A buried trampoline raises belt tension and motor load until the belts can fail; a PLC current trip can stop a sustained overload, but it cannot make an undersized or poorly protected drive safe. Use the 0.5-second startup interval as a configurable delay before evaluating the running-current threshold, then validate the threshold against normal starts, normal travel, and a controlled obstruction test. A VFD with an appropriately configured current or torque limit can reduce the force applied during a jam, while current alone cannot confirm that the trampoline is moving.
Read current, elapsed time, and travel together
The key quantity is motor current after the startup transient, interpreted alongside elapsed time and a separate indication of motion where practical. Current above a calibrated limit for the allowed duration can indicate a jam or excessive mechanical drag. Current below a calibrated limit can indicate a disconnected motor, broken transmission, or unloaded condition, but only when the command and operating state make that interpretation meaningful.
| Observation | Possible causes | What to check |
|---|---|---|
| Current rises above the running limit after startup | Trampoline buried or obstructed; binding belt or mechanism; abnormal load; electrical or motor problem | Compare current by direction and travel condition; inspect the mechanism and drive diagnostics before resetting. |
| Current is high only during startup | Normal inrush or acceleration demand; excessive acceleration load; mechanical resistance | Record the current profile and startup time. Confirm that the evaluation delay covers the transient without masking a jam that persists after the delay. |
| Current is low during a run command | Broken belt or coupling; motor not energized; sensor or scaling fault; genuinely low load | Check motor power, the current signal, command state, and independent motion feedback. |
| PLC detects the condition but motion continues | Input filtering, task scan, logic, output update, contactor or drive response, or stopping behavior adds delay | Measure the time from threshold crossing to actual motor torque removal or controlled stop. |
These observations are diagnostic clues, not unique fault signatures. A current measurement can identify load changes, but it does not identify their mechanical cause. In this installation, absent position feedback also means a command to return to the top does not prove the trampoline reached the top—or moved at all.
Understand why current protection may not save the belts
Motor torque and current are related through the motor and drive operating conditions, while the belt experiences mechanical force through the transmission geometry. The allowable belt load therefore cannot be derived from a current threshold alone. Select a trip or limiting level using the motor and drive data, the belt and gearbox limits, and measured current during representative operation. If those limits are unknown, obtain the relevant equipment ratings before setting a protective threshold.
A current transformer or other current sensor feeding a 4–20 mA signal can provide a PLC with a usable load measurement. The sensor range, current-to-output scaling, analog input range, and wiring determine what current each PLC count represents. The source gives no motor current, sensor range, analog module model, or input resolution, so the numerical scaling and trip value must come from the installed hardware documentation and commissioning measurements.
End-to-end response matters as much as analog input update time. The measurement must pass through sensor response, analog conversion and filtering, PLC input update, program execution, output update, and the stopping element's response. A nominal 0.5-second persistence criterion does not mean the motor stops within 0.5 seconds; it means the overload is accepted only after the condition persists through that interval, with additional system delays possible.
Use the startup delay without masking a jam
Implement separate startup and running states. On a start command, inhibit the running overload comparison during the specified 0.5-second startup interval, but continue to monitor and record current if the hardware permits. Once the interval expires, evaluate the running current against a threshold and persistence timer. Treat 0.5 seconds as the requested starting value for testing, not a universal motor-start setting: adjust it only from observed startup behavior and the required mechanical protection response.
Use hysteresis or a reset policy to prevent noisy current values near the trip point from repeatedly setting and clearing the condition. Latch a trip when the overload criterion is met, remove or stop the drive command through the intended control path, and require a deliberate reset after inspection. Keep undercurrent logic distinct: enable it only during a commanded run and after startup, and define what the machine should do when it detects low current. A low-current indication should not automatically be treated as a broken belt until verified against the actual mechanism.
Configure a VFD for controlled jam response
A VFD can limit current or torque and can start the motor at a reduced speed. The proposed approach in this application was to use a low-speed startup and then raise speed after detecting successful movement. This can reduce mechanical shock and may limit the force available during a jam, but reduced speed by itself is not a dependable torque limit. In V/f operation, torque capability depends on motor flux, drive behavior, and load; configure and verify the drive's current or torque limits rather than assuming that a low speed command guarantees low torque.
A VFD current-limit setting also needs coordination with motor and transmission ratings. A limit low enough to protect a belt may prevent normal travel under heavy but valid loads. Commission the limit using measured normal operation and a controlled obstruction test, while checking the drive's documented response when the limit is reached. A drive may reduce output or stall under limiting; determine how the installed drive reports that state and how the PLC should respond.
Current monitoring or a drive-reported startup-complete condition can support a transition from safe startup speed to operating speed, but it does not establish actual travel. In V/f control, commanded frequency is not independent proof of motor or trampoline speed. Add a suitable motion sensor or position feedback if the sequence must distinguish a moving assembly from a stalled motor. Validate sensor mounting and operation in the bunker environment, where product and debris may affect sensing.
Commission the current loop and stopping path
- Identify the motor, drive or starter, current sensor, 4–20 mA range, analog input configuration, and belt or transmission ratings. Confirm the current signal represents the intended motor conductor measurement and check sensor and input wiring.
- With the mechanism in a known safe condition, record the PLC raw input and scaled current during startup, normal travel in each direction, stopping, and representative load conditions. Verify scaling against an independent suitable current measurement or the drive's documented current display.
- Set the startup inhibit to 0.5 seconds for initial evaluation. Compare measured starts to that interval. If current remains elevated after it expires, confirm that the selected persistence time and threshold stop the motor before belt or mechanism damage can occur.
- Test the overload and undercurrent conditions separately under controlled conditions. Verify the PLC detects each condition only in the intended state, records or latches it as designed, and commands the correct stop response.
- Measure total response from current crossing the limit through actual stopping action. Include input filtering, PLC scan and output timing, and the drive, contactor, or other stopping device. Compare the measured response with the mechanical protection objective.
- Repeat tests after changing filters, thresholds, drive limits, or motion sensing. Document the final operating values, test conditions, and reset procedure in the machine records.
Retain the motor's required overload and short-circuit protection independently of PLC analog logic. The PLC threshold is process or equipment protection logic; it does not replace properly selected electrical protection. Use the drive or starter diagnostics to separate an electrical trip from a mechanical overload indication.
Verify that a trip protects the mechanism
Acceptance testing should prove more than a changing PLC value. Verify the analog reading against the expected current range, confirm the delay and persistence behavior with a controlled test, and observe the motor's actual response when the logic trips. Inspect the belts and transmission after the test; a PLC bit changing state does not establish that the mechanical load was removed quickly enough.
Where motion feedback is added, test that it changes state during genuine travel and remains in the expected state during a stopped or obstructed condition. Cross-check motion and current together: high current with no motion is a strong jam indication, while current with motion may indicate normal loaded travel or excessive drag. Define the machine response to disagreement, such as current indicating a run while the motion sensor reports no movement.
Avoid common current-protection pitfalls
- Setting a threshold from motor nameplate current alone: nameplate data does not state the allowable belt force. Relate the trip threshold to the drive, motor, transmission and measured operating envelope.
- Using a startup timer as the only protection: a delay suppresses nuisance trips during inrush but also postpones overload action. Measure the startup profile and overall stopping time.
- Treating current as position feedback: current indicates electrical load, not trampoline height. Use a limit, position, or motion sensor for the sequence condition it must prove.
- Assuming low speed always means low torque: verify the actual VFD current or torque limiting behavior and test it under the relevant load.
- Ignoring false trips from variable resistance: bunker conditions and uneven travel can raise current without a complete stall. Trend normal operation, then choose a setting that distinguishes acceptable load from damaging load.
- Ignoring undercurrent context: only evaluate low current when a run is expected and startup is complete; otherwise normal idle or stopping states can look like faults.
FAQ
How do I detect a motor overload after startup in a PLC?
Inhibit the running-current comparison for the configured startup interval—in this case, 0.5 seconds—then compare scaled current with a calibrated running threshold and require the condition to persist for the selected trip time. Validate both values against measured starts, normal travel, and a controlled obstruction test.
How do I scale a 4–20 mA motor-current signal?
Use the current sensor's stated measurement range and the analog input's configured range to map the PLC value to amperes. Verify the mapping against a suitable independent measurement; the sensor range and input model determine the actual scaling.
Can a VFD's low-speed start protect a jammed belt?
Low speed alone does not guarantee low torque. Configure the drive's current or torque limit based on motor and transmission limits, verify its limiting behavior, and use motion feedback if the PLC must prove the assembly is moving.
When should I stop testing and escalate a motor-current trip?
Stop commissioning if the belts or mechanism show damage, the motor fails to stop as expected, or the drive current-limit behavior remains unclear. Have a qualified controls or electrical engineer review the measured response and consult the official drive, motor, sensor, or PLC manufacturer support channel for equipment-specific configuration and ratings.