4.9.4 Derating for running at low speedWhen a motor is connected to an adjustable frequency drive, it is necessary to make sure that the cooling of the motor is adequate.The level of heating depends on the load on the motor as well as the operating speed and time.Constant torque applications (CT mode)A problem may occur at low RPM values in constant torque applications. In a constant torque application, a motor may overheat at low speeds due toless cooling air from the motor integral fan.Therefore, if the motor is to be run continuously at an RPM value lower than half of the rated value, the motor must be supplied with additional air-cooling(or a motor designed for this type of operation may be used).An alternative is to reduce the load level of the motor by choosing a larger motor. However, the design of the adjustable frequency drive limits the motorsize.Variable (quadratic) torque applications (VT)In VT applications such as centrifugal pumps and fans, where the torque is proportional to the square of the speed and the power is proportional to thecube of the speed, there is no need for additional cooling or de-rating of the motor.In the graphs shown below, the typical VT curve is below the maximum torque with de-rating and maximum torque with forced cooling at all speeds.Maximum load for a standard motor at 104°F [40°C] driven by an adjustable frequency drive type VLT FCxxxLegend: ─ ─ ─ ─Typical torque at VT load ─•─•─•─Max torque with forced cooling ‒‒‒‒‒Max torqueNote 1) Oversyncronous speed operation will result in the available motor torque decreasing inversely proportional to the increase in speed. Thismust be considered during the design phase to avoid overloading the motor.4.9.5 Automatic Adaptations to Ensure PerformanceThe adjustable frequency drive constantly checks for critical levels of internal temperature, load current, high voltage on the intermediate circuit and lowmotor speeds. As a response to a critical level, the adjustable frequency drive can adjust the switching frequency and/or change the switching patternin order to ensure the performance of the drive.FC 300 Design Guide 4 AutomationDrive FC 300 SelectionMG.33.BC.22 - VLT® is a registered Danfoss trademark 4-394
