Siemens 810T DRIVE AXIS FAULT: Power Path, Not Position

The X-axis refuses to move and the control immediately reports DRIVE AXIS FAULT. On this Siemens 810T installation, the commissioning path separated an open semiconductor fuse and a failing amplifier power stage from later DAU-limit reached, contouring, and clamping faults. Those messages represent different failure layers and require different tests.

Fault-Layer Classification

DRIVE AXIS FAULT means the axis drive did not reach or retain its ready state. Diagnose the power path first: supply, fuse, amplifier, armature circuit, and drive-ready interface. The absence of even slight motor motion strengthens that priority, but it does not identify which element opened the path.

DAU-limit reached occurs later in the control chain. The term here means the commanded velocity output has reached its limit while the measured response remains inadequate. The installation used a +/-10 V velocity command. Its manual classified this as a maximum-speed error and directed the technician to reduce speed, check the pulse generator, check the tachogenerator or drive actuator, and inspect MD268, MD364, and MD368.

Contouring and clamping errors are also closed-loop symptoms. A contouring error means actual position failed to track commanded position within the permitted deviation. A clamping error can appear when a nominally stationary axis is displaced; manually turning this ballscrew produced that response. Either message indicates that the control progressed farther than an immediate drive-not-ready fault, but it does not prove that the amplifier, motor, feedback, or mechanics are healthy.

Observed condition	Failure layer to test first	What the symptom establishes
DRIVE AXIS FAULT, no motion	Fuse, power stage, armature wiring, drive-ready circuit	The drive cannot become or remain ready
DAU-limit reached after some travel	Velocity feedback, available torque, mechanical load, speed parameters	The loop commands motion but cannot obtain the expected response
Contouring error during motion	Following error, feedback integrity, acceleration and load	Actual position lags the commanded trajectory
Clamping error after manual displacement	Stationary-axis monitoring and feedback	The control detects motion or position deviation while clamped

Check 1: Record the first alarm after a full power cycle and the point at which it occurs. Expect DRIVE AXIS FAULT before motion for a power-path problem; expect DAU-limit reached or a contouring error only after the drive accepts a motion command.

Motor-Protection Baseline

Repeated resets are wrong practice when the armature condition is unknown. A damaged power stage can apply current without commanded motion. On this machine, the original amplifier produced approximately 0.010 A at one observed standstill condition and 0.100 A to 0.150 A after the control was enabled. Another observation found a constant 100 mA. The motor later smoked; inspection found bubbled and charred rotor encapsulant and commutator bars welded together.

Those measured values describe this installation, not a universal acceptable-current specification. A DC servo may use holding current to oppose load, so a nonzero reading alone does not condemn an amplifier. The deciding tests are whether current agrees with the command and mechanical load, whether it settles after enable, and whether the amplifier applies unintended armature voltage. Motor heating, smoke, ticking, abnormal odor, or current that persists with no torque demand requires immediate shutdown.

1. Isolate machine motion and arrange the ammeter and voltmeter for the expected DC armature circuit without opening a live circuit.
2. Inspect the meter ranges, lead ratings, and polarity before applying control power.
3. Enable the control without commanding motion and observe armature current and voltage only long enough to obtain stable readings.
4. Remove power immediately if current rises unexpectedly or the motor shows thermal or mechanical distress.

During one controlled attempt, power was removed within approximately 5 to 10 seconds to protect the replacement motor. That limit was an intervention used during diagnosis, not a permissible heating interval.

Check 2: With no motion command, expect no unexplained rise in armature current, no smoke, no ticking that develops under power, and no rapid temperature increase. Stop here if any appears.

Fuse and Supply-Path Inspection

The X-axis semiconductor fuse measured open while the Z-axis fuse produced the meter's normal continuity indication. Removing both fuses from their circuits confirmed that the result belonged to the fuse rather than a parallel circuit. This was the first conclusive break in the X-axis power path.

Test semiconductor fuses out of circuit because connected power electronics can produce misleading resistance readings. Use the exact approved fuse type and rating from the machine documentation. A semiconductor fuse protects faster power devices and is not interchangeable merely because another fuse fits its holder. The working installation later demonstrated that a 4 A fuse could operate either tested axis, but that observation does not define the design rating for every Siemens 810T or 6RB2025 installation.

Moving a known-good fuse from the Z circuit into the faulty X circuit caused that fuse to open when used with the original amplifier. That test sacrificed protection for the healthy axis and should not be repeated as a routine diagnostic. Fit a correctly specified replacement and isolate the load before energizing a circuit that has already opened a fuse.

1. Remove machine power and verify the DC link and associated supplies are discharged by the machine's documented method.
2. Remove the X-axis fuse from its holder.
3. Measure continuity directly across the fuse.
4. Inspect the holder, terminals, and conductors for heat damage, looseness, or contact contamination.
5. Trace the circuit against the full schematic before fitting the approved replacement.

Check 3: Expect continuity through the removed serviceable fuse and an open reading through the failed fuse. If a correct replacement opens again at enable, treat the event as excess current or a shorted path rather than as a second random fuse failure.

Armature Wiring Isolation

A grounded or pinched armature conductor can open the drive fuse and imitate an amplifier failure. The X-axis conductors pass through moving machine structure and a cable track, so flexing, pinching, and disturbed terminal connections belong in the test boundary. A broken terminal-block wire was found and reconnected during this repair, and a separate small yellow turret-encoder wire was accidentally dislodged during diagnosis. Both discoveries show why every disturbed conductor must be logged and rechecked before power returns.

1. Remove power and identify the two X-axis motor power leads from the schematic and terminal markings.
2. Disconnect the motor power leads at the motor-side terminal bar while leaving the protective ground intact.
3. Inspect the disconnected leads, cable track, strain reliefs, and terminal block for crushed insulation, exposed copper, broken strands, or contact with the chassis.
4. Measure each isolated power conductor to the chassis using the method and limits specified for the connected equipment.
5. Verify end-to-end conductor continuity and confirm that the two power leads are not shorted together.

Do not apply an ordinary high-voltage insulation test through connected amplifier or feedback electronics. Disconnect sensitive components exactly as the machine documentation specifies.

Check 4: Expect an intact armature pair with no chassis short, the protective ground still bonded, and every terminal restored to its documented position. Repair any wiring defect before judging the amplifier.

Unloaded Amplifier Discrimination

With the armature leads isolated, a fuse that still opens during drive enable points upstream of the motor: the amplifier power stage, its supply wiring, or another connected load. If it remains intact only when the motor circuit is disconnected, the fault follows the armature cable or motor unless the amplifier fails specifically under load.

Open-armature motion commands require caution. A velocity amplifier receiving a command without motor motion or tach feedback can drive its output to a limit and create a secondary alarm. Perform such a test only if the 6RB2025 documentation permits it. Otherwise, restrict the unloaded test to drive enable and static output measurement.

The decisive comparison on this machine was repeatable. The original amplifier opened a replacement fuse. With another amplifier and another fuse, the X-axis moved. The replacement amplifier initially showed no observed standstill motor current, whereas the original unit had produced the small current readings described earlier. Together, fuse survival and restored motion localized the hard DRIVE AXIS FAULT to the original amplifier path rather than to CNC position state.

Motor isolated	Fuse result	Diagnostic direction
Drive enabled, no command	Fuse opens	Amplifier power stage or upstream drive wiring
Drive enabled, no command	Fuse remains intact	Continue with static output checks
Fuse survives isolated but opens after reconnecting motor	Load-dependent overcurrent	Armature cable, motor, commutator, or amplifier under load

Check 5: With a correct fuse, isolated armature, and no motion command, expect the fuse to remain intact and the amplifier to reach its documented ready condition. A fuse that opens at this point blocks further motor testing.

Motor and Tachogenerator Qualification

A motor that rotates from an external DC supply has armature continuity and can produce torque, but that test does not qualify its commutator, insulation, brushes, bearings, or operation under servo current. The first rotor on this machine was visibly destroyed even though earlier testing had focused on drive behavior.

Inspect both commutator brushes, brush movement, spring action, commutator bars, and accumulated conductive carbon. Heavy graphite contamination can create leakage or a short path. Clean only by a method compatible with the motor materials, and do not disturb factory alignment by casually dismantling the motor. A ticking sound after installation requires inspection of the commutator and bearings before further powered cycling.

The tachogenerator measured 20 V per 1000 rpm. Qualification requires more than one voltage point: output polarity must reverse when shaft direction reverses, voltage must change smoothly with speed, and the signal must reach the control without intermittent conductors. Reversed polarity turns negative feedback into positive feedback; an open or noisy signal makes the control increase command while measured speed remains absent or unstable.

1. Inspect the motor and brushes with power removed.
2. Rotate the shaft by hand and feel for repeatable binding, scraping, or a localized tight point.
3. Drive the shaft by an approved external method and measure tachogenerator voltage in both directions.
4. Compare measured speed and voltage against 20 V per 1000 rpm.
5. Check continuity and polarity from the tachogenerator terminals to the drive input.

Check 6: Expect smooth bidirectional tachogenerator output, reversed polarity with reversed rotation, and a ratio of 20 V per 1000 rpm. Expect no charred commutator bars, welded segments, binding, or intermittent feedback wiring.

Mechanical Load Qualification

After the replacement amplifier restored movement, the X-axis completed several full-speed jog reversals before DAU-limit reached returned. That transition matters: the hard power-stage fault had been cleared, and the remaining fault occurred while the velocity loop was operating.

A severe crash preceded the failures. Mechanical drag therefore had to be separated from an electrical torque deficit. Belt tension, ballscrew condition, support bearings, carriage lubrication, and motor bearings all affect the acceleration achieved for a given armature current. Excessive belt tension can load the motor bearings and increase starting torque. A damaged ballscrew or support bearing can create position-dependent drag. Cold or degraded lubricant can increase breakaway force, but ambient heating is not a repair for damaged lubrication components.

The ballscrew and its bearings were later inspected and reported to turn smoothly. A comparison with another 1HU5044 motor showed that the installed DC servo motor normally had some rotational resistance and did not freewheel. Free-spinning behavior therefore was not the acceptance criterion; smooth, uniform resistance without a tight spot was.

1. Decouple or unload the axis according to the machine procedure.
2. Rotate the ballscrew through its usable travel and note any position-dependent torque change.
3. Check support bearings, belt tracking, belt tension, pulley alignment, and carriage lubrication.
4. Rotate the motor separately and distinguish uniform magnetic or brush drag from scraping or cyclic binding.
5. Reassemble and begin motion at a reduced feedrate rather than at rapid traverse.

Check 7: Expect smooth ballscrew travel, uniform motor resistance, correct belt alignment, and no localized increase in effort. Correct mechanical drag before changing control parameters.

Velocity-Loop and Parameter Checks

Once the fuse, amplifier, wiring, motor, feedback, and mechanics pass, diagnose DAU-limit reached as a velocity-loop commissioning problem. The control raises its analog command toward the +/-10 V limit to obtain requested speed. If feedback speed remains too low, the command saturates and the alarm follows. Possible causes include reduced available torque, incorrect tachogenerator scaling or polarity, a pulse-generator problem, excessive requested speed, and mismatched machine data.

The manual's listed checks were lower operating speed, pulse generator, tachogenerator or drive actuator, MD268, MD364, and MD368. Read and record the existing values before making any change. The parameter meanings and valid values must come from the matching Siemens 810T machine documentation; parameter numbers alone do not define a safe setting. Changing several values together destroys the diagnostic relationship between a change and its result.

1. Back up or transcribe MD268, MD364, and MD368.
2. Verify feedback polarity, scaling, and continuity before editing machine data.
3. Command a low-speed jog and record command voltage, tachogenerator voltage, actual response, and alarm state.
4. Increase speed in controlled increments while watching where command voltage approaches its limit.
5. If the alarm repeats, compare requested velocity with measured feedback and inspect the manual definition of the three machine-data entries.

Check 8: Expect actual speed to rise proportionally with command and remain stable in both directions. At the speed where DAU-limit reached occurs, record whether the command is at its limit and whether tachogenerator voltage remains below the value predicted by 20 V per 1000 rpm.

Position Reference and End-to-End Commissioning

Removing a motor, rotating a ballscrew, or disturbing encoder wiring can invalidate the control's position relationship even after the velocity fault is repaired. Home the machine using its documented reference procedure before program execution. A position-reference problem does not explain a fuse that opens at enable, but it can create limit, contouring, or clamping alarms after the axis begins moving.

The staged return used a reduced feedrate of 70%, followed by gradual increases toward 100% or the next DAU-limit reached event. This approach preserves the speed threshold as diagnostic data. Do not clear an alarm and immediately resume at rapid speed; first record direction, position, commanded speed, command voltage, tachogenerator voltage, armature current, and whether the fuse survived.

1. Confirm the correct X-axis semiconductor fuse remains serviceable after drive enable.
2. Verify zero-command current and voltage remain stable and the replacement amplifier reports ready.
3. Home the machine and confirm the position display follows motion in the correct direction.
4. Jog X in both directions at low speed, then repeat through several reversals while observing current and tachogenerator response.
5. Run at 70% feedrate and increase gradually toward 100%.
6. Execute coordinated motion only after single-axis operation remains stable.
7. After the test, remove power and inspect the fuse holder, amplifier connections, motor, belt, and terminals for heat, odor, loosening, or insulation damage.

Check 9: Expect the axis to enable without opening the fuse, hold without unexplained current rise, home correctly, move smoothly in both directions, follow command without contouring or clamping alarms, and complete operation from 70% through 100% without DAU-limit reached or DRIVE AXIS FAULT.

Frequently Asked Questions