Troubleshooting Motoman XRC Alarm 4303: Converter Ready Signal Error

Problem Details

Alarm 4303 – Converter Ready Signal Error on Motoman XRC, DX, and YRC1000 controller families latches during the servo ON handshake. The controller asserts the servo ON request, but the converter (or a downstream axis servo pack) never returns a valid charge-completion / servo-ready acknowledgment within the controller's polling window. The fault typically appears during mode transitions (manual → automatic, teach → play, or remote → auto) when contactors re-close and the DC bus recharges.

Three operational signatures are documented in the Yaskawa alarm help text and reproduce the fault field reports describe:

1. The servo-ready signal is intercepted while servo ON is being asserted.
2. The servo-ready signal is not intercepted during servo OFF commanded by an emergency stop.
3. While turning servo OFF (at E-stop), the servo-ready signal is incorrectly turned back ON.

The same alarm code is published for the XRC, DX100, DX200, and YRC1000micro platforms. See the official Yaskawa Motoman knowledge base entries for the cross-platform definitions:

• YRC1000micro Alarm Code 4303 – Converter Ready Signal Error
• DX100 Alarm Code 4303 – Converter Ready Signal Error
• DX200 Alarm Code 4303 – Converter Ready Signal Error

Root Cause Analysis

Four root causes are catalogued in the controller alarm help text:

Why the Alarm Latches at Mode Change

The mode transition is the most common trigger because the controller momentarily drops the servo-on request to re-initialize axis groups, then re-asserts it. This re-energizes the main contactor(s), which closes the three-phase path to the converter. The converter's soft-charge resistors are bypassed, and the bulk DC bus climbs from the pre-charge level to the full rectified line voltage. Any of the following during that 200–600 ms transition causes the ready handshake to drop:

• Chattering main contactor (mechanically worn, coil voltage marginal, aux contact failing).
• Voltage sag under capacitor inrush.
• Phase imbalance that prevents the converter's three-phase rectifier from achieving rated DC link.
• Stale WRCA firmware that does not match the system version, causing the ready-line debounce to misfire.

Affected Hardware and Signal Path

The ready handshake travels the following physical path on an XRC system:

1. Controller backplane (CPU board) → WRCA01 board (backplane I/O).
2. WRCA01 → WRCF01 (E-stop / converter interface) board.
3. WRCF01 → Converter SRDA-COA series (main axis) or external-axis servo pack.
4. Converter → AC contactor coil (K1, K2) which closes the three-phase feed.
5. Converter → Ready output contact closes → WRCF01 reads it → WRCA01 → CPU.

Any single open in this chain causes 4303. The fault is most often attributed to the converter, but in field experience a significant fraction of 4303 events trace back to:

• Contactor timing mismatch. Main contactors (often Cutler-Hammer/Westinghouse or Yaskawa-branded) that pull in asynchronously. The phase that closes last briefly back-feeds the DC bus through the rectifying diodes of the other phases, producing a brief imbalance and a momentary ready-signal drop.
• Auxiliary contact oxidation. The contactor's NO aux contact that signals "contactor closed" oxidizes and develops resistance, slowing the rising edge of the ready signal below the controller's debounce time.
• WRCA board firmware revision mismatch. After board swaps the new WRCA carries a different firmware revision that interprets the ready signal with a different debounce constant.

Diagnostic Procedure

Step 1 – Capture Alarm History and Subsystem Tag

From the teach pendant:

1. Navigate to {SYSTEM} → {ALARM} → {HISTORY}.
2. Record the date/time, the subsystem tag (e.g., S1, S2, S3, or R1), and the motion program that was active.
3. Open the system version screen and capture the controller type, software version, and the WRCA board software version.

The subsystem tag narrows the search to a specific servo pack. S1–S3 typically point to the robot's three axes; an external-axis-only system (where the manipulator is electrically disconnected but its servo pack remains in the cabinet) usually shows the alarm on the tag of the external-axis pack.

Step 2 – Verify Primary Voltage Under Load

Measure phase-to-phase voltage at the converter input terminals with a true-RMS meter while the alarm is active:

Step 3 – Inspect Cable Sizing and Length

For the primary feed, use the following guidance (verify against the controller's installation manual):

Voltage drop under inrush must remain below 3% of nominal. Use:

V_drop = I_inrush × R_cable × L × 2 (round-trip)

where I_inrush is the peak capacitor charging current (typically 2–3× the steady-state input current for 100–300 ms), R_cable is resistance per meter at 20 °C, and L is the one-way cable length.

Step 4 – Inspect the Main Contactor

1. De-energize and lock-out the controller.
2. Locate the main contactor(s) feeding the converter input. On XRC systems this is typically inside the Yaskawa Power Unit (YPU) cabinet, often labelled K1 / K2.
3. Measure contact resistance across each pole: >50 mΩ indicates pitting and replacement is justified.
4. Inspect the coil for discoloration, swelling, or burnt varnish.
5. Apply control voltage and listen/feel for simultaneous pull-in. Asynchronous pull-in is the most common field finding for a 4303 that only appears at mode transitions.

Step 5 – Inspect the WRCA01 / WRCF01 Boards

With power removed:

1. Pull the WRCA01 board and inspect the PCB for burnt traces near the converter interface connector (typically CN-series).
2. Inspect the WRCF01 board for capacitor leakage, especially electrolytic capacitors in the E-stop and converter interface sections.
3. Verify the firmware sticker on the new board (if a spare is available) matches the system version in {SYSTEM} → {VERSION}.

Step 6 – Isolating External-Axis vs. Robot Converter

On systems with three external axes where the manipulator servo pack remains in the cabinet but the manipulator is mechanically disconnected, isolate the failing converter:

1. Disconnect the robot's main axis servo pack from the backplane and the DC bus.
2. Power the controller with only the external-axis converter and packs connected.
3. Repeat the manual → automatic transition 20 times.
4. If 4303 still fires, the converter or interface in the external-axis path is the source.
5. If 4303 clears, restore the robot pack and disconnect the external-axis pack; repeat.

This isolation procedure is essential because the alarm help text directs you to "the converter" but does not distinguish robot vs. external-axis. The subsystem tag captured in Step 1 confirms which path.

Solution Tree

Apply the following remediation in order of cost and invasiveness. Each step includes the verification check that confirms you can move to the next.

Solution A – Reset and Re-arm

From the pendant: {ALARM} → {RESET} → {SERVO ON}.

Verification: If the alarm does not recur within ten mode transitions, treat it as a transient and document the time/conditions. Persistent recurrence proceeds to Solution B.

Solution B – Tighten Primary Power Tolerance

1. Measure and log the no-load and inrush voltages per Step 2.
2. Adjust the upstream transformer tap if a sag is detected.
3. Replace undersized primary cabling per the table in Step 3.
4. Verify the building's supply transformer is rated for the inrush of the controller plus all other loads on the same feeder.

Verification: Re-run the voltage measurement under load. Voltage must remain within +10% / -10% of nominal throughout the inrush window.

Solution C – Replace Main Contactor(s)

1. Replace K1 (and K2 if present) with the OEM-specified equivalent. Yaskawa typically uses Sprecher+Schuh CA7 or equivalent; verify by the cabinet wiring diagram inside the door.
2. Replace the contactor's aux contact block if it is separately available.
3. Tighten all power lugs to the manufacturer's torque spec (typically 26–35 in-lb for 10 AWG terminations).

Verification: Use a clamp-on ammeter on each phase of the contactor output during a servo ON cycle. The three phases must reach steady-state current within 50 ms of each other. Asymmetry beyond 10% indicates contact wear and the contactor must be replaced again.

Solution D – Replace the WRCA01 Board

1. Order the WRCA01 with the firmware revision matching the existing system version. Reference: Yaskawa spare-part catalog.
2. Power down and remove the WRCA01 from the backplane.
3. Install the replacement; verify seating is fully latched.
4. Power up and verify {SYSTEM} → {VERSION} shows the same revision.

See the published DX100 Alarm Code 4303 Converter Ready Signal Error entry for additional board-replacement context applicable to XRC and DX200 systems.

Verification: Perform 50 manual → automatic mode transitions with a representative motion program loaded. Zero 4303 alarms is the acceptance criterion.

Solution E – Replace the Converter

1. Identify the converter part number from the system nameplate (typical XRC converters: SRDA-COA series for the main axis pack).
2. Replace as a unit; do not attempt component-level repair of the IGBT module.
3. Re-flash or reload the converter parameters from the controller's backup file before servo ON.

Verification: Run the system through the full motion envelope for four hours, including repeated E-stop and reset cycles. Monitor for any ready-signal glitches on the pendant's diagnostic screen.

Verification Procedure

After applying any solution, perform the following comprehensive check:

1. Reset the alarm history.
2. Cycle the controller power (cold start).
3. Run a no-motion program that asserts servo ON, holds for 5 s, then de-asserts. Repeat 10 times. Monitor the ready signal with a scope or the pendant's I/O monitor.
4. Run a representative motion program in {AUTO} mode for at least 30 minutes, including a minimum of five E-stop → reset cycles.
5. Switch between {TEACH} and {AUTO} modes 20 times. This is the highest-stress transition and the most likely to expose intermittent contactor or WRCA faults.
6. Confirm zero 4303 events in {ALARM} → {HISTORY} after the test sequence.

Troubleshooting Matrix

Related Alarm Codes

4303 frequently appears in conjunction with or is misdiagnosed alongside the following codes. Treat them as a fault cluster:

Preventive Maintenance

• Quarterly: Measure and log the primary voltage under load. Compare against the baseline; drift >2% triggers a transformer tap review.
• Semi-annually: Inspect the main contactor aux contacts for oxidation; clean or replace as required.
• Annually: Pull and inspect the WRCA01 and WRCF01 boards for capacitor leakage and burnt traces.
• On every WRCA replacement: Document the firmware revision of the board removed and installed. Keep a board-firmware history per controller.
• After any cabinet modification: Re-run the verification procedure (50 mode transitions) before returning the cell to production.

Safety Notes