VFD Receives a Run Command but Won\’t Start? Trace the Command Source
A true run bit in the PLC does not prove a drive has accepted a valid start request. The drive may be listening to another command source, waiting for a permissive, or reporting an active inhibit. Read its actual status and selected control source before changing wiring or parameters.
General command-path method; parameter names and sequencing depend on the drive.
What is happening?
A drive typically has separate concepts for command selection, reference selection, readiness, and output operation. Their names and priority rules vary by model. A network command can be correct at the PLC while the drive is configured for terminal or keypad control, and a ready indication may still have a defined start sequence.
Check these five things
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Record the exact model, firmware, active fault, warning, and status word. Use the corresponding parameter manual.
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Compare the intended run source with the source actually selected by local/remote and control-mode settings.
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Trace the request from the PLC tag through the communication mapping to the drive's received command indication.
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Inspect ordinary permissives and drive inhibits without bypassing protective functions. Have safety-related conditions checked through the approved procedure.
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Check the selected speed reference and the documented start transition, then test only under the authorized commissioning plan.
Worked example
A bench drive shows a network run request, but its active command source is the keypad. The PLC bit can change repeatedly without controlling the drive. Correcting the approved source configuration resolves the mismatch; forcing the PLC output would not address it.
Quick diagnostic reference
| Layer | Evidence |
|---|---|
| PLC request | Command tag |
| Drive reception | Received control word |
| Drive acceptance | Selected source and status |
| Motor operation | Output and feedback |
The mistake to avoid
Do not reset faults repeatedly without recording them. The first code and operating conditions may contain the best explanation of the inhibit.
How to verify the fix
Confirm start, normal stop, communication loss, and recovery follow the documented machine behavior. Save the accepted drive parameter set.
Field-work boundary: These are educational diagnostic guides. Use the exact equipment manuals and approved site procedures. Electrical testing and machinery changes belong to qualified, authorized personnel; control hazardous energy and validate affected functions before release.
Technical reference
Siemens: SINAMICS S120 commissioning with Startdrive. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.