Figure 2-32b shows a family of load lines superposed on the torque-speed curve of a PM motor. These load lines represent a time-varying load applied to the driven mechanism. The problem comes from the fact that as the required load torque increases, the motor must reduce speed to supply it. Thus, the input speed will vary in response to load variations in most motors, regardless of their design. * If constant speed is required, this may be unacceptable. Other types of DC motors have either more or less speed sensitivity to load than the PM motor. A motor is typically selected based on its torque-speed
curve