238-1004

Check input voltage at main disconnect

Measure incoming 3-phase voltage at the main power disconnect. Verify voltage is within ±10% of rated voltage (typically 400V/480V) and all phases are balanced.

Use proper PPE and lockout procedures when working with high voltage

Monitor voltage during operation

Use oscilloscope or power quality analyzer to check for voltage sags, spikes, or dropouts during machine operation cycles that could cause momentary DC-link undervoltage.

Check voltage during high-load operations like rapid traverse or spindle acceleration

Verify power supply input connections

Inspect all input power connections to the TNC power supply module for loose connections, corrosion, or overheating signs at terminal blocks.

Use thermal imaging to identify hot connections under load

Check building power quality

Coordinate with facility maintenance to verify building power quality, transformer loading, and any recent electrical work that might affect voltage stability.

Power down and discharge DC-link

Follow proper lockout procedures and allow DC-link capacitors to fully discharge (minimum 5 minutes) before accessing power supply components.

DC-link voltages can exceed 600V and remain dangerous after power removal

Visual inspection of capacitors

Inspect DC-link capacitors for physical signs of failure including bulging tops, electrolyte leakage, or discoloration around the base.

Look for white crystalline deposits around capacitor terminals indicating electrolyte leakage

Measure capacitor ESR and capacitance

Use ESR meter to test capacitor equivalent series resistance and capacitance values. Compare against nameplate ratings - typically should be within 20% of rated capacitance.

ESR values above 0.5 ohms typically indicate capacitor degradation

Replace capacitor bank if degraded

Replace entire DC-link capacitor bank with HEIDENHAIN approved components maintaining proper polarity and torque specifications.

Measure DC-link voltage at test points

With power applied, measure DC-link voltage at designated test points on the power supply module. Should typically read 540-600VDC for 400V input systems.

Use proper high-voltage test equipment and maintain safe working distances

Check rectifier bridge diodes

Power down system and test rectifier bridge diodes for forward/reverse bias characteristics using diode test function on multimeter.

A shorted diode will show low resistance in both directions, an open diode shows infinite resistance

Inspect for thermal damage

Examine rectifier components and heat sinks for signs of overheating, discoloration, or thermal stress that indicates component failure.

Check thermal compound between semiconductors and heat sinks

Replace power supply module

If rectifier bridge failure is confirmed, replace the complete power supply module with HEIDENHAIN part number specific to your TNC model and power rating.

Check regenerative resistor operation

Verify regenerative (brake) resistors are functioning and properly connected. Measure resistance values and inspect for physical damage or overheating.

Regenerative resistors should show consistent resistance across all elements

Monitor DC-link during deceleration

Use oscilloscope to monitor DC-link voltage during rapid deceleration cycles to identify voltage drops coinciding with regenerative energy events.

Look for voltage dips during spindle deceleration or rapid axis reversals

Adjust deceleration parameters

Access Machine Parameters and review axis deceleration rates. Reduce deceleration values to limit regenerative energy peaks if within acceptable cycle time requirements.

Parameter MP1420 controls axis deceleration limits on most TNC systems

Verify regenerative control circuit

Test regenerative control circuit operation including switching devices and control logic that manages energy dissipation during regenerative conditions.

Verify actual DC-link voltage

Measure actual DC-link voltage with calibrated multimeter while alarm is active to confirm if voltage is actually low or if monitoring circuit is faulty.

Compare readings at multiple test points to verify measurement accuracy

Check UVR threshold settings

Access Service Mode and verify UVR threshold parameters are set correctly for your system voltage rating. Typical threshold is 85% of nominal DC-link voltage.

Only qualified service personnel should modify UVR threshold parameters

Test monitoring circuit components

Test voltage divider networks, comparator circuits, and reference voltage sources in the UVR monitoring circuit for proper operation and calibration.

Use precision voltage reference to verify comparator switching points

Replace monitoring circuit board

If monitoring circuit malfunction is confirmed, replace the power supply monitoring board or complete power supply module as required.