Diagnosing VFD Rotation Conflicts Between Hand and Auto

The Problem

A blower runs backward in Hand during startup. The reflex is muscle memory from across-the-line motors: open the MCC bucket and swap two leads. But on a VFD-driven machine that reflex can be exactly wrong — if the drive's Hand reference is inverted in software, the motor may run correctly in Auto, and swapping leads “fixes” Hand while quietly breaking Auto. The crew leaves site with the machine verified in the mode they tested and wrong in the mode it will actually run in.

The Approach

The tool is a state matrix: add an equipment ID, record observed rotation in Hand and in Auto (Forward / Reverse / Untested), and let one piece of logic issue the verdict. The logic rests on a single fact worth internalizing:

Physical wiring affects Hand and Auto identically. The same three leads feed the same motor in either mode. So if rotation differs between modes, it cannot be wiring — one of the two references is inverted in software. Only when rotation is wrong in both modes is it a lead problem.

That yields the verdict table: Hand-Reverse/Auto-Forward flags yellow — software conflict, do not swap physical leads, find the drive parameter. Hand-Forward/Auto-Reverse is the same conflict on the Auto path (PLC reference sign or terminal mapping). Reverse in both flags red — hardware: swap two leads at the MCC and retest both modes. Forward in both goes green, and anything Untested gets no verdict at all — the tool refuses to conclude from partial data.

Equipment cards with Hand and Auto rotation radios, one highlighted yellow with a software-conflict warning
Figure 1: A card in the yellow state — rotation differs between modes, so the leads are innocent.
Test discipline that rides along
  • Bump-test at low speed with the drive train ready for it — some machines (screw pumps, certain gearboxes) must never run backward even briefly. Check the O&M first.
  • Verify against the rotation arrow on the casing, not against what sounds right — a centrifugal machine running backward delivers roughly half flow and head and can sound nearly normal.
  • After any fix — parameter or leads — retest both modes. That's the entire point of tracking them as a matrix.

The matrix persists in localStorage and exports to CSV (with verdicts) for the commissioning record. Single file, vanilla JS, no dependencies [1][2].

The tool: VFD “Hand vs. Auto” Rotation Matrix.

References

  1. [1] The drive manufacturer's parameter manual — direction/reference parameters (governing document for the software side).
  2. [2] Motor and driven-equipment O&M — rotation arrow and reverse-rotation restrictions.