Inductive and Capacitive Reactance

 Inductive and Capacitive Reactance

In a purely resistive AC circuit, opposition to current flow is
called resistance. In an AC circuit containing only inductance,
capacitance, or both, opposition to current flow is called
reactance. Total opposition to current flow in an AC circuit that
contains both reactance and resistance is called impedance,
designated by the symbol “Z”. Reactance and impedance are
expressed in ohms.

Inductive Reactance

Inductance only affects current flow when the current is
changing. Inductance produces a self-induced voltage (counter
emf) that opposes changes in current. In an AC circuit, current
is changing constantly. Inductance in an AC circuit, therefore,
causes a continual opposition. This opposition to current flow is
called inductive reactance and is designated by the symbol XL

Inductive reactance is dependent on the amount of inductance
and frequency. If frequency is low, current has more time
to reach a higher value before the polarity of the sine wave
reverses. If frequency is high, current has less time to reach
a higher value. In the following illustration, voltage remains
constant. Current rises to a higher value at a lower frequency
than a higher frequency

In a 60 hertz, 10 volt circuit containing a 10 mh inductor, the inductive reactance would be

XL = 2πfL

XL = 2 x 3.14 x 60 x 0.10

XL = 3.768 Ω

Phase Relationship between Current and Voltage in an Inductive Circuit

 Current does not rise at the same time as the source voltage in an inductive circuit. Current is delayed depending on the amount of inductance. In a purely resistive circuit, current and voltage rise and fall at the same time. They are said to be “in phase.” In this circuit there is no inductance. Resistance and impedance are the same

In a purely inductive circuit, current lags behind voltage by 90 degrees. Current and voltage are said to be “out of phase”. In this circuit, impedance and inductive reactance are the same

All inductive circuits have some amount of resistance. AC
current will lag somewhere between a purely resistive circuit,
and a purely inductive circuit. The exact amount of lag depends
on the ratio of resistance and inductive reactance. The more
resistive a circuit is, the closer it is to being in phase. The more
inductive a circuit is, the more out of phase it is. In the following
illustration, resistance and inductive reactance are equal.
Current lags voltage by 45 degrees

Calculating Impedance in a Capacitive Circuit The following formula is used to calculate impedance in a capacitive circuit

In the circuit illustrated above, resistance and capacitive reactance are each 10 ohms. Impedance is 14.1421 ohms.

The following vector illustrates the relationship between resistance and capacitive reactance of a circuit containing equal values of each. The angle between the vectors is the phase angle represented by the symbol θ. When capacitive reactance is equal to resistance the resultant angle is -45 degrees. It is this angle that determines how much current will lead voltage.

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