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.