One of the most common points of confusion with any VFD is the difference between Local control and External control.
If your spindle:
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Runs from the keypad but not from your controller
-
Runs from your controller but not from the keypad
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Suddenly stops responding after a parameter change
…it almost always comes down to control mode.
This article explains what’s happening and how to think about it clearly.
The VFD Has Two Separate Decisions to Make
Every VFD must decide two things:
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Where does the RUN command come from?
-
Where does the speed (frequency) command come from?
These are separate settings.
If either one is wrong, the spindle will not behave as expected.
1️⃣ Run Source (Start / Stop Control)
This determines who is allowed to turn the spindle on and off.
🔹 Local (Keypad Mode)
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You press RUN on the VFD itself
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The controller cannot start the spindle
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The keypad controls start/stop
This is used for:
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Testing
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Break-in
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Manual diagnostics
🔹 External Mode
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The CNC controller sends the RUN signal
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The keypad RUN button does nothing
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Start/stop is controlled by M3 / M4 / M5 in G-code
This is normal operating mode for CNC machines.
Why People Get Confused
Customers often:
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Test the spindle in Local mode
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Confirm it works
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Then switch to controller use
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Forget to change the run source to External
Result:
The controller sends M3… and nothing happens.
The VFD is not broken.
It’s simply still listening to the keypad.
2️⃣ Frequency Source (Speed Control)
This determines who sets the RPM.
🔹 Local (Keypad Knob)
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Speed set by the dial or keypad
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Controller RPM commands are ignored
-
Spindle runs at whatever the VFD dial says
🔹 External (Controller Controlled)
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Speed set by:
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PWM
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0–10V analog
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Modbus
-
-
G-code S commands control RPM
Example:
M3 S17000
The controller sends a speed request.
The VFD follows it.
Important: These Two Settings Are Independent
You can have:
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Local RUN + External Frequency
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External RUN + Local Frequency
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Both Local
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Both External
If only one is set correctly, the spindle may:
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Start but not change RPM
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Change RPM but not start
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Appear “partially working”
This creates panic.
It is not hardware failure. It is configuration mismatch.
Why Switching Modes Causes Panic
Here’s the typical scenario:
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Customer tests spindle in keypad mode
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Everything works
-
They switch one parameter
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Now nothing responds
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They believe something is broken
What actually happened:
The VFD is now waiting for commands from a different source.
The spindle is doing exactly what it was told to do.
Normal Operating State for CNC Use
For most CNC installations:
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RUN Source → External
-
Frequency Source → External
The controller:
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Starts the spindle
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Sets the RPM
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Stops the spindle
The VFD becomes a power amplifier, not a manual controller.
When to Use Local Mode
Use Local mode when:
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Performing spindle break-in
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Verifying motor direction
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Testing without a controller
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Diagnosing wiring
After testing, switch back to External mode before running jobs.
Quick Mental Model
Think of the VFD like this:
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Local Mode → “I’ll control myself.”
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External Mode → “I’ll listen to the CNC.”
If the spindle isn’t responding, ask:
Who is the VFD currently listening to?
That question solves most control issues immediately.
Key Takeaway
Local vs External control is not a fault.
It is simply a choice of command source.
Understanding:
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Run source
-
Frequency source
…prevents most startup confusion and eliminates unnecessary troubleshooting.
