Quick answer
If a cement screw conveyor draws higher motor current while the material throughput remains unchanged, the most common causes are increased mechanical resistance (e.g., wear, buildup, mis‑alignment), electrical supply issues, or control system drift. Begin by confirming safe isolation, then verify motor voltage, inspect the screw and trough for obstruction or wear, check bearing temperatures, and review drive‑train alignment. Correct the identified fault and re‑measure current to ensure it returns to the baseline range established for the specific equipment.
Problem / Job
A cement plant reports that a screw conveyor feeding raw meal to the kiln is drawing a higher motor current than normal, yet the measured throughput (tons per hour) has not increased. The operation team needs a systematic diagnostic approach to isolate the root cause, restore efficient power usage, and avoid unplanned downtime.
Evidence
- Motor current increased by 15‑30 % over the last 48 hours.
- Throughput sensor reading unchanged (within ±2 %).
- No recent changes to feed rate, raw material composition, or process set‑points.
- Ambient temperature stable; no reported power quality events.
- Vibration levels nominal on the motor bearing housing.
- No alarms from the PLC or motor protection relay.
Decision
Proceed with a step‑by‑step diagnostic that starts with electrical verification, moves to mechanical inspection, and ends with control‑system validation. Prioritize safe isolation before any hands‑on work, and document each measurement against the equipment’s OEM baseline values.
7‑step diagnostic workflow
- Secure the equipment. Initiate lock‑out/tag‑out (LOTO) per the plant’s electrical safety procedure. Verify zero voltage on the motor terminals with a calibrated tester before removing any covers.
- Record baseline electrical data. Using a power quality analyzer, capture line voltage, line current, power factor, and harmonic distortion under the current operating condition. Compare these values to the most recent OEM or site‑specific baseline.
- Check supply integrity. Confirm that the incoming three‑phase voltage is within the design tolerance (typically ±5 % of nominal). Look for unbalanced phases, voltage sag, or harmonic spikes that could force the motor to draw extra current.
-
Inspect the screw and trough. After LOTO verification, remove the conveyor housing. Look for:
- Material buildup or cement paste bridging the flight pitch.
- Worn or broken flight edges that increase slip.
- Corrosion or deformation of the trough that reduces clearance.
Clean any buildup and note wear patterns for future maintenance planning.
- Examine bearings and shaft alignment. Rotate the screw manually; feel for binding or rough spots. Measure bearing temperature with an infrared thermometer; temperatures above the OEM‑specified limit indicate lubrication failure or overload. Use a dial indicator or laser alignment tool to verify that the screw shaft is concentric with the motor shaft and that the coupling is correctly centered.
- Validate control‑system set‑points. Review the PLC or VFD parameters governing motor speed, torque limit, and overload protection. Ensure that any recent software updates have not altered the default ramp‑up profile or introduced a drift in the speed reference.
- Re‑energize and re‑measure. Remove LOTO, start the conveyor at normal feed, and monitor motor current, speed, and throughput simultaneously. The current should settle within the OEM‑specified operating band for the measured load. Document the final readings and any corrective actions taken.
Diagnostic matrix
| Evidence observed | Likely direction of fault | Next check / action |
|---|---|---|
| Current ↑, voltage stable, power factor unchanged | Mechanical load increase | Inspect screw flight condition and trough clearance (Step 4) |
| Current ↑, voltage ↓ or unbalanced | Electrical supply issue | Measure line voltage on each phase, check transformer taps (Step 3) |
| Current ↑, motor temperature ↑, vibration normal | Bearing overload or lubrication loss | Check bearing temperature, oil level, and run‑out (Step 5) |
| Current ↑, speed reference unchanged, VFD alarm logged | Control‑system drift or torque limit reduction | Review VFD/PLC parameters, reset to OEM defaults (Step 6) |
| Current ↑, material feed unchanged, occasional spikes | Intermittent obstruction (e.g., material bridge) | Perform visual inspection during operation, clean trough (Step 4) |
| Current normal, but throughput ↓ | Under‑feeding or upstream blockage | Verify feed hopper level and discharge gate position |
Acceptance / verification criteria
After corrective actions, the following measurable conditions must be met before the conveyor is returned to production:
- Motor current measured at the same load point is within ±5 % of the OEM‑published nominal current for that operating speed.
- Line voltage on all three phases is within the plant’s acceptable range (verify against the site‑specific voltage tolerance table).
- Motor temperature, as read by the built‑in sensor or infrared scan, does not exceed the OEM‑specified maximum for continuous operation.
- Vibration amplitude on the motor bearing housing remains below the threshold defined in the plant’s vibration monitoring program.
- Throughput reading matches the target rate within the plant’s normal process variance (typically ±2 %).
- All safety interlocks, LOTO tags, and lock devices are removed only after a final visual inspection confirms that no tools, debris, or temporary supports remain in the conveyor housing.
Document the before‑and‑after measurements in the maintenance log, and schedule a follow‑up inspection within the next 48 hours to confirm stability.
Common mistakes and repeat‑failure traps
- Skipping LOTO. Performing visual checks or cleaning without proper isolation can lead to accidental motor start‑up, causing injury and masking the true current draw.
- Assuming voltage is correct without measurement. Even if the utility meter reads nominal, a loose connection or phase loss can create an apparent current increase.
- Cleaning only the visible portion of the trough. Cement paste can harden deep in the flight pitch, creating hidden drag that re‑appears after the next run.
- Neglecting bearing lubrication. Over‑time, grease can migrate or become contaminated, increasing friction without obvious vibration changes.
- Resetting VFD parameters without documenting the original values. This makes it difficult to trace whether a software change introduced the fault.
- Relying solely on a single current reading. Motor load can fluctuate with raw material moisture; trend the current over several cycles before concluding.
- Failing to verify alignment after a bearing replacement. Even a small mis‑alignment can cause a permanent increase in torque demand.
Addressing these pitfalls early reduces repeat‑failure cycles and extends the service life of the screw conveyor.
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