MM0704, Lesson 3
GENERATORS AND MOTORS
Task. The skills and knowledge taught in this lesson are common to all missile repairer tasks.
Objectives. When you have completed this lesson, you should be able to explain the purpose and function of AC and
DC motors and generators and describe their uses.
Conditions. You will have this subcourse book and work without supervision.
Standard. You must score at least 70 on the end-of-subcourse examination that covers this lesson and lesson 1, 2, and
4 (answer 23 of the 30 questions correctly).
DC GENERATORS AND MOTORS
The basic DC generator is similar to the basic AC generator except in the system of taking the output from the rotating
coil. In the AC generator, the rotating coil is connected to the external circuit by the sliprings that convey the AC
current in the coil to the external circuit. In the DC generator, the rotating coil is connected to the external circuit by a
device called a commutator. The commutator converts the AC current from the rotating coil to current that flows in a
single direction through the external circuit.
A commutator is a metal ring (usually copper) that is divided into a number of segments. These segments are insulated
from each other and from the shaft upon which they and the rotating coil are mounted. In the basic generator shown in
figure 3-1A, the commutator consists of two segments separated by air, with each segment connected to one side of the
rotating coil. Two brushes, mounted on opposite sides of the commutator, bear on its surface so that electrical contact
is made between the coil and the external circuit.
Figure 3-1A shows the commutator positions during one revolution of the coil. For coil positions a through e, the
current in the rotating coil flows in the direction indicated by the arrows and undergoes the changes in magnitude
shown by the corresponding points on the curve in figure 3-1B. Verify this by using the left-hand rule for generators.
At position e, the coil has completed one alternation, and the induced voltage is zero.
During the second alternation, the coil sides cut the field in the opposite direction. Current flow is thus reversed as
shown in figure 3-1B. This means that at the moment when the current in the coil sides changes its direction, the
commutator segments reverse their connections to the brushes. It also means that the current flow through the load is
in the same direction during the second alternation as it was during the first alternation. This is indicated by points e to
i on the curve in figure 3-1B.