Siemens 840D Axis Motor Humming: Kv Factor and MD1407 Tuning

A 2006-vintage SINUMERIK 840D (Powerline) with SIMODRIVE 611D drives developed a loud, sustained audible hum on a Nikken 4th/5th-axis rotary immediately after the backlash compensation was tightened. The motor had not been mechanically binding prior to the change, and the hum persisted even after the backlash value was relaxed. This reference documents the full root-cause map for that class of fault, the alarm codes it produces (21612, 25201, 300608), and the field-proven tuning sequence used to silence the drive without losing follow-on accuracy. The procedures apply to SINUMERIK 840D Powerline with 611D, 840D sl with 611D, and 840D sl with SINAMICS S120, with callouts where drive-MD semantics differ.

1. Symptom Classification

840D operators and service engineers consistently mis-label the noise class. The acoustic signature points directly at the loop that is mis-tuned, so classify the noise first.

If the hum is unchanged when the controller enable is removed via PLC (DB38.DBX2.1 = 0), the source is mechanical or electrical (cable, brake). If the hum dies within 200 ms of enable removal, the source is in the drive control loops.

2. System Identification and Drive Type

MD numbering and effective drive MD scope differ between the three relevant combinations. Confirm what is in the cabinet before you start changing values.

For the 2006 vintage described, expect SIMODRIVE 611D modules (6SN1123-xAx0x-xHAx series for 1FE1 / 1FK6 motors; 6SN1145 / 6SN1146 for spindles). The 1FK6 feed motor with built-in holding brake is the most common cause of a post-backlash hum because the brake pad spring-pre-load is sensitive to axial shaft loading.

3. Alarm Code Reference

Three alarms typically appear in the alarm log when a hum is accompanied by instability. Each one points to a different layer of the enable chain and must be cleared in order; do not skip ahead to the drive parameter while a high-priority alarm is still pending.

4. Enable Chain and Brake Verification (DB38.DBX2.1)

The 840D uses the DB31–DB38 (8 axes per NCU) block as the axis interface to the PLC. Each axis has 32 bytes of process data. The signals relevant to a hum that started immediately after a mechanical change are concentrated in the first six bytes.

Procedure:

1. Open the PLC online (Step 7) and force a VAT view on DB38.
2. Apply axis enable, jog command, and confirm DB38.DBX2.1 goes to 1 within 100 ms.
3. Read the brake feedback bit. On a 1FK6, this is DBX62.5 or the equivalent mapped to the drive telegram.
4. If the brake is dragging, the motor current will rise 8–15% at standstill and the audible hum is the PWM struggling against the brake torque. Releasing or adjusting the brake shim is the mechanical fix; do not chase this with a Kv change.

5. Mechanical Root Causes

Mechanical binding produces a hum because the speed controller fights a constant disturbance torque. The Kv increase you try to compensate actually amplifies the audible result.

1. Backlash over-tightened. The Nikken CNB and CBN rotaries use a worm-and-wheel or planetary reduction. A backlash value below the published minimum (typically 0.02 mm or 3 arc-min) pre-loads the gear train. Pull the backlash back to the manufacturer value and re-test.
2. Coupling alignment. Concentricity and angularity tolerances for a typical 1FK6-to-Nikken input coupling are 0.05 mm. A lateral offset of 0.1 mm produces a sinusoidal speed disturbance at 1× mechanical revolution that the speed controller amplifies into a hum.
3. Bearing pre-load. After backlash adjustment, the rotary table bearings (e.g., Nikken cross-roller) can be pre-loaded off their factory setting. Re-shim to published preload.
4. Holding brake drag. Common failure mode on 1FK6 and 1PH8 motors after 15+ years of service. The armature plate is a clearance fit; a worn plate drags the rotor.

6. Electrical Root Causes (Cable, Winding, Encoder)

If the decoupling test produces a clean motor, move to electrical isolation. A damaged cable or winding imbalance drives an asymmetric current that the controller cannot fully reject.

6.1 Power Cable Resistance Test

With the motor leads disconnected from the drive (always confirm lockout), measure resistance phase-to-phase at the drive-end of the cable:

6.2 Winding Resistance

Measured at the motor junction box (cable disconnected from motor):

• Phase-to-phase 0.3–1.5 Ω typical for 1FK6 family; balance ±3%
• Phase-to-ground > 100 MΩ
• If balance exceeds 5%, suspect a turn-to-turn short; replace motor

6.3 Encoder Cable

Common failure mode on rotaries that have been moved. Inspect for pinch points at the slip ring or rotary union. Shield continuity must be one-piece from encoder to drive end; a shield break produces velocity-dependent noise that masquerades as a hum.

7. Drive Parameter Root Causes (MD32200, MD1407)

If mechanical and electrical are both clean, the hum is in the drive control loops. Two MDs are responsible for almost every audibility issue on a healthy motor-cable pair.

7.1 MD32200 $MA_POSCTRL_GAIN (Kv)

The Kv factor is the proportional gain of the position controller, in units of (feed/min) per mm (or (rev/min) per mm for a rotary). The default of 2.7 is typical for a small rotary; it is not a universal value. Increasing Kv does not always make the hum louder. A Kv that is too high for a high-inertia rotary (Nikken CNB-160 weighs 60 kg) will produce a low-frequency oscillation that the drive tries to cancel, resulting in a noticeable hum. Lowering Kv below the system natural frequency can silence it but increases following error at corners.

Effective Kv range for a 60 kg rotary: 0.5 to 4.0. Start with 1.0, raise in 0.5 increments while monitoring following error.

7.2 MD1407 $MD_SPEEDCTRL_GAIN_V (Speed Controller P-Gain)

On 611D, this is the P-gain of the speed controller. Units are Nms/rad (Nm·s/rad) on the torque/velocity plane. On SINAMICS S120 the equivalent is p1460[0]. The 611D default is determined by the motor type at commissioning; on a 1FK6, values between 0.5 and 3.0 are typical.

MD1407 that is too high produces the whistle; MD1407 that is too low produces a low-frequency moan at low speeds. The audible signature of the original machine was a whistle, which is the more common 611D failure mode and a strong indicator that MD1407 needs to come down.

7.3 Speed Controller Integral Time MD1408

The integral component of the speed controller (MD1408) should be roughly 10–20× the value of MD1407 in ms for a healthy motor. If MD1407 is reduced without checking MD1408, the integral term can become dominant and produce a low-frequency drift hum. The relationship is:

MD1408 [ms] ≈ 10 × MD1407 [Nm·s/rad] (rough field starting point; refine by following-error plot)

8. Position Loop Gain (MD32200) Tuning Procedure

1. Power on the machine, enter user area, navigate to StartUp → Machine Data → Axis MD. Filter to MD32200 for the affected axis.
2. Note the current value (typical 2.7 on a Nikken small rotary).
3. Lower the value by 30% (e.g., 2.7 → 1.9). The HMI prompts whether the change is effective immediately or after POWER ON; on 611D, MD32200 is generally effective immediately on the affected axis once the drive is re-initialized, but the standard commissioning practice is to apply at next POWER ON.
4. Power on the machine, jog the axis at 50% rapid, monitor the HMI Service → Axis → Following error trace. A healthy Kv produces a following error proportional to velocity (the slope equals 1/Kv). If the trace shows oscillation at a fixed period, Kv is too high.
5. Iterate in 0.3 increments. The target is no audible noise, following error below the value set in MD36400 $MA_CONTOUR_TOL, and no fault within 30 minutes of part-program run.
6. Save the machine data to the CF card (StartUp → Series Commissioning → Save).

9. Speed Controller (MD1407) Tuning Procedure

The 1.850 → 1.4 reduction documented in the source fault is in the right direction but overshoots the safe range. Use the following stepped procedure to avoid the "lower MD1407 too far and start faulting" trap:

1. Open the drive MD view: StartUp → Machine Data → Drive MD. Filter to MD1407 for the affected drive (drive 8 in the alarm log, but for the rotary axis you are working on, the drive address is the axis's drive address; do not assume the drive number is the same as the axis number).
2. Record the current value, MD1407 = 1.850.
3. Reduce by 10% only: MD1407 = 1.665. Save. Wait for the drive to re-initialize. Reset the controller enable.
4. Run the rotary axis in JOG at 50% rapid, 30° swing. Listen for the whistle. Check for any 300608 alarm in the alarm log.
5. If still whistling, drop another 10%: 1.50. Repeat.
6. Continue in 0.15 steps until the whistle disappears or the first 300608 alarm appears. The sweet spot is the highest MD1407 value that does not produce audible noise.
7. Once MD1407 is fixed, adjust MD1408 (TN) to maintain the 10–20× ratio described in Section 7.3.

9.1 Verification by Following-Error Trace

Use the HMI integrated trace to record following error during a JOG move. Set the trigger threshold to 2× the steady-state following error. A healthy Kv/MD1407 combination shows:

• Steady-state following error: F_ss = V / K_v (linear axis); or F_ss = V / K_v (rotary, where V is in deg/s and F is in deg).
• Acceleration following error: F_acc = J / K_v where J is axis jerk, dominated by mechanical inertia reflected back through the gear.
• Oscillation period in the trace should be < 50 ms. A 200–400 ms period oscillation indicates mechanical resonance not yet addressed.

10. Auto-Optimization on Limited-Travel Axes

The rotary in question has ~60° of total swing. The standard 840D auto-optimization routine (StartUp → Optimize Axis) requires a configured travel range large enough to reach a stable speed. For a 60° swing, the routine will refuse to start or will produce garbage if the axis is commanded beyond its software limit. Two practical workarounds:

10.1 Manual Circularity Test

The HMI circularity test (under Commissioning → Optimization → Circularity) requires two linear axes to be moved in a circle. Use the rotary as the index axis and a single linear as the radial. With the backlash and Kv dialed in, the circle should round-trip within the configured position window (MD32250 $MA_RATED_AMP).

10.2 Step-and-Listen Optimization

1. Set MD1407 to the current value, write a 5-line part program that moves the axis +20°, settles 2 s, −20°, settles 2 s, M30.
2. Run in single-block, listen for the whistle in the dwell period.
3. Change MD1407, repeat. This is essentially what the 840D auto-optimization does internally, but with much finer control over each step.

10.3 What the Auto-Optimization Will Travel

The 840D optimization sequence uses the following logic for travel bounds:

T_travel = min(SW_limit_high − SW_limit_low − safety, configured_optimize_range)

Where configured_optimize_range is from the HMI prompt. For a 60° swing, enter the value 50° in the prompt; the drive will run multiple moves within that 50° span. This is sufficient to estimate the moment of inertia and tune the speed controller.

11. Motor and Cable Field Testing

Use a digital multimeter with a low-Ohms scale (resolution 0.01 Ω) and a 500 V insulation tester (megger). De-energize the drive, lock the main disconnect, and verify absence of voltage at the drive output terminals.

12. Parameter Backup, Restore, and Change Activation

Before any optimization, take a series-commissioning backup to a CF card. The 840D stores machine data in three logical areas: NCK MD, PLC, and drives. A full backup covers all three.

StartUp → Series Commissioning → Create Series Start-up File → select all three areas → save to CF card

To restore a single MD after a failed optimization:

1. Edit MD on the HMI: StartUp → Machine Data, find the MD, set to known good value.
2. Press Save; the HMI prompts whether to write to CF card. Confirm.
3. Reset the axis. Verify no alarm.

To restore a full backup:

1. Boot the NCU in service mode (CNC-STARTUP switch on the front of the NCU box, position 1).
2. Insert the CF card with the series start-up file.
3. Select StartUp → Series Commissioning → Load and follow the prompts.
4. Normal boot. The drive may re-initialize twice.

13. Safety and High-Energy Considerations

Drive MD tuning is not free of risk. The current at standstill on a stalled 1FK6 can exceed rated within 200 ms if the controller fights a mis-wired encoder.

1. Confirm the emergency-stop circuit is functional before applying the first command.
2. Keep one hand on the E-stop button during the first move after any change.
3. Reduce the rapid override (MD32010 $MA_JOG_VELO_RAPID or feedrate override 0%) for the first verification move.
4. Set MD36610 $MA_AX_EMERGENCY_STOP_TIME to a small value (e.g., 200 ms) to ensure the drive de-energizes quickly on a fault.
5. Monitor drive fault LED (red on 611D modules) throughout. Any trip means back off the previous change by 30%.

14. Field-Proven Sequence (End-to-End)

The following 12-step sequence is the recommended field workflow for a 2006-class 840D that developed an audible hum after backlash adjustment. Total time: 60 to 90 minutes including verification.

1. Capture the alarm log. Note 21612, 25201, 300608 status and timestamps.
2. Verify the mechanical change. Re-read the backlash value; if < 0.02 mm on a Nikken, restore to 0.03 mm.
3. Decouple the motor from the rotary. Run motor unloaded, listen. If hum persists, go to step 5; if not, mechanical is binding, fix and re-test.
4. Re-couple. Verify coupling concentricity < 0.05 mm with a dial indicator.
5. Megger the power cable and the motor windings. Replace cable if insulation < 10 MΩ or resistance imbalance > 5%.
6. Inspect the encoder cable at the rotary's slip ring. Replace if any shield break is visible.
7. Confirm the holding brake is releasing. Read DB38.DBX62.5 in the PLC. If 0, the brake is dragging; service the brake.
8. Set MD1407 to 1.65 (10% below the original 1.85). Save. Reset axis. Jog in 30° moves. Listen for whistle.
9. If still whistling, reduce MD1407 by another 0.15 (to 1.50). Repeat. Stop at the first sign of 300608 alarm.
10. Adjust MD1408 to maintain ~10× the MD1407 value in ms (e.g., MD1407 = 1.5 → MD1408 ≈ 15 ms).
11. Reduce MD32200 (Kv) by 30% from original (e.g., 2.7 → 1.9). Verify following error stays below MD36400 setting.
12. Run a part program for 30 minutes. Confirm no alarms and no audible noise.

15. Speed/Position Control Loop (Block Diagram)

16. Verification Checklist

17. Reference: Common 611D Drive MDs

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