Challenges and solutions of protecting variable speed drive motors

Variable frequency drives (VFDs), also known as variable speed drives, provide significant advantages for the operation of induction motors. These advantages include reduced starting currents, adjustable speed control, and improved energy efficiency. However, VFDs create additional challenges for motor operation and protection that are not present in line-connected motors. The protection challenges are solved by new protection elements available in an electronic motor protection relay. Traditional motor protection elements use fundamental frequency measurements for operating current. VFD-operated motors have a fundamental frequency that changes rapidly in response to speed adjustments. Additionally, the synthesized sine waves produced by the VFD contain significant harmonic content. The new protection elements use true rms (fundamental plus harmonics) operating current. True rms measurements are not dependent on having a fixed fundamental frequency. True rms measurements properly account for the motor heating caused by harmonic currents. Conventional single-speed motor protection uses a fixed value for the full-load amperes (FLA) of the motor. This single FLA assumes that a fixed cooling rate is available to the motor. Commonly used low-voltage motors have shaft-coupled cooling fans that spin at the same speed as the motor. At reduced operating speeds, the cooling air provided by the fan is significantly reduced. In order to prevent overheating of the motor, the protection elements should compensate for the reduced cooling. The new protection elements dynamically compensate for the reduced cooling available at reduced speed operation. VFDs are typically installed in motor control centers (MCCs). Because of the large available fault current, MCCs can have significant arc-flash hazard potential. The safety of VFD-operated motors can be improved using arc-flash protection elements. VFDs are often installed in large numbers at industrial facilities. This makes it impractical to manually monitor and control each motor as a standalone device. A centralized motor management system solves this problem. This paper describes novel protection elements that accommodate the unique protection, monitoring, and control challenges of VFD-operated motors.