Fanuc SVM Alarm b / c / d (DC Link Current Alarm) — Causes, Diagnosis & Fixes

Alarm bJJ Automation

On a Fanuc servo amplifier (SVM), a 7-segment display showing ‘b’, ‘c’ or ‘d’ means a DC link current alarm — the inverter measured abnormal current flowing on the DC link side for one of the module's axes (the letter identifies which axis of a multi-axis module, the same way 8./9./A. identify axes for the direct overcurrent alarms). The current drawn from the shared DC bus exceeded what the amplifier allows — caused by wrong servo parameters, an over-harsh duty cycle, degraded insulation, or a faulty amplifier. Fanuc's own maintenance manual gives a clean elimination sequence for this alarm; here it is expanded with the workshop detail the manual leaves out.

Step 1 — Check the servo parameters first (free, and often the answer)

Wrong current-loop parameters make the drive command impossible current. Check that these hold their correct (motor-specific default) values against the FANUC AC Servo Motor αi Parameter Manual (B-65270EN):

  • Series 16i / 18i / 20i / 21i / 0i / Power Mate i: parameters 2004 (control cycle/loop setting), 2040 (current loop integral gain PK1) and 2041 (current loop proportional gain PK2).
  • Series 15i: the same three live at 1809, 1852 and 1853.

These get corrupted by botched parameter restores from another machine, manual ‘tuning’, or a wrong motor ID (parameter 2020 / motor code). The fastest fix: re-enter the motor ID and let the control auto-set the current loop values, or restore a known-good parameter file for this machine — see our wrong-data guide for why sister-machine files cause exactly this.

Step 2 — Does it only trip on rapid accel/decel?

If the alarm appears only during aggressive acceleration or deceleration — new heavier fixture, raised rapid override, fresh program with tighter time constants — the motor is simply being worked beyond its thermal/current envelope. Increase the acceleration/deceleration time constant and retest. If the alarm disappears, the machine is telling you the duty was too harsh: either keep the gentler time constant or size up the motor/amplifier — do not chase it with parameter overrides.

Step 3 — Reseat the control board faceplate

Straight from the manual and easy to dismiss: push the faceplate (control printed-circuit board) fully home. A half-seated board-to-board connector between the control and power PCBs corrupts current sensing and raises phantom current alarms — we see this after transport vibration and after previous repair attempts. Power OFF, lock out, wait for the charge LED to go out, then press the faceplate as far as it will go.

Step 4 — The elimination sequence: amplifier → insulation → motor vs cable

  1. Cable-off test: disconnect the motor power leads (U/V/W) at the amplifier, release the E-stop and enable.
    • Alarm still present with no motor connected → the amplifier itself is faulty — its current sensing or power stage. Bench repair or exchange unit.
    • Alarm gone → the problem is downstream. Continue.
  2. Insulation check, leads + motor together: with the leads still off the amplifier, megger between PE (earth) and each motor lead U, V, W (500 V DC). Healthy insulation with the alarm present in operation → paradoxically, back to the amplifier (marginal sensing that passes the static test) — bench testing settles it.
  3. Insulation deteriorated → split motor from leads at the motor connector and test each separately:
    • Motor insulation bad → motor rewind/replacement. Coolant ingress at the connector and moisture in an old motor are the usual causes — re-test warm if the cold reading is borderline.
    • Cable insulation bad → replace the power lead — drag-chain crush and oil-rotted jackets are the classic finds. Replace, don't tape: a taped repair fails at the next flood.
Tip: Follow the order. The parameter check and faceplate reseat cost nothing and fix a surprising share of b/c/d alarms; the cable-off test then splits hardware cleanly in ten minutes. Jumping straight to ‘replace the amplifier’ without the insulation checks is how a shorted motor kills a freshly repaired unit on day one.

Diagnosis checklist (in order)

  1. Verify current-loop parameters 2004/2040/2041 (15i: 1809/1852/1853) and motor ID 2020 — especially after any parameter restore or amplifier/motor swap.
  2. Alarm only on hard accel/decel → lengthen the time constant and retest.
  3. Reseat the control-board faceplate fully.
  4. Cable-off test → amplifier vs downstream.
  5. Megger PE-to-U/V/W; if degraded, split motor from cable and megger each.
  6. All external checks pass → amplifier to the bench for load testing.

Related guides

SVM showing b, c or d and the checks point at the amplifier? JJ Automation repairs Fanuc servo amplifiers at component level — current sensing, gate drives, IGBT stages — with real load testing before dispatch, and can megger-test motors and cables on site in NCR. Tested exchange SVMs in stock. WhatsApp us or call +91 88022 12394.
Tags: Alarm b, Alarm c, Alarm d, Dc link current, Fanuc, Servo amplifier, Svm, Troubleshooting