239-0005

Check DC Bus Voltage Display

Press MOD key, navigate to Service > MC Diagnostics > Voltage Monitor. Observe DC bus voltage for axis 5 section - should read 560-600V nominal for 3x480V supply systems. Note any fluctuations or low readings.

The voltage display updates in real-time. Watch during machine operation for transient drops that occur under load conditions.

Verify Supply Voltage at Transformer

Using MOD > Service > PLC > Inputs, check input voltage monitoring values. Verify incoming 3-phase supply is balanced and within ±10% of nominal. Check for phase loss indicators.

Only qualified electricians should measure high voltage at transformer terminals. Use appropriate PPE and lock-out/tag-out procedures.

Inspect Regenerative Unit Operation

If equipped with regenerative power supply, check regeneration resistor and chopper operation. Look for fault LEDs on the regenerative module. Verify brake resistor connections are tight and resistor is not damaged.

Regenerative units can fail in a mode where they cannot absorb energy during deceleration, causing bus voltage spikes or control circuit faults.

Test Under No-Load Condition

Clear alarm and attempt to enable axis 5 with no mechanical load. If voltage stabilizes, issue may be load-related (weak supply, undersized transformer, or excessive regenerative energy). If alarm persists, fault is in monitoring or power distribution circuit.

Compare axis 5 voltage behavior with other axes during identical moves to isolate whether problem is global (supply) or axis-specific (inverter section).

Compare Measured vs Displayed Voltage

Access MOD > Service > MC Diagnostics and record displayed DC bus voltage. Using a calibrated true-RMS multimeter, measure actual DC bus voltage at the inverter test points. If readings differ by more than 5%, suspect voltage sensing circuit fault.

DC bus carries high voltage (typically 600V DC). Only qualified personnel should access inverter test points with proper insulated test equipment rated for high voltage DC.

Check FSUC Board Status Indicators

Visually inspect the MC main computer FSUC board for diagnostic LEDs. Refer to the MC hardware manual for your specific control version. Look for voltage monitoring fault indicators or board communication errors.

On iTNC 530 and TNC 620/640 systems, the FSUC status is also reflected in PLC diagnostic bits accessible through MOD > Service > PLC > Markers.

Review Voltage Sensor Calibration

In MOD > Service > MC Test Functions, access voltage sensor calibration display if available on your control version. Compare calibration values against factory specifications in the MC configuration documentation.

Voltage sensor drift is uncommon but can occur after extended operation in high-temperature environments or after power supply events like lightning strikes.

Perform FSUC Self-Test

Execute the MC built-in self-test function via MOD > Service > MC Test > Self Test. This runs diagnostics on the FSUC monitoring circuits. Document any reported failures and consult with Heidenhain service for interpretation.

Self-test results are logged in the service file. Export this log via data interface before contacting Heidenhain support for faster diagnosis.

Isolate Axis 5 Inverter Section

Power down the control completely. Disconnect the motor cable from axis 5 inverter output terminals. Restart control and attempt to clear alarm. If alarm does not recur without motor connected, fault is likely in inverter power stage.

Ensure complete lockout/tagout before disconnecting motor cables. Discharge DC bus capacitors per manufacturer procedure (wait minimum 5 minutes after power-down).