2-11
The interpole coil in a motor is connected to carry the armature current the same as in a generator.
As the load varies, the interpole flux varies, and commutation is automatically corrected as the load
changes. It is not necessary to shift the brushes when there is an increase or decrease in load. The brushes
are located on the no-load neutral plane. They remain in that position for all conditions of load.
Q15. What current flows in the interpole windings?
The dc motor is reversed by reversing the direction of the current in the armature. When the armature
current is reversed, the current through the interpole is also reversed. Therefore, the interpole still has the
proper polarity to provide automatic commutation.
MANUAL AND AUTOMATIC STARTERS
Because the dc resistance of most motor armatures is low (0.05 to 0.5 ohm), and because the counter
emf does not exist until the armature begins to turn, it is necessary to use an external starting resistance in
series with the armature of a dc motor to keep the initial armature current to a safe value. As the armature
begins to turn, counter emf increases; and, since the counter emf opposes the applied voltage, the
armature current is reduced. The external resistance in series with the armature is decreased or eliminated
as the motor comes up to normal speed and full voltage is applied across the armature.
Controlling the starting resistance in a dc motor is accomplished either manually, by an operator, or
by any of several automatic devices. The automatic devices are usually just switches controlled by motor
speed sensors. Automatic starters are not covered in detail in this module.
Q16. What is the purpose of starting resistors?
SUMMARY
This chapter presented the operating principles and characteristics of direct-current motors. The
following information provides a summary of the main subjects for review.
The main PRINCIPLE OF A DC MOTOR is that current flow through the armature coil causes the
armature to act as a magnet. The armature poles are attracted to field poles of opposite polarity, causing
the armature to rotate.
The CONSTRUCTION of a dc motor is almost identical to that of a dc generator, both physically
and electrically. In fact, most dc generators can be made to act as dc motors, and vice versa.
COMMUTATION IN A DC MOTOR is the process of reversing armature current at the moment
when unlike poles of the armature and field are facing each other, thereby reversing the polarity of the
armature field. Like poles of the armature and field then repel each other, causing armature rotation to
continue.
