1. Current
Influenced by electric field force, free electrons do regular
movements, forming the current.
The direction at which positive charge moves is regarded as the
direction of the current. The speed is 300,000km per second. The current
intensity is represented by the quantity of electricity flowing through the
conductor section per second. The unit is ampere (represented by the letter A).
1A current (I) means that the quantity of electricity (Q) flowing through 1C
conductor section per second (T).
Calculation formula:
I = Q/T
1A = 1C/1S
2. Voltage
The voltage is the difference in electric potential between two points
in the electric field, which is used to measure the energy difference generated
by unit charge owing to different electric potentials in the static electric
field. It equals to the work done by unit positive load moving from point A to
point B. In terms of voltage direction, it is regulated that high electric
potential points to low potential.
The voltage is represented by U. The Volt (V for short) is the unit of
voltage in the international system of units. Common units consist of mV, uV
and kV etc. This definition is similar to the "water pressure" definition. However, in most cases, the "voltage" is used in the circuit while the "electric potential difference" is generally applied in all electrical phenomena.
Calculation formula:
1Volt = 1J/1C
U = W/Q
3. Electrical Resistance
The electrical resistance of an electrical conductor is a measure of
the difficulty to pass an electric current through that conductor. It is
represented by the letter R. Its calculation unit is Ω.
The conductor's numerical value of the resistance is proportional to
the length of the conductor (L) and is inversely proportional to cross section
area (S). Moreover, it is related to metal variety.
R = ρL/S
Where:
R — resistance (Ω)
L — length (m)
R — cross section area (cm2)
ρ— resistance rate of the conductor, also referred
to as resistance coefficient (Ω·m)
4. Ohm's Law
Ohm's law deals with the relation among voltage, current and
resistance in the circuit. It states that the current I flowing through the
resistance R is proportional to the voltage across two ends of the resistance.
However, the current is inversely proportional to the resistance in the
circuit.
I = U/R
Where:
U — voltage (V)
I — current (A)
R — resistance (Ω)
5. Electrical Work
Electric work refers to the work done by power source force within
certain time, which is represented by W.
W = IUt
Where:
W — electrical work (KW·h)
I — current (A)
U — voltage (V)
t — time (h)
6. Electric Power
The work done by electric field force per unit time is called electric
power, which reflects the speed at which electric field force moves the charge.
It is represented by P.
P = W/t = IU
Where:
P — electric power (W)
W— (KW·h)
t — time (h)
The calculation unit is W.
7. Sine AC
The sine AC refers to voltage and current, the value and direction of
which can vary in a periodic manner according to sine regular pattern. Three
main factors of sine AC are as follows: maximum value (or effective value),
frequency (or cycle) and initial phase (angle).
The cycle of AC refers to the time needed to
complete one cycle, which is represented by the letter T. The unit is second.
Frequency: it refers to the number of complete cycles per second. It
is represented by the letter f. The unit is Hz. The frequency is set 50Hz in
China. The relation between cycle and frequency is in the following:
T =1/f or f = 1/T
Angular frequency: The change of AC for one cycle is equal to 360°. The angular frequency of AC refers to the angular displacement per unit time,
which is represented by ω. Its unit is radian/second. The relation among angular
frequency, cycle and frequency is as follows:
ω =2π/T=2πf
Maximum value: it is also referred to as
amplitude value. It means the maximum value of AC within one cycle.
Effective value: One AC current and one DC current pass through the
same resistor. If the same energy is generated within the same time, the
numerical value of DC current is called effective value of AC current.
Relation between maximum value and effective value: effective value =
1/√2 maximum value or maximum value =√2 effective value.
In the formula u = √2Usin(ωt+φ), (ωt+φ) is an angle and also the function of the time. Certain time
corresponds to certain angle. It shows the progress of sine AC change so we
call the angle of AC at some time as phase.
Initial phase: when t = 0, the phase is called
initial phase. It is represented by φ.
Phase difference refers to the phase difference
of two sine AC with same frequency.
8. Three-phase AC
Three sine AC potential, current and voltage with same frequency and
amplitude and 120°phase discrepancy is called three-phase AC.
In the three-phase power source, the voltage of each phase to earth is
called phase voltage. The current flowing each phase is named as phase current.
In the three-phase power source, line voltage refers to the voltage
between any lines. The current flowing three-phase line is called as line
current.
(1) Three-phase load connected in a star shape:
Correlation between phase voltage and line voltage:
Uphase = Uline/√3 or Uline =√3Uphase
Correlation between phase current and line current:
Iphase = Iline
(2) Three-phase load connected in a triangle shape
Correlation between phase voltage and line voltage:
Uphase = Uline
Correlation between phase current and line current:
Iphase = Iline/√3 or Iline =√3Iphase
9. Inductive Reactance
When the AC passes through the inductance coil, inductive potential
will be generated to stop the flow of the current, which is called inductive
reactance. In quantitative terms, the capacitive reactance is the ratio of
effective value of voltage to effective value of current on the inductance
coil.
XL = UL/IL
Where:
XL — inductive reactance (Ω)
UL — effective value of the voltage on the inductance coil (V)
IL — effective value of the current which flows through the inductance
coil (A)
The inductive reactance is related to frequency (f) and the inductance
of the coil (L). The correlation is as follows:
XL = ωL = 2πfL
Where:
L — coil inductance
The common unit of the inductance is H.
10. Capacitive Reactance
When the AC passes through the capacitor, the capacitor will prevent
the AC from passing, which is called capacitive reactance. In quantitative terms,
the capacitive reactance is the ratio of effective value of voltage to effective
value of current. Namely:
Xc = Uc/Ic
Where:
Xc — capacitive reactance (Ω)
Uc — effective value of the voltage on the capacitor (V)
Ic — effective value of the current which flows through the capacitor (A)
The relation among capacitive reactance (Xc), power source frequency
(f) and the capacitance of the capacitor (C) is in the following:
Xc = 1/( ωC) = 1/(2πfC)
Where:
C — the capacitance of the capacitor
The unit of the capacitance is F.
11. Electrical Impedance
The impedance is the measure of the opposition composed of resistance,
inductance and capacitance when the AC flows through the circuit.
In quantitative terms, it is the complex ratio of total voltage to total
current in an alternating current (AC) circuit.
Z = U/I
Where:
Z — impedance (Ω)
U — effective value of total voltage in AC circuit (V)
I — effective value of total current in AC circuit (A)
In the AC circuit, the impedance includes two parts: resistance R and
reactance X. X = XL-XC. The relation between resistance, reactance and
impedance is in the following:
Z = √R²+X²
12. Real Power
In the AC circuit, the power consumed by the resistance is called real
power, which is represented by the letter P. Calculation unit is represented by
the symbol W:
Single phase: P = UI cosφ
Where:
P — active power (W)
U — phase voltage (V)
I — phase current (A)
cosφ — power coefficient
Three-phase: P = √3UI cosφ
Where:
P — active power (W)
U — line voltage (V)
I — line current (A)
cosφ — power coefficient
13. Reactive Power
In the AC circuit, the inductance (capacitance) does not consume the
power. It only exchanges the energy with power source. The power for exchanging
the energy with power source is called reactive power, which is represented by
the symbol Q. The calculation unit is represented by var.
Single phase: Q = UI sinφ
Where:
Q = reactive power (var)
U — phase voltage (V)
I — phase current (A)
Three-phase: Q =√3UI sinφ
Where:
Q = reactive power (var)
U — line voltage (V)
I — line current (A)
14. Apparent Power
The apparent power is the product of voltage and current in the
circuit, which is expressed in S. Unit: vA, KvA. The capacity of transformer is
expressed by apparent power.
Single phase: S = UI
Where:
S — capacity (VA)
U — phase voltage (V)
I — phase current (A)
Three-phase: S =√3UI
Where:
S — capacity (vA)
U — line voltage (V)
I — line current (A)
Relation between apparent power and active power, reactive power:
S2 = P2+Q2
