Variable Frequency Drives

A variable frequency drive (VFD) controls the speed of an electric motor by changing the frequency and voltage it supplies. VFDs run pumps, fans, conveyors and HVAC systems. Inside is high voltage, and some of it stays after the power is shut off.

• Electricians
• Maintenance
• HVAC & Industrial

Hazards

• Stored energy: large capacitors in the DC bus can hold a dangerous charge for several minutes after power is off. See Capacitors.
• Shock and arc flash when the cabinet is opened with power on. See Arc Flash.
• Unexpected motor start from a remote command, auto-restart setting or control signal.
• Back-feed and multiple power sources (control power, bypass, external circuits) that can leave terminals live.
• High leakage currents if the drive is not properly grounded.
• Hot heat sinks and cooling fans that are still moving.

Safe work steps

1. Read the drive's manual and the warning labels; note the wait time required after shutoff, often 5 to 15 minutes or more.
2. Stop the drive and the motor. Open and lock out the upstream disconnect and any other power sources, including control power and bypass. See Lockout/Tagout.
3. Wait the full time listed.
4. Test for absence of voltage with a rated meter on line terminals, load terminals and the DC bus. Confirm the meter works before and after. See Absence of Voltage Testing.
5. Discharge or verify low DC bus voltage as the maker instructs, only if you are qualified.
6. Prevent the motor from being turned by the load (such as a windmilling fan), since it can generate voltage back into the drive.

Installation and commissioning

• Use only qualified electricians, following the manufacturer's installation manual and the National Electrical Code
• Ground the drive and motor correctly with proper bonding; use shielded motor cable where specified
• Install guards, enclosures and doors, and keep cabinet doors closed
• Set restart behavior safely and tell operators if auto-restart is enabled
• Use arc-rated PPE and insulated tools when work on live parts is justified and permitted. See Energized Work Permits
• Label drives, disconnects and motor circuits clearly

Last updated: October 8, 2026