Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

12.6 Lighting Systems 363

Duty Cycle

Manufacturers of motors classify them according to the amount of time the motor needs to be

operated. The motors are generally classified ascontinuous dutyorintermittent duty. The con-

tinuous duty motors are used in applications where the motor is expected to operate over a

period of an hour or longer. In some applications, continuous operation of the motor may be

required. In applications where the motor is expected to operate for short periods of time and

then rest, the intermittent duty motors are found. The continuous duty motors are more expen-

sive than the intermittent duty motors.

In this chapter, we introduced you to electricity and some basic electrical components.

Most of you will take a basic electrical circuit class where you will learn in more detail about

electricity, electrical components, and motors, so you now see the importance of paying atten-

tion and studying carefully.

12.6 Lighting Systems

In this section, we will provide a brief introduction to lighting systems. Lighting systems account

for a major portion of electricity use in buildings, and have received much attention recently

due to the energy and sustainability concerns. Energy could be saved by reducing illumination

levels, increasing lighting efficiency, or by taking advantage of daylighting. Daylighting refers

to using windows and skylight to bring light into a building to reduce the need for artificial

lighting. As is the case with any new areas you explore, the lightinghas its own terminology.

Therefore, make sure you spend a little time to familiarize yourself with the terminology, so you

can follow the example problems later.

Let us begin by defining illumination. Illuminationrefers to distribution of light on a hor-

izontal surface, and the amount of light emitted by a lamp is expressed in lumens. As a refer-

ence, a 100-watt incandescent lamp may emit 1700 lumens. Another important lighting

characteristic is the intensity of illumination. The intensity of illuminationis a measure of how

light is distributed over an area. A common unit of illumination intensity is called footcandle

and is equal to one lumen distributed over an area of 1-square-foot. To give you an idea of what

a footcandle represents, to find your way around at night you would need between 5 to 20

footcandles. As another example, 30 to 50 footcandles will be needed for office work. If you have

to do detailed work such as fixing electronic equipment or a spring-driven watch, you would

then need around 200 footcandles of illumination intensity.

Efficacy is another term used by lighting engineers. Efficacyis the ratio of how much light

is produced by a lamp (in lumens) to how much energy is consumed by the lamp (in watts).

Efficacy is used by lighting engineers when designing an optimal lighting system for a building

or by an engineer who is doing an energy audit of a building to determine if the lighting sys-

tem is energy efficient. When engineers design a lighting system for a building, they consider

Efficacy

Light Produced 1 lumens 2

Energy Consumed by the Lamp 1 watts 2

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