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Heidenhain Controller
Drive · Intermediate · 30-120 minutes
HEIDENHAIN 231-8680 - 8680 DQ inverter: Maximum current limiting
The inverter section of the drive module is being asked to supply more continuous DC bus current than its thermal design allows, triggering protective current limiting. This typically results from cumulative demand from multiple axes fed by the same inverter exceeding its capacity during simultaneous acceleration, heavy cutting loads, or regenerative braking. The inverter hardware is protecting itself from thermal damage.
231-8680
HEIDENHAIN 231-8680 - 8680 DQ inverter: Maximum current limiting
8680 DQ inverter: Maximum current limiting
This TNC 640 error indicates excessive current draw in the drive system. The control has detected overload conditions that could damage the motor or inverter.
Root Cause Summary
The inverter section of the drive module is being asked to supply more continuous DC bus current than its thermal design allows, triggering protective current limiting. This typically results from cumulative demand from multiple axes fed by the same inverter exceeding its capacity during simultaneous acceleration, heavy cutting loads, or regenerative braking. The inverter hardware is protecting itself from thermal damage.
Safety
- Refer to Heidenhain service manual
- Ensure qualified technician performs repairs
Causes and Fixes
- 1
Identify affected inverter and connected axes
Press MOD key, select PlcDiag, navigate to 'Drive' section, then 'UM Modules'. Note which UM number is shown in %1 and document all axis motor modules (X, Y, Z, etc.) connected to this inverter bus.
The UM module assignment is typically documented in the machine electrical diagrams under 'Drive Bus Topology' - cross-reference this to confirm axis grouping.
- 2
Monitor real-time current demand
In PlcDiag, remain in the UM module display for the affected unit. Run the problematic NC program in single-block mode and observe the 'DC Bus Current' value during simultaneous moves to confirm it approaches or exceeds rated capacity.
Typical UM inverter modules are rated 10A, 20A, or 40A continuous depending on model - compare observed peaks against your specific module rating found in the drive configuration documentation.
- 3
Analyze program motion overlap
Open the NC program and identify blocks where multiple axes on the same inverter execute simultaneous linear (L) or circular (C) moves. Look especially for short positioning moves with high feedrates (FMAX) that cause rapid acceleration.
Use the Test Run graphical simulation with 'Show velocities' active to visualize where multiple axes reach peak acceleration simultaneously.
- 4
Implement sequential motion strategy
Restructure critical program sections to stagger axis movements using intermediate positioning blocks or M01 optional stops between major moves. Alternatively, reduce feedrate override for testing to confirm current limiting stops.
If process requires simultaneous motion, consider using FUNCTION FEEDRATE FACTOR to programmatically reduce speed during high-current sections without modifying all coordinates.
Call a technician if:
- ·Servo amplifier hardware replacement needed
- ·Complex motor parameter calculations required
- ·Mechanical binding cannot be resolved
Prevention
- Regular mechanical maintenance and lubrication
- Monitor servo current trends during operation
- Verify PWM frequency settings during commissioning
Common Mistakes
- Ignoring mechanical binding symptoms
- Incorrectly setting motor parameters without verification
- Overlooking PWM frequency derating effects
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Always verify with your machine manual or a certified technician before performing service procedures.