A variable frequency drive takes an AC input, converts it to DC, then by controlling the output of an array of insulated-gate bipolar transistors (IGBTs), provides an (approximately) sine-wave AC output to an electric motor, typically a 3 phase synchronous AC motor.

Changing the timing of the outputs changes the frequency of the AC output and $\therefore$ the speed, torque, and/or #power output of the electric motor.

topologies

all VFDs share common components:

1. a FULL BRIDGE RECTIFYER!!!
2. A high-voltage DC bus with a electronic choke.
3. An array of inverters controlled by a microcontroller. -> 6 inverters (as shown above) is a typical configuration (6 pulse output).

voltage-source inverter (VSI)

This is the most common type of VFD, a VSI drive with PWM output voltage.

As shown at the top of the page, the output of the rectifier feeds a capacitor to supply constant DC voltage to the IGBT Inverters

Current-source inverter (CSI)

A current source inverter functions similarly to a VSI, with the notable exception that the capacitor is replaced by a large inductor with the goal of supplying a constant current to the motor when the circuit is driven. This drive topology is more common for larger motors with high current requirements (say, 200A).

motor drive ratings

according to #Kendall Electric:

failures and faults

• overvoltage faults (e.g., CF004 on PowerFlex 520 series) occur when too much energy is fed back into the drive, often due to rapid deceleration of high-inertia loads. Kendall Electric recommends using braking resistors or adjusting deceleration times to mitigate the issue.

• overcurrent faults typically result from attempting to start heavy loads too quickly. Kendall advises users to increase acceleration times and check for component failures if faults occur after normal operation.

• Environmental and Maintenance Issues: drives must be protected from heat, water, and debris. Kendall Electric recommends regular cleaning of fans and filters as part of preventive maintenance to ensure reliable operation.

• Capacitor Degradation: Capacitors degrade when power is removed for extended periods, leading to drives failing to power up after long downtimes.

  • Don't overthink these failures
    • they are common in older equipment;
    • for drives more than 10 years old, Kendall Electric recommends biasing towards replacing drives instead of attempting to troubleshoot.
  • A typical failure mode would be that (especially after a prolonged power-off period) a old (10+ years old) drive will fail to start. This occurs because drive capacitors degrade over time and those chemical changes acclerate when the system is unpowered.

control software

• Connected Components Workbench (CCW)
  • Troubleshooting and Logging: CCW can be used to log faults and alarms, compare program versions, and assist in troubleshooting recurring issues reported by end users.
  • Batch Configuration: CCW enables efficient batch programming for large-scale deployments, allowing users to download the same configuration to multiple drives quickly.
  • Transition to Rockwell Advisor: CCW is currently at version 23 and will eventually be replaced by Rockwell Advisor, a web-based tool, but sales reps assured me that CCW will remain available for the foreseeable future and that the interface will remain similar.

off label uses

• VFDs CAN be used as power supplies non-motor equipment BUT due to output quality (pseudo-sinewave power), it's not recommended for sensitive equipment or for machines requiring clean power.

• Single-Phase to Three-Phase Conversion: PowerFlex drives (and probably most others) can convert single-phase input to three-phase output at lower horsepower.

  • make sure you
  • considerations: running a VFD designed for three phase power on single-phase power WILL result in greater current draw for the same power. Consequently, you'd need a more expensive drive with a larger current rating.