Chapter 16

Inductive AC Circuits

• Objectives– After completing this chapter, the student

should be able to:• Describe the phase relationship between current

and voltage in an inductive AC circuit.

• Determine the inductive reactance in an AC circuit.

• Explain impedance and its effect on inductive circuits.

• Describe how an inductor-resistor network can be used for filtering and phase shifting.

• Explain how low-pass and high-pass inductive circuits operate.

• Inductance in AC circuits– Inductors offer opposition to current flow.

• Voltage placed across an inductor creates a magnetic field.

• When AC voltage changes polarity, it causes the magnetic field to expand and collapse.

• Voltage is induced in the inductor coil called a counter-electromotive force (CEMF).

• CEMF– 180 degrees out of phase with the applied

voltage.– Opposes the applied voltage.– Opposition is as effective in reducing current

flow as a resistor.

• Inductive reactance– The opposition offered to current flow by an

inductor.– Measured in ohms.– Depends on its inductance and the frequency of

the applied voltage.

– Expressed by the symbol XL.

• The formula for determining inductive reactance is:

XL= 2fL

where: = pi or 3.14.

f = frequency in hertz.

L = inductance in henries.

• Applications of inductive circuits– Inductors are widely used in electronics.

• Compete with capacitors for filtering and phase shift applications.

– Inductors have fewer applications than capacitors because they are:

• larger.• heavier.• more expensive.

• Inductors provide a reactive effect while still completing a DC circuit path.

• Capacitors provide a reactive effect, but block the DC elements.

• Inductors and capacitors are sometimes combined to improve the performance of a circuit.

• Series RL networks are used as high- and low-pass filters.

• The frequency above or below the frequencies passed or attenuated is called the cut-off frequency.– Symbol is fco.

– Can be determined by the formula:

fL2R

C

where fco = cut-off frequency in hertz.

R = resistance in ohms.

= 3.14.

f = frequency in hertz.

L = inductance in henries.

• In Summary– In a pure inductive circuit, the current lags the

applied voltage by 90 degrees.– Inductive reactance

• the opposition to current flow offered by an inductor in an AC circuit.

• symbol is XL.• measured in ohms.• Formula: XL = 2fL.

• Impedance

• RL circuits used for:– High-pass filters.– Low-pass filters.