Back electromotive force is generated by opposing the tendency of the current in the winding to change. Back electromotive force is generated in the following situations: (1) when an alternating current is passed through the coil; (2) when a conductor is placed in an alternating magnetic field; (3) when a conductor cuts through the magnetic field. When electrical appliances such as relay coils, electromagnetic valves, contactor coils, and motor windings are working, they all generate induced electromotive force.
The generation of steady-state current requires two necessary conditions: first, a closed conductive loop. Second, back electromotive force. We can understand the phenomenon of induced electromotive force from the induction motor: three-phase symmetrical voltages are applied to the stator windings of the motor with a difference of 120 degrees, generating a circular rotating magnetic field, so that the rotor bars placed in this rotating magnetic field are subjected to electromagnetic force, changing from static to rotating motion, generating induced potential in the bars, and induced current flows through the closed loop of the bars connected by the conductive end rings. In this way, an electric potential or electromotive force is generated in the rotor bars, and this electromotive force is the so-called back electromotive force. In a wound rotor motor, the rotor open circuit voltage is a typical back electromotive force.
Different types of motors have completely different changes in the size of the back electromotive force. The size of the back electromotive force of an asynchronous motor changes with the load size at any time, resulting in very different efficiency indicators under different load conditions; in a permanent magnet motor, as long as the speed remains unchanged, the size of the back electromotive force remains unchanged, so the efficiency indicators under different load conditions remain basically unchanged.
The physical meaning of back electromotive force is the electromotive force that opposes the passage of current or the change of current. In the electric energy conversion relationship UIt=ε逆It+I2Rt, UIt is the input electric energy, such as the input electric energy to a battery, motor or transformer; I2Rt is the heat loss energy in each circuit, which is a kind of heat loss energy, the smaller the better; the difference between the input electric energy and the heat loss electric energy is the part of the useful energy ε逆It corresponding to the back electromotive force. In other words, the back electromotive force is used to generate useful energy and is inversely correlated with the heat loss. The greater the heat loss energy, the smaller the achievable useful energy.
Objectively speaking, the back EMF consumes the electrical energy in the circuit, but it is not a "loss". The part of the electrical energy corresponding to the back EMF will be converted into useful energy for the electrical equipment, such as the mechanical energy of the motor and the chemical energy of the battery.
It can be seen that the size of the back EMF means the strength of the electrical equipment's ability to convert the total input energy into useful energy, reflecting the level of the electrical equipment's conversion ability.
Factors that determine the back EMF For motor products, the number of stator winding turns, the rotor angular velocity, the magnetic field generated by the rotor magnet, and the air gap between the stator and rotor are factors that determine the back EMF of the motor. When the motor is designed, the rotor magnetic field and the number of turns of the stator winding are determined. Therefore, the only factor that determines the back EMF is the rotor angular velocity, or the rotor speed. As the rotor speed increases, the back EMF also increases. The difference between the stator inner diameter and the rotor outer diameter will affect the size of the winding's magnetic flux, which will also affect the back EMF.
Things to note when the motor is running ● If the motor stops rotating due to excessive mechanical resistance, there is no back electromotive force at this time. The coil with very small resistance is directly connected to the two ends of the power supply. The current will be very large, which can easily burn the motor. This state will be encountered in the test of the motor. For example, the stall test 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. At present, most motor manufacturers use instantaneous value collection for the stall test, which basically avoids the problem of motor burning caused by long stall time. However, since each motor is affected by various factors such as assembly, the collected values are quite different and cannot accurately reflect the starting state of the motor.
● When the power supply voltage connected to the motor is much lower than the normal voltage, the motor coil will not rotate, no back electromotive force will be generated, and the motor will easily burn out. This problem often occurs in motors used in temporary lines. For example, temporary lines use power supply lines. Because they are one-time use and to prevent theft, most of them will use aluminum core wires for cost control. In this way, the voltage drop on the line will be very large, resulting in insufficient input voltage for the motor. Naturally, the back electromotive force should be relatively small. In severe cases, the motor will be difficult to start or even unable to start. Even if the motor starts, it will run at a large current in an abnormal state, so the motor will be easily burned out.
low voltage electric motor, Ex motor, Motor manufacturers in China, three phase induction motor, SIMO motor