Unit 17
Inductance in AC Circuits
Unit 17 Inductance in AC Circuits
Objectives:
• Discuss the properties of inductance in an
alternating current circuit.
• Discuss inductive reactance.
• Compute values of inductive reactance and
inductance.
• Discuss the relationship of voltage and
current in a pure inductive circuit.
Unit 17 Inductance in AC Circuits
Objectives:
• Be able to compute values for inductors
connected in series or parallel.
• Discuss reactive power (VARs).
Unit 17 Inductance in AC Circuits
A continually changing magnetic field induces a
voltage into any conductor.
Unit 17 Inductance in AC Circuits
As current flow
increases through
a coil, a magnetic
field is created
around the coil.
Unit 17 Inductance in AC Circuits
As current flow
decreases
through a coil,
the magnetic
field collapses.
Unit 17 Inductance in AC Circuits
The applied voltage and induced voltage are 180°
out of phase.
Unit 17 Inductance in AC Circuits
• Inductive reactance is the current-limiting
property of an inductor.
• The inductive reactance symbol is XL .
• The unit of measurement is in ohms, just
like resistance, and Ohm’s law formulas can
be used.
Unit 17 Inductance in AC Circuits
Inductive Reactance
XL = 2πfL
XL = inductive reactance
2 = a constant
π = 3.1416
F = frequency in hertz (Hz)
L = inductance in henrys (H)
Unit 17 Inductance in AC Circuits
Coils with turns close together produce more
inductance than coils with turns far apart.
Unit 17 Inductance in AC Circuits
Three factors that determine induced voltage:
1. The number of turns of wire.
2. The strength of the magnetic field.
3. The speed of the cutting action.
Unit 17 Inductance in AC Circuits
Circuit current is limited by inductive reactance.
XL = 2πfL = 377 x .8 = 301.6 Ω
I = E / XL = 120 V / 301.594 Ω = 0.398 A
Unit 17 Inductance in AC Circuits
Inductors connected in series.
LT = L1 + L2 + L3
XLT = XL1 + XL2 + XL3
Unit 17 Inductance in AC Circuits
Inductors connected in parallel.
1/LT = 1/L1 + 1/L2 + 1/L3
1/XLT = 1/XL1 + 1/XL2 + 1/XL3
Unit 17 Inductance in AC Circuits
Applied Voltage
Current Flow
In a pure inductive circuit, the current lags the
applied voltage by 90°.
Unit 17 Inductance in AC Circuits
Reactive Power
VARs = EL x IL
VARs = EL2 / XL
VARs = IL2 x XL
EL = voltage applied to an inductor
IL = current flow through an inductor
XL = inductive reactance
Unit 17 Inductance in AC Circuits
Review:
1. Induced voltage is proportional to the rate of
change of current.
2. Induced voltage is always opposite in polarity to
the applied voltage.
3. Inductive reactance is a countervoltage that
limits the flow of current, as does resistance.
Unit 17 Inductance in AC Circuits
Review:
4. Inductive reactance is measured in ohms.
5. Inductive reactance is proportional to the
inductance of the coil and the frequency
of the line.
6. Inductive reactance is symbolized by XL.
7. Inductance is measured in henrys (H) and
is symbolized by the letter L.
Unit 17 Inductance in AC Circuits
Review:
8. When inductors are connected in series,
the total inductance is equal to the sum of
all the inductors.
9. When inductors are connected in parallel,
the reciprocal of the total inductance is
equal to the sum of the reciprocals of all
the inductors.
Unit 17 Inductance in AC Circuits
Review:
10. The current lags the applied voltage by 90°
in a pure inductive circuit.
11. Reactive power is measured in VARs.