Speed Control Circuits

Speed control circuits are used on the vehicle to control the speed of motors or accessories. A common use is the control of the fan speed in the vehicle’s HVAC system and the electric radiator fans. Essentially, the speed control circuit manages the speed of a fan motor to control the amount of air movement through the system.

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FIGURE 36-52
Pulse-width modulation. A. 25% duty cycle. B. 50% duty cycle. C. 75% duty cycle.

Most speed control systems use pulse-width modulation to control motor speed. Pulse width (or duty cycle) refers to the percentage of time a circuit is fully on, versus fully off. A pulse width of 50% means that the circuit is on 50% of the time and off 50% of the time Figure 36-52. This provides about 50% of the electrical power to the motor, which allows it to operate at about 50% power. Likewise, a 25% pulse width means that the electrical power is on 25% of the time. Since the pulse width is cycled on and off very fast (sometimes exceeding 10,000 cycles per second), power to the motor appears very smooth and the motor power is controlled very accurately. Like the time-delay circuit, speed control circuits are constructed from electronic components that can be in dedicated modules or integrated into the vehicle’s power train control module that controls the circuit.