text: Electric motors are utilized in countless types of systems, such as control systems in manufacturing environments, consumer appliances like washers and dryers, computer peripheral devices like printers and scanners, and power supplies for computer and other types of electronic systems. A typical computer power supply includes a direct current (DC) fan and a temperature sensor coupled to a motor controller and other electronic circuitry for generating a supply voltage. The temperature sensor develops a signal indicating the temperature of the electronic circuitry and possibly the temperature of other circuitry contained in the computer system. In response to the signal from the temperature sensor, the motor controller develops control signals to control the speed of the DC motor. The DC motor operates in response to the control signals to circulate air across the circuitry and thereby remove heat generated by the circuitry.
  The motor controller develops the control signals to adjust the speed of the DC motor as a function of the value of the signal from the temperature sensor. When the signal from the temperature sensor indicates the temperature is above a predetermined value, the motor controller develops the control signals to increase the speed of the DC motor. Conversely, the motor controller develops the control signals to decrease the speed of the DC motor as the indicated temperature decreases. Typically, the control signals developed by the motor controller correspond to a pulse width modulated (PWM) signal having a duty cycle that the controller adjusts to control the speed of the DC motor. As will be appreciated by those skilled in the art, such a PWM signal corresponds to a square wave having an ON value for a portion of a period of the PWM signal and an OFF value for the remainder of the period. The duty cycle of the PWM signal is defined by the ratio of the portion of the period for which the PWM signal has the ON value divided by the total period of the PWM signal.
  The ON value of the PWM signal corresponds to a maximum DC voltage that is applied to energize the DC motor and the OFF value of the PWM signal corresponds to an approximately zero DC voltage. As a result, when the ON value is applied to the DC motor the motor is energized and therefore rotates and when the OFF value is applied the DC motor is deenergized or turned off. The motor controller controls the speed of the motor bike adjusting the duty cycle of the PWM signal and in this way controls the portion of each period for which the PWM signal has the ON value and the portion of each period for which the PWM signal has the OFF value. The DC motor is thus repeatedly energized or turned on and deenergized or turned off as determined by the PWM signal, with the duty cycle of the PWM signal determining the speed of the motor.
  While the speed of many DC motors may be controlled using a PWM signal has just described, some DC motors are intended to be operated at a relatively constant speed or at least at the same speed for a relatively long period of time. With such DC motors, the use of the PWM signal puts stress on various components in the motor and thereby decreases the operational life of the motor, as will be understood by those skilled in the art. For example, a PWM signal may be turning a DC motor on and off 300 times a second and thus will reduce the operational life of such DC motors. These types of DC motors must therefore be controlled with a different type of signal, necessitating the design of a custom motor controller. While this may be done, PWM motor controllers are commonplace thus would be the most cost effective option many situations. For example, in some situations a third-party purchases the motor controller and the DC motor to be controlled, and because of the prevalence of PWM motor controllers it would be convenient and cost effective for the third party to utilize a PWM motor controller to control the DC motor. Currently, the use of a PWM motor controller with at least with some DC motors may decrease the operational life of the DC motor below an acceptable minimum lifetime.
  There is a need for controlling the speed of all types of DC motors with a PWM signal without adversely affecting the operational life of the motor.