Nearly all practical generators use electromagnetic poles instead of the permanent magnets used in
our elementary generator. The electromagnetic field poles consist of coils of insulated copper wire wound
on soft iron cores, as shown in figure 1-6. The main advantages of using electromagnetic poles are (1)
increased field strength and (2) a means of controlling the strength of the fields. By varying the input
voltage, the field strength is varied. By varying the field strength, the output voltage of the generator can
Figure 1-6.Four-pole generator (without armature).
Q9. How can field strength be varied in a practical dc generator?
Commutation is the process by which a dc voltage output is taken from an armature that has an ac
voltage induced in it. You should remember from our discussion of the elementary dc generator that the
commutator mechanically reverses the armature loop connections to the external circuit. This occurs at
the same instant that the voltage polarity in the armature loop reverses. A dc voltage is applied to the load
because the output connections are reversed as each commutator segment passes under a brush. The
segments are insulated from each other.
In figure 1-7, commutation occurs simultaneously in the two coils that are briefly short-circuited by
the brushes. Coil B is short-circuited by the negative brush. Coil Y, the opposite coil, is short-circuited by
the positive brush. The brushes are positioned on the commutator so that each coil is short-circuited as it
moves through its own electrical neutral plane. As you have seen previously, there is no voltage generated
in the coil at that time. Therefore, no sparking can occur between the commutator and the brush. Sparking
between the brushes and the commutator is an indication of improper commutation. Improper brush
placement is the main cause of improper commutation.