1.) Series Field DC Motor - The series field DC motor contains field coils composed of a few turns of wire connected in series with the armature as shown in Figure 1 below. This motor has high starting torque and a variable speed characteristic. The greater the load the motor carries, the lower the speed.
At light loads, the motor accelerates to a high speed because of the effect of the counter EMF (Electromotive Force). Large DC motors will usually race to destruction if not loaded, but fractional horsepower DC series motors generally are designed to withstand these high speeds. The series field DC motor is generally used in cranes, winches, trains, etc.
|Figure 1: Field and Armature connection of a Series Field Direct Current (DC) Motor|
2.) Shunt Field DC Motor - The DC shunt field motor contains a field composed of many turns of wire. This is connected in parallel with the armature, as shown in Figure 2 below. The shunt field DC motor has medium torque and constant speed characteristic. However, the operating speed can be adjusted by adjusting the strength of the shunt field, either by insertion of a rheostat in series with it or by adjusting the voltage across the field. Weakening the field increases the speed while strengthening the field decreases the speed.
Shunt field DC motors may have a light series field added and are called stabilized shunt motors. A stabilized shunt motor is a DC motor in which the shunt field is connected in parallel with the armature circuit and which also has a light series field added to prevent a rise in speed or to obtain a slight reduction in speed with increase in load.
|Figure 2: Field and Armature connection of a Shunt Field Direct Current (DC) Motor|
3.) Compound DC Motor - Compound-wound DC motor, figure 3, employs both a series and a shunt field (see illustration), and has speed characteristics intermediate between those of the shunt and the series DC motors, depending upon the amount of the compounding. Compound-wound DC motors are normally known as varying-speed motors.
|Figure 3: Field and Armature connection of a Compound Wound Direct Current (DC) Motor|