308 Electrical Power Systems Technology
BASIC HEATING LOADS
Most loads that are connected to electrical power systems produce a
certain amount of heat, mainly as the result of current flow through resis-
tive devices. In many instances, heat represents a power loss in the circuit,
since heat energy is not the type of energy that the system was intended to
produce. Lights, for instance, produce heat energy as well as light energy.
The conversion of electrical energy to heat energy in a light-producing
load reduces the efficiency of that load device, since not all of the available
source energy is converted to light energy. There are, however, several
types of power conversion systems that are mainly heating loads. Their
primary function is to convert electrical energy into heat energy. Some
basic systems include resistance heating, inductive heating, and dielectric
(capacitive) heating.
Resistance Heating
Heat energy is produced when an electrical current flows through a
resistive material. In many instances, the heat energy produced by an elec-
trical current is undesirable; however, certain applications require con-
trolled resistance heating. Useful heat may be transferred from a resistive
element to a point of utilization by the common methods of heat trans-
fer—convection, radiation, or conduction. A heating-element enclosure is
needed to control the transfer of heat by convection and radiation. For
heat transfer by conduction, the heating element is in direct contact with
the material to be heated. Actual heat transfer usually involves a combina-
tion of these methods.
Figure 12-1 illustrates the principle of resistance heating. The self-
contained heating element uses a coiled resistance wire, which is placed
inside a heat-conducting material and enclosed in a metal sheath. This
principle may be used to heat water, oil, the surrounding atmosphere, or
various other media. This type of heater may be employed in the open air
or immersed in the media to be heated. The useful life of the resistance el-
ements depends mainly upon the operating temperature. As the tempera-
ture increases, the heat output also increases. Basically, the heat energy
produced is dependent upon the current flow and the resistance of the
element; it can be calculated as current squared times resistance (I^2 R).
Induction Heating
The principle of induction heating is illustrated in Figure 12-2. Heat