SERIES-WOUND DC MOTORS have a torque-speed characteristic like that shown in Figure 2-33b. This type is more speed-sensitive than the shunt or PM configurations.However, its starting torque can be as high as 800% of full-load rated torque. It also does not have any theoretical maximum no-load speed which makes it tend to run away if the
load is removed. Actually, friction and windage losses will limit its maximum speed which can be as high as 20,000 to 30,000 revolutions per minute (rpm). Overspeed detectors are sometimes fitted to limit its unloaded speed. Series-wound motors are used in sewing machines and portable electric drills where their speed variability can be an advantage as it can be controlled, to a degree, with voltage variation. They are also used in heavy-duty applications such as vehicle traction drives where their high starting torque
is an advantage. Also their speed sensitivity (large slope) is advantageous in high-load
applications as it gives a "soft-start" when moving high-inertia loads. The motor's tendency to slow down when the load is applied cushions the shock that would be felt if a
large step in torque were suddenly applied to the mechanical elements