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Inductors Sheet 1

Copyright 2002 Kilowatt Classroom, LLC.
Inductors
Inductors

IND1
Inductor
Adjustable Inductor
Iron Core Inductor
Powdered Iron Core
Schematic Symbols
17 mh power supply
smoothing reactor.
One-half actual size.
Above: Large DC link reactors
used in 4160 volt 5000 hp VFD.
Left: Reactor used in conjunction
with capacitors for harmonic filtering.
Radio frequency choke
coil wound on ceramic
powdered iron core.
Shown actual size.
Arc Welder Slope Control
Inductor Characteristics


An inductor is created when a conductor is wound into a coil.


The unit of inductance is the Henry - named after the American inventor Joseph Henry. By definition: an in-
ductor has an inductance of one (1) henry if an electromotive force of one (1) volt is induced in the inductor
when the current through the inductor changes at the rate of one (1) ampere per second. The abbreviation for
the Henry is h and mh stands for millihenry.


The inductance of a coil is affected by a number of factors including: the type and size of the core material, the
size of the conductor, and the way in which the coil is wound.


In an electrical circuit, an inductor opposes a change in current. This characteristic has resulted in the term
choke coil, particularly in radio work.



Adjustable inductors are made by changing the amount of core material within the coil. The drawing below
left illustrates a common method of achieving slope control in a welder by raising or lowering the iron core
within the coil. The the AM broadcast band antenna coil pictured below right is tuned by moving the position
of the powdered iron core within the coil form; a non-magnetic tuning wand is required for this adjustment.

Loopstick Antenna Coil
Shown with core slug removed from coil form.
Shown one-half actual size.
Coils (3)

Adjustable Powdered
Iron Core

Non-magnetic
Threaded Rod

Threaded clip fits into
bottom of coil form.
Tuning Wand Adjustment Slot Sheet 2

Copyright 2002 Kilowatt Classroom, LLC.
AC Theory
Inductive Reactance - Page 1
AC Theory

XL1

Inductive Reactance is the opposition to the flow of current in an electrical circuit due to inductance and is
measured in ohms.


The symbol for reactance is X; inductive reactance is represented by the symbol X
L
.


The formula for inductive reactance is: X
L
= 2 f L



Where: XL = Inductive Reactance in ohms, f = Frequency in hertz, L = Inductance in henrys, 2 = 6.28.


As illustrated by the formula above, inductive reactance is directly proportional to frequency. When an alter-
nating current is applied to an inductor, the inductive reactance will increase as the frequency increases.


The opposition offered to the flow of steady-state Direct Current (DC) by an inductor is equal to the resis-
tance of the inductor only (the ohmic value of the conductor with which the coil is wound ). During the appli-
cation of DC to an inductor, during any fluctuations or ripple, or during de-energization of the coil, inductive
reactance becomes a factor.


An inductor opposes a change in current. The mechanical analogy of inductance is inertia.
Voltage of Self Inductance

When a changing current is applied to an
inductor, a counter electromotive force
(cemf) is generated. This generated voltage
is termed a counter or back emf because
it is in a direction which opposes the applied
voltage.

Figure A of the drawing at the right illus-
trates how this counter emf is generated. As
current is applied to a coil and flows through
the conductors of that coil, an expanding
magnetic field will be established that sur-
rounds each of the conductors. This expand-
ing flux cuts through the adjacent conductors
and induces a voltage in these adjacent con-
ductors. Using the Left Hand Rule, it can be
seen that the direction of this induced volt-
age is in a direction that opposes the applied
DC voltage.

When the switch is opened (or the level of
the applied voltage is reduced), the reverse
effect takes place. The magnetic field will
collapse and effectively cut through the adja-
cent conductors in the opposite direction
than was previously described. The counter
emf will reverse and will oppose the reduc-
tion on the applied voltage. This directional
change is illustrated in Figure B, on the right.
Lenzs Law

The induced EMF in any circuit is always
in a direction to oppose the effect that pro-
duced it.
Remember - To Generate a Voltage:

A conductor can be moved so as to cut the lines of force of a
magnetic field .
Or
An expanding or collapsing magnetic field can cut through
a stationary conductor. Sheet 3

Copyright 2002 Kilowatt Classroom, LLC.
AC Theory
Inductive Reactance - Page 2
AC Theory

XL2
Continued from Page 1
In a purely inductive circuit, the circuit current will lag the applied voltage by 90
o
. This is a theoretical condition,
since any circuit will have some value of resistance or capacitive reactance in addition to the inductance.

In this circuit the current is all reactive and no work will be done. Single-phase power in watts in an AC circuit is:
P =E x I x Cos 0. The phase angle in this case is 90
o
. Since

Cos 90
o
= 0, the circuit power therefore equals zero.

Remember:

There are 360 degrees in a sine wave.

Electrical Phasors rotate counter-clockwise (CCW).

Phasors (electrical vectors) show two things: (1) magnitude, and (2) direction.
(Reference Voltage @ 0
o
)



E
REF
X Observer
CCW Phasor Rotation
(Circuit Current) I
0 = 90
o
Angle of lag
Phasor Diagram
If the observer stands a point X above and watches the phasors rotate CCW,
the voltage phasor will appear first, followed 90
o
later by the current phasor.
Phasor
Axis of
Rotation
In the above drawing, the voltage crosses zero and goes positive 90
o
before the current crosses zero and goes
positive.
0
Phase Angle 0 = 90
0
Lagging (Voltage is Reference)
Sine Wave Relationship
Red - Current
Black - Voltage
Positive 1/2 Cycle
Negative 1/2 Cycle
Zero Amplitude
















































































AC
Circuit Diagram
L
0
o

90
o

180
o

T
0
Time Increasing
Degrees shown for voltage waveform