Wiring 7 — PLC Analog Output Driving a VFD Speed Reference

PLC Analog Output Driving a VFD Speed Reference

A Variable Frequency Drive (VFD) controls the speed of a 3-phase induction motor by varying the frequency and voltage of the AC power it delivers to the motor windings. This replaces mechanical speed-control methods (gearboxes, belt drives, throttling valves) with an electronic solution that is more precise, more energy-efficient, and easier to integrate with a PLC.

On variable-torque loads such as pumps and fans, motor speed and power draw are related by the affinity laws: reducing speed to 80 % reduces power consumption to roughly 51 % (the cube law). VFDs are therefore the single most impactful energy-saving measure in most facilities.

0-10 V vs 4-20 mA speed references

Most VFDs accept either a 0-10 V voltage reference or a 4-20 mA current reference at their analog input. The choice is usually made via a jumper or parameter on the VFD itself. This lesson uses voltage: 0 V commands 0 % speed, 10 V commands 100 % speed (maximum frequency). Current references are preferable on long cable runs where voltage drop could introduce error — the same reason 4-20 mA dominates in process instrumentation. For a panel-mount VFD reachable by a short cable run, 0-10 V is entirely appropriate.

The signal-and-common pair

The 0-10 V signal is measured as a differential voltage between the analog input terminal (terminal 2 on this VFD) and the analog common (terminal 5). Both wires must run in the same cable — ideally a twisted pair — so that any noise induced on the cable affects both conductors equally and cancels at the VFD's differential input.

Never route the signal wire alone and rely on a separate common wire routed by a different path. The common wire carries the return current for the signal; separating them creates a loop that acts as an antenna.

Single-point shield grounding

Instrument cable shields must connect to earth at one end only. In this topology the PLC analog output card is the signal source, so the shield terminates at the card's SHIELD terminal. The VFD end of the shield stays floating (unterminated or taped back). Grounding both ends would create a ground loop: small potential differences between the two earth points drive a circulating current through the shield, which couples directly into the signal pair as noise.

PE bonding — VFD and motor separately

Both the VFD chassis and the motor frame must be bonded to protective earth (PE), but they are wired as separate bonds to the cabinet ground bar — not daisy-chained. Each bond provides an independent low- impedance fault-current return path. Industrial safety practice requires that removing one item of equipment does not break the earth continuity of another.

Out of scope

VFD parameter setup (acceleration/deceleration ramps, motor nameplate data, P-code configuration) is a commissioning concern, not a wiring concern. This lesson covers the cable connections only.