The basic difference that stands between reactance and impedance is that reactance represents the opposition to an alternating current flow of a circuit but impedance is a combined effect of resistance and reactance of a circuit.
So, reactance is only a part of impedance and impedance is an extended form that represents both the resistance and reactance. Reactance is represented with X and impedance is expressed as Z and both possess the same unit which is the ohm(Ω).
Resistance, Reactance, and Impedance
In the case of a DC circuit, we consider the opposition to current flow termed as resistance but when it comes to AC circuits, there exists frequency. Frequency is the reason for the production of inductance and capacitance in a circuit with inductors and capacitors respectively. Inductance and capacitance are the reasons why the alternating current is opposed in a circuit and thus, reactance is generated in an AC circuit.
In the case of an AC circuit combining resistors, capacitors, and inductors, the total opposition to the flow of current is impedance, which is the vector sum of the resistance, capacitive reactance, and inductive reactance of a circuit.
Impedance
Impedance measures the overall opposition of a circuit to current. As mentioned above, impedance is like resistance but also takes into account the effects of capacitance and inductance. The difference between resistance and impedance is that impedance takes capacitive and inductive reactance which vary with the frequency of the current.
This means impedance varies with frequency. In electrical engineering, impedance is expressed in phasor form. It is the ratio of AC voltage to current expressed in the frequency domain. Thus impedance is a complex number consisting of real and imaginary parts:
Z=R+jX
Here,
Z= Complex impedance
R= Resistance
X= Reactance
The value of impedance Z is given by the following equation:
Z=R2+X2
And the phase angle, θ=XR
Impedance Triangle
The geometrical representation of the impedance of an electrical circuit is the impedance triangle of that circuit. It is a right-angled triangle whose base is the resistance, perpendicular is the reactance and hypotenuse is the impedance of the circuit. The impedance triangle of a circuit having reactance, X and resistance, R with impedance, Z is drawn below:
Now the calculation of the circuit’s impedance is much easier. The triangle helps to calculate the magnitude of the impedance as well as its angle.
Here, the magnitude of impedance, Z = √(R2 + X2)
The angle between R and Z is termed the angle of impedance, θ
From the impedance triangle, tanθ = (X/R)
Thus we can calculate the angle of impedance, θ
The impedance triangle is clear evidence of the fact that the impedance has two components and the reactance is one of the components.
Inductive reactance and impedance
The circuit has both a resistor and an inductor with resistance and inductance, R and L respectively. So, the resistance and the inductive reactance of the circuit oppose the current flowing through it. The total opposition offered to the current is termed the impedance of this circuit. Here, if the angular frequency of this circuit is ω and the inductive reactance of this circuit is XL,
XL=ωL
Here, XL and R both are opposing the current flowing through the circuit.
The impedance of the circuit=Z= jXL+R
The magnitude of the impedance= Z = √(R2+XL2)
The same equation can be derived from its impedance triangle as we’ve seen before. Both the ways of deriving the impedance of a circuit prove that the reactance of the circuit is nothing but a component of the circuit’s impedance.
As current always lags voltage in an inductive circuit, the power factor, in this case, will be lagging power factor.
Capacitive reactance and impedance
Here, the resistance and capacitive reactance of the circuit opposes the AC flowing through it. So, to calculate its total opposition to the alternating current, we need impedance.
Let, the capacitive reactance of the circuit be XC and C be the capacitance of the capacitor.
XC=1/(ωC)
So, the impedance will be jXC+R
The magnitude of the impedance=√(R2+XC2)
As current always leads voltage in a capacitive circuit, the power factor, in this case, will be leading power factor. The power factor derivation from its impedance triangle is the same as we’ve seen before.
Power Factor and Impedance
Suppose a circuit has both inductors and capacitors. The resulting reactance of that circuit will be the difference between inductive reactance (XL) and capacitive reactance(XC) of it.
So, the resulting reactance would be, X= |XL – XC|
If Z is the impedance of this circuit, Z = √(R2 + (|XL – XC|)2)
The angle of impedance is equal to the power factor angle of the circuit. The angle of impedance is the angle between resistance and impedance of the circuit’s impedance triangle. The figures below show the power factor angle and the angle of the impedance of a circuit:
From the impedance triangle(figure-b), tanθ = (X/R)
θ is also the power factor angle (figure-a).
Thus, if we know the angle of the impedance of a circuit we can calculate its power factor, cosθ. The reactance of a circuit is the reason why impedances have angles.
Complex Power Equation for Impedance and Reactance
Complex power is used to find the total effect of parallel loads. It contains all the information about the power absorbed by a given load. Complex power(S) is given by the following equation:
S=I2Z=I2R+jX=P+jQ
Here, P is the real part and Q is the imaginary part of complex power.
P=I2R
Q=I2X
S=I2Z
P is the average power or real power caused by load resistance R. Q is called reactive or quadrature power caused by load reactance X. And S is the complex power caused by load impedance Z.
Can reactance and impedance be equal?
Reactance and impedance can be equal if the resistive component of the circuit is equal to zero. But that doesn’t mean they are the same.
Conclusion
The reactance and resistance show individual effects and to know the total outcome of these effects, we must calculate impedance. Impedance is the effective opposition to the alternating current of a circuit that is generated from the individual effects of resistance and reactance of that circuit.
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