Resolving SINAMICS S120 F30015 Phase Failure Motor Cable Fault

Overview

The SINAMICS S120 drive fault F30015 (N, A) Power unit: Phase failure motor cable is reported on SERVO, SERVO_AC and SERVO_I_AC drive objects when the Motor Module detects that one of the three motor feeder phases is missing, unbalanced, or carrying a current that is inconsistent with the demanded torque. The fault triggers an OFF2 stop (with NONE, OFF1, OFF3 as configurable alternatives) and can be acknowledged with IMMEDIATELY (POWER ON) in default configuration. On chassis-format Motor Modules, the phase-failure monitoring is intentionally disabled because the cabling is integrated into a fixed power bus.

On machine tools running a Sinumerik 840D sl or 840Di sl, F30015 is frequently seen on axes that are mechanically coupled to a high-inertia load such as a spindle or boring bar, even when the affected axis itself is commanded to remain stationary. The drive interprets the oscillating torque produced by a marginally stable speed/position control loop as a missing-phase condition and trips. The same fault code and message value semantics apply across the SINAMICS drive family, including the SINAMICS G220 (ID 109825375) and the SINAMICS G120X (ID 109781534) converter lines, so the diagnostic procedure below is portable to any SINAMICS platform that uses the SERVO control structure.

Affected Products and Drive Objects

How the Drive Detects a Phase Failure

The phase-failure monitor in a SINAMICS Motor Module evaluates the three-phase current measurement on every PWM cycle. Two complementary checks are implemented in firmware:

1. Zero-sequence (sum) current check. In a balanced three-phase star, the instantaneous sum i_a(t) + i_b(t) + i_c(t) must equal zero at all times (within the resolution of the ADC and the small zero-offset of the transducers). If the magnitude of the sum exceeds a configurable threshold for longer than the debounce time, a phase is flagged as open. The threshold is normalized against the rated module current and the threshold is typically 5% of rated, internally scaled.
2. Phase-current symmetry vs. torque demand. A second check compares the measured phase current magnitudes with the torque-producing current i_q and the field-producing current i_d reconstructed from the Park transform. If the drive demands a non-zero torque setpoint but one phase remains near zero while the other two balance the demanded current, the firmware concludes a phase is open. This is the path that triggers on control-loop oscillation: the position controller demands an oscillating i_q, and the speed controller produces large, swinging currents that exceed the symmetry tolerance.

Root Cause Classification

Use the following decision tree to classify F30015 on a Sinumerik-controlled axis before changing any parameters.

Diagnostic Procedure: Capture the Trip in Real Time

The fastest way to separate a wiring fault from a control-loop fault is to record a STARTER / Startdrive / SINUMERIK Operate trace covering the last 500 ms before F30015 latches. The following points must be recorded at 4 ms or finer:

• r0021 actual speed, smoothed
• r0027 absolute actual current (motor current magnitude)
• r0031 actual torque
• r0078[0..2] phase currents i_a, i_b, i_c
• r0079[0..2] torque-producing current components
• r1480 speed-controller I-component
• r1490 speed-controller P-gain times error (actual trace of controller output)
• r0080 torque setpoint before current limiting
• r2135 status word, faults 1

Interpretation rule:

1. If any one of i_a, i_b, i_c is persistently near zero across the trace while the other two phases show balanced sinusoidal current proportional to demanded torque, the fault is in the cable or motor. Stop and inspect the wiring.
2. If all three phases carry non-zero current but the amplitudes oscillate at a frequency between 30 Hz and 250 Hz and the oscillation amplitude grows over several cycles, the trip is control-induced. The phase-failure monitor is reacting to the rapidly varying, asymmetric current waveform produced by an unstable speed loop. Proceed to the tuning section.

Step-by-Step: Inspecting the Motor Feeder Cable and Motor

1. De-energize and lock out. Open the line contactor, wait for the DC bus to drop below 50 V (visible on r0046[0] = 0), verify absence of voltage at the Motor Module output terminals with a Cat IV meter.
2. Insulation test (megger). Disconnect the motor leads from the Motor Module. Apply 500 V DC between each phase conductor and PE for 60 seconds. Resistance must be > 200 MΩ for a healthy cable; a reading below 1 MΩ indicates insulation damage or moisture ingress, both of which can produce intermittent F30015.
3. Continuity and phase-identity test. With the motor leads still disconnected, ring out U→U, V→V, W→W at the motor terminal box. A phase-to-phase swap will not produce F30015 directly, but a phase that is open or high-resistance will. Look for resistance > 0.5 Ω in any one leg with the others below 0.2 Ω.
4. Connector inspection. Power connectors (Q8/0.4, M23, M40 Speedtec) on S120 power cables are a known source of intermittent opens. Inspect each pin for deformation, oxidation, or retraction. Apply the manufacturer-specified torque on the connector coupling nut (typically 1.5–2.5 Nm for M23).
5. Terminal box on the motor. Re-torque the terminal box bolts to the motor manufacturer's specification (typical 2.5–4 Nm for 1FK7/1FT7). Vibration will loosen improperly torqued lugs and produce exactly the intermittent F30015 reported on drilling applications.
6. Shield termination. Verify the cable shield is bonded at both ends with 360° EMC glands. A floating shield can inject common-mode noise that fools the symmetry check.

Step-by-Step: Speed Controller Tuning to Eliminate False Trips

If the wiring has been verified and the trace shows oscillating current (case 2 above), the speed controller must be stabilised. The default speed loop on a SERVO axis is set up by the one-button auto-tuning in p3900 or by SINUMERIK's circularity test, but neither captures instabilities caused by external vibration from a coupled load.

1. Read the current proportional gain. r0070 shows the active speed-controller P-gain Kp_n. Note the value in N·m·s/rad.
2. Read the integral time constant. r0072 shows Tn_n in seconds.
3. Reduce Kp_n by 30%. Set p1460 = 0.7 × r0070. This widens the phase margin at the cost of a small following-error increase; for a positioning axis with a holding-position loop, this is acceptable.
4. Increase Tn_n to at least 3 × the mechanical oscillation period. Measure the oscillation period T_osc from the trace (peak-to-peak time in r0021). Then set p1462 = max(0.030, 3 × T_osc). The integral action cannot correct oscillation periods faster than itself; raising Tn above 3× the dominant mechanical resonance makes the loop a proportional-only regulator at that frequency, which damps the resonance.
5. Add a current-setpoint filter. Enable a band-stop filter at the dominant mechanical resonance using the current-setpoint filter structure:
   • Set p1656[0..3] = 1 (band-stop, type 2) for filter slots 1–4 as needed.
   • Configure the centre frequency in p1666 (numerator natural frequency) and p1667 (denominator natural frequency) to the measured oscillation frequency f_osc = 1 / T_osc.
   • Set the bandwidth p1668 = 0.1 × f_osc (narrow notch to avoid excessive phase loss).
6. Activate the reference model. The reference model softens the speed setpoint and limits overshoot. Set p1433 = reference model natural frequency in rad/s = 2π × (f_osc / 2) and p1434 = 1 to enable. This is the most effective single change for vibration-driven instability.
7. Lower the speed-controller sampling if needed. For 1FK7/1FT7 motors above 16 kHz PWM, the speed controller runs at 125 µs. If the mechanical resonance sits above 500 Hz, switch to 8 kHz PWM (p1800) to give the current loop a faster update.
8. Repeat the trace and confirm. Record the same trace points as in the diagnostic step and verify the oscillation amplitude has decayed by at least 10× before re-enabling production.

Mechanical Vibration and Cross-Axis Coupling

On a drilling machine with mechanically ganged axes (spindle + axial feed + lateral slide), vibration from the cutting edge of the drill is transmitted through the workpiece, the fixture, and the machine structure to every servo on the machine. The lateral-slide axis, which is commanded to hold position, will see a position error proportional to the relative deflection between the linear encoder and the motor. The position controller in the NC (MD 32200 POSCTRL_GAIN in 1/s and MD 32400 AX_JERK_TIME in s) tries to correct this, and the result is a torque setpoint that oscillates at the drill's natural frequency. When this frequency sits inside the speed controller's bandwidth, the controller amplifies the disturbance rather than rejecting it.

Recommended mechanical mitigations, in order of cost:

1. Sharpening the drill and reducing feed per revolution so the cutting force and its harmonics drop.
2. Re-grinding the workpiece fixture to increase stiffness in the path between the cutting edge and the lateral slide.
3. Adding a flexible coupling on the lateral-slide driveshaft if the coupling is currently a rigid bellows type.
4. Active vibration damping with the NCK MD 32900 … 32950 filters; configure the same frequency as in the drive-side reference model.

Key Parameters Reference

Verification Checklist

• F30015 no longer appears during 30 minutes of normal production on the worst-case part.
• Captured trace shows phase currents within ±5% of each other while torque is demanded.
• Speed-controller output (r1490) shows no oscillation > 2% of rated torque while holding position.
• Megger reading on motor feeder is > 200 MΩ at 500 V DC.
• Motor terminal box lugs re-torqued to specification and pull-tested.
• Reference model enabled and reference frequency recorded in the machine commissioning sheet.
• Spare drive project archived with the new p1460, p1462, p1433, and filter values.

Frequently Asked Questions