Troubleshooting MASTERDRIVES MC F008 on 1FK7 Synchronous Motors

Overview

The Siemens MASTERDRIVES MC (Motion Control) family is a field-proven AC/AC drive platform used for high-dynamic servo applications. When a unit configured for an asynchronous induction motor is suddenly connected to a permanent-magnet synchronous servo motor from the 1FK7 series, the drive can trip immediately with fault F008 (DC link undervoltage) and abort the automatic motor identification with F111. The two symptoms share a common root cause family: the power section, the IVI (current/voltage interface) board, or the motor/encoder feedback path cannot build the expected magnetising/injectable current.

This reference walks through the field-tested diagnostics for the 6SE7021-4EP70-Z drive with a 1FK7044-7AF71-1AG0 motor, including parameter-level checks (r006, P096, P949), PSU/IVI board verification, cable and insulation tests, and the synchronous-motor commissioning procedure. The procedure generalises to any MASTERDRIVES MC/VC pair running a 1FK6/1FK7 servo.

Identifying the Drive and Motor Variant

Before diagnosing, confirm that the order numbers actually match and that the firmware is approved for synchronous operation. The drive nameplate must agree with the inverter type programmed in P096, otherwise the controller uses an asynchronous motor model and the current regulator never stabilises.

F008 — Definition, Typical Root Causes, and What It Does Not Mean

Fault F008 on MASTERDRIVES MC/VC is DC link undervoltage. It is raised when the smoothed DC bus voltage falls below the monitoring threshold stored in parameter P071 (line undervoltage threshold) for longer than the time in P070. It is not a generic "drive not ready" code, nor is it directly an overcurrent fault.

For an induction motor drawing tens of amps, an undervoltage on the bus may go unnoticed because the magnetising current is low and the I²t in the bus caps masks the dip. A synchronous servo motor, however, draws its field current in phase with rotor position; the regulator requests a sharp current pulse the instant the encoder reports a non-zero speed, and if the bus collapses under that pulse the regulator flags F008 within a fraction of a second.

F111 — Current Controller Optimisation Fault

F111 is raised when the drive cannot complete the automatic current-controller measurement during motor identification (the rotating and/or stationary measurement run started by P115 = 1 or P115 = 2). F111 itself does not pinpoint the root cause; the diagnostic information sits in P949.

The combination of F008 + F111 with P949 = 1 observed on this drive is a strong indicator that the drive cannot push current through the motor windings at all — either because the output stage is shut down by the undervoltage trip before the identification routine can complete, or because the motor/encoder feedback chain is broken and the regulator never enables the output stage.

P949 = 1: Current Does Not Build Up When Pulses Are Applied

This is the single most informative parameter in the entire fault scenario. During the stationary measurement run (P115 = 2) the drive injects short, low-energy current pulses into each phase pair (U-V, V-W, W-U) and watches the current ramp on its own IVI board. If the measured current never crosses a defined threshold within the pulse window, P949 = 1 is latched.

The same fault is raised when:

1. An IGBT in the output stage has failed open (no current can flow even with full gate drive).
2. The IVI board has lost its ±15 V supply or its hall transducers have a broken wire.
3. An output cable is open-circuited (broken lug, wrong terminal torque, severed conductor in a drag chain).
4. The motor has an internal phase-to-phase or phase-to-ground short that opens the fuse in the IVI sense path.
5. The encoder is unreadable: the current regulator is gated by the position controller, and without a valid encoder the regulator never enables the output bridge.

Motor Code P096 = 85 — Compatibility Verification

For the 1FK7044-7AF71-1 motor family, Siemens assigns code 85 in P096 (motor code). P096 is the single parameter that tells the drive which motor model is connected; it selects the matching stator resistance, leakage inductance, torque constant, and encoder model. Loading the wrong code is one of the most common sources of F111/F008 on synchronous motors.

Verification procedure:

1. From the PMU (parameterisation unit) keypad, navigate to P096 and read the value. Confirm it equals 85.
2. Cross-check the motor rating plate: 1FK7044-7AF71-1* must match the datasheet reference for code 85 in the MASTERDRIVES MC motor list (Compendium, chapter "Synchronous servomotors 1FK7").
3. If the drive was previously configured for an induction motor, the technician may have left P096 at the asynchronous code (e.g. 17, 19, 31 depending on the older motor). The asynchronous code typically requests a sinusoidal flux model and a magnetising current that the 1FK7 cannot deliver.
4. Save the parameter set to the operator panel (PMU) or to a PC using DriveMonitor / STARTER for MASTERDRIVES, then read it back to verify the value was retained after power-down.

Step-by-Step Diagnostic Procedure

Apply the procedure in the order listed. Each step must be confirmed before moving on, otherwise the next step will inherit a misleading result.

1. De-energise and lock out the drive. Wait five minutes for the DC bus to self-discharge below 50 V DC; verify with a meter at the DC+ / DC- terminals on X3.
2. Inspect the motor and encoder cables end-to-end. For drag-chain installations, open the chain at the mid-point and inspect for crushed insulation, broken shields, and missing strain relief.
3. Measure the motor winding resistance phase-to-phase (U-V, V-W, W-U) and each phase to PE with a 4-wire milliohmmeter or a high-quality DMM. A 1FK7044 stator typically shows between 0.4 Ω and 1.6 Ω phase-to-phase at 20 °C; consult the motor's datasheet for the exact value. Asymmetry of more than 10 % between phases indicates a turn-to-turn short or a loose terminal.
4. Measure the insulation resistance phase-to-PE at 500 V DC (megger or equivalent). Anything below 1 MΩ on a 1FK7 winding is a rejection criterion.
5. Inspect the encoder cable and connectors at both ends. Resolver cables (1FK7 with resolver feedback) are 4-wire plus shield; absolute-encoder cables (1FK7 with multi-turn encoder) are 8-wire plus shield. Confirm pin-out against the connector drawing.
6. Power the drive with no motor connected. Read r006 (DC bus voltage) and r008 (line voltage) on the PMU. For a 400 V class drive fed with 400 V three-phase, the no-load bus should settle around 540–565 V DC. If r006 is substantially below r008 × √2, the PSU or IVI board is suspect.
7. Read r006 with the motor connected and the drive in standby (not enabled). Compare with the no-motor value. A drop of more than 5 % indicates DC bus leakage, a partially shorted IGBT, or a bleed resistor that has failed.
8. Reset the drive with P097 = 1, then enter P115 = 2 to start the stationary motor identification. Watch the PMU for the live P949 value — the fault is latched into P949 only after the routine aborts.
9. If F111 recurs with P949 = 1, the IVI board cannot sense motor current. Move to the hardware inspection.

Hardware Inspection — PSU and IVI Boards

The MASTERDRIVES MC uses two distinct boards inside the electronics box: the PSU (power supply unit) and the IVI (current/voltage interface). On frame-size 2 units like the 6SE7021-4EP70, both boards are accessible from the front once the operator panel is hinged open.

Field checks:

• Power the drive with the line on; measure +15 V and −15 V at the PSU test pins (typically TP3/TP4). Both rails must be within ±5 %.
• Inspect the IVI board for discoloured current transducers (the black plastic blocks around the output bus bars). Any with cracked housings or burn marks must be replaced.
• Swap the encoder slot module between this drive and a known-good spare (only on a de-energised unit). If the fault follows the module, replace it.

Encoder Feedback Chain

A synchronous motor's current regulator is gated by the position feedback. The drive will refuse to apply meaningful torque without a healthy encoder signal. On a MASTERDRIVES MC driving a 1FK7044-7AF71-1AG0, the encoder is the integrated multi-turn absolute encoder (EnDat interface on SMC30, or SSI on older firmware).

• Check r141 (encoder status word). Bits indicating "signal too small", "frequency too high" or "CRC error" point at cable damage or a failing encoder head.
• For 1FK7 motors with resolver feedback, confirm the resolver is excited: r140 should report a stable carrier of about 8 kHz.
• Spin the motor shaft by hand with the drive enabled but not started (P100 = 0). r140 / r141 values must change smoothly. If the values jitter or freeze, the cable is the prime suspect.

Cable, Connector, and Winding Workarounds

Several field-proven mitigations are worth applying before swapping boards:

1. Re-torque the motor terminal box screws to the published torque (typically 2.5 Nm for a 1FK7 size 4 terminal box). Loose terminals are a documented cause of P949 = 1.
2. Re-crimp the power connector pins. Crimp fatigue at the motor end is common after years of thermal cycling.
3. Replace the encoder cable outright — shielded, twisted-pair, with 360° backshells at both ends. A 1FK7 encoder cable run longer than 50 m must use the Siemens-specified cable (e.g. 6FX8002-…); a generic cable is the single most common source of unstable identification.
4. Confirm the shield is bonded to PE at the drive end and the motor end, and that there is no parallel bond that creates a ground loop.
5. Verify the holding brake is being released by the drive (BR+ / BR− terminals) — a 1FK7 with the brake engaged draws stator current but cannot rotate, and the regulator interprets this as a fault during the rotating measurement run.

Bringing the Drive Back Online

After resolving the root cause:

1. Acknowledge the faults with P097 = 1 on the PMU. Watch the LEDs: green READY, red FAULT cleared.
2. Verify that r006 matches the DC bus voltage measured at X3 within ±3 %.
3. Re-run the stationary identification with P115 = 2. The drive should report the measured stator resistance (r038) and leakage inductance (r039) on the PMU.
4. If a rotating identification is required (P115 = 1), uncouple the load from the motor shaft — the rotating run moves the shaft several revolutions in each direction.
5. Run the motor open-loop in speed control with P100 = 3 and a small speed setpoint; monitor r019 (actual speed), r020 (output frequency), and r028 (current). The current should settle within 200 ms of the speed step.
6. Save the parameter set with the PMU SAVE key (or P977 = 1 on older firmware).

Troubleshooting Matrix

Standards and References

• EN 61800-3 — Adjustable speed electrical power drive systems — EMC requirements (referenced for cable and grounding practice).
• EN 60204-1 — Safety of machinery — Electrical equipment of machines (earthing and PE bonding).
• Siemens MASTERDRIVES MC Compendium — Operating instructions and parameter list for the 6SE70 series; the P096 motor code table for 1FK7 motors lists code 85 for the 1FK7044-7AF71-1* family.
• Siemens 1FK7 synchronous servo motor datasheet — for the rated winding resistance, insulation class F (155 °C), and holding brake parameters.

FAQ