For this type application, adding an external blower (or auxiliary fan) provides sufficient cooling for the motor to operate below rated speed. However, if the existing motor is installed in an application that has constant torque (such as a reciprocating compressor or pump, mill, or extruder), then the same load will remain on the motor as the speed is reduced, and the motor will overheat. If the load is a standard variable torque load, such as a fan or pump (load on the motor decreases exponentially with speed), additional cooling may not be needed. When the motor is operated with a VFD, the motor shaft speed (motor speed) decreases and will cause decreased cooling from the shaft-driven fan. Most motors running across the line depend on an internal, shaft-driven fan for cooling. This article details some of the factors to consider when installing a medium-voltage (MV) VFD on an existing MV motor in order to ensure safe and efficient operations. When a variable frequency drive (VFD) is added to an existing system, many potential problems may arise.
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