“When applications generate excessive heat, DC fans are a common cooling option, keeping equipment components within extreme operating temperature ranges and improving thermal performance. After some basic thermal analysis, running fans continuously is certainly a good cooling option, but more advanced controls and protections allow fans to last longer and be more efficient. CUI Devices has a comprehensive portfolio of DC fans and blowers available in a variety of sizes, airflows, speeds and controls to simplify user selection.
By Jeff Smoot, Vice President, Applications Engineering and Motion Control, CUI Devices
A DC fan is a well-known and widely used thermal management device that can be used alone, in series, or in parallel to provide forced air convection cooling. It is the versatility and relative simplicity of DC fans that have made them a reliable choice for end-application temperature control technology for many years. According to the basic laws of physics, the airflow generated by the fan will absorb the heat of the device to be dissipated and take the heat away from the device to be dissipated, thereby effectively cooling the device components. However, cooling effectiveness is affected by several factors; engineers would benefit from a better understanding of the DC fan’s existing capabilities and options to improve reliability and efficiency.
Figure 1: Natural convection versus forced air convection cooling (Image credit: CUI Devices)
Before selecting a DC fan, engineers need to perform some basic thermal analysis in order to calculate minimum air flow requirements. A typical thermal analysis might include modeling of heat sources, ambient conditions, and temperature rise. In addition, other factors such as fan size, direction of rotation, and airflow paths within the application need to be considered to ensure a suitable solution. The CUI Devices blog, Airflow Basics You Should Know Before Choosing the Right DC Fan, provides details on thermal analysis and selection process.
Once the thermal analysis is done and a fan of the right size and rating is selected, the next step is to power up the fan and make it work, right? While continuously running fans can serve their purpose in some cases, continuous forced air cooling often does not achieve energy savings or provide a long-term solution. Today’s DC fans offer designers a range of control, monitoring and protection options for enhanced thermal management. The rest of this article will cover these features, allowing designers to benefit from more advanced fan control techniques.
As mentioned above, continuous operation of the fan will certainly keep the temperature-sensitive components cool, but ignores its power consumption and the fact that the fan’s moving parts have a limited lifespan. Fans can also generate audible noise when operating, which may be prohibitive in various applications and environments.
Fan start/stop operation based on temperature setpoint is an alternative that removes some of the drawbacks of continuous fan operation. Using fan start/stop control technology, you can save energy by limiting run time, reducing stress on the moving parts of the fan, and reducing audible noise by stopping the fan when the temperature drops below a set point.
However, start/stop fan control also oversimplifies the forced air cooling method in many ways and has its own drawbacks. First, the start/stop control technology implements a heat-cool cycle for temperature-sensitive components. Thermal cycling is even more detrimental to critical components than operating at constant high temperatures. This is because thermal cycling can create temperature coefficient differences that cause additional stress on the material and solder joints, resulting in premature failure.
The second is the inevitable thermal overshoot factor. This is the time delay between when the fan starts and the forced air flow from the fan actually starts to cool. During this time delay, unless the “Fan On” set point is lowered, overheating of the components may result. Also, by lowering the set point, the amount of time that the fan is powered on and producing audible noise also increases. Finally, hysteresis is required to avoid rapid start and stop near the set point (often referred to as “jitter”).
The diagram below helps to illustrate the thermal overshoot problem caused by starting/stopping fans to control thermal lag in an application. The graph plots the desired set temperature in step change (light blue), fan on/off cycle (green), and actual temperature (dark blue).
Figure 2: Fan start/stop cycling can cause thermal overshoot and lag (Image credit: CUI Devices)
Today’s Fan Control Options
Today’s DC fans offer designers a range of control and protection options for more sophisticated thermal management systems. These advanced designs take basic start/stop fan control to a whole new level of performance, efficiency and reliability. In addition, protection options are available to detect problems before they cause damage to fans and fan cooling components. Some of the most common fan control and protection options are described below:
・ Pulse width modulation
Pulse Width Modulation (PWM) is a common method used to control and change fan speed based on changing thermal conditions. When used in conjunction with advanced control algorithms, PWM-based variable speed control increases operating efficiency and adapts to operating dynamics that match fan speed to thermal load.
Fan start/stop control can also be upgraded by employing proportional integral derivative (PI and PID) closed loop control techniques. These strategies ensure airflow conditions at the desired temperature set point, which helps avoid thermal overshoot or undershoot during load changes.
・ Embedded tachometer signal
An embedded tachometer for closed-loop feedback and more advanced fan control detects and reports fan speed by measuring the frequency of the pulsed output signal. The tachometer can also be used as a lockout sensor, alerting the user if the fan stops running due to power outages, blockages, etc. Being able to detect these issues as quickly as possible is a major advantage of system operation and allows for timely shutdown to protect temperature-sensitive components.
・ Auto restart protection
Auto-restart protection detects when the fan motor is blocked from spinning and automatically cuts off drive current. This protects the fan drive circuit and notifies the fan Controller of urgent problems due to drive current cutoff.
・ Rotation detection/lock sensor
The rotation detection/lock sensor is used to detect if the fan motor is running or stopped, preventing problems during startup or operation.
When applications generate excessive heat, DC fans are a common cooling option, keeping equipment components within extreme operating temperature ranges and improving thermal performance. After some basic thermal analysis, running fans continuously is certainly a good cooling option, but more advanced controls and protections allow fans to last longer and be more efficient. CUI Devices has a comprehensive portfolio of DC fans and blowers available in a variety of sizes, airflows, speeds and controls to simplify user selection.
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