The back electromotive force is generated by the trend of changing the current magnitude in the resistance winding. The situation of generating back electromotive force includes:  alternating current is introduced into the coil; Place the conductor in an alternating magnetic field; Conductors cut magnetic fields. 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 generation of steady-state current requires two necessary conditions: first, to close the conductive circuit. Secondly, the back electromotive force. We can understand the phenomenon of generating induced electromotive force from an induction motor: 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 conductor placed in this rotating magnetic field is subjected to electromagnetic force, changing from static to rotating motion. The induced electromotive force is generated inside the conductor, and the induced current flows through the closed circuit of the conductor connected by the conductive end ring. In this way, an electromotive force or electromotive force is generated inside the rotor conductor, which is known as 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 greatly for different types of motors. The magnitude of the back electromotive force of an asynchronous motor varies 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 unchanged, so the efficiency index remains basically unchanged under different load conditions.
The physical significance of back electromotive force

The electromotive force that resists the passage or change of current is called the back electromotive force. In the energy conversion relationship UIt=ε anti It+I2Rt, UIt refers to the input electrical energy, such as the input electrical energy into a battery, electric 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 input electrical energy and heat loss electrical energy consumption is the useful energy ε anti It corresponding to the back electromotive force. In other words, the back electromotive force is used to generate useful energy, which is inversely correlated with heat loss. The larger the heat loss energy, the smaller the achievable useful energy.

Objectively speaking, the back electromotive force consumes 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 equipment, such as the mechanical energy of the motor and the chemical energy of the battery.

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.

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 stator and rotor are the factors that determine the back electromotive force of the motor. When the motor design is completed, the rotor magnetic field and the number of turns in the stator winding are both determined. Therefore, the only factor that determines the back electromotive force is the rotor angular velocity, which can be said to be 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 can affect the magnetic flux of the winding, thereby also affecting the back electromotive force.

Precautions during motor operation

If the motor stops rotating due to excessive mechanical resistance during operation, there is no back electromotive force. The coil with low resistance is directly connected to both ends of the power supply, and the current will be high, making it easy to burn out the motor. This state may be encountered in motor testing, such as the locked rotor test, which requires the motor rotor to be in a stationary state. At this time, the motor is very large and it is easy to burn the motor. Currently, most motor manufacturers use instantaneous value collection for the locked rotor test, which basically avoids the problem of motor burning caused by long locked rotor time. However, due to various factors such as assembly, the collected values have significant differences and cannot accurately reflect the starting status 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, and there is no back electromotive force generated. The motor is also prone to burnout. This problem often occurs in motors used in temporary circuits. For example, temporary circuits use power lines, and aluminum core wires are mostly used for cost control due to disposable use and prevention of theft. As a result, the voltage drop on the circuit will be large, resulting in insufficient input voltage of the motor. The natural back electromotive force should be relatively small, and in severe cases, the motor may be difficult to start or even unable to start. Even if the motor starts, it operates under abnormal high current conditions, making it easy for the motor to be burned out.