Diagnose Masterdrives VC F011 CT Drift on STS Crane Hoist Drives

Problem Summary: F011 Trips With Output CT Feedback Drift on an STS Crane Hoist

On a ZPMC ship-to-shore (STS) crane hoist, two 1100 A Siemens SIMOVERT VC chassis drives (MASTERDRIVES VC, 6SE70 series) are configured as a master-slave pair. They run two identical Siemens cage-induction motors that share a common gearbox. The reported symptom is that the slave motor winding temperature climbs above 100 °C during normal operation, even after the motor has been replaced with a brand-new unit. The drive returns no thermal fault, which proves the heat is generated by a control or feedback anomaly rather than a mechanical defect.

The diagnostic thread that resolves the issue is observation of a non-zero current transformer (CT) feedback at drive standstill. Parameters r831 (CT feedback for the slave torque reference) and r832 (scaled CT feedback on the master) should return 0.0 A when the drive is commanded to zero speed. A standing reading of approximately −0.5 A that fluctuates is a direct indication that the analog path between the output CT and the CUVC control board (or its analog option card) has developed a DC offset, an open circuit, a dry solder joint, or an EMI ground loop. This is a strong predictor of subsequent F011 overcurrent trips, motor overheating, and torque-sharing instability in the master-slave configuration.

The final corrective action in the case study is replacement of the ABO analog I/O option card in the master drive. This article documents the diagnostic path, the field test method for the output CT, the function of the ABO option card, and the verification steps that confirm the repair.

System Configuration: 1100 A MASTERDRIVES VC Master-Slave on STS Crane Hoist

The system described in the source is a textbook master-slave hoist. The following table summarizes the equipment involved:

In a properly configured master-slave pair, the master regulates hoist speed and total torque while the slave is commanded to follow the master’s torque reference. CTs on the output of each drive provide isolated current feedback to the CUVC. The CTs must be installed in the same orientation and wired so that positive motor current produces a positive connector value at the CUVC. Any asymmetry, drift, or noise in the CT signal corrupts the torque-sharing loop. With a rigid mechanical tie, this manifests as one motor doing more work than the other — and a temperature imbalance that the drive’s own internal current sensor cannot see.

F011 Overcurrent Fault: Definition and Trip Behavior

F011 is the overcurrent trip of the CUVC firmware. The fault is raised when the measured output current exceeds the parameterized trip threshold. The default threshold is typically ~1.5 × drive rated output current, and the absolute level is scaled by the value written to P380 (motor rated current) and the drive’s current scaling block.

On a 1100 A chassis, an F011 trip drops the line contactor, blocks the IGBT firing pulses, and latches the fault in the fault memory. Reset requires the OFF1 → ON command sequence. Persistent F011 with no apparent mechanical cause is the strongest indicator of a CT feedback issue in a master-slave configuration. Review the fault history with the OP1S operator panel or Drive ES / DriveMonitor via the serial RS-232 port on the CUVC.

Output-Side Current Transformers: Function and Wiring

The 1100 A chassis already includes internal current sensors (typically Hall-effect or current-transformer based) on each phase leg for the closed-loop current control. The external CTs at the drive output serve a different purpose in this installation:

1. Provide a secondary current signal for the master-slave torque-sharing loop on the CUVC.
2. Allow the CUVC to read the actual motor current on the slave drive (used to detect a slave malfunction).
3. Enable external protection relays (Sepam, 7UT, or similar) to monitor hoist motor current independently.

The CT is typically wound around the output cable of each phase and has a turns ratio such that the secondary current is in the 1 A or 5 A range for a 50/60 Hz sine wave. Because the drive output is a PWM waveform with a fundamental frequency that varies from 0 Hz to 50/60 Hz and a switching frequency of typically 2.5 kHz to 8 kHz, the CT must have sufficient bandwidth to measure the fundamental component without saturation at the switching harmonics.

Diagnostic Procedure: Reading r831 and r832 at Drive Standstill

The CT feedback is observed on parameters r831 and r832 in the CUVC parameter set. These are connector-source parameters; their meaning depends on the parameterization (BICO wiring) in the parameter tree. In the case described, the user has wired the CT feedback connector into the slave torque-sharing block, so r831 and r832 represent the actual CT-derived current in amperes.

Diagnostic steps:

1. Connect the OP1S operator panel or a DriveMonitor (Drive ES) PC link to the drive’s serial RS-232 port (X500 on the CUVC).
2. Command the drive to OFF1 (stop).
3. Wait 5 seconds for the gate pulses to inhibit.
4. Read r831 and r832 on the operator panel.
5. Expected value: 0.0 A ± 0.1 A.
6. Observed value in the fault scenario: −0.5 A, fluctuating.

A standing offset greater than 0.2 A on a 1100 A drive is significant. The fluctuation indicates noise pickup or a bias drift on the analog input to the ABO card — not on the CT itself, because the CT primary current is zero (the drive is at standstill, no motor current flows).

Field Test Method for Output-Mounted CTs

To verify that the CT itself is functional (separate from the CUVC / ABO input stage), perform the following bench or field test. The goal is to isolate the CT from the card and inject a known current.

Test 1 — Secondary Injection (Preferred)

1. Isolate the drive (lock-out, tag-out) and disconnect the CT secondary leads from the CUVC / ABO input terminals.
2. Connect a calibrated secondary injection source (e.g., OMICRON CTA, Megger MRCT, or equivalent) to the CT secondary terminals.
3. Inject 5 A secondary (or 1 A for 1 A CTs) at 50 Hz.
4. Measure the output of the CUVC/ABO input circuit with a multimeter, or read the value on the operator panel (this requires re-connection).
5. Expected: linear output proportional to the injected current.
6. Inject 0.0 A, then 1.0 A, then 5.0 A, and verify linearity. A non-zero output at 0.0 A injection indicates card offset; a non-linear response indicates card failure.

Test 2 — Primary Injection (Clamp-On Verification)

1. With the drive still isolated, route a single primary turn through the CT window using a heavy test cable.
2. Connect a variable AC current source (e.g., primary injection set rated 0–2000 A) to the test cable.
3. Monitor the CT secondary with a clamp meter or ammeter.
4. Ramp the primary current from 0 A to 200 A (or higher, as available) in 50 A steps.
5. Record the secondary current at each step.
6. Calculate the measured turns ratio: I_primary / I_secondary.
7. Expected ratio: 1100:5 → 220:1, or 1100:1 → 1100:1.
8. A deviation of more than 2% from the rated ratio, or a non-linear ratio curve, indicates CT saturation or winding damage.

Test 3 — Compare with r004 During No-Load Run

1. Restore the drive to a safe state and run the motor at 50 Hz no-load (no load on the hook).
2. Clamp a calibrated current clamp around one output phase cable.
3. Compare the clamp reading to the drive display (r004 or r005 — the internal measurement).
4. If r004 and the clamp agree within 5%, the internal current sensing is good. Any deviation between the internal measurement and the CT measurement (r831 / r832) is then attributable to the external CT path or the ABO card.

ABO Card Function and Failure Modes

The ABO option card in a MASTERDRIVES VC system provides additional analog and digital I/O for the CUVC. In the master-slave hoist configuration described, the ABO card typically hosts:

The CT secondary is typically terminated on the ABO card’s analog input through a burden resistor. The card scales the input, applies a low-pass filter, and outputs a connector value that the CUVC reads via the BICO system. Common failure modes of the ABO card:

In the fault scenario, the standing −0.5 A offset and the fluctuation point strongly to a noisy input stage on the ABO card, most likely a dry solder joint or a failing op-amp. The repair is to replace the ABO card with a known-good spare from the spare parts inventory. Confirm firmware compatibility with the CUVC board before re-energizing.

Root Cause: Why the ABO Card Failure Caused Motor Overheating

In the master-slave hoist, the slave drive’s torque command is derived from the master drive’s measured current. If the master drive’s CT feedback drifts to −0.5 A at standstill, the torque-sharing loop interprets this as a small negative current on the master. The slave’s torque command is then adjusted upward to compensate, which means the slave is commanded to deliver slightly more torque than the master.

The mechanical consequence: the common gearbox is rigid, so the motor speeds must match exactly. The slave motor, commanded to deliver more torque, will pull against the master. Both motors remain synchronized, but the slave is now doing more of the work for a given load demand. Over time, the slave motor’s I²R losses are higher than the master’s, and the winding temperature climbs above the design limit (typically 155 °C for Class F insulation, or 180 °C for Class H).

The thermal protection in the CUVC is based on the slave drive’s internal current measurement, not the CT feedback. So even though the slave motor is overheating, the drive does not see an overload condition. The motor winding’s embedded PTC or PT100 thermistor may be wired to a separate input and eventually trips an external thermal relay — but by then the motor insulation has already been stressed. The replacement motor in the source case had the same fate because the feedback was still drifting.

A secondary risk: if the CT offset is large enough, the master drive’s torque loop may interpret the input as a fault condition and trigger F011. This is the F011 trip observed in the diagnostic thread. With a 1100 A chassis, even a few-percent torque-sharing error is a significant thermal load on the slave motor.

Replacement Procedure and Post-Replacement Verification

Replacement Steps

1. Isolate the drive at the line contactor. Lock-out and tag-out. Verify zero voltage at the DC bus (wait at least 5 minutes after isolation for the DC-link capacitors to discharge).
2. Open the electronics bay of the drive (front cover or top cover, depending on the chassis variant).
3. Locate the option card slot. The ABO card typically lives in slot A or slot B on the CUVC backplane.
4. Note the position of the terminal blocks, ribbon cables, and any jumpers on the card. Photograph if possible.
5. Depress the card retainers and remove the ABO card.
6. Install the replacement card. Verify the firmware version (typically indicated by a label or by reading the card’s identifier parameter) matches the original, or is compatible with the CUVC firmware version.
7. Reconnect the terminal blocks and ribbon cables exactly as removed.
8. Restore power. Run the drive commissioning tool (DriveMonitor) and verify the card is recognized.
9. Re-enter any card-specific parameters (scaling, offset, filter time constant) that were stored in the parameter backup.

Verification Procedure

Master-Slave Considerations: Torque Sharing and CT Placement

For 1100 A master-slave drives on an STS crane hoist, the CTs should be installed with the following engineering considerations:

1. Polarity. All CTs must be installed with P1 toward the drive and P2 toward the motor. Reversed polarity will produce a 180° phase shift, which the CUVC interprets as negative torque, breaking the master-slave loop.
2. Symmetry. The CTs on both drives must be of the same type, ratio, and burden. Mismatched CTs cause torque-sharing errors that compound under load.
3. Cable routing. The CT secondary cable should be shielded and routed away from the IGBT switching noise. Twisted pair and proper grounding at one end only are essential to avoid ground loops.
4. Burden matching. The burden resistor on the CUVC / ABO input must match the CT rating. An undersized burden saturates the CT; an oversized burden reduces signal level and increases noise susceptibility.
5. Calibration. After any CT or card replacement, the master and slave current scaling must be verified by lifting a known test load and confirming both motors draw similar current (within 10%).

Preventive Measures

To prevent recurrence of the F011 / motor overheating issue on MASTERDRIVES VC hoist installations:

• Annual CT secondary injection test on all hoist drives. Use a calibrated secondary injection source and verify the ratio, polarity, and phase.
• Quarterly review of r831 / r832 at standstill. A drift greater than 0.2 A triggers investigation.
• Keep a spare ABO card on site, with the same firmware version as the installed units.
• Document the CT wiring (polarity, ratio, burden) in the drive’s parameter sheet and the crane’s electrical drawing set.
• When replacing motors, also re-verify the master-slave scaling. A new motor with slightly different slip can mask or amplify a CT drift.
• Train operators and electricians on the meaning of F011. F011 is not always a real overcurrent — it can be a feedback problem, and replacing the motor will not fix it.

Related Diagnostics: When to Suspect the CT vs the Card vs the Motor

The following matrix summarizes the diagnostic decision tree for the symptoms in this case study:

Documentation and Standards References

The following documents and standards apply to the work described in this article. Verify the latest revision against your local jurisdiction.

• Siemens MASTERDRIVES VC Operating Instructions (6SE70 chassis series) — available from the Siemens Industry Online Support portal.
• Siemens MASTERDRIVES VC Compendium (function block and parameter reference) — available from the Siemens Industry Online Support portal.
• Siemens SIMOVERT VC Parameter List (r-parameters and P-parameters) — available from the Siemens Industry Online Support portal.
• IEC 61800-1: Adjustable speed electrical power drive systems — General requirements.
• IEC 61800-2: Adjustable speed electrical power drive systems — General requirements — Rating specifications for low voltage adjustable speed d.c. power drive systems.
• IEC 60204-1: Safety of machinery — Electrical equipment of machines.
• NEMA ICS 7.1: Safety standards for construction and guide for selection, installation, and operation of adjustable-speed drive systems.