Basics concept of : AC current alternating current

1 - The concept of alternating current

AC power line dimensions and values ​​change over time, these changes often circulating under certain cycle.

Above is the AC sine, square pulses and spikes.

Cycle and frequency of the alternating current.
The cycle of alternating current is denoted T time that AC repeat the old location, the cycle is measured in seconds (s)
AC Frequency: The number of repetitions of the old status of alternating current in a second denoted F unit is Hz

F = 1 / T

Phase alternating current:
Speaking to the alternating current phase is often said to the comparison between two alternating current with the same frequency.
* Two-phase AC electric current is two times the voltage increases and the same reduction:

Two-phase alternating current

* Two AC phase shift: the two currents have the voltage rise time reducing mismatches.

Two out of phase AC

* Two reverse-phase alternating current: two 180 degree out of phase current, this current when the current increases, the other decreases and vice versa.

Two reverse-phase alternating current

The amplitude of the AC
The amplitude of the alternating current is the peak value of the line voltage dien.xoay afternoon, usually a higher amplitude voltage that we measured from the clock

The effective value of AC
Usually measured values ​​from the meter and also the value stated on the zig-voltage power adapter of electronic equipment., For example 220V AC that we are using is the only root mean square, real International peak amplitude of about 220V AC voltage 220V x 1.4 = about 300V

The capacity of the AC.
AC Output depends on the intensity, voltage and phase difference between two quantities above, capacity is calculated by the formula:

P = UIcosα

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  In which U: voltage


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  I is the current


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  α is the angle of phase difference between U and I

=> If an alternating current passes through the resistor, the phase difference between original U and I is α = 0 then cos = 1 and P = UI

=> If an alternating current passes through the coil or capacitor, the phase difference between U and I is +90 degrees or -90do, then cos = 0 and P = 0 (the output of the alternating current passing through the capacitor electricity or coils is = 0)

2 - AC current passing through R, C, L

1. The alternating current passes through resistor
Alternating current passes through the current resistance and voltage in phase with each other, that is when the voltage increases, the current peaks through increased maximum return. Such an alternating current with direct current nature as passing back thuan.do can apply the formula of direct current to alternating current passes through resistor

I = U / R or R = U / I Ohm's law formula

The calculation formula P = UI capacity

2. Alternating current passes through the capacitor.
Alternating current passes through the capacitor, the current will soon phase voltage than 90 degrees

AC power line soon
phase voltage than 90 degrees as it passes through capacitors

* Field alternating passing makes capacitors will be converged with a resistance called Zc, and Zc is calculated by formula

Zc = 1 / (2 x 3.14 x F x C)

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  In which Zc is resistant capacity (unit is Ohm)


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  F is the frequency of alternating current (unit is Hz)


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  C is the capacitance of the capacitor (unit is μ Fara)

This formula shows that the capacitor capacitance is inversely proportional to frequency alternating current (ie, the higher frequency capacitors easily pass through) and the rate of convergence Paradoxes with capacitive (ie capacitors with capacitance large, the alternating current passes through more easily)

=> Series One-way line frequency F = 0 = ∞ so Zc so direct current does not pass through the capacitor.

3. The alternating current passes through the coil.
When an alternating current passes through the coil creates magnetic field and magnetic field sensors turn up the winding voltage which a sensor may reverse, so that the coil tends to resist flow AC passing through it, this is the fight of the coil inductance denoted Z L

Z L = 2 x 3.14 x F x L

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  Where Z L is inductance (unit is Ohm)


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  L is the inductance of the coil (unit is Henry) L depends on the number of turns winding and core materials.


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  F is the frequency of alternating current (unit is Hz)

From this formula we see, inductance of the coil is proportional to the frequency and inductance of the coil, the higher frequency coil passing increasingly difficult => properties of reverse coil with capacitors Power.

=> With DC, Z L = 0 ohm coil, so the direct current passes through the coil is only subjected to pure resistor R alone (or pure resistance of the coil is measured by the clock multimeter), if the coil becomes relatively small net direct current through the coil will be short-circuited.

* The alternating current passes through the coil, the current phase is delayed compared to the 90-degree voltage means the voltage rise faster when the current through the coil.

AC power line delayed
phase voltage than 90 degrees as it passes through the coil

= >> Due to the phase difference between current and voltage while passing capacitors and coils, so we can not apply Ohm's law to AC circuit as the participation of L and C are.

= >> About the export, the AC was not born when we go through L and C despite U> 0 and I> 0.

4. Synthesis of two AC on the same circuit
* On the same circuit, if appear two-phase alternating current, the voltage amplitude of the voltage will be equal to the sum of two components.

Two phase currents and the amplitude will increase.

* If on the same circuit, if there are two reverse-phase alternating current, the voltage amplitude of the voltage will be equal to two component signal.

Two reverse-phase current, amplitude decreases