Page 101 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

AC MACHINE FUNDAMENTALS

5.2 CHAPTER FIVE

FIGURE 5.1 A simple three-phase stator. Currents
in this stator are assumed positive if they flow into the
unprimed and out of the primed ends of the coils.

An examination of the currents and their corresponding magnetic flux densities at spe-
cific times is used to determine the resulting net magnetic flux density. For example, at time
t 0°, the magnetic field from coil aa′ will be
B 0
aa'
The magnetic field from coil bb′ will be
B B sin ( 120°) 120°
bb' M
and the magnetic field from coil cc′ will be
B B sin ( 240°) 240°
cc' M
The total magnetic field from all three coils added together will be
B B B B
net aa' bb' cc'
3 3

0 B M 120° B 240°
M
2
2
1.5B 90°
M
As another example, look at the magnetic field at time t 90°. At that time, the cur-
rents are
i I sin 90° A
aa' M
i I sin ( 30°) A
bb' M
i I sin ( 150°) A
cc' M
and the magnetic fields are

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.