8.3. INDUCTORS
ELECTRICAL CHARACTERISTICS OF INDUCTORS
A) Inductors at dc
If we connect an inductor to a dc-voltage-source we will find:
it takes some time till the current has reachted its full value. The amount of
current flowing at last will be depending only on the voltage connected and the
ohmic resistance (the resistance of the wire).
fig. 63
B) Inductor at ac
If we connect the same inductor to an ac-voltage of the same
magnitude than the dc was, we find a much smaller current flowing.
We know, that the resistance of the copperwire will not have
changed. But we know too, that in every circuit the current is governed by Ohm's
Law.
So the decreased current is only possible if the ac-voltage has
somehow decreased in the circuit.
We know too that in a coil in case of change of current is
produced a so called BACK-EMF.
Applying this knowledge we can assume the shape of the back emf.
It will be slightly smaller but exactly inverse to the original voltage.
The rest of the voltage left over is necessary for the resistance of the
copperwire. We know too that the amount of back emf is depending on the change
of current in the coil. Applying this we can state: the current must have its
fastest changes when the back emf has its maximum. So the back emf has an
opposite direction the current must change positively to produce a negative back
emf and vice versa. Also we can state: the current must not change anyhow while
the back emf is “0”. We have got now four conditions for the shape of
the current flowing in the circuit of an inductor connected to an ac-voltage. We
can easily foresee, that the current will flow in the shape of a sinewave.
fig. 64
We can also predict some facts about the phase relation between
voltage and current.
fig. 67
PHASE RELATION
As we concluded above at an inductor the current must be LAGGING
BEHIND THE VOLTAGE. The biggest phaseshift possible is 90 degrees. But this
value cannot be reached in practice.
FREQUENCY RESPONSE
As we found the back emf depends on the change of the current.
If the frequency of the sinewave is higher we will find a faster change of
current, and therefore there will be produced a bigger back emf.
So we can derive: as higher the frequency as lower will be the
current - this means as higher the frequency as higher the impedance of the
inductor.
The impedance of the inductor is to be calculated by the
formula:
And the inductive reactance of the inductor can be calculated by
the formula:
MAIN FUNCTIONS OF INDUCTORS
Inductors can be divided into two main groups:
A) LOW- (or audio-) FREQUENCY INDUCTORS. They are
used for smoothing the dc of the supply or for letting through only audio
frequency and to cut off high frequencies.
fig. 68
B) HIGH- (or radio-) FREQUENCY INDUCTORS They are used in tuned
circuits and for aerial coils onferrite rods. They are small compared with those
of group A and often they do not even have an ironcore.
figure
If the frequency is very high the wires are mode out of a lot of
extremly thin strands in order to avoid the so called “skin
effect”.
SPECIALITIES:
A) Sometimes we find in superhet receivers for cars
instead of variable capacitors so called variable inductors used for the tuned
circuits.
B) To cut off very high frequencies in wires entering a special
part of a radio or another electronic equipment, there are used simple pearls of
ferrite which are just put on the wire.
CHECKING AND HANDLING OF INDUCTORS
The possibilities to check an inductor by simple means are very
limitted.
With a normal AVO-meter we can just measure the ohmic resistance
across the two terminals. But we will hardly read more than 10 Ohms. Mostly we
will measure values near “0” Ohms and if we find that, we can be sure
that the wire is at least not broken, but we do not know
- if there is short circuited winding, or
- if
there is anything wrong with the ironcore.
These kinds of faults can cause very considerable changes of the
inductivity of the inductor.
If we suspect a fault like that we can only replace the inductor
by one of the same type, or we have to measure the impedance of it at the
frequencies it is meant for.
PLEASE KEEP IN MIND: There are always found inductors with an
iron core which resembles a screw. They are meant for adjustment but....
NEVER TRY TO ADJUST THESE INDUCTORS ONLY FOR FUN AND WITHOUT
MEASURING THE SIGNALS PRODUCED! YOU WILL MISALIGN THE RADIO!!
NEVER TOUCH THE IRON CORES WITH A NORMAL SCREWDRIVER. YOU MIGHT
MAGNETIZE IT, AND THAN IT WILL NOT WORK PROPERLY ANYMORE.
CHECK YOURSELF
1. What are passive components
2. Compare the behaviour of a) resistors, b) capacitors, d)
inductors at ac voltages with different frequencies.
3. Explain the term phaseshift and state which kind of
phaseshift we find at a) resistors, b) capacitors and c)
inductors.
