To increase the efficiency of the application-specific operating point, it makes sense to control the speed of the fan. At the same time, speed control can be used to realize several operating points within the application with the same fan.
Due to lower prices for asynchronous standard motors, which leads to their preferred use in the industrial sector, speed control is carried out with frequency converters. The frequency inverter allows stepless speed control up to the rated speed without a drop in torque. However, it must be noted that frequency converters generate electrical noise signals, which can
affect the efficiency of the motor and cause tonalities in the high-frequency spectrum. The advantage is the energy savings due to operation close to the efficiency optimum and the associated reduction in the sound power level.
Generally, a distinction is made between current-controlled and voltage-controlled converters. Current-controlled converters ensure a constant ratio of current to frequency (I/f) and are frequently used for drives in the high megawatt range. In fan technology, the frequency is usually controlled by voltage-controlled frequency converters, which rectify and smooth the AC voltage fed in and ensure an AC voltage in line with demand via the output-side inverter. This results in a constant relationship between voltage and frequency, from which the corresponding motor speed is calculated.