4–20 mA Speed Reference Signals and Drive-to-PLC Integration

Why Current Loops Instead of Voltage?

A variable-frequency drive (VFD) can accept an analog speed reference as either a voltage signal (typically 0–10 V) or a current signal (typically 4–20 mA). Current loops are preferred in industrial environments because current does not drop across long cable runs the way voltage does. A 4–20 mA loop maintains the same current at both ends of the wire regardless of lead resistance, making it far more noise-immune and accurate over distances of hundreds of feet. Most industrial PLC analog output modules, sensors, and process controllers use 4–20 mA as the standard.

The signal range starts at 4 mA, not 0 mA. That offset is intentional: a live zero of 4 mA lets the system distinguish between a commanded zero-speed condition and a broken wire or failed transmitter (which would produce 0 mA). A reading below 3.6 mA typically triggers a fault in the drive, alerting maintenance before a process problem occurs.

The Linear Scaling Formula

Because 4–20 mA maps linearly to output frequency, you can calculate the expected drive output frequency for any current value using:

Output frequency = [(mA − 4) / 16] × (f_max − f_min) + f_min

• mA is the actual loop current you are commanding.
• 4 is subtracted to normalize the signal to start at zero.
• 16 is the full usable span (20 − 4 = 16 mA).
• f_max and f_min are the programmed maximum and minimum output frequencies in the drive parameters (commonly 60 Hz and 0 Hz, but they can be set differently).

At 4 mA the formula yields f_min. At 20 mA it yields f_max. At any value in between, the result is proportional. This is the same scaling concept used throughout process instrumentation and is worth memorizing.

Drive Parameter Setup

Before the current loop will work, two drive parameters must be configured correctly (exact parameter numbers differ by manufacturer — always consult the drive's parameter manual):

1. Reference source selection — Set the analog input type to current (4–20 mA). Many drives have a physical DIP switch on the control board that must also match; both the switch and the parameter must agree or the input will not read correctly.

2. Analog input scaling — Confirm that the drive maps 4 mA to f_min and 20 mA to f_max, or enter the values you need. Some drives allow you to invert the relationship (20 mA = f_min) for specific process applications.

Record every parameter change in a drive commissioning log. This practice is expected in industrial workplaces and is essential for troubleshooting.

Wiring a Current-Loop Circuit

A basic 4–20 mA reference circuit has three key points: a source (the PLC analog output module or current-loop simulator), the VFD current input terminal, and a common reference terminal. Wire the positive output of the source to the designated current input terminal (often labeled AI1 or AIN+) and the negative return to the common terminal (AIN− or AGND). Keep analog signal wiring away from power conductors; use shielded twisted-pair cable and connect the shield at one end only (typically the drive end) to prevent ground loops.

Safety — Before You Touch Any Terminal

Follow these steps every time:

• Lockout/tagout (LOTO): Verify the drive and PLC are de-energized under OSHA 29 CFR 1910.147 before opening enclosures or connecting wiring. Attach your personal lock. Test — do not assume.
• Residual charge: VFD DC bus capacitors can hold lethal voltage for several minutes after power is removed. Wait the time listed in the drive manual (often 5 minutes or more), then verify with a DMM set to DC voltage before touching internal terminals.
• Electrical PPE: Per NFPA 70E, wear appropriately rated PPE. For low-voltage control wiring with the drive confirmed de-energized, safety glasses and insulated hand tools are the minimum. Verify absence of voltage with a properly rated meter before contact.
• Machine guarding: Per OSHA 1910.212, ensure all guards are in place before running a connected motor. Confirm the test area is clear before enabling the drive output.
• Never work inside a live drive cabinet unless you are qualified, the work is justified, and an energized-work permit is in place per NFPA 70E.

Measuring Loop Current with a Clamp Ammeter

A standard clamp ammeter measures AC current magnetically and cannot measure a DC 4–20 mA signal correctly. For this lab, use a true DC clamp ammeter or, preferably, place your DMM in series with the loop (set to DC milliamps, 200 mA range) to measure the actual loop current precisely. Compare your measured value to your commanded value and your calculated frequency to the drive display. Tolerance for this lab is ±1 Hz.

Drive-to-PLC Integration and Network Alternatives

The 4–20 mA analog method is simple and reliable but provides only one speed command and no feedback data. Modern installations increasingly use industrial networks that pass multiple variables over a single cable:

• EtherNet/IP (Common Industrial Protocol over standard Ethernet) is widely used in Rockwell/Allen-Bradley systems.
• PROFINET is the standard in Siemens environments.
• Modbus RTU is a legacy serial protocol still common on smaller drives.

Networked drives can report output frequency, motor current, fault codes, and drive temperature back to the PLC in real time — information that a simple analog loop cannot provide. Understanding when analog control is sufficient and when a network interface adds value is an important technician judgment.

Connecting to the Bigger Picture

The scaling formula you practice today appears throughout mechatronics: pressure transmitters, flow meters, temperature sensors, and valve positioners all use 4–20 mA. Mastering one application gives you a transferable skill for every analog instrument you will encounter on the job.