Duty Cycle
It is the ratio of the time the pulse stays on within one PWM period to the total period, given as a percentage. The average voltage applied to the motor is directly proportional to this ratio: on a 24 V supply, a 50 percent duty cycle means approximately 12 V average armature voltage.
Duty cycle is the mathematical counterpart of speed setting. When you turn a potentiometer the controller converts this into a target duty cycle, and the power stage sets the pulse width accordingly. What the motor sees is discontinuous pulses; however, because the winding inductance smooths the current, in practice the motor responds to the average voltage.
The point to note is that the relationship between duty cycle and speed is not independent of the load. In an open-loop driver a 50 percent duty cycle gives a certain speed with no load, but when load is applied the speed drops at the same ratio. If constant speed is wanted, feedback and closed-loop control are required.
There are special cases at the two extremes. At very low ratios (typically below 5–15 percent) the motor may fail to produce torque and only vibrate; this is called the dead zone. At ratios very close to 100 percent, in drivers with bootstrap supply the gate voltage of the upper switch weakens, so designs typically limit the duty to 95–99 percent.
The context in which this term is used: DC Motor Driver Selection Guide