What causes fault code VFD-AF1 (VFD Analog Input Fault)?

4-20mA signal wire broken — current reads 0mA instead of minimum 4mA. Signal source (PLC analog output or potentiometer) powered off or faulted. Shield wire on analog signal cable broken or grounded at both ends — noise causing erratic readings. Wrong signal type selected — VFD configured for 4-20mA but receiving 0-10V or vice versa. Analog input terminal corroded or loose — high resistance contact

How do I fix fault code VFD-AF1?

Measure signal at VFD analog input terminals with multimeter — 4-20mA or 0-10V expected. If 0mA, trace wire back to source — check for broken wire or loose terminal. Verify VFD analog input configuration matches source type (mA vs V, unipolar vs bipolar). Check analog signal cable shield — ground at VFD end only, not at source end. Set VFD signal loss action parameter — fault, hold last speed, or go to preset speed. If using 4-20mA, the live-zero (4mA = 0 speed) provides built-in wire break detection

VFD Analog Input Fault

VFD-AF1

Analog speed reference input (4-20mA or 0-10V) signal is out of range or lost. Drive cannot determine commanded speed.

medium vfd plc

vfd analog 4-20mA signal loss speed reference

Probable Causes (ranked)

4-20mA signal wire broken — current reads 0mA instead of minimum 4mA
Signal source (PLC analog output or potentiometer) powered off or faulted
Shield wire on analog signal cable broken or grounded at both ends — noise causing erratic readings
Wrong signal type selected — VFD configured for 4-20mA but receiving 0-10V or vice versa
Analog input terminal corroded or loose — high resistance contact

Fix Steps

1 Measure signal at VFD analog input terminals with multimeter — 4-20mA or 0-10V expected
2 If 0mA, trace wire back to source — check for broken wire or loose terminal
3 Verify VFD analog input configuration matches source type (mA vs V, unipolar vs bipolar)
4 Check analog signal cable shield — ground at VFD end only, not at source end
5 Set VFD signal loss action parameter — fault, hold last speed, or go to preset speed
6 If using 4-20mA, the live-zero (4mA = 0 speed) provides built-in wire break detection

Field Trick

Always use 4-20mA over 0-10V for speed reference. With 4-20mA, 0mA means the wire is broken — you get automatic fault detection for free. With 0-10V, a broken wire looks like 0V which means 'run at zero speed' — you'll never know it's broken.

Suggested Replacement Parts

Parts matched to fault category: Vfd

Allen-Bradley PowerFlex 525 AC Drive 1HP P/N: 25B-D2P3N114 1HP, 480V, 3-phase, built-in EtherNet/IP, EMC filter

AutomationDirect $395.00 Amazon $450.00 Grainger $520.00

ABB ACS580 General Purpose Drive 2HP P/N: ACS580-01-05A7-4 2HP, 480V, built-in safe torque off, adaptive programming

AutomationDirect $425.00 Amazon $480.00 Grainger $550.00

GS20 AC Drive 1HP (Budget Option) P/N: GS20-20P5 1HP, 230V, sensorless vector, compact size — good replacement for testing

AutomationDirect $149.00 Amazon $175.00

Prices are estimates. Verify part numbers match your equipment before ordering.

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