Simodrive 6RB2101 X-Axis Runaway: Tacho Feedback Polarity Diagnosis and Correction

1. System Overview

The Simodrive 6RB2101 is a modular four-axis transistorised DC chopper (pulse-width modulated armature converter) intended for DC feed-drive applications in CNC machine tools. The unit integrates a shared G0 power board with per-axis controller cards and per-axis power cards. It is commonly paired with the Sinumerik 850T turning control on late-1980s and early-1990s lathes. The official Siemens product documentation, including the 6RB21 ordering data and configuration sheets, is published on the Siemens Industry Online Support (SIOS) portal under document 22073597 — Simodrive 6RB21 Transistor DC Chopper for DC Feed Drives.

2. Reported Symptom Set

The reported symptoms, captured verbatim from the field, are:

• On machine power-up, axis 1 (X) runs away in either positive or negative direction with no NC setpoint issued.
• The Sinumerik 850T then drops into emergency stop after a contour-monitoring (following-error) alarm trips, and the drive unit is shut down.
• LEDs V2 on the controller card and V1 + V4 on the G0 power board are illuminated prior to shutdown.
• The condition appears immediately when the enable signal (terminals 9 and 65) is applied to the X-axis controller card, regardless of whether the setpoint input (terminals 56 and 14) is shorted, open, or driven normally.
• Removing either enable wire (9 or 65) on the X-axis controller card and re-powering allows the drive unit to stay on; the other three axes jog normally.
• The enables for all four axes energise at power-on as a normal machine state.

3. LED Interpretation on the 6RB2101

LEDs on the G0 power board and on each controller card provide the first diagnostic layer. Always power down, open the unit, and observe the LED pattern with the unit in the failing state (enables applied, before the E-stop drops them out).

The combination of V1 (rail OK) + V2 (controller enabled) + V4 (drive warning) plus immediate armature current at enable time is the classic pattern of a velocity-loop error caused by incorrect tacho feedback. The V4 indication should be cross-checked against the exact label set on the specific G0 board revision in the machine, because board revisions relocate the warning LED mapping.

4. Eliminating Non-Feedback Causes First

Before touching the tacho, rule out the four other causes that produce identical-looking runaways on a Simodrive 6RB:

1. Setpoint wiring error. Already ruled out in this case by shorting 56/14 — runaway persists.
2. Controller card failure. Ruled out — the X-axis controller card was swapped for a known good one with no change.
3. Power card failure. Ruled out — the X-axis power card was swapped for a known good one with no change.
4. Complete drive unit failure. Ruled out — the entire 6RB2101 was replaced with a known working unit and the fault reappeared.

All four drive-internal sources are eliminated. The fault is therefore in the wiring between the drive and the X-axis motor, in the motor itself, or in the tacho generator that rides on the motor shaft. Of those, the tacho path is by far the most common failure mode and is non-destructive to test.

5. Tacho Feedback Theory of Operation

The DC feed motor in this class of lathe carries a small permanent-magnet or iron-core DC tacho generator coupled to the armature shaft. The tacho output is a DC voltage proportional to motor speed, with polarity set by the direction of rotation. The 6RB2101 controller card sums this voltage against the NC setpoint at the speed-loop input, then feeds the current loop. The polarity convention is fixed:

• Motor rotating in the positive (commanded +) direction must produce a tacho voltage of the defined sign at the controller card input.
• If the sign is reversed, the speed loop sees a negative-velocity error and commands full positive armature current, accelerating the motor until the current limit or the mechanical stops are reached.
• On a vertical or gravity-loaded axis, the runaway may be in the "wrong" direction because gravity now supplies the apparent velocity; the loop still drives the armature to oppose the (incorrectly sensed) motion.

The defining diagnostic feature of an inverted-polarity tacho is that the runaway is present with the NC setpoint held at zero, because the loop is driven entirely by the polarity-inverted feedback offset. A noise spike on a healthy tacho would not produce a sustained, unidirectional run.

6. Tacho Diagnostic Procedure (Field-Proven)

The following sequence was used in the captured case to isolate the tacho path. It is destructive only in the sense that it requires swapping wires at the drive terminals — the motor, tacho, and power stages are not disturbed.

6.1 Continuity and Insulation Check

1. With the machine isolated and locked out, disconnect the X-axis tacho leads at both the drive terminals and the tacho body.
2. Measure conductor-to-conductor resistance. For a typical small DC tacho on a 1FT / 1HU feed motor the field-side resistance is in the tens of ohms; an open circuit (infinite ohms) indicates a broken brush lead or shunt wire.
3. Measure each conductor to earth with a 500 V insulation tester. Anything below 1 MΩ is suspect; below 100 kΩ is a definitive insulation failure.
4. Spin the motor by hand (with shaft decoupled if necessary) and confirm a clean DC voltage at the tacho terminals with no spikes or dropouts.

6.2 Tacho Swap Test (Cross-Axis)

This is the most discriminating test in the procedure:

1. Identify a known-good axis of similar size (Z is the typical match on a lathe).
2. Disconnect the Z-axis tacho leads at both the drive terminals and the tacho body.
3. Connect the X-axis tacho to the Z-axis drive terminals and the Z-axis tacho to the X-axis drive terminals. Keep the cable screens and ground bonds on their original axes.
4. Power the machine up and observe. If the runaway follows the X-axis tacho (i.e., now the Z axis runs away), the X-axis tacho is at fault. If the runaway stays on the X-axis regardless of which tacho is connected, the fault is in the X-axis cable harness or in the drive-side wiring.

In the captured case, swapping the tachos moved the runaway to the new axis (Z), confirming the X-axis tacho signal — and therefore the tacho cable or the tacho body — was the source.

6.3 Cable Swap Test (Same Axis, Reversed Polarity)

The final step in the diagnostic is to test the harness with the original tacho reconnected:

1. Restore the X-axis tacho to its own terminals.
2. At the drive end only, swap the two tacho signal conductors (reverse polarity at the controller card input).
3. Power up the machine.
4. If the runaway disappears, the X-axis tacho harness has its polarity inverted relative to the controller card's expectation. The runaway returns the moment the conductors are restored to their original positions.

This is the test that resolved the captured case. The fault was the X-axis tacho harness being wired in the wrong polarity convention at the drive terminal strip.

7. Root Cause: Polarity Inversion in the X-Axis Tacho Harness

The runaway on the X axis of the captured Simodrive 6RB2101 / Sinumerik 850T lathe was caused by the tacho-generator feedback leads being connected to the controller card with the wrong polarity. The Sinumerik 850T contour monitor detected the resulting position error and dropped the machine into emergency stop before the X axis could reach its mechanical endstop.

Three pieces of evidence converge on this conclusion:

• Swap test 1 (tacho for tacho): Fault followed the X-axis tacho, isolating the fault to the tacho body or its harness.
• Swap test 2 (cable pair reversed at drive end): Reversing the two conductors at the drive terminals eliminated the runaway; restoring the original wiring re-introduced it.
• Setpoint independence: The runaway was present with the setpoint shorted, which is the textbook signature of a polarity-inverted feedback rather than a noisy or offset feedback signal.

Mechanically, the tacho itself was not damaged and the motor armature was healthy. The fault was a wiring / commissioning issue inside the harness connecting the tacho to the controller card.

8. Permanent Solution: Correct the Tacho Polarity Convention

The permanent repair is to land the tacho harness on the controller card in the polarity the speed loop expects. Two safe methods are available, both consistent with Siemens commissioning practice for the 6RB family:

8.1 Method A — Re-land the conductors at the drive terminal strip

1. Isolate and lock out the machine.
2. At the X-axis controller card tacho terminals, swap the two tacho conductors so the polarity matches the controller card's defined convention for a positive-going motor rotation.
3. Re-torque the terminal screws to the Siemens-specified value for that terminal type (typically 0.6–0.8 N·m for the 6RB21 cage-clamp terminals — verify against the wiring diagram for the specific card revision).
4. Record the change on the machine's electrical drawing and commissioning sheet.

8.2 Method B — Reverse the tacho rotation direction

If the controller card supports it and the machine drawing allows, the tacho can be re-coupled in the opposite sense. On Siemens 1FT/1HU feed motors the tacho mounting is keyed; reversing the direction requires remounting the tacho body or fitting a reversing gear. In most field cases, Method A is preferred because it is a wiring change at a known terminal.

9. Verification Procedure

After applying the polarity correction, run the following acceptance sequence before returning the machine to production:

1. Static check. Power up with all enables applied. Confirm no axis moves at idle and that V1, V2 are on while V4 remains off.
2. Manual jog, both directions. Use the Sinumerik 850T handwheel or jog keys to drive X in + and −. Confirm commanded direction matches machine-printed direction and that the position display tracks the actual motion.
3. Reference (datum) return. Run the X-axis reference cycle from both directions. Confirm consistent reference-point capture and no following-error alarm.
4. Repeat power cycle. Power down, wait 30 seconds for DC-link bleed, power up. Repeat the jog and reference checks. This catches intermittent wiring or terminal problems that a single power-up can mask.
5. Programmed move. Run a short G-code move (e.g. G0 X10. and G0 X-10. at low rapid override) to confirm the position loop is stable at speed.
6. Contouring test. Run a known-good part program with circular interpolation (G2/G3) and verify no contour-monitoring alarm over at least one full spindle revolution.

10. Preventive Measures for the 6RB2101 / 850T Drive Lineup

11. Diagnostic Decision Matrix

12. Field Notes and Lessons Learned

• Shorting the setpoint is the key discriminator. If shorting 56/14 does not stop a runaway, the fault is not in the NC setpoint path. This single test eliminates 50% of the diagnostic tree for a runaway at enable.
• Swapping the whole drive did not fix it. A field-replacement 6RB2101 exhibited the same fault because the fault was in the tacho harness, not the drive. Drive swapping is therefore not a definitive test — it is a process-of-elimination step that has to be followed by harness-level testing.
• The tacho swap test is non-destructive. Swapping the X and Z tacho leads at the drive terminal strip does not touch the motor, the power stages, or the NC. It is safe to perform on a production machine during a planned stop.
• Polarity inversion can be intermittent. A tacho lead that is marginal — corroded, partially broken, or held by a loose terminal — can change effective polarity briefly as the motor moves. Always verify with a controlled swap, not a single observation.
• Document any polarity change. When reversing tacho leads at the drive terminal, mark the change on both the wiring diagram and the commissioning sheet. The next person to swap a controller card will not know the polarity has been reversed and may chase a phantom fault for hours.