Alternating current (AC) is widely used in power transmission because AC transmission is more efficient than DC transmission under the previous technical conditions. The dissipation power of the current transmitted on the conductor can be calculated by Joule's law (P =I? R) It is found that it is obviously necessary to reduce the transmission current or the resistance of the conductor to reduce the energy loss.
Due to the limitation of cost and technology, it is difficult to reduce the resistance of the transmission line (such as copper wire) currently used, so reducing the transmission current is the only effective method. According to P=IU (actually active power), increasing the voltage of the power grid can reduce the current in the conductor, thus achieving the purpose of energy saving.
Alternating current is easy to step up and step down, which is just suitable for high voltage transmission. With a simple step-up transformer, the alternating current can be raised to several thousand to several hundred thousand volts, so the power loss on the conductor is very small. In cities, step-down transformers are generally used to reduce the voltage to tens of thousands to thousands of volts to ensure safety, and then to the mains voltage or the applicable voltage of electrical appliances before entering the house.
Extended data
Excitation mode refers to the way that the rotating motor generates magnetic field, and the excitation mode of DC motor can be divided into four types:
1, separately excited DC motor
The excitation winding has nothing to do with the armature winding. DC motors powered by other DC sources are called separately excited DC motors, and permanent magnet DC motors can also be regarded as separately excited or self-excited DC motors, which are generally directly called permanent magnets.
2. Shunt DC motor
The excitation winding of shunt DC motor is connected in parallel with the armature winding. As a parallel generator, the terminal voltage from the motor itself supplies power to the excitation winding. As a shunt motor, the excitation winding and armature use the same power supply, and its performance is the same as that of separately excited DC motor.
3.DC series motors
The excitation winding of series-excited DC motor is connected in series with armature winding, and then connected to DC power supply. The excitation current of this DC motor is armature current.
4. Compound excitation DC motor
Compound excitation DC motor has two excitation windings, parallel excitation and series excitation. If the magnetic flux potential produced by series winding is in the same direction as that produced by parallel winding, it is called compound excitation. If the two magnetic flux potentials are in opposite directions, it is called differential compound excitation.
DC motors with different excitation modes have different characteristics. Generally speaking, the main excitation modes of DC motor are shunt excitation, series excitation and compound excitation, while the main excitation modes of DC generator are separate excitation, shunt excitation and compound excitation.
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