Number of views: 4077 Author: Aili Industry Release time: 2026-09-28 Source: Ai Li Industrial
When debugging some projects and finding that the air volume is insufficient, the first reaction is to increase the speed. After adjusting the air volume, the air volume did increase, but the noise also increased, the motor current was close to the upper limit, and the bearing developed a problem after running for several months. The higher the speed, the better. It must find a suitable position between the fan curve and the system resistance, and it must cover the entire operating range.
The starting point for determining the motor speed is the design working condition. After the design air volume and corresponding external static pressure are determined, check which speed line this operating point falls on the fan performance curve. Fan curves are usually drawn at different speeds, and each curve represents the relationship between air volume and pressure at a certain speed. Whichever curve the design operating point falls near, corresponds to which speed.
When checking the speed, you should also look at three parameters: shaft power at the speed, noise level and motor temperature rise. The shaft power determines whether the motor capacity is sufficient, the noise level determines whether silencer measures are needed, and the temperature rise determines whether the motor can run at this speed for a long time. Just look at whether the air volume is sufficient without looking at the other three, and the selection is incomplete.
If the design working point falls between two speed curves, the curve corresponding to the higher speed should be selected, and then reduced to the target point through speed regulation. This leaves a margin so that when the filter accumulates dust and the system resistance increases, there is still room for upward adjustment. However, the margin cannot be left indefinitely. The higher the rotational speed, the faster the shaft power increases, and the noise will also increase significantly.
When you need to adjust the rotational speed during the debugging stage, you cannot just focus on the air volume reading. After increasing the speed, the amount of increase in terminal air volume must be confirmed through on-site measurement, while observing noise, vibration and current.
In some cases, the increase in air volume is not obvious after increasing the speed, but the current and noise increase significantly. This shows that the system resistance may have exceeded the usable range of the fan at this speed. Continuing to accelerate will only increase energy consumption and will not effectively increase the air volume. At this time, you should check whether the air duct resistance is abnormal, whether the valve is not fully opened, and whether the filter is heavily dusted. Continuous acceleration cannot replace system troubleshooting.
There is another situation: after increasing the speed, the air volume increases, but it is still not enough at the far end. This is usually not a speed problem, but a branch balance or air leakage problem. The total air volume has increased, but the distribution is uneven, and the far end still cannot get enough air. At this time, the branch should be rebalanced instead of continuing to increase the speed.
If the project requires two levels of working and duty, or needs to operate under partial load, the speed adjustment range must be determined. The lower limit and upper limit of the speed regulation range must be based on evidence.
The upper limit is determined by the design conditions and necessary margins. The required rotation speed under the design working conditions, plus the allowance for increased resistance such as dust accumulation on the filter, constitute the upper limit. The upper limit is also limited by the available range of motors, impellers and drives. The driver has a maximum output frequency, the motor has a maximum allowable speed, and the impeller has a strength limit. Take the lowest value among the three.
The lower limit is determined by end demand. When on duty at night, when equipment is on standby, or when zones are closed, the minimum speed must ensure the required air volume or pressure difference at the critical end. The minimum speed cannot be set arbitrarily according to the controller percentage and should be verified on site. If the speed drops too low, the fan may enter an unstable zone or be unable to overcome the resistance of the filter section. For the pressure difference control system, the sensor position and pressure difference target should be confirmed during the commissioning stage to avoid adjusting speed based on only one measuring point, resulting in insufficient air volume in the remote area.
When retrofitting an existing fan box, it is sometimes considered to increase the air volume by increasing the speed. This idea may work in some cases, but there are a few extra things to check.
The first is the capacity of the motor and drive. The original motor runs at rated speed, and the current and temperature rise are within the design range. After the speed is increased, the shaft power increases according to the cube of the speed, and the motor current increases accordingly. It needs to be checked whether the original motor still has margin and whether the driver supports higher frequencies.
Second is mechanical strength. The impeller has a strength margin at the design speed, but the margin is limited. If the rotational speed exceeds the design range, the impeller may vibrate, deform or even be damaged. Bearings also have a maximum speed limit, which will accelerate wear if exceeded.
The third is noise and vibration. After the speed is increased, the noise usually increases significantly. If the existing cabinet and air duct are close to the resonance range, they need to be checked step by step during debugging. If the operating point is unreasonable, optimizing the pipeline or rematching the fan may be more effective than continuing to increase the speed.
After the rotation speed is determined, the rotation speed, air volume, static pressure, current and noise of each gear should be recorded during the commissioning phase. These data are the baseline for subsequent maintenance. If the same speed but air volume decreases after running for a period of time, this baseline can help distinguish control failure, rising system resistance and air duct leakage.
For systems using DC speed regulation, the actual air volume and static pressure should also be measured at several typical control command points to form a correspondence table between commands and air volume. The controller gives a command of 50%, which does not mean that the actual air volume must be half of the rated value. This correspondence table is more valuable than the theoretical curve in the driver manual because it includes the resistance characteristics of the actual duct and filter section.
The determination of the motor speed ultimately comes back to matching the entire machine. Whether the impeller and motor torque characteristics correspond, whether the driver parameters match the fan curve, and whether the air path loss inside the box is controllable will all affect whether the speed plan can be implemented. Aili Industry has independent supporting capabilities in fans, motors, controls and sheet metal structures. It can check the impeller, motor, driver and box at the same working point, determine the speed range according to the design, common and duty working conditions, and verify whether the shaft power, temperature rise and noise are within the allowable range. This kind of internal collaboration from the power end to the complete machine structure can reduce the common problems of mismatch between rotation speed and fan curve and insufficient speed range coverage in outsourced assembly solutions.
If the system end configurationFor HEPA/ULPA high-efficiency filters , the initial resistance of the filter and the resistance at the end of use will affect the working point. The rotation speed plan should be calculated based on the allowable pressure difference before maintenance. When the process requirements are higher, it can be evaluatedDoes the ULPA ultra-high efficiency filter meet the requirements? Clean room ceiling air supply is comparableFFU fan filter unit ; centralized management of multiple devices can be matchedThe FFU computer group control system realizes zone speed regulation and status monitoring. Protection of single process equipment can be consideredEquipment-side fan filter unit (EFU) . The final filtered air supply at the end of the air duct can beHigh-efficiency air supply outlet (HEPA BOX)。
Specific structure and configuration options can be found inIt is confirmed in the fan box product catalog that non-standard projects should provide equipment or machine room drawings, and then finalize them based on the determined working points and on-site interfaces.
