Evidence to watch
The function is proved by changed behavior, not by a parameter value alone.
- Run input or control-word bit
- Latched run demand
- Ready/running status and motor speed
F-20 · vendor-neutral function
VFD 2-Wire vs 3-Wire Control
A 2-wire VFD command follows a maintained contact: closed means run and open means stop. A 3-wire circuit uses momentary start and stop inputs; the drive latches the run request after the start pulse is released.
Why it matters
The behavior behind the parameter.
The choice changes how the drive reacts to a released button, a broken conductor and power restoration. Treating both schemes as the same toggle creates unsafe or confusing restart behavior.
- 2-wire uses a maintained run input.
- 3-wire uses momentary start and stop inputs.
- The speed reference is a separate selection from the command scheme.
Interactive proof
Test vfd 2-wire vs 3-wire control in three passes.
Keep the motor, load and starting condition constant. Change one function at a time, then use the electrical and mechanical evidence to explain the result.
- 01
Select 2-wire, issue RUN, then release the run input and observe the motor stop.
- 02
Select 3-wire, pulse START, release it and confirm the command remains latched.
- 03
Pulse STOP and confirm the run bit clears before changing direction.
Common commissioning mistakes
These errors can make a healthy drive appear faulty or create a misleading test.
- Using a selector switch with a 3-wire configuration.
- Omitting the dedicated stop path in a 3-wire circuit.
- Assuming STO and an ordinary stop input perform the same safety function.
Primary technical references
Transfer the concept; verify the exact drive.
This guide uses vendor-neutral terminology. For real equipment, use the manual for the installed model, option cards and firmware, plus the machine risk assessment and approved commissioning procedure.
Related VFD functions
F-20
VFD Run Command Sources
A VFD run command can come from the local keypad, hardwired terminals or a fieldbus control word. The selected source must match the signal the PLC or operator is actually sending.
Read and test F-22VFD Stop Modes: Ramp, Coast, Quick Stop and DC Brake
A VFD can ramp the frequency down, remove torque and coast, use a faster quick-stop ramp, or apply DC injection braking. Each produces a different current, torque, stopping time and DC-bus response.
Read and test S-01VFD Safe Torque Off (STO)
Safe torque off prevents the drive from generating motor torque when its safety channels are de-energized. It does not isolate mains power and does not necessarily stop a moving machine quickly.
Read and testFrom explanation to evidence