Quick answer
Apron-feeder speed feedback that does not match commanded speed should be diagnosed by comparing actual drive/shaft speed with the feedback signal and the VFD command. Common causes include encoder/tachometer coupling slip, wrong scaling, target runout, gearbox ratio configuration, sensor wiring, VFD feedback mismatch or real mechanical slip in the drive chain. Do not retune crusher feed control until actual feeder speed is proven.
Problem / job
Crusher feed rate is unstable or the DCS shows a feeder speed different from the commanded percentage.
Evidence
Capture VFD command/frequency, actual motor rpm, feeder shaft speed, encoder/tach signal, gearbox ratio, material feed rate and drive-chain condition.
Decision
Separate measurement/scaling faults from VFD/motor problems, coupling/drive slip and actual mechanical speed loss.
As-found evidence capture
Freeze the controller settings before changing scaling or PID values. Record VFD command/frequency, motor current, motor rpm if available, independently measured feeder shaft or sprocket speed, DCS/HMI feedback, crusher load, and actual material feed behavior at the same moment. Note whether the mismatch is constant, proportional across the operating range, intermittent, or appears only at low speed.
Inspect the complete feedback path: sensor/encoder mounting, coupling or target wheel, sensing gap, wiring, junction boxes, shielding/grounding, converter modules, input type and PLC/DCS scaling. Also document gearbox ratio, chain/sprocket condition and any recent sensor, gearbox, VFD or PLC work.
Seven-step diagnostic workflow
Safety boundary: Collect operating trends remotely or from approved guarded positions. Do not approach moving chains, sprockets, couplings or rotating shafts for speed measurement. Before touching sensors, removing guards or inspecting drive components, stop the feeder and crusher as required, apply the site-approved lockout/tagout procedure to all relevant energy sources, and verify zero energy. Restore guards and obtain authorization before controlled restart.
- Measure actual feeder shaft or sprocket speed using an approved independent method.
- Compare actual speed with VFD output frequency and motor speed.
- Inspect encoder/tachometer coupling, target wheel, bracket and runout.
- Verify pulses-per-revolution, gearbox ratio and PLC/DCS scaling.
- Check sensor power, wiring and shielding for intermittent dropout/noise.
- Inspect drive chain, coupling, gearbox and sprockets for slip or lost motion if motor speed is correct but feeder speed is low.
- After correction, confirm command, feedback and actual material feed track consistently through the operating range.
Diagnostic decision matrix
| Evidence | Likely direction | Next check |
|---|---|---|
| Motor speed correct, shaft speed low | Mechanical drive slip | Gearbox, chain, coupling, sprocket |
| Actual speed correct, feedback wrong | Sensor/scaling issue | Encoder and PLC configuration |
| Feedback drops once per revolution | Target/runout problem | Target wheel and gap |
| VFD command and motor speed both low | Drive/control limit | VFD reference, torque/current limits |
Use mismatch shape to localize the fault
Constant percentage error across the full speed range usually points toward scaling, pulses-per-revolution, gearbox ratio or engineering-unit configuration. Intermittent dropouts point more toward sensor gap, loose wiring, damaged cable, EMI or an unstable converter/input. Feedback error mainly at low speed can indicate pulse-rate resolution, excessive sensor gap, target runout or a filtering/watchdog issue. Motor speed correct but feeder shaft speed low moves the investigation into the mechanical drive path—coupling, gearbox, chain and sprocket engagement.
Do not use material feed rate alone as proof of feeder speed because wet material, hopper head pressure and crusher acceptance can change tonnes per hour at the same mechanical speed.
Speed-feedback inspection checklist
- Measure actual shaft/sprocket speed with an approved independent method before changing PLC/DCS scaling.
- Confirm the sensor/encoder is mechanically coupled to the intended shaft and is not slipping.
- Inspect target wheel/runout and sensing gap through a complete revolution.
- Verify PNP/NPN or encoder input type matches the PLC/VFD input hardware.
- Check cable continuity, shield termination, junctions and separation from VFD output cables.
- Confirm pulses per revolution, gearbox ratio and unit conversion from raw pulse frequency to displayed speed.
- Compare VFD command, output frequency and actual motor rpm to separate reference/control issues from downstream mechanical loss.
- Inspect drive coupling, gearbox, chain and sprockets if motor speed is correct but apron speed is not.
Acceptance criteria
Verify the repaired feedback path across the normal operating range:
- Check at several commanded speeds rather than one point.
- At each point, record VFD command/output frequency, independent shaft speed and DCS/HMI feedback together.
- Confirm no pulse dropout or feedback hunting occurs at low speed.
- Verify crusher feed remains stable and no false under-speed/over-speed or feedback-loss alarms appear.
- Inspect the mechanical drive after the test and confirm no coupling, gearbox, chain or sprocket slip is present.
- Save the verified scaling, pulses-per-revolution, gear ratio and sensor configuration as the maintenance baseline.
Use the apron-feeder/VFD/OEM documentation for allowable speed error and protection thresholds; this guide deliberately avoids a generic tolerance.
Common mistakes and repeat-failure traps
- Retuning crusher-feed PID first: control tuning cannot fix a false speed measurement.
- Trusting displayed speed without an independent measurement: scaling can be perfectly stable and still be wrong.
- Changing pulses-per-revolution until the number “looks right”: derive it from the real target/encoder geometry and gearbox path.
- Ignoring sensor output type: PNP/NPN or encoder-input mismatch can produce missing or invalid feedback.
- Ignoring EMI: VFD output cables can corrupt low-level feedback wiring if routing/shielding is poor.
- Assuming motor rpm equals apron speed: mechanical slip, gearbox problems or chain/sprocket loss can occur downstream of the motor.
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