FieldBench
How To
Electrical14 min · 5 steps

Troubleshoot a rooftop indoor-fan VFD

Enable 24V, then 0–10 V, then the drive code. OC is static or a seized wheel. Do not jumper fan-prove. Do not throw a dual cap at a VFD motor.

Still failing after this procedure?

How To stays open. Diagnose walks the next measurement.

Before you start

  • VFD DC bus holds a charge after disconnect. Wait the drive’s posted time before you open it.
  • Lock the roof disconnect. 3-phase at the drive is line voltage.
  • Do not megger a motor with the VFD still connected — you will take out the output transistors.
  • Do not jumper fan-prove. Heat will fire into a near-dead blower.

On the truck

  • True RMS meter (VAC and VDC)
  • Manometer for total external static
  • The drive keypad in this cabinet (EcoBlue / Reliatel / Yaskawa / ABB / aftermarket)
  • Megohmmeter if the drive shows ground fault

Steps

  1. 1 Photograph the drive code

    OC, LU, OU, GF, OH, or a unit-board fan alarm (Reliatel fan fail, EcoBlue fan). Power-cycle wipes it. Confirm L1/L2/L3 at the drive under load.

    Expect: Live code written. Three legs present if this drive is 3-phase.

    If it fails: No display and no line = disconnect / fuses, not a motor.

  2. 2 Enable, then speed reference

    24 VAC (or this drive’s digital enable) while G / occupied / a heat or cool call is on. Then 0–10 VDC at the analog pair under a high-fan call. 0 V enable is the unit board. 0 V analog with enable is a wire or a dead analog output.

    Expect: Enable present. Analog not stuck at 0. Occupied cool is often not 10 V — use this fan table.

    If it fails: Fix the board / harness before you change a drive.

  3. 3 OC — wheel, static, FLA

    Power off, spin the wheel. TES vs this blower table. FLA / motor-data on the drive vs the motor nameplate. A rooftop at 1.4" with a 0.8" motor is OC waiting to happen. Do not raise FLA to hide a dirty coil.

    Expect: Wheel free. Static legal. FLA matches the motor plate.

    If it fails: Clean filters/coil and fix duct static before you condemn the drive.

  4. 4 Limp / min Hz vs fan-prove

    Some Reliatel / VFD cabinets drop into limp (~min Hz). That CFM will not close a 0.07" prove switch, and the board reports fan fail. Clean first, then min-frequency. Never leave a jumper on prove.

    Expect: Prove switch closes at a real occupied CFM.

    If it fails: Load the rooftop plate in Diagnose — YSC fan-fail is not a 48FC EcoBlue menu.

  5. 5 GF — megger with the drive off

    Disconnect U/V/W from the drive. Megger the motor. Wet rooftop motors after rain and cables rubbed through the curb are this code. Reset-and-run a GF and you buy a drive.

    Expect: Insulation in spec. Cable undamaged.

    If it fails: Replace the motor or the cable. Do not keep resetting GF.

Field notes

  • Carrier EcoBlue, Trane Reliatel VFD, and an aftermarket Yaskawa in the same curb do not share a parameter list. Read THIS keypad.
  • Type the live keypad string in Diagnose. Examples: oC (overcurrent), Uv (undervoltage), ov (overvoltage), GF (ground), oH (heatsink), PF (input phase).
  • A dual run cap belongs on a PSC outdoor fan, not on this indoor drive.

Related

Sources

Practice distilled from public HVACR School and AC Service Tech material. OEM I/O and EPA 608 still win.