In Part 1, we explored the internal “Anatomy” of the drive—the rectifiers and inverters that bend the laws of physics. But a VFD sitting in a box is just an expensive heater. To make it useful, we must establish a Handshake between the brain (the PLC) and the muscle.
As we move toward the high-density automation of late 2026, the way we command these drives is shifting from miles of copper wire to sophisticated digital packets. However, to troubleshoot a system, you must understand all three methods of the “Command Handshake.”
1. The Physical Handshake: Wiring the I/O
Before a VFD can spin a motor, it needs two things: a Run Command (Logic) and a Speed Reference (Frequency).
Discrete Control (The “Go” Signal)
We typically use 24VDC digital signals to tell the drive to move.
- 2-Wire Control: A maintained switch. If the wire has 24V, the motor runs. If the wire is cut, it stops. Simple, but can be dangerous if a machine restarts unexpectedly after a power failure.
- 3-Wire Control: Momentary “Start” and “Stop” buttons. The VFD “latches” the command internally (just like the Relay Seal-In circuit we studied earlier).
Analog Control (The “How Fast” Signal)
To set the speed, we use the 4-20mA signals we mastered in our Analog series.
- 4mA = Minimum Frequency (usually 0Hz).
- 20mA = Maximum Frequency (usually 60Hz).
2. The Digital Handshake: VFD as a Network Node
In the Intelligence Economy, wiring every individual drive with six different copper wires is seen as “Legacy” thinking. Modern 2026 installations use Ethernet/IP or PROFINET.
By connecting a single Category 6 cable to the VFD, the PLC can:
- Send Start/Stop/Speed commands.
- Read real-time torque, current, and temperature.
- Modify parameters “on the fly” based on production recipes.
- Access HART or IO-Link data from sensors connected directly to the drive.
3. The “Big 5” Parameters
When you first power up a VFD, it is “dumb.” You must program it to match the motor it is connected to. While there are thousands of parameters, these are the Big 5 that every technician must master:
- Motor Nameplate Data: You must enter the Voltage, Full Load Amps (FLA), and base RPM. If this is wrong, the drive cannot protect the motor from burning out.
- Acceleration/Deceleration Time: How many seconds should it take to get to full speed? Setting this too short on a heavy load will cause an “Overcurrent” trip.
- Frequency Limits: Defining the Min (usually 0Hz) and Max (usually 60Hz or 120Hz for high-speed spindles).
- Stop Mode: Do you want the motor to “Coast” to a stop (like a bicycle) or “Ramp” to a stop (like a car with brakes)?
- Control Source: Telling the drive where to look for instructions (Terminals, Keypad, or Network).
4. Safety Integrated: Safe Torque Off (STO)
In 2026, we no longer just cut the main power to stop a motor for minor maintenance. We use Safe Torque Off (STO). This is a dedicated hardware circuit within the VFD that physically prevents the IGBTs from firing, even if the PLC tells it to run. This allows for faster restarts and higher system uptime while maintaining Category 3 safety standards.
Summary for the Commissioning Engineer
The “Handshake” is where your logic meets the high-power world. If a drive won’t run:
- Verify the Control Source (Is it looking at the right wires?).
- Check the Enable signal (Is the safety circuit/STO closed?).
- Verify the Nameplate Data (Does the drive trust the motor?).
Lab Exercise: Use the VFD Workbench below. Try to run the motor using “Analog” control. What happens if your Accel time is set to 0.1 seconds? Watch the Current monitor for a “Trip” condition.
VFD Workbench
Commissioning the Digital Handshake between PLC and Motor.