Back to main page
Inductive Reactance

Read pages 163 to 166 of your textbook.

Reactance is the property of an inductor or capacitor that opposes AC current flow.

Reactance has the symbol X, and the unit is Ohms.

The symbol for Inductive Reactance is XL .

Inductive reactance is frequency dependent, and is given by the following equation

XL = 2pfL

Where:

XL = inductive reactance (ohm)

f = frequency (Hz)

L = inductance (H)

Example: Find the reactance of a 1mH inductor at 2kHz.

On your calculator you'd do this
XL = 2´p´2exp3´1exp-3 = 12.57W

where exp is your exponent key

You might like to try the onscreen calculator below to verify this. For p use PI, for multiplication use * and for exponents use e. Press the calc button for your answer, which will be expressed in engineering notation.

Also try the following calculations
(a) A 15H inductor at 50Hz  (ans = 4.71kW)
(b) A 2.5mH inductor at 3.5kHz  (ans = 55W)
(c) A 550mH inductor at 1.278MHz  (ans = 4.42kW)

Once the reactance of an inductor is calculated, Ohm's Law can be used in exactly the same manner as with resistors, except that we use XL instead of R.  E.g for the following circuit, find the RMS current i.

Firstly, calculate the inductive reactance XL = 3.77W
Then we calculate i = V/XL = 10V/3.77W = 2.65A
 

Self-help Exercises  Answer questions 1 to 6 on page 171 of your textbook.
Also answer the following questions:

1. Define the term inductive reactance.

2. A 100 mH inductor is connected to a 1VRMS AC 10 kHz signal. Calculate
(i) the inductor's reactance
(ii) the RMS current

3. If the frequency in question 2 (above) is doubled, what would be the effect on the current?

Feel free to email your responses to your tutor if you would like clarification.