A good place to start is the tabulation of all electric devices (lamps,
appliances, equipment, etc.) that consumers can connect to their supply
system. The ratings of the devices at specified voltages (and sometimes
frequency and temperature) limits are usually contained in the nameplate
or other published data accompanying the devices. The devices can
be classified into four broad general categories: lighting, power, heating,
and electronic. Each of these has different characteristics and requirements.
Lighting Loads
Included under lighting are incandescent and fluorescent lamps,
neon lights, and mercury vapor, sodium vapor, and metal halide lights.
Nominal voltages specified for lighting are usually 120, 240, and 277
Volts (variations may exist from the base 120-V value, e.g., 115 and 125
V). All operate with dc or single-phase ac; the discussion will be in terms
of ac, with comments concerning dc operation where applicable.
Incandescent Lighting
Incandescent lamps operate at essentially unity power factor.
Their light output drops considerably at reduced voltage, being some
16 percent less with a 5 percent lowered voltage, and decreasing at a
geometrically faster rate from then on. They are also sensitive to sudden
rapid voltage variations, producing a noticeable (and annoying) flicker
at variations of as little as 3 Volts (on a 120-V base). Street lighting of the
incandescent type can be operated in a multiple or a series fashion. The
former operates as other lighting in a multiple or parallel circuit, while
the light output for the series type depends on the amount of deviation
from the standard value of current flowing through it (usually 6.6, 15,
or 20 A); it is sensitive to variations of as little as 1 percent in the value
of the current. The life of incandescent lamps is considerably reduced at
voltages appreciably above normal.
Fluorescent and Neon Lighting
Fluorescent lamps and neon lights operate at power factors of
about 50 percent, but usually have corrective capacitors included so
that, for planning purposes, they may also be considered to operate at
100 percent or unity power factor. Their light output, per unit input of
electrical energy, is considerably greater (25 percent or more) than that
of a similarly rated incandescent lamp. The life of fluorescent lamps
and neon lights is affected by the number of switching operations they
undergo. If fluorescent lamps are used on dc circuits, special auxiliaries
and series resistance must be employed; operation is inferior to that on
ac, with much less light produced per unit of energy and rated life reduced
20 percent. Neon lights are not usually employed on dc circuits.
Fluorescent lamps, neon lights, mercury and sodium vapor, and metal
halide lights may, if improperly installed or when deteriorating, cause
radio and TV interference.
High-intensity Vapor Lighting
Mercury vapor (high pressure) and sodium vapor (high and low
pressure) and metal halide lights operate at power factors of 70 to 80 percent,
but also are associated with capacitors to raise the effective value
to 100 percent. They are not as susceptible to voltage variations as are
incandescent lamps.
Their light output and life expectancy are greater
than those for fluorescent lamps. They may be employed on dc circuits,
but require additional starting auxiliaries. They are generally restricted
to applications where large amounts of lighting are desirable, such as
on expressways, in large manufacturing areas, or in photographic work;
they are somewhat more expensive than other types and have the disadvantage
of taking some time after being energized before maximum
light output occurs.
Power Loads
Generally included in power loads are motors of all sizes: direct
current shunt, compound and series types; alternating current singlephase
and polyphase, induction and synchronous types; and universal
(series) for both dc and ac operation. Table 3-1 summarizes the characteristics
and general application of these various types of motors.
Single-phase Fractional-horsepower Motors
The majority of fractional horsepower motors, generally used in
appliances of various kinds, are single-phase and operate at power factor
values of 50 to 70 percent, but many have corrective capacitors associated
with them. When they operate without speed controls or starters,
their starting currents may cause lights on the same circuit to flicker;
where starts are relatively frequent, as with refrigerators and oil burners,
the flicker may be annoying.
Induction Motors
Most commercial and industrial ac motors are of the induction
type; limited speed control may be obtained in some types by varying
the applied voltage. Where accurate speed control is desirable, such as
for elevators and printing presses, dc motors are employed, sometimes
served from ac sources through motor-generator sets. Induction motors
may operate at power factors of 50 to 95 percent but generally operate
on the order of 80 to 90 percent; at less than full load, the power factors
may drop to 50 to 60 percent. Most large motors for industrial loads
(from about 2 hp and larger) are usually three-phase (although many
older two-phase motors still exist). Voltage variations of about—10 percent
can be accommodated with little lowering of motor efficiency and
power factor values.
Synchronous Motors
Synchronous motors, usually of large sizes, can operate at power
factors leading or lagging 100 percent by adjusting their excitation: overexcitement
draws leading current, under-excitement lagging current.
Often this type of motor is used for power factor correction for the entire
installation.
Since larger motors are apt to cause voltages to dip when starting,
circuits separate from lighting circuits are provided to eliminate flicker
problems; sometimes separate supply transformers are also provided.
Also causing similar flicker problems are chemical and electrolytic devices
and mechanical devices operated by coils or solenoids.
Heating Loads
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
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
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.
Electronic Loads
The electronic load category includes radio, television, x-rays,
laser equipment, computers, digital time and timing devices, rectifiers,
oscillators for high-frequency current production, and many other elec
tronically operated devices. In general, these employ electron tubes or
solid-state devices such as transistors, semiconductors, etc. Practically
all of these devices operate at voltages lower than the commercial power
sources and employ transformers or other devices to obtain their specific
voltages of operation.
They are all affected by voltage variations.
Voltage variations may have a marked effect on electron tubes, affecting
their current-carrying abilities or emissions as well as their life
expectancy. Because of the reduced life of the heater element and higher
rate of evaporation of active materials from the cathode surface, the cathode
life of electron tubes may be reduced as much as one-half by only a
5 percent rise in cathode voltage. Industrial-type tubes are normally designed
to operate with a voltage tolerance of ± 5 percent, though closer
tolerances are often specified.
While voltage variations also affect the operation of solid-state devices,
the effect on their life expectancy is not as serious as in the case of
electron tubes. On the other hand, variations in frequency of the power
supply have little effect on electron tubes but may have a pronounced
effect on solid-state devices.
Both types of devices are very sensitive to voltage dips, and, from
the power supply viewpoint, operate at essentially unity power factor.
Some applications, such as computers, may require an uninterrupted
source of supply, and various schemes are employed to achieve this, including
the use of motor-generator sets capable of running on batteries
for a limited time; the motor-generator set also eliminates the problems
of voltage dips on the commercial power supply.
Except for some rectifier applications, most of these devices operate
from single-phase ac supply circuits; large rectifiers may be supplied
from three-phase sources.
Oscillators for commercial purposes employ industrial-type electron
tubes in conjunction with capacitors and inductances that may be
varied to produce the desired high-frequency sources. The regular tolerances
in voltage supply from commercial power sources are suitable for
this application.
