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ation that cannot be controlled—once it’s on, it’s on at a set power level.
One may then wonder how the cooking temperature and duration can be
controlled. This is accomplished through a controlling network that
determines the amount of off and on time during the input cycle of the
120-V supply. Higher temperatures are achieved by setting a high ratio
of on to off time, while low temperatures are set by the reverse action.
One unfortunate characteristic of the magnetron is that in the conversion
process, it generates a great deal of heat that does not go toward the
heating of the food and that must be absorbed by heat sinks or dispersed
by a cooling fan. Typical conversion efficiencies are between 55% and
75%. Considering other losses inherent in any operating system, it is
reasonable to assume that most microwaves are between 50% and 60%
efficient. However, the conventional oven with its continually operating
exhaust fan and heating of the oven, cookware, surrounding air, and so
on, also has significant losses, even if it is less sensitive to the amount
of food to be cooked. All in all, the convenience factor is probably the
other factor that weighs the heaviest in this discussion. It also leaves the
question of how our time is figured into the efficiency equation.
For specific numbers, let us consider the energy associated with baking
a 5-oz potato in a 1200-W microwave oven for 5 min if the conversion
efficiency is an average value of 55%. First, it is important to realize
that when a unit is rated as 1200 W, that is the rated power drawn
from the line during the cooking process. If the microwave is plugged
into a 120-V outlet, the current drawn is
I � P/V � 1200 W/120 V � 10.0 A
which is a significant level of current. Next, we can determine the
amount of power dedicated solely to the cooking process by using the
efficiency level. That is,
P o � hP i � (0.55)(1200 W) � 600 W
The energy transferred to the potato over a period of 5 min can then be
determined from
W � Pt � (660 W)(5 min)(60 s/1 min) � 198 kJ
which is about half of the energy (nutritional value) derived from eating
a 5-oz potato. The number of kilowatthours drawn by the unit is determined
from
W � Pt/1000 � (1200 W)(5/60 h)/1000 � 0.1 kWh
At a rate of 10¢/kWh we find that we can cook the potato for 1 penny—
relatively speaking, pretty cheap. A typical 1550-W toaster oven would
take an hour to heat the same potato, resulting in 1.55 kWh and a cost
of 15.5 cents—a significant increase in cost.
Household Wiring
A number of facets of household wiring can be discussed without
examining the manner in which they are physically connected. In the
chapters to follow, additional coverage will be provided to ensure that
you develop a solid fundamental understanding of the overall household
wiring system. At the very least you will establish a background that
will permit you to answer questions that you should be able to answer
as a student of this field.
APPLICATIONS ⏐⏐⏐ 115