Fans are often controlled with a frequency converter, which gives the fan a wider performance range.
When controlling with a frequency converter, the user must take into account the fan’s resonant speed and ensure that the fan is not at these frequencies for longer periods.
These resonant frequencies must be blocked in the control so that they are passed quickly.
When checking vibrations due to resonance, it is important that the vibration amplitude is measured on the motor and not on the fan cover.
At low speeds, i.e. at low engine torque, care must be taken so that the fan cannot be stopped by an opposing air current, which can lead to overheating of the engine.
Change of direction of rotation and free rotation (so-called “Wind milling”) of axial fans can create large current pulses that can cause disturbances or wear on connections and electrical networks. These pulses can also damage impellers and electric motors.
Therefore, a sufficient deceleration time must always be allowed before changing the direction of rotation.
In case of major problems with free rotation, the recommendation is to install a motor brake that releases just before the motor stalls.
When a “star-delta” start is used, the changeover must not be done too early, this to avoid large current pulses.
