Motor Tripping After Bearing Replacement
A 480 V, 3-phase, 30 kW, 4-pole TEFC motor drives a screw conveyor. The motor nameplate FLA is 48 A, service factor 1.15, ambient rating 40°C. The class 20 overload relay is set at 68 A. Supply voltage is 477, 481, and 479 V; line currents are 41, 43, and 66 A at about 60% mechanical load. The motor trips after 8–12 minutes. Insulation resistance is 500 MΩ at 500 V. Winding resistance line-to-line at the motor terminals is 0.41, 0.42, and 0.78 Ω. Ambient at the starter is 32°C. The bearing was replaced last week and alignment was checked. The maintenance technician wants to raise the overload to 75 A and change the trip class to 30.
1. Interpret the voltage, current, insulation, and winding-resistance readings. Where is the fault likely located, and why is it not simply an overload setting problem?
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The supply voltages are balanced within about 1%, so the current unbalance is not being caused by the supply. The line currents are not balanced: 41, 43, and 66 A. The 66 A line is 137% of the 48 A nameplate FLA, which explains why the overload trips after several minutes. The insulation resistance of 500 MΩ is healthy, so there is no clear ground fault. However, the line-to-line winding readings of 0.41, 0.42, and 0.78 Ω are far outside a normal balance. The 0.78 Ω reading is roughly 90% above the lowest reading, which points to a motor winding or lead-connection defect rather than a load or overload-setting issue. A bad internal connection, damaged lead, or partial winding fault can produce the current unbalance and localized heating. The exact fault type should be confirmed with a surge comparison test and a careful inspection of the terminal box and motor leads.
2. Evaluate the technician's proposal to raise the overload to 75 A and change the trip class to 30.
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The proposal is unacceptable. A 75 A setting is 156% of the 48 A nameplate FLA and well above the service-factor allowance. Raising the setting and slowing the trip class would allow the motor to run longer with an internal fault, increasing the risk of winding damage, fire, and unexpected shutdown. The overload is doing its job by tripping on the unbalanced current. The correct response is to remove the motor from service and find the cause of the unbalance, not to defeat the protection. After repair or replacement, the overload should be set according to the nameplate FLA, service factor, and actual ambient conditions, and the trip class should match the starting duty.
3. List the next tests and the repair decision process you would follow before returning the conveyor to service.
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Lock out and isolate the motor. Recheck the line-to-line winding resistance with a calibrated meter and good lead contact, then perform a surge comparison test across all three phases. Inspect the terminal box, motor leads, and connections for heat, arcing, loose lugs, or damaged insulation. Megger each phase to ground and phase to phase. Check the conveyor for mechanical binding or a changed load, even though the load is reported at 60%. Compare the motor current to the nameplate and verify the overload heater or electronic setting against the actual FLA. If the winding or lead fault is confirmed, repair or replace the motor. After repair, run the conveyor uncoupled if practical, record voltage and current on all phases, verify balance, and set the overload to the correct value. Document the readings and the repair.