231-806B

Verify AC Input Voltage Stability

Using a true-RMS multimeter or power quality analyzer, measure all three phases (L1, L2, L3) at the main disconnect feeding the drive controller cabinet. Verify voltage is within 380-420VAC for 400V systems or 440-480VAC for 460V systems. Check for phase imbalance greater than 2%.

Use the multimeter's MIN/MAX recording function over several minutes to catch intermittent voltage sags that may not be obvious during spot checks.

Only qualified electricians should work inside the main electrical cabinet. Lethal voltages are present.

Check Building Power Quality

Consult with facility maintenance to identify if other equipment experienced power disruptions at the same time. Check for heavy loads on the same circuit (welders, large motors) that could cause voltage sags during startup.

806B alarms occurring at consistent times (morning startup, shift changes) often point to facility-wide power quality issues rather than machine-specific faults.

Inspect Main Contactor and Circuit Breakers

With power OFF, inspect the main line contactor (typically labeled K1 or similar) for contact wear, pitting, or burn marks. Check all circuit breakers for loose mounting or internal failure. Verify proper torque on all AC input terminals per manufacturer specifications.

Use proper lockout/tagout procedures before opening any electrical panels.

Measure DC Bus Voltage

Access the drive controller cabinet and locate the DC bus test points (usually marked + and - or P and N). With the system powered ON but not running, measure DC voltage. For 400VAC input, expect 565-600VDC nominal. Below 500VDC indicates rectifier or capacitor problems.

Compare the measured DC bus voltage to the value displayed on the drive controller's diagnostic panel if available - significant discrepancy indicates metering or feedback issues.

DC bus voltages exceed 600VDC. Use appropriate high-voltage test equipment and PPE. Do not touch any components while powered.

Inspect Rectifier Module and Connections

With power OFF and locked out, visually inspect the rectifier module for discoloration, bulging capacitors, or burnt components. Check all DC bus bar connections for tightness and signs of overheating. Look for white or brown residue indicating electrolyte leakage from capacitors.

Even with power OFF, DC bus capacitors can retain lethal voltage for several minutes. Verify 0VDC before touching any components.

Review Drive Controller Error History

If your drive controller has an integrated diagnostic display (Siemens Simodrive, Indramat/Rexroth, etc.), access the internal error log to check for undervoltage warnings, rectifier overtemperature alarms, or DC bus monitoring faults that preceded the 806B alarm.

The drive controller often logs internal faults several seconds before the TNC sees the 806B alarm - this timing information helps distinguish between power supply failure and momentary power loss.

Perform DC Bus Ripple Measurement

Using an oscilloscope with differential high-voltage probes, measure the AC ripple voltage on the DC bus during normal operation. Ripple should typically be less than 5% of DC voltage (30-35V peak-to-peak on a 600VDC bus). Excessive ripple indicates capacitor degradation.

Compare ripple measurements taken at light load versus heavy machining operations - capacitor failure shows dramatically increased ripple under load.

This measurement requires specialized high-voltage differential probes rated for at least 1000V. Do not use standard oscilloscope probes.

Check Capacitor Physical Condition

With power OFF and locked out, visually inspect all DC bus capacitors for bulging tops, case swelling, or electrolyte leakage. Capacitors are typically large cylindrical components mounted near the rectifier. Note manufacturing date codes - capacitors over 10 years old should be considered for preventive replacement.

In three-phase systems, capacitor banks often consist of multiple units. A single failed capacitor in the bank significantly reduces total capacitance and can cause 806B alarms.