Lt / LL Low Torque / Underload fault in delta drive

Lt / LL Low Torque / Underload fault in delta drive troubleshooting

Description

Lt (Low Torque) or LL (Low Load) is the opposite of Ot. It triggers when the motor current or torque drops below a set threshold while running. This is commonly used in pumping applications to detect a "Dry Run" condition or in fan applications to detect a broken belt.

Causes

1. Broken Belt: The V-belt connecting the motor to the fan has snapped; the motor is spinning freely with no load.
2. Pump Dry Run: The pump has lost prime, the intake is empty, or there is air in the line. The load drops significantly.
3. Coupling Shear: The coupling between motor and load has broken.
4. Parameter Sensitivity: The threshold is set too high (e.g., setting low-load detection at 40%, but the machine idles at 30%).

Solution

Mechanical verification:
1. Inspect Transmission: Check belts, chains, and couplings immediately.
2. Check Fluids: If this is a pump, ensure liquid is present at the inlet. Running dry destroys seals.
3. Tune Parameters: Run the machine at normal "light" load. Read the Amps/Torque % on the display. Set the "Low Torque Detection Level" slightly below this running value (e.g., if running at 30%, set detect at 20%).
4. Delay Timer: Increase the detection time to prevent tripping during brief load fluctuations.