromotive force.

The generation of steady-state current requires two essential conditions: first, closing the conductive circuit. Secondly, the back electromotive force. We can understand the phenomenon of generating induced electromotive force from induction motors: a three-phase symmetrical voltage is applied to the stator winding of a motor with a mutual difference of 120 degrees, generating a circular rotating magnetic field. The rotor bars placed in this rotating magnetic field are subjected to electromagnetic force, transforming from stationary to rotating motion. Induced potential is generated inside the bars, and induced current flows through the closed circuit of the bars connected by the conductive end rings. In this way, an electric potential or electromotive force is generated inside the rotor conductor, which is called the back electromotive force. In a wound rotor motor, the open circuit voltage of the rotor is a typical back electromotive force.

The magnitude of the back electromotive force varies significantly for different types of motors. The magnitude of the back electromotive force of asynchronous motors is constantly changing with the size of the load, resulting in significant differences in efficiency indicators under different load conditions; In permanent magnet motors, as long as the speed remains constant, the magnitude of the back electromotive force remains constant, so the efficiency index under different load conditions remains basically unchanged.

Objectively speaking, the back electromotive force consumes the electrical energy in the circuit, but it is not a "loss". The portion of electrical energy corresponding to the back electromotive force will be converted into useful energy for electrical devices, such as the mechanical energy of electric motors and the chemical energy of batteries.

From this, it can be seen that the magnitude of the back electromotive force indicates the strength of the ability of the electrical device to convert the total input energy into useful energy, reflecting the level of the electrical device's conversion ability.

On December 3rd, the Ministry of Industry and Information Technology issued a notice on the "14th Five Year Plan for Industrial Green Development".

The "Plan" points out that the "14th Five Year Plan" period is a critical and window period for China to address climate change and achieve carbon peak goals, as well as a key five-year period for industry to achieve green and low-carbon transformation. In the face of new situations, new tasks, and new requirements, we must elevate our political stance, face difficulties head-on, overcome them, and firmly adhere to the high-quality development path of ecological priority, green and low-carbon.

The "Plan" emphasizes that the green development of industry during the 14th Five Year Plan period should be led by the goal of carbon peak and carbon neutrality, with the overall focus on reducing pollution, carbon reduction, and enhancing efficiency. It should coordinate development with green and low-carbon transformation, deeply implement green manufacturing, accelerate industrial structure optimization and upgrading, vigorously promote industrial energy conservation and carbon reduction, comprehensively improve resource utilization efficiency, actively promote clean production transformation, enhance the supply capacity of green and low-carbon technologies, green products, and services, and build a modern industrial pattern that promotes and deeply integrates industrial green and low-carbon transformation and industrial empowerment of green development, supporting the timely achievement of carbon peak and carbon neutrality goals.

Assist in achieving carbon peak and carbon neutrality with the release of the "14th Five Year Plan" for industrial green development

Author: Published on December 9, 2021

On December 3rd, the Ministry of Industry and Information Technology issued a notice on the "14th Five Year Plan for Industrial Green Development".

The "Plan" points out that the "14th Five Year Plan" period is a critical and window period for China to address climate change and achieve carbon peak goals, as well as a key five-year period for industry to achieve green and low-carbon transformation. In the face of new situations, new tasks, and new requirements, we must elevate our political stance, face difficulties head-on, overcome them, and firmly adhere to the high-quality development path of ecological priority, green and low-carbon.

The "Plan" emphasizes that the green development of industry during the 14th Five Year Plan period should be led by the goal of carbon peak and carbon neutrality, with the overall focus on reducing pollution, carbon reduction, and enhancing efficiency. It should coordinate development with green and low-carbon transformation, deeply implement green manufacturing, accelerate industrial structure optimization and upgrading, vigorously promote industrial energy conservation and carbon reduction, comprehensively improve resource utilization efficiency, actively promote clean production transformation, enhance the supply capacity of green and low-carbon technologies, green products, and services, and build a modern industrial pattern that promotes and deeply integrates industrial green and low-carbon transformation and industrial empowerment of green development, supporting the timely achievement of carbon peak and carbon neutrality goals.

By 2025, significant achievements will be made in the green and low-carbon transformation of industrial structure and production methods. Green and low-carbon technology and equipment will be widely applied, energy and resource utilization efficiency will be significantly improved, and the level of green manufacturing will be comprehensively enhanced, laying a solid foundation for achieving carbon peak in the industrial sector by 2030. Among them, the carbon emission intensity continues to decline, and the carbon dioxide emissions per unit of industrial added value decrease by 18%. Energy efficiency has steadily improved, and the energy consumption per unit of added value of industrial enterprises above designated size has decreased by 13.5%. The level of resource utilization has significantly improved, with a comprehensive utilization rate of 57% for bulk industrial solid waste and a recycling and utilization volume of 480 million tons of major renewable resources per unit