Velocity Frequency Drives (VFDs) primarily control the speed and torque of standard AC induction motors in an open-loop system, with advanced VFDs supporting encoder feedback. VFDs generally do not provide as good control as servo drives (see EtherCAT Servo Drives) but are often the only option for very large motors.
VFDs can be controlled by RMCs via EtherCAT. Look for VFDs that support CiA402. Delta Motion has found that many VFDs seem to have added EtherCAT support as an afterthought, and configuration is sometimes complicated.
Delta Motion has tested the following drives and has created instructions on how to configure them to work with the RMC EtherCAT: