Page 85 - Electric Machinery Fundamentals

INTRODUCTION TO MACHINERY PRINCIPLES 61

T
IOem
_I

------
------ --.l_
-
,..-----k N = 1,000 turns O.05Cffi 30cm
T

----1------

(
IDcm
~

L 10 em ~1-----30 em------II I
I I 5em
Depth =5 em
FIGURE PI- II
The core of Problem 1-14.

1- 15. A transformer core with an effective mean path length of 6 in has a 200-turn coil
2
wrapped around one leg. Its cross-sectional area is 0.25 in , and its magnetization
curve is shown in Figure 1-10c. If current of 0.3 A is flowing in the coil, what is the
total flux in the core? What is the flux density?
1- 16. The core shown in Figure Pl-2 has the flux (tJ shown in Figure PI-l2. Sketch the
voltage present at the terminals of the coiL
1-17. Figure PI- 13 shows the core of a simple de motor. The magneti.zation curve for the
metal in this core is given by Figure l-lOe and d. Assume that the cross-sectional
2
area of each air gap is 18 cm and that the width of each air gap is 0.05 cm. The ef-
fective diameter of the rotor core is 5 cm.
(a) We wish to build a machine w ith as great a flux density as possible while avoid-
ing excessive saturation in the core. What would be a reasonable maximum flux
density for this core?
(b) What would be the total flux in the core at the flux density of part (a)?
(e) The maximum possible field current for this machine is 1 A. Select a reasonable
number of turns of wire to provide the desired flux density while not exceeding
the maximum available current.
(
1-18. Assume that the voltage applied to a load is V = 208L -30 V and the CUlTenr flow-
0
ing through the IO<ld is I = 2L20° A.
(a) Calculate the complex power S consumed by this load.
(b) Is this load inductive or capacitive?
(c) Calculate the power factor of this load.