The heating category may be conveniently divided into residential (small) and industrial (large) applications.

Residential Heating
Residential heating includes ranges for cooking; hot water heaters; toasters, irons, clothes dryers, and other such appliances; and house heating. These are all resistance loads, varying from a relatively few watts to several kilowatts, most of which operate at 120 V, while the larger ones are served at 240 V; all are single-phase.

The power factor of such devices is essentially unity. The resistance of the elements involved is practically constant; hence current will vary directly as the applied voltage. The effect of reduced voltage and accompanying reduced current is merely to cause a corresponding reduction in the heat produced or a
slowing down of the operation of the appliance or device.

While voltage variation, therefore, is not critical, it is usually kept to small values since very often the smaller devices are connected to the same circuits as are lighting loads, although hot water heaters, ranges, and other larger loads are usually supplied from separate circuits. (Microwave ovens employ high-frequency induction heating and are described below.)

Industrial Heating
Industrial heating may include large space heaters, ovens (baking, heat-treating, enameling, etc.), furnaces (steel, brass, etc.), welders, and high-frequency heating devices. The first two are resistance-type loads and operate much as the smaller residential devices, with operation at 120 or 240 V, single-phase, and at unity power factor. Ovens, however, may be operated almost continuously for reasons of economy, and some may be three-phase units.

Electric Furnaces
Furnaces may draw heavy currents more or less intermittently during part of the heat process and a fairly steady lesser current for the rest; on the whole, the power factor will be fairly high since continuous operation is indicated for economy reasons. The power factor of a furnace load varies with the type of furnace from as low as 60 percent to as high as 95 percent, with the greater number about 75 or 80 percent.

Sizes of furnaces vary widely; smaller units with a rating of several hundred kilowatts are single-phase, while the larger, of several thousand kilowatts, are usually three-phase. Voltage regulation, while not critical, should be fairly close because of its possible effect on the material in the furnace.

Welders draw very large currents for very short intermittent periods of time. They operate at a comparatively low voltage of 30 to 50 V, served from a separate transformer having a high current capacity.

Larger welders may employ a motor-generator set between the welder and the power system to prevent annoying voltage dips. The power fac tor of welder loads is relatively low, varying with the load. The timing of the weld is of great importance and may be regulated by electronic timing devices.

High-Frequency Heating
High-frequency heating generates heat in materials by high-frequency sources of electric power derived from the normal (60-Hz) power supply. High-frequency heating is of two types: induction and dielectric.

Induction heating. In induction heating, the material is conducting (metals, etc.) and is placed inside a coil connected to a high-frequency source of power; the high-frequency magnetic field induces in the material high-frequency eddy currents which heat it.

Because of the skin effect, the induced currents will tend to crowd near the surface; as the frequency is increased, the depth of the currents induced will decrease, thus providing a method of controlling the depth to which an object is heating.

Dielectric heating
In dielectric heating, a poor conducting material (plastic, plywood, etc.) is placed between two electrodes connected to a high-frequency source; the arrangement constitutes a capacitor, and an alternating electrostatic field will be set up in the material.

(Some slight heating will also be set up from the induction effect described above, depending on the conducting ability of the material.) The alternating field passing uniformly through the material displaces or stresses the molecules, first in one direction and then in the other as the field reverses its polarity.

Friction between the molecules occurs and generates heat uniformly throughout the material. Such friction and heat are proportional to the rate of field reversals; hence, the higher the frequency, the faster the heating.

Because of heat radiation from the surface, however, the center may be hotter than the outside layers. Residential-type microwave ovens are an application of dielectric heating.

Oscillators are used as the source of high-frequency power required for both induction and dielectric heating. This is an electronic application, and its characteristics and requirements are described in the following section.

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