MM0704, Lesson 3
Figure 3-3. Shunt Generator.
field current is made as small as possible. So that the field magnets may be sufficiently excited by a small current, they
are wound with many turns of wire.
In a cumulative-compound generator both series and shunt fields are used. The shunt field is usually the stronger, but,
since both contribute to the magnetic field, they aid each other. When the load increases, the armature voltage decrease
and the voltage applied to the shunt field decreases. The increased load current flowing through the series increases its
field. By proportioning the two fields so that the series fields compensate for the undesirable effects of the armature
and shunt fields, the output voltage may be kept constant.
In a differential-compound generator, the two fields are wound so that they oppose each other; that is, the magnetic
field of the series field is in opposite direction to that of the shunt field. For any increase in load, there is an increase in
the strength of the series field. Also, because its direction is opposite to the shunt field, the total combined field
strength is decreased. Therefore, the output voltage decreases as the load increases. Differential-compound generators
are used in welding equipment where the output terminals are sometimes short circuited.
From these general types of generators, engineers have developed many modifications to satisfy particular
requirements and to increase generator efficiency.
A machine that changes electrical energy into mechanical energy is called a motor. Around any current-carrying
conductor, a magnetic field exists whose strength depends on the amount of current. A current-carrying conductor
(supplying electrical energy), when placed in a magnetic field, has a force exerted on the conductor which causes it to
move out of the field (mechanical energy) (figure 3-4). In this figure, you can see that a current-carrying conductor in a
magnet field tends to move at right angles to the field. If a coil