Motoman XRC Alarm 4305: Converter Charge Error Troubleshooting

System: UP6 dual-robot Arcworld cell. Sub-code: 0000_0000_0000_0010 (SV#1). Symptom: Intermittent 4305, clears on reset, returns within hours or days. Likely causes: Primary power sag below 170 V, marginal WRCA01 or WRCF01 board, converter capacitor drift, amplifier internal short, or an undersized shared transformer in a dual-robot cell. This article is structured for maintenance technicians, controls engineers, and robot operators who need a complete diagnostic pass on the fault.

1. Alarm 4305 Definition and Sub-Code Decoding

Alarm 4305 on the Motoman XRC controller is a converter circuit charge error. The controller raises 4305 when the soft-charge cycle on the DC bus fails to complete or is interrupted. The alarm is reported with a 16-bit sub-code that maps one bit per physical converter in the cabinet. A set bit identifies the specific converter that failed to assert its ready-2 feedback.

The reported sub-code 0000_0000_0000_0010 sets bit 1, which on the XRC corresponds to SV#1 (servo amplifier 1, the first axis of the primary robot in a dual-UP6 cell). Decoding the sub-code is a mandatory first step because it tells the technician which physical module to target during diagnosis.

Ready-2 Signal - The Heart of Alarm 4305

The XRC controller commands a main contactor to close, which connects the three-phase primary to the soft-charge resistor network. The soft-charge resistors limit the inrush current to the DC bus capacitors. When the bus voltage reaches the operating threshold (typically 280-300 V DC), the converter asserts the ready-2 signal (also abbreviated RDY2 or RD2). The controller then closes the main run contactor, bypassing the soft-charge resistors and applying full primary power to the amplifiers.

Alarm 4305 is raised in four scenarios involving the ready-2 signal:

1. Ready-2 never asserts within the controller's charge time window after a servo ON command.
2. Ready-2 de-asserts after the controller has already commanded servo ON (charge interrupted mid-cycle).
3. Ready-2 remains asserted at emergency stop (the controller expects the converter to drop ready-2 on servo OFF).
4. Ready-2 toggles ON while the controller is in the process of commanding servo OFF.

2. Affected Controllers and Cross-References

Alarm 4305 is documented across the entire Motoman controller lineup that uses a converter-based power section. The XRC platform is the earliest, but the underlying fault conditions (ready-2 missing, interrupted, or stuck) are identical in every generation. The differences are the alarm code naming, sub-code width, and the part numbers of the suspect board assemblies.

The mechanical and electrical steps to replace the converter control board are functionally identical across the entire family. The Motoman knowledge base articles linked above are the authoritative reference for the replacement procedure on each controller. The XRC WRCA Board Replacement Procedure is cross-referenced in the YRC1000 alarm article.

3. Root Cause 1 - Primary Power Supply

The most common cause of an intermittent 4305, and the one the manufacturer documents first in the alarm description, is the primary power supply. Four specific primary-power faults can trigger 4305:

3.1 Wiring Fault on Primary Side

A missed or loose connection on the 200/220 V three-phase input to the cabinet or to the converter module. Symptoms include:

• 4305 raised only on first servo ON after a long power-off (cold start)
• 4305 raised only when the second robot in a dual cell is commanded to move (the first cabinet's contactor closure sags the shared secondary)
• Visible discoloration on the input lugs of the cabinet or transformer

Corrective action: with the disconnect locked out, re-torque all input lugs to the XRC-specified value (typically 2.5-3.0 N·m for the XRC input terminal block). Meg the phase-to-phase insulation before re-energizing.

3.2 Primary Power Sag Below 170 V

The XRC's lower operating threshold is 170 V line-to-line RMS. The nominal primary is 200 V or 220 V. Below 170 V, the converter cannot complete the charge cycle, and ready-2 will not assert. The 170 V limit is not a hard cut-off; the converter will attempt to charge at lower voltages but the soft-charge time will exceed the controller's 500 ms timeout, raising 4305.

3.3 Undersized Primary Cabling

The cable cross-section between the step-down transformer and the cabinet must be sized for the inrush. The voltage drop on the cable during inrush is:

V_drop = sqrt(3) × I_inrush × R_cable_per_km × L / 1000

Where:

• I_inrush = peak inrush current per phase (typically 30-60 A peak for an XRC cabinet, depending on the soft-charge resistor value)
• R_cable_per_km = cable AC resistance per km at 75°C (from cable manufacturer data)
• L = one-way cable length in meters

For a 50 m run of 6 AWG copper (R ≈ 1.6 Ω/km at 75°C) with 50 A peak inrush:

V_drop = 1.732 × 50 × 1.6 × 50 / 1000 = 6.9 V

This is acceptable on a 220 V nominal tap (3.1% drop) but problematic on a 200 V nominal tap (3.5% drop) and unacceptable on a 180 V nominal tap (3.8% drop). The dual-robot cell doubles the inrush, so two cabinets drawing 50 A each simultaneously on a shared transformer will see 100 A peak per phase, pushing the drop to 13.9 V - well past the 5% allowance.

3.4 Cable Length Too Long

Even with properly sized cable, an excessively long run between the transformer and the cabinet will cause the same inrush drop. For a 100 m run, even 4/0 AWG cable will drop 4-5 V at 100 A peak inrush. The XRC manual does not specify a maximum cable length directly; the maximum is determined by the voltage-drop calculation above, not by a wire manufacturer's ampacity chart. A short, undersized cable is just as bad as a long, properly sized one.

4. Root Cause 2 - Defective WRCA01 Board

The WRCA01 is the converter control board. It hosts the microcontroller that monitors the DC bus voltage, drives the soft-charge and run contactors, and reports ready-2 back to the XRC CPU. Failure modes that cause 4305 include:

• Marginal optocoupler on the ready-2 output - the signal gets through most cycles but drops on temperature or vibration
• Drifted voltage reference - the bus voltage is read slightly low, so the controller thinks the charge is still in progress when it has actually completed
• Cold solder joint on the contactor coil driver - contactor chatters on the first cycle, briefly dropping ready-2
• Failed EEPROM - board cannot retain its calibration across power cycles, raising 4305 on every cold start

The WRCA01 is the most likely hardware cause of an intermittent 4305 that clears on the first reset. The repair is board replacement, not component-level.

5. Root Cause 3 - Defective WRCF01 Board

The WRCF01 is the gate-driver / interface board that sits between the WRCA01 and the IGBT power stage. It provides isolation and level-shifting for the IGBT gate drives, and it carries the ready-2 feedback path. A marginal optocoupler or a cracked solder joint on the WRCF01 will produce symptoms very similar to a marginal WRCA01.

Field practice is to replace the WRCA01 and WRCF01 as a pair. The two boards share the same failure mechanisms (thermal cycling, vibration, contamination), and replacing only one means the other is likely to fail shortly after.

6. Root Cause 4 - Defective Converter

The converter itself (the IGBT power stage) is the most expensive replaceable. For the reported system, the part is JUSP-ACP35JAA, a Yaskawa converter module. The JUSP-ACP family includes multiple current ratings; the exact suffix is determined by the cabinet variant and the robot model.

A converter with marginal DC bus capacitors can show 4305 only during the first charge cycle of the day, then run cleanly until the cabinet cools. The symptom is identical to a thermal fault on the WRCA01. Distinguish by swapping the WRCA01/WRCF01 first; if the 4305 returns with known-good control boards, the converter itself is the cause.

7. Root Cause 5 - Defective Amplifier

A servo amplifier with an internal power-circuit short will pull the DC bus voltage down at the moment the contactor closes, preventing the converter from ever declaring ready-2. The amplifier is the last component to suspect because the replacement cost is high; rule out all other causes first.

An amplifier with an internal short typically also reports amplifier-specific alarms (4004, 4101, 4110, 4107, depending on axis) before the 4305 is raised. If the XRC raises a 4305 with no preceding amplifier alarm, the amplifier is unlikely to be the cause.

8. Intermittent vs. Continuous Alarm - Diagnostic Table

The mismatch between the user's reported intermittent 4305 and the manufacturer's expectation of a continuous (latching) fault is itself a diagnostic clue. The table below maps symptom patterns to root cause.

For a dual-robot Arcworld cell, the most common cause of intermittent 4305 is a shared step-down transformer that is undersized for the combined inrush of both robots. The dual-robot configuration doubles the inrush current at the moment both cabinets are commanded to servo ON. If the transformer secondary is already near 200 V on a 220 V nominal tap, the inrush will pull it below 170 V and trip 4305. After the first robot's capacitors charge, the second cabinet's inrush can briefly drag the first cabinet's input below threshold, even if the first cabinet had already completed its own charge. Once the secondary voltage recovers, the alarm clears and normal operation resumes. This matches the pattern of 4305 raising only on the first motion after a long pause, or at the start of a shift.

9. Diagnostic Procedure - Step by Step

Follow this ordered procedure. Do not skip primary power verification, as it is the cheapest and most common cause. A full diagnostic pass on a dual UP6 cell takes 2-4 hours including power quality logging.

9.1 Save CMOS.BIN

Before any work on the controller, back up the CMOS.BIN. The XRC will not load a saved job file from a non-matching CMOS.BIN, so a corrupted CMOS during troubleshooting will block the cell indefinitely. In Maintenance Mode, navigate to FILE → CMOS → SAVE. Confirm the destination (typically a USB stick or PC104 card slot on the XRC). Wait for the complete file to write. Eject the media safely.

9.2 Capture Alarm History

On the XRC teach pendant, navigate to MAIN MENU → ALARM → HISTORY. Scroll through the last 200 alarms. Note the timestamp and sub-code of every 4305. Cross-reference with a power quality log or the upstream facility's power quality record. The user-provided history (clustered on 9/7 and 9/12) is consistent with a thermal or contactor-wear mechanism rather than a random primary power event.

9.3 Measure Primary Power

With the controller energized and the main disconnect closed, measure the following at the cabinet primary terminal block:

• L1-L2, L2-L3, L1-L3 (line-to-line RMS)
• All under no-load and again under servo-ON inrush (use a DMM with min/max capture or a power quality logger such as a Fluke 1730 or Dranetz HDPQ)

9.4 Inspect Contactor and Soft-Charge Resistors

With the main disconnect OFF and locked out, open the cabinet and inspect:

• The main contactor: pitting, discoloration, loose lugs
• The soft-charge resistors: case cracking, discoloration, lead oxidation
• The bus capacitors: any sign of venting, swelling, or electrolyte residue
• The DC bus bars: loose hardware, corrosion, arcing marks

A soft-charge resistor that has drifted high in resistance will lengthen the charge time. If the charge time exceeds the controller's timeout, 4305 is raised even with clean primary power. Resistance check (with disconnect OFF): a typical XRC soft-charge resistor bank reads 20-50 Ω total, depending on cabinet variant. Replace the entire bank if any single resistor reads 20% above nominal.

9.5 Verify Ready-2 Signal

The ready-2 signal is available on the converter's status connector. With an oscilloscope or a DMM with a fast capture, monitor the ready-2 line while issuing a servo ON command from the teach pendant. The signal should:

• Be LOW (not ready) at power-up
• Go HIGH (ready) within 200-500 ms of servo ON, with no chatter
• Stay HIGH throughout the run
• Go LOW within 100 ms of servo OFF or emergency stop

If ready-2 is noisy, has a slow rise, or toggles, the issue is at the converter / WRCF01 interface. If ready-2 is clean but the controller still reports 4305, the issue is at the WRCA01 or the controller's input stage.

9.6 Swap Suspect Boards

If primary power and the ready-2 signal are clean, swap boards in this order:

1. WRCA01 (with known-good spare or known-good from the other cabinet if dual-cell)
2. WRCF01 (paired with WRCA01)
3. Converter module (JUSP-ACP35JAA or equivalent)
4. Amplifier (last resort)

The XRC supports a hot-swap of the converter module in some cabinet variants, but board-level replacement requires a power cycle.

10. WRCA Board Replacement Procedure

The XRC WRCA board replacement procedure is functionally identical to the procedure documented for the later controller families (DX100, YRC1000, NX100, FS100) in the Motoman knowledge base. The abbreviated steps are:

1. Save CMOS.BIN to external storage
2. Power down the controller, lock out the disconnect, wait 5 minutes for the bus to bleed through the bleeder resistors
3. Verify zero energy on the DC bus with a CAT III 600 V meter
4. Open the cabinet and locate the WRCA01 board (top row, leftmost in most XRC cabinets)
5. Label all ribbon cables and connectors; the XRC uses high-density insulation-displacement connectors that are easy to misalign
6. Remove the board
7. Install the replacement WRCA01, re-seat all connectors
8. Power up, verify the controller boots without alarms
9. Reload CMOS.BIN if required
10. Run a full mastering and home-position check
11. Run a low-speed program in step mode to verify axis integrity

If the replacement WRCA01 was a used unit pulled from a different controller, the mastering values may not match; re-mastering is required. The YRC1000 / NX100 / DX100 / FS100 XRC WRCA Board Replacement Procedure documentation in the Motoman knowledge base is the authoritative reference.

11. CMOS.BIN Backup and Restoration

The CMOS.BIN file contains the controller's calibration, mastering, and configuration data. It is stored on a battery-backed SRAM. The Motoman standard for any board replacement is to save CMOS.BIN before working on the cabinet and to verify it can be loaded back before commissioning the cell.

A corrupted CMOS will raise additional alarms (typically 4107, 4204, 4300-class) that compound the 4305 and make diagnosis difficult. Always save before working on the cabinet.

To restore CMOS.BIN after a board replacement:

1. In Maintenance Mode, navigate to FILE → CMOS → LOAD
2. Select the file
3. Confirm the load
4. Cycle power
5. Verify all axes are mastered

12. Cabling and Wiring Best Practices

For a dual-robot Arcworld cell, the primary cabling between the step-down transformer and the two cabinets must be sized for the combined inrush. The conservative sizing rule for the XRC is:

• Cable cross-section: ≥ 6 AWG (13.3 mm²) for runs up to 30 m
• Cable cross-section: ≥ 4 AWG (21.1 mm²) for runs 30-60 m
• Cable cross-section: ≥ 2 AWG (33.6 mm²) for runs 60-100 m
• Voltage drop at full load: ≤ 3% of nominal
• Voltage drop at inrush: ≤ 5% of nominal (not 3%, due to the soft-charge resistors drawing a controlled current spike)

The XRC manual also requires that the cabinet's ground be bonded to the same ground point as the transformer secondary neutral (for wye systems) or the same ground electrode as the transformer. A floating or high-impedance ground is a hidden source of converter faults and can also cause 4305 by allowing common-mode noise to corrupt the ready-2 signal.

13. Field Commissioning Checklist (Post-Repair)

After any repair that involves the converter, WRCA01, WRCF01, or amplifier, run through the following checklist before returning the cell to production:

1. Verify all alarms cleared on power-up
2. Master all axes per the XRC mastering procedure
3. Run a low-speed program through the full work envelope
4. Run a high-speed program for at least 30 minutes
5. Run a thermal test: leave the cabinet powered but idle for 2 hours, then issue servo ON and verify clean charge
6. Capture a 24-hour alarm history with a clean run
7. Document all measurements (line voltage, bus voltage, ready-2 timing) for the maintenance log

14. XRC Primary Power Specification

15. Frequently Asked Questions