VFD control — terminals and fieldbus
Two- and three-wire control from digital inputs, analog reference, network control via control and status words, and reference normalisation.
Terminal control
Two-wire control: one "Run" input (level). While the input is closed the motor runs; when it opens the drive stops. Simple, but after a power loss and restoration the drive may restart by itself if the input stayed closed — this must be covered by the risk assessment.
Three-wire control: momentary "Start" (NO) and "Stop" (NC) buttons. The drive latches the run state itself; opening "Stop" stops the motor. After power restoration a new start command is required.
On PowerFlex 520, for example, the mode is selected with P046 [Start Source 1] and the digital input assignments ("2-Wire", "3-Wire Start" etc.); on Siemens G120 and ABB ACS580 with setup macros and command source parameters.
A speed reference from terminals is an analog input (0–10 V, 4–20 mA), preset speeds (a combination of digital inputs) or a motor potentiometer ("up/down" inputs).
Network control
Over a network the drive receives a control word (commands) and a reference, and returns a status word and an actual value. Example — the PROFIdrive profile on Siemens G120, control word STW1:
| Bit | Meaning |
|---|---|
| 0 | ON / OFF1 (start / ramp stop) |
| 1 | OFF2 — coast (0 = active) |
| 2 | OFF3 — quick stop (0 = active) |
| 3 | enable operation |
| 4–6 | enable ramp generator, continue ramp, enable setpoint |
| 7 | acknowledge fault (on a 0 → 1 edge) |
| 10 | master control by PLC |
| 11 | setpoint reversal |
Hence the typical values: 16#047E — ready (OFF1, bit 0 = 0), 16#047F — run. Bit 10 must be 1, otherwise received data is not processed. The network reference is normalised: 16#4000 equals 100 % of the reference speed.
Other manufacturers use different bits (ABB, Danfoss and Rockwell have their own profiles or ODVA/DS402), but the principle is the same: command bits + reference + status word.
Network start sequence
- Check the status word shows no fault and the drive is ready to switch on.
- Send the "ready" word (e.g.
047E) and wait for "ready to operate". - Send the reference and the run command (
047F). - Monitor the "running", "fault" and "warning" bits and supervise communication: on loss of communication the drive must go to the intended safe state (the timeout reaction parameter).
When the drive does not follow the PLC
- Reference written in Hz or rpm directly instead of the normalised value (
16#4000= 100 %). - "Control by PLC" bit missing — the drive ignores commands.
- Acknowledge sent as a level instead of an edge — the fault does not reset.
- Byte/word order of 32-bit values over Modbus — see byte and word order.
Tools
- Bit / Byte Tool — compose and decode control and status words.
- Modbus Data Converter — drive values from registers.
- Function Block Generator — motor block with feedback monitoring.
Connections
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Sources
Updated 2026-10-01