1-19
high Q (discussed briefly at the end of this section) are generally required. These coils usually are single-
layered with air or metallic cores. Since comparatively low values of inductance are required, this type of
coil is very compact, and relatively high values of Q are obtained.
At frequencies in the lf and mf regions of the frequency spectrum, single-layered, universal, spiral,
and other types of windings are used. When size is a factor, the more compact windings are preferred to
the single-layered type of coil. At frequencies below 500 kilohertz, the single-layered type is too large for
practical use; therefore, the more compact types are used exclusively.
The inherent resistance of the conductor with which an inductor is wound is the most important
factor contributing to the losses of the inductor. Losses caused by this resistance increase with frequency.
This results in a concentration of current near the outer surface of the wire, called SKIN EFFECT. Skin
effect is negligible at low frequencies, but can be an important factor at high frequencies. Other
contributing factors to inductor losses are (1) eddy currents set up in the core and surrounding objects (if
they are conductors); (2) the dielectric properties of the form used for the coil and surrounding objects;
and (3) hysteresis in the core and surrounding objects, if they are magnetic metals. Losses occur as a
result of the dielectric properties of the coil form because of the distributed capacitance of the inductor
(for example, between turns and between the terminals and leads). To some extent the core and
surrounding objects serve as a dielectric of the distributed capacitance, and the resulting dielectric losses
contribute to the overall losses of the inductor.
As we discussed earlier, an inductor has the ability to act as a storehouse of magnetic energy.
However, because of the various loss factors described above, all of the energy stored in the magnetic
field is not returned to the source when the applied voltage decreases to zero. The losses of an inductor
may be represented by an equivalent series resistance. The value that it would dissipate would be an
amount of energy equal to the total amount dissipated by the inductor. The losses of an inductor may be
expressed in terms of the ratio of its inductive reactance to its equivalent series resistance. This ratio is
referred to as the Q of the inductor and is stated in equation form as shown below:
Q-14.
What type of core produces the greatest inductance?
HAY BRIDGE
Inductance measurements are seldom required in the course of troubleshooting. However, in some
cases inductance measurements are useful and instruments are available for making this test. Many
capacitance test sets can be used to measure inductance. Most manufacturers of capacitance test sets
furnish inductance conversion charts if the test equipment scale is not calibrated to read the value of
inductance directly. For the measurement of inductance, the following basic types of test equipment
circuitry are used: (1) the bridge-circuit type, which is the most accurate, and (2) the reactance type,
which is often an additional test circuit incorporated into another piece of test equipment to increase its
utility. The measurement of capacitance using the capacitance-inductance-resistance bridge instrument
was discussed. Since the measurement of capacitance and inductance are interrelated, the existing
capacitance standards and loss controls of this test equipment are used whenever possible. A wider range
of dissipation must be provided to accommodate the practical value of inductors. The 250DE+1325 (view
A of fig. 1-14), a typical rcl bridge and our reference in this discussion, uses two basic bridge circuits
(Hay bridge and Maxwell bridge) to accommodate the extensive range in inductor loss factors. You
should take time to review the bridges in NEETS, module 16, or other bridge-circuit descriptions before
continuing.
