As a design engineer, you are asked to design a lighting system consisting of a 70-W power supply and two light bulbs as shown in Fig. 2.124. You must select the two bulbs from the following three available bulbs. R 1 = 80 Ω , cost = $ 0.60 ( standard size ) R 2 = 90 Ω , cost = $ 0.90 ( standard size ) R 3 = 100 Ω , cost = $ 0.75 ( nonstandard size ) The system should be designed for minimum cost such that I lies within the range I = 1.2 A ± 5 percent. Figure 2.124
As a design engineer, you are asked to design a lighting system consisting of a 70-W power supply and two light bulbs as shown in Fig. 2.124. You must select the two bulbs from the following three available bulbs. R 1 = 80 Ω , cost = $ 0.60 ( standard size ) R 2 = 90 Ω , cost = $ 0.90 ( standard size ) R 3 = 100 Ω , cost = $ 0.75 ( nonstandard size ) The system should be designed for minimum cost such that I lies within the range I = 1.2 A ± 5 percent. Figure 2.124
Solution Summary: The author explains how to select two bulbs in three available bulbs, such that system should be designed for minimum cost and current range.
As a design engineer, you are asked to design a lighting system consisting of a 70-W power supply and two light bulbs as shown in Fig. 2.124. You must select the two bulbs from the following three available bulbs.
R
1
=
80
Ω
,
cost
=
$
0.60
(
standard
size
)
R
2
=
90
Ω
,
cost
=
$
0.90
(
standard
size
)
R
3
=
100
Ω
,
cost
=
$
0.75
(
nonstandard
size
)
The system should be designed for minimum cost such that I lies within the range I = 1.2 A ± 5 percent.
A dc compound motor having a rating of 10 kW,
1150 r/min, 230 V, 50 A, has the following losses at
full-load:
bearing friction loss
40 W
brush friction loss
==
50 W
windage loss
=
200 W
(1)
total mechanical losses
=
290 W
(2)
iron losses
=
420 W
(3)
copper loss in the shunt field
=
120 W
copper losses at full-load:
(4)
a. in the armature
b. in the series field
c. in the commutating winding
total copper loss in the
500 W
25 W
70 W
armature circuit at full-load
=
595 W
4 What determines the power rating of a ma-
chine?
-5 If we cover up the vents in a motor, its out-
put power must be reduced. Explain.
-6 If a motor operates in a cold environment,
may we load it above its rated power? Why?
An electric motor driving a skip hoist with-
draws 1.5 metric tons of minerals from a
trench 20 m deep every 30 seconds. If the
hoist has an overall efficiency of 94 percent,
calculate the power output of the motor in
horsepower and in kilowatts.
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