Crane Hoist Weak Lifting & Motor Overheating: 12 Fault Checks
Key Point Weak hoisting and motor overheating are two interrelated crane faults that often occur simultaneously. Six causes of weak hoisting: ① Low voltage (more than 10% below rated value); ② Single-phasing of the motor; ③ Brake not fully released (dragging); ④ Seized pulley/drum; ⑤ Broken rotor bars; ⑥ Overload. Six causes of motor overheating: ① Overload operation; ② Poor heat dissipation (blocked fan cover/damaged fan); ③ Excessive start-stop cycles beyond the duty rating; ④ Three-phase voltage unbalance exceeding 5%; ⑤ Bearing seizure; ⑥ Ambient temperature above 40°C without derating. Troubleshooting should follow the principle of "electrical first, mechanical second; simple first, complex second." Kelude heavy industry crane motors are designed with Insulation Class F and a temperature rise limit of 105K.
Weak Hoisting: Root Causes Explained
① Low Voltage
Motor torque is proportional to the square of the voltage (T∝U²). When voltage drops by 10%, torque falls to 81%, resulting in noticeably weak hoisting. This issue is common in remote factories or at the end of long power supply lines. Use a multimeter to measure the voltage at the motor input terminals, comparing readings under no-load and full-load conditions. Solutions include adjusting the transformer tap or installing a voltage stabilizer. Kelude recommends keeping the crane supply voltage fluctuation within ±5%.
② Single-Phasing
A blown fuse in one phase or poor contactor contact can cause the motor to run on a single phase. Symptoms include a noticeably slower lifting speed, a humming sound from the motor, and one phase reading zero on the ammeter. Single-phasing can quickly burn out the motor, so shut down immediately upon detection. Inspect the fuses, contactor contacts, and terminal block.
③ Brake Not Fully Released
Improper brake clearance adjustment or insufficient thruster stroke prevents the brake from fully releasing, leaving the friction lining in light contact with the brake wheel (dragging). Dragging not only increases hoisting resistance (causing weak lifting) but also generates heat that conducts from the brake wheel to the motor. Adjust the brake clearance to the characteristic value of 0.6–1.5 mm.
④ Seized Pulley or Drum
Damaged sheave bearings or wire rope derailment increase mechanical resistance. Check that the pulleys rotate freely and that the wire rope is properly seated on the pulleys and drum. When the wire rope on the wire rope drum becomes entangled, the ropes press against each other, causing a sharp increase in resistance.
⑤ Broken Rotor Bars
When rotor bars in a squirrel cage motor fracture, torque drops, current fluctuates, and speed decreases significantly under load — the motor may even stall. The motor overheats and emits a periodic humming sound. A motor with broken rotor bars must be replaced.
⑥ Overload
When the actual suspended load exceeds the rated lifting capacity, the motor cannot deliver sufficient torque. The overload limiter triggers an alarm at 110% of rated load and automatically shuts down the hoisting circuit at 130%.
Motor Overheating: Root Causes Explained
① Overload Operation
When a motor runs continuously above its rated power, the sustained high current causes the temperature rise to exceed limits. Temperature rise limits are 80K for Class B insulation, 105K for Class F, and 125K for Class H. Kelude crane motors are designed with Class F insulation (temperature rise limit 105K) and can operate at 1.2 times the rated load for 2 minutes.
② Poor Heat Dissipation
Dust accumulation in the motor fan cover, damaged fan blades, or a thick layer of dirt on the motor surface all reduce heat dissipation capacity. Regularly cleaning the motor fan cover and fan is essential for preventing overheating. Kelude recommends cleaning the motor cooling fins and fan cover screen once a month.
③ Frequent Start-Stop Cycles
Cranes with S3 duty classification have a specified Cyclic Duration Factor (e.g., S3 40%). When start-stop frequency exceeds the designed duty cycles, heat accumulates in the motor faster than it can dissipate. Verify whether the actual working cycle exceeds the design parameters.
④ Three-Phase Voltage Unbalance
When the voltage difference between phases exceeds 5%, negative-sequence currents intensify motor overheating. Measure the three-phase voltages: the maximum difference among Uab, Ubc, and Uca should not exceed 5%.
⑤ Bearing Seizure
Bearings that are under-lubricated, worn, or damaged increase frictional resistance and cause the motor to overheat. The bearing area will be noticeably hotter than other parts of the motor. Replace bearings promptly if abnormal noise is heard.
⑥ High Ambient Temperature
Motor rated power corresponds to an ambient temperature of 40°C. When ambient temperature exceeds 40°C, the motor must be derated (10% derating for every 5°C rise).
Voltage & Current Fault Diagnosis Parameter Table
| fault symptom | Testingpoint | normal value | abnormal value | fault cause |
|---|---|---|---|---|
| Hoisting / Liftingloss of power | Motorthree-phase Voltage | 380V±38V | ＜320Vor three-phase not | power supply Voltagelow/phase loss |
| Motoroverheating | Motorenclosure temperature | ≤80℃(Fclassinsulation) | ＞95℃ | overload/poor heat dissipation/dragging |
| operation Currenthigh | clamp metermeasure each phase | ≤rated current×1.1 | ＞1.3timesrated current | Brakingdragging/Machinerysticking |
Kelude Heavy Industry's after-sales technical support offers phone-based diagnostic guidance. If the issue cannot be resolved remotely, an engineer will be dispatched to your site within 48 hours.
Troubleshooting and Precautions
FAQ
Q: Hoisting is weak but the motor isn't hot — what should I check first?
A: Check the voltage first. When voltage is insufficient, motor output drops but the motor may not overheat (current may stay within limits). Measure the three-phase voltage and confirm it's within ±5% of the rated voltage. Next, check for a missing phase (one phase reading zero on the ammeter).
Q: The motor is too hot to touch — can I keep running it?
A: If the motor housing feels hot to the touch (approximately 60°C or above), stop the unit and inspect it. A Class F insulated motor can have a housing temperature of up to roughly 90°C (40°C ambient + 50K temperature rise). Kelude recommends using an infrared thermometer for accurate readings: if the housing temperature exceeds 90°C or runs more than 30°C above normal operating temperature, shut down and investigate the root cause.
Q: How do I diagnose a broken rotor bar in the motor?
A: A broken rotor bar typically causes the motor to vibrate, produce a periodic humming sound, and draw fluctuating current under load. Use an ammeter to observe the current — if it swings periodically at full load, the rotor is likely damaged. Confirm with a rotor bar inspection or an AC winding test, and contact Kelude technical support for a replacement rotor if needed.
A: The typical characteristics of broken rotor bars are: normal no-load operation, but under load the speed drops noticeably, motor vibration increases, and the ammeter needle swings periodically. In severe cases, the motor cannot drive the rated load. To confirm, remove the motor and test it with a short-circuit tester, or send it for repair.
Q: Is a faulty brake the cause of weak hoisting?
A: Weak hoisting caused by brake issues is usually due to "incomplete brake release" (dragging), not "brake failure." Symptoms include slower-than-normal lifting speed, motor overheating, and a hot brake wheel. After energizing to release the brake, manually turn the brake wheel to check for smooth rotation. Kelude recommends inspecting the brake clearance (0.6–1.5 mm) and confirming the thruster stroke is fully engaged.
This completes our guide to diagnosing and resolving weak hoisting and motor overheating in overhead cranes. Kelude crane motors feature Class F insulation and dual brake redundancy, and we offer motor inspection and replacement services. If you're experiencing weak hoisting or motor overheating, please contact the Kelude after-sales technical team.
