A theory: The electric power technology revolution originated in Europe and was completed in the United States.
In 1866, after Wei Siemens invented the motor, he wrote to his brother in London: "Electricity technology is very promising, and it will usher in a new era." Later, facts proved his prediction.
Following Siemens’ motor, Bell (1847-1922) invented the telephone in 1876, and Edison invented the electric light in 1879. These three inventions illuminated the road to electrification for mankind. In the early 1880s, the motor It is already relatively complete in structure, and the need for further improvement has promoted theoretical research.
Because the power source was only DC power provided by batteries, most motors at that time were still DC, and generators for electrolysis, electroplating and other purposes also had to be DC.
The alternating current generated by electromagnetic induction must be converted into direct current by the commutator on the motor before it can be used.
The earliest large-scale use of alternating current was in electric lighting in 1876.
The power plant built by H. Yablochikov of Russia for lighting sent alternating current.
In 1883, the British L. Golard (1850～1888) and I. Dickson Gibbs made a transformer with taps and several windings, and used the method of changing the wiring to transform all the transformers. The required voltage still uses an open magnetic circuit.
This kind of transformer was exhibited at the London Expo in the UK, with a capacity of 5kVA per unit.
In 1885, Hungarian engineer Maxwell (Max Weri 1851-1934) developed a dry-type transformer using a closed magnetic circuit, which greatly improved the efficiency and obtained a German patent.
Another characteristic of alternating current is that a stationary coil can produce a rotating magnetic field.
It had a significant impact on later motors.
Italian scientist Galileo Ferrais (1847-1897) reported at the Turin Academy of Sciences in the spring of 1888 that he discovered in 1885 that using alternating currents of different phases to several stationary coils could produce a rotating magnetic field.
Almost at the same time, Yugoslavian-American engineer Tesla (Nicola Tesla 1856 ~ 1943) also reported the discovery of a rotating magnetic field in the United States, and in 1882 made an AC motor without slip rings.
In the autumn of 1888, the young Russian engineer Dolivo Dobrowski (1862-1919) noticed that during the dynamic braking experiment of the motor, if the armature coil of the motor was short-circuited, it would cause Produce strong braking effect.
From this, he quickly realized that if the resistance of the coil on the armature is reduced and the induced current is increased, it is not used for braking, but rotates with the rotating magnetic field, which can provide a certain torque.
Based on this idea, he passed a copper bar through the iron column, short-circuited the end to serve as a rotor, and placed it in a rotating magnetic field to create a squirrel-cage induction motor.
This kind of motor does not need to introduce excitation current to the rotor, thus eliminating the need for sliding contact rings. It has a simple and solid structure, low cost, and smooth operation. It is still widely used as a power source until now.
He also changed the two-phase to three-phase, so that the space on the circumference of the motor can be fully utilized.
The three-phase alternating current has a phase difference of 120 electrical degrees from each other. The sum of the three sinusoidal currents of equal magnitude is exactly zero.
In other words, to supply three coils with three-phase current, you do not need to use six wires. You only need to connect the other ends of the coils together to become the midpoint, so that only three wires are needed.
In 1889, he made a 100W electric motor, and in 1891, he made a 3.7kW electric motor.
Dolivo-Dobrowski also made a three-phase transformer.
He proposed that several structures are feasible, including the core being shell type, core type, or Japanese-shaped.
It was found that the measured results of energy losses in AC motors differed greatly from the calculated results.
J.A. Ewing from the UK pointed out that this may be due to the fact that hysteresis loss is not taken into account.
German-American Charles Proteus Steinmetz (1865～1923) gave an empirical formula for calculating hysteresis loss, that is, the loss is proportional to the magnetic flux density B raised to the power of 1.6 to 2, according to materials in different ways.
This formula is very effective and has been applied to this day.
The use of alternating current has promoted the development of AC circuit theory.
There is a big difference between AC circuits and DC circuits. Not only do the electromotive force and current change positively and negatively over time, but the circuit not only has the effect of resistance, but also the effects of inductance and capacitance must be considered.
As early as 1847, Y.
In 1877, II.H. Yablochikov observed that the AC voltage on the capacitor was also in phase with the current.
In the 1880s, J.C. Maxwell proposed the full impedance representation of AC in a circuit.
Kapp (King *** urgKapp1852～1922) introduced the formula for calculating the average value of the induced electromotive force E generated by the transformer in 1887:
E＝4.44wfΦ10-8
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where f is the frequency, W is the number of turns, and Φ is the magnetic flux.
According to this formula, the relationship between magnetic flux and magnetizing current in the transformer can be determined.
M.O. Dolivo-Dobrowski developed Karp's theory.
In 1891, he presented a report on the theory of alternating current at the Frankfurt Electrician Academic Conference: "The magnetic flux is determined by the magnitude of the applied voltage, not by the magnetic resistance.
The change in reluctance only affects the magnitude of the magnetizing current.
If the change in magnetic flux is sinusoidal, the electromotive force or voltage is also sinusoidal, but the phase difference between the two is 90 degrees. The magnetizing current is divided into two components, namely the "active component" and the "magnetizing component".
He proposed that the basic waveform of alternating current is sinusoidal, and divided the current in the coil into two components, which were later used.
An important development in the calculation method of AC circuits is C.P. Steinmetz's complex symbol method.
He used Timerway's theorem in mathematics to use complex numbers to represent the magnitude and phase of the sinusoidal quantity.
At a given frequency, the operation of trigonometric functions is simplified to the algebraic operation of complex numbers.
He also used vectors to represent complex numbers based on the Swiss mathematician Jean Robert Argand (1768-1813) proposed in 1806, and then the vectors on the plane can be used to represent the magnitude and phase of the alternating current, so it can Call it "phasor".
The phasor concept has become a powerful tool for analyzing alternating current because of its intuitiveness and easy understanding.
Manage Jiaotong University Electrical