Easy Guide on Open Loop vs. Closed Loop Control in DC Cooling Fans
Closed loop fan control has often been associated with external controllers to modify a DC fan’s performance. However, many SANYO DENKI Fans utilize an internal closed loop speed control to enhance performance.
This brief article will explain the differences between open and closed loop control and how to tell what type of loop control the fan model has.
Open Loop Control
Open loop control is basically just a free running motor and is the simplest design. It has no internal feedback circuitry and so the speed (RPM) can be affected by changes in pressure or temperature. Most of our older fan models are open loop controlled.
In order to tell if a DC fan is open loop controlled, look at the Speed vs PWM duty cycle graph provided in the data sheet. As you can see below, the nominal speed (black line) is not linear over the PWM duty cycle range.
Advantage: Lowest cost because there is no feedback circuitry.
Disadvantage: Will not self-correct the RPMs when conditions change.
Closed Loop Control
Over the last decade, many of our new fan models have closed loop control which keeps the speed (RPMs) constant regardless of temperature or back pressure variations. This is accomplished by adding a feedback circuit to the motor control via an internal tachometer signal so that the feedback circuit automatically makes corrections to bring the speed back to nominal.
For example, below is a Speed vs PWM duty cycle graph showing a fan with closed loop control. As you can see, the RPMs are linear over the PWM duty cycle range and will autocorrect the speed up to +/- 5% of the nominal value over the duty cycle range. This percentage can vary based on the fan model.
Advantage: Will autocorrect the RPMs based on external changes to maintain a constant airflow and thus provides the desired temperature in a system.
Disadvantage: Adds a small cost increase since additional internal feedback circuitry is needed.
In closing, DC fans with closed loop control features are a great way to maintain constant fan performance over the PWM duty cycle even when changes in system pressure or temperature occur.
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