The next step in our consideration of DC motors is to develop an equivalent circuit which can be used to better understand motor operation. The armatures in real motors usually consist of many windings of relatively thin wire. Recall that thin wires have larger resistance than thick wires. The equivalent circuit then must include a resistor Ra which accounts for the total resistance of the armature winding. Figure 1 shows the equivalent circuit for a DC motor. VDC represents the applied voltage which causes the armature current to flow, Ra is the resistance of the armature, and Ea is the generated or induced
“back EMF” in the armature.

Linear motors have some practical applications, but rotating DC motors are much more prolific. The principles
which explain the operation of linear motors are the same as those which explain the operation of practical DC motors. The fundamental difference between linear motors and practical DC motors is that DC motors rotate rather than move in a straight line. The same forces that cause a linear motor to move “right or left” in a straight line cause the DC motor to rotate. This chapter will examine how the linear motor principles can be used to make a practical DC motor spin.