Download Inductor Basics notes

Introductory Analog
Electronics
Ctec 101.
Inductor Basics
Supplement
Prepared by Mike Crompton. (Rev. 31 March 2005)
Inductance
An inductor is a series of turns of wire around a core material. The core can be air, soft
iron in solid of powdered form, graphite or alloys of different metals chosen for their
magnetic properties. The inductance of a coil (inductor) is dependant on:
1. Number of turns
2. Area (How close the turns are to each other)
3. Core material
The theoretical D.C. resistance of a pure inductor is 0, therefore they pass (offer no
resistance) to DC. In practice there is some resistance due to the resistance of the wire.
The principal of induction relies on two theories.
1. When current flows through a wire, a magnetic field is established around the
wire. The size of the field is determined by the amount of current. If the current
varies, the field varies.
2. When a moving magnetic field “cuts” a conductor, a voltage is induced.
(Generator Principle).
In 2 above, if the moving magnetic field is the result of varying current flow through a
conductor, then the varying current is created by a varying voltage source (VSUPPLY)
somewhere in the circuit. The induced voltage in 2 above is always the opposite polarity
to VSUPPLY.
Inductance is measured in Henry’s (H, mH or H) and the letter ‘L’ is used to denote an
inductor.
The simple circuit of Fig 1 will be used to explain
induction. The coil ‘L’ has very few turns, the first
turn (T1) and the last turn (T5) are deliberately
spaced far apart.
Fig. 1
L1
T1 T2 T3 T4 T5
S1
12V
At the instant S1 is closed the l2VDC will appear
across the coil and current will start to increase
from 0A and flow into the first turn (T1). In doing
so it creates an expanding (moving) magnetic field
around T1. This expanding field cuts the other turns (T2-T5) and induces a voltage in
them.
The induced voltage is the opposite polarity to the DC source and tries to force current
the opposite way. This of course stops the source current from increasing. (Doesn’t stop
it, just stops it from increasing further). Once the source current stops increasing the
magnetic field stops moving (still there, just isn’t moving). With no movement of the
2
I Vind in each turn (T1-T5)
field the induced voltage disappears, the
stops current from
current starts to increase again and flows
rising at each of
into T2. The increasing current forms an
T5
these
points
expanding magnetic field that induces a
T4
voltage into the other turns (T1 & T3 –
T5). This induced voltage is in opposition
T3
to the DC source and tries to force current
T2
the opposite way ..etc…etc. Eventually the
current (which actually increases in an
T1
exponential curve, not in steps as shown in
Fig 2) will reach maximum value but will
Imax
Switch
have been delayed considerably by
reached
Closed
comparison to the circuit voltage, which
reached max. the instant SW1 closed.
Notice that the delay or “opposition to current flow” is created by the induced voltage
forcing current to flow in the opposite direction, and that the greater, or quicker the
current changes the greater the induced voltage and the greater the opposition to current
flow.
When the switch is opened again, the current will drop to zero almost instantly and the
magnetic field will collapse very quickly. This very fast collapse can induce an extremely
large voltage in the coil that can be in the many thousands of volts, creating spectacular
effects or doing severe damage. The amount of voltage induced in an inductor, some
times called “Back EMF” or “Kick-Back” is calculated by the formula:
VIND = L(dI / dt)
Where L is inductance in Henrys
dI is the CHANGE in current.
dt is the CHANGE in time.
Notice that the amount of induced voltage has little or nothing to do with the supply
voltage and everything to do with the rate of change of current.
The foregoing is an over-simplified description of “self inductance” but should help to
create a “mental picture” of what happens every time the current tries to change in any
way (increase or decrease) when there is an inductor in the circuit.
When an AC is applied to an inductor the circuit current will change with the voltage of
the applied AC. This means there will always be a changing magnetic field and always an
induced voltage that will always oppose the current flow. This is one of the major
attributes of inductors, they tend to block or oppose AC and pass DC. This will be
studied at length in a later course.
3
Fig.2
time
Switch
Opened