Troubleshooting a Schneider Modicon-to-Altivar Drive Loop: 4-20 mA Control and DMM Fault Isolation

The System and the Symptom

A variable-frequency drive feeds AC power to an induction motor and sets motor speed. A Schneider Modicon PLC with an analog output card sends the speed command as a 4–20 mA DC signal. Here, 4 mA means zero speed (no power to the motor) and 20 mA means full speed (60 Hz to the motor).

The symptom: the pump does not run regardless of what the operator commands from the touch screen. The VFD faceplate shows a few lit LEDs, but those LEDs neither confirm nor deny power at the motor. With only a digital multimeter available, the task is to prove or disprove each stage in order: PLC analog output → control wiring → drive input → drive output stage → motor.

Q: How Do You Isolate the Fault With Only a DMM?

Work from source to load. This prevents wasted parts swaps and false conclusions.

• Step 1 — PLC output: Command a speed and measure the analog current at the PLC output card. A correct card shows the expected mA. If you read 4 mA on a 60% command, the fault is upstream in logic or scaling.
• Step 2 — Drive input: Measure the same mA at the drive analog input terminals. Current present at the PLC but absent at the drive proves an open loop, a loose terminal, or a short.
• Step 3 — DC bus: With power safely confirmed, check the DC bus voltage inside the drive. A dead bus points to incoming power or a blown fuse.
• Step 4 — Drive output: Measure AC volts at the motor terminals during a run command. De-energize and lock out before touching conductors — use the drive’s own display and safe probe points for live checks.
• Step 5 — Motor windings: Verify motor winding resistance and insulation. A grounded or open winding trips the drive on output fault.

Q: What Are the Most Common Culprits?

Field experience narrows this fault to a few causes:

• Control-wire disconnection: The mA loop can lose its path through a stray terminal. This is the most frequent cause.
• Drive in Local mode: The drive may be ignoring the PLC command entirely. The operator touch screen still shows speed, hiding the mismatch. Always confirm the drive’s control-source setting.
• Latched protection trip: An Altivar holds a fault until manually reset. Check the fault log before replacing hardware.
• Scaling error: A 4 mA floor with a wrong range can scale a ‘60%’ command back to zero. Verify the drive’s analog input range configuration.

Q: When Should You Use Modbus Instead of 4-20 mA?

A 4–20 mA loop is simple yet fragile — sensitive to noise, opens, and scaling errors. On the same Modicon and Altivar pair, you can run a Modbus RTU link over RS-485 using a Schneider Modbus communication module. The PLC then writes a speed word directly to a drive register, returning exact speed, current, and fault codes — not just an analog shadow.

• Set the drive to serial control and assign a unique Modbus slave address (e.g., 1).
• Configure matching baud rate, parity, and stop bits on both master and slave.
• Write run and speed commands to the mapped registers.
• Read back actual speed and fault status to verify the loop.

For diagnostics-rich pump applications, prefer Modbus RTU and keep the analog loop as a fallback.

A Repeatable Troubleshooting Checklist

• Prove command mA at the PLC analog output card, then at the drive input terminals.
• Confirm drive control source, operating mode, and active fault state.
• Verify DC bus voltage, then safe AC output voltage, then motor winding resistance.
• Re-test the pump against a known good command signal.

Conclusion & Action Advice

A drive that will not start is a chain, and a chain breaks at one link. Move from the PLC analog output toward the motor, proving each stage with a digital multimeter. Compare mA at the card and at the drive input to split logic faults from wiring faults. Confirm control source and latched faults before blaming hardware. Watch scaling: a 4 mA floor can silently zero your command.

Where diagnostics matter, run Modbus RTU to the Altivar for exact speed and fault data. Lock out safely for every live power check. Follow this order and the pump fault becomes a five-minute fix instead of a full-shift mystery.

Author: Deng Zhiyuan is an industrial automation engineer with over 10 years of experience in PLC, DCS, and control systems.