Series DC Motor
As shown in Figure 4.9, the motor has the armature and field winding connected in series, therefore, its name – series motor. Field winding is made up of a relatively few turns of a large diameter wire so that the field resistance remains low.
In the series-wound DC motor, the field windings are fixed in the stator frame and the armature windings are placed around the rotor. These are connected in series. The current passing through the armature also passes through the field. In a series-wound motor, any increase in the load results in more current passing through the armature and the field windings.
As the increased current strengthens the field, the motor speed decreases.
Conversely, if the load is decreased, the field is weakened and the speed increases. For lighter loads, the speed increase may be excessive and undesirable. In order to control this, the series-wound DC motors are usually directly connected or geared to the load to prevent runaway.
At times, a series-wound motor designated as a series-shunt wound, is provided with a light shunt field winding, to prevent the dangerously high speeds at light loads. The increase in armature current with the increasing load produces an increased torque, so that the series-wound motor is suited to heavy staring duty. The motor speed can be adjusted with a variable resistance placed in series with the motor, but due to the variation with the load, the speed cannot be maintained at a constant. The series-wound DC motors are used for hoists, cranes, and elevators.