4.9.4 Derating for running at low speedWhen a motor is connected to a frequency converter, it is necessary to check 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 s a motor may over-heat 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 frequency converter puts a limit to themotor size.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 40 °C driven by a frequency converter type VLT FCxxxLegend: ─ ─ ─ ─Typical torque at VT load ─•─•─•─Max torque with forced cooling ‒‒‒‒‒Max torqueNote 1) Over-syncronous speed operation will result in the available motor torque decreasing inversely proportional with the increase in speed. Thismust be considered during the design phase to avoid over-loading of the motor.4.9.5 Automatic Adaptations to Ensure PerformanceThe frequency converter constantly checks for critical levels of internal temperature, load current, high voltage on the intermediate circuit and low motorspeeds. As a response to a critical level, the frequency converter can adjust the switching frequency and/ or change the switching pattern in order toensure the performance of the drive.4 AutomationDrive FC 300 Selection FC 300 Design Guide96 MG.33.BC.02 - VLT® is a registered Danfoss trademark4
