Page 100 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

Source: ELECTRICAL EQUIPMENT HANDBOOK

CHAPTER 5
AC MACHINE

FUNDAMENTALS

AC machines are motors that convert ac electric energy to mechanical energy and genera-
tors that convert mechanical energy to ac electric energy. The two major classes of ac
machines are synchronous and induction machines. The field current of synchronous
machines (motors and generators) is supplied by a separate dc power source while the field
current of induction machines is supplied by magnetic induction (transformer action) into
the field windings.
AC machines differ from dc machines by having their armature windings almost always
located on the stator while their field windings are located on the rotor. A set of three-phase
ac voltages is induced into the stator armature windings of an ac machine by the rotating
magnetic field from the rotor field windings (generator action). Conversely, a set of three-
phase currents flowing in the stator armature windings produces a rotating magnetic field
within the stator. This magnetic field interacts with the rotor magnetic field to produce the
torque in the machine (motor action).

THE ROTATING MAGNETIC FIELD

The main principle of ac machine operation is this: A three-phase set of currents, flowing
in an armature windings, each of equal magnitude and differing in phase by 120°, produces
a rotating magnetic field of constant magnitude.
The stator shown in Fig. 5.1 has three coils, each 120° apart.
The currents flowing in the stator are given by
i (t) I sin t A
aa'
M
i (t) I sin ( t 120°) A
bb' M
i (t) I sin ( t 240°) A
M
cc'
The resulting magnetic flux densities are
B (t) B sin t 0° Wb/m 2
M
aa'
B (t) B sin ( t 120°) 120° Wb/m 2
bb' M
B (t) B sin ( t 240°) 240° Wb/m 2
cc'
M
The directions of these fluxes are given by the right-hand rule. When the fingers of the right
hand curl in the direction of the current in a coil, the thumb points in the direction of the
resulting magnetic flux density.
5.1
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.