What strategies do you recommend for overload protection when employing variable-frequency drives?
Chip McDaniel, technical marketing at AutomationDirect: Overload and other forms of protection for the VFD, the motor, factory personnel, and the surrounding equipment include a properly sized power supply, the right grounding system, fusing, disconnects, and contactors. Input fuses protect the AC drive from excessive input current due to line surges, short circuits, and ground faults. They are recommended for all installations and may be required for UL-listed installations. See the attached diagram for an overview of VFD protection in all its forms.
Chris Jaszczolt, industrial drive product management at Yaskawa: A variable frequency drive will provide electronic motor overload protection. This means the drive will measure motor current and monitor it electronically to limit overheating of the motor. The drive must be set up properly for the motor's full load current to operate correctly. There are typically several levels of thermal overload protection that the drive can provide. Graphs are available that depict the various thermal overloads a drive can provide (Figure 1). These overloads are typically classified by motor type. Certain motor's cannot operate at constant torque (rated current conditions) throughout their speed range. These motors have reduced speed ranges (typically 10:1 or 40:1) and follow the current required to meet the demands of a variable torque applications. A VFD can be programmed to provide thermal protection for each of these motor designs. VFDs also provide ground fault, phase loss, and over current protection for the motor. Moreover, VFDs have additional user defined protection methods. These methods can detect pre-set over/under current/torque conditions to provide quicker motor/load protection than the drive's thermal overload. Finally, external thermal overload protection should be used when operating more than one motor with a singular drive.
Chris Cusick, senior VFD product specialist at Mitsubishi Electric Automation: Generally the drives own thermal overload protection system is the simplest and best way to accomplish protection. Separate overload relays would be used for multiple motor applications. The drive is then set up so that it will trip if any of the overload relays open—a circuit wired in series.
Tim Albers, director of marketing and product management at Nidec Motor: All VFDs come with both some level of alerting, shut down and droop modes in the case of overload. The process demands and safety should drive how those are implemented. In some cases shutdown may be the only solution, in others, droop may be the only option. Being aware of the control options is key to employing the most optimal solution. Having external thermal protection on the motor that is directly connected to the VFD is also recommended.
Mark Harshman, director, systems engineering, at Siemens: If the drive is always operating the motor, typically the VFD control will have its own inverse thermal protection algorithm and instantaneous current overload protection built in. However, if the VFD is used to start the motor and then transfer it to the utility, a commercial motor protection relay (MPR) must be supplied, usually as part of the switchgear used to connect the motor to the utility once the VFD has accelerated the motor up to full speed and synchronized it to the utility. There is also the capability to use a commercial MPR with the VFD if additional motor protection features in the MPR are desired. In this case, the MPR trip function is connected to the VFD control allowing the VFD control to trip if a motor condition is detected by the MPR.