The SEL-849 Motor Management Relay enhances VFD protection and provides redundant backup control for VFD normal operation, malfunction, soft start bypass, maintenance, or replacement. |
By Alan Saldivar
The biggest challenge for continuous process industries is operating without any unscheduled interruptions. Any process in the oil, gas, petrochemical, water, wastewater, metals, or mining markets that involves heavy use of rotary machinery needs to have reliable monitoring, protection and control devices. Even though most VFD manufacturers offer unique features such as component bypass, component redundancy, cooling capability, communications, and diagnostics, they cannot guarantee that their VFDs will not fail. From a reliability and risk standpoint, the need for a redundant device is evident.
A VFD bypass is commonly used to ensure that the fans, pumps, compressors, and other equipment are kept running in an industrial plant even if the VFD is taken out of the control loop. Both electronic and conventional bypasses are commonly used to take the VFD out of the control loop. This is concerning because the motor is only controlled by a VFD bypass contactor when there is no intelligent electronic device (IED) providing protection, monitoring or control. This automatically excludes the possibility of having any type of motor start report or waveform capture for motor data analysis during normal or abnormal operation.
Even though the VFD bypass fulfills the purpose of keeping the process running, an IED is needed to properly ensure the reliability and safety of the industrial process. Another evident challenge is the fact that VFDs are typically installed in MCCs. MCCs have a significant arc-flash hazard potential due to the large available fault current.
SEL Solution
The SEL-849 provides complete motor protection for a low-voltage motor. It can be applied to enhance VFD protection when the motor is running with the VFD or when the VFD is bypassed. Figure 2a shows a typical VFD bypass diagram where the SEL-849 is enhancing VFD protection and control, as well as acting as a redundant protective device.