2-1
CHAPTER 2
DIRECT CURRENT MOTORS
LEARNING OBJECTIVES
Upon completion of this chapter you will be able to:
1. State the factors that determine the direction of rotation in a dc motor.
2. State the right-hand rule for motors.
3. Describe the main differences and similarities between a dc generator and a dc motor.
4. Describe the cause and effect of counter emf in a dc motor.
5. Explain the term "load" as it pertains to an electric motor.
6. List the advantages and disadvantages of the different types of dc motors.
7. Compare the types of armatures and uses for each.
8. Discuss the means of controlling the speed and direction of a dc motor.
9. Describe the effect of armature reaction in a dc motor.
10. Explain the need for a starting resistor in a dc motor.
INTRODUCTION
The dc motor is a mechanical workhorse, that can be used in many different ways. Many large pieces
of equipment depend on a dc motor for their power to move. The speed and direction of rotation of a dc
motor are easily controlled. This makes it especially useful for operating equipment, such as winches,
cranes, and missile launchers, which must move in different directions and at varying speeds.
PRINCIPLES OF OPERATION
The operation of a dc motor is based on the following principle:
A current-carrying conductor placed in a magnetic field, perpendicular to the lines of flux, tends to
move in a direction perpendicular to the magnetic lines of flux.
There is a definite relationship between the direction of the magnetic field, the direction of current in
the conductor, and the direction in which the conductor tends to move. This relationship is best explained
by using the RIGHT-HAND RULE FOR MOTORS (fig. 2-1).
