Conveyor Motor Tripping on Overload
A 50 hp, 480 V, 3-phase conveyor motor (FLA 65 A) trips its thermal overload after 20 to 40 minutes of running. The overload is a Class 20 set at 65 A. Line voltages measured at the starter: L1-L2 487 V, L2-L3 479 V, L1-L3 469 V. Clamp readings on the motor leads under load: 62 A, 60 A, 74 A. Insulation resistance to ground is 5 MΩ at 500 V. The motor frame is warm but not hot, the conveyor is not jammed, and the gearbox turns freely by hand. The electrician wants to raise the overload setting to 75 A to stop the nuisance trips.
1. Calculate the voltage unbalance and the current unbalance. Explain why "the voltage is within ±10%, so the supply is fine" is the wrong conclusion, and evaluate the electrician's proposal.
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Voltage unbalance (NEMA method: maximum deviation from average ÷ average × 100):
- Average = (487 + 479 + 469)/3 = 478.3 V
- Largest deviation = 487 − 478.3 = 8.7 V
- Unbalance = 8.7/478.3 × 100 ≈ 1.8%
Current unbalance:
- Average = (62 + 60 + 74)/3 = 65.3 A
- Largest deviation = 74 − 65.3 = 8.7 A
- Unbalance = 8.7/65.3 × 100 ≈ 13.3%
Why the ±10% argument fails: the ±10% figure is a service-voltage tolerance for equipment in general. Motor heating is driven by unbalance, not by whether each phase sits inside a band. NEMA guidance derates a motor to roughly 95% of rating at about 2% voltage unbalance, and the resulting current unbalance can be several times larger than the voltage unbalance - here about 13%. That is exactly what the clamp readings show: one leg at 74 A is above the 65 A nameplate FLA while the other two are below it. The overload is doing its job.
The proposal: raising the setting to 75 A is wrong. It masks the symptom and lets the high-current phase run above nameplate until the winding insulation fails. It also voids the motor's listing basis and removes protection from a genuine mechanical overload later. A Class 20 overload set at nameplate FLA is correct for this motor.
2. What would you check next, in what order, and what would make you suspect the problem is upstream in the plant rather than at this motor?
Read the worked answer
Work from the source toward the motor, and record everything under load:
- At the MCC bus and the starter line side: measure all three phase-to-phase voltages and check for a loose lug, corroded connection, or a partially open contact. A high-resistance connection shows up as a voltage drop that grows with load - measure line-side and load-side voltage at the same time and compare.
- At the motor terminals: repeat the voltage and current readings. If the unbalance exists at the MCC, the problem is upstream; if it only appears at the motor, the cable or a termination is the suspect.
- Infrared scan of the starter, lugs, and disconnect under load - looking for a hot joint, not just a loose screw.
- Look upstream: single-phase loads on the same bus, a failed capacitor bank fuse or contactor on the plant power-factor correction equipment, an open delta transformer configuration, or a utility supply issue. A blown capacitor fuse is a classic cause of a few percent voltage unbalance that appears only on part of the plant.
- Confirm the mechanical side is genuinely clear: amp readings on all three phases at no load versus loaded, gearbox oil level and temperature, and chain/belt tension. Low current on two phases plus high on one is electrical; high current on all three is usually mechanical.
Upstream indicators: the unbalance is present at the MCC with the conveyor off; other motors on the same bus show similar current unbalance; the unbalance changes with plant load or time of day; a thermal scan shows nothing hot at this starter.
3. What do you make of the 5 MΩ insulation resistance reading, and how does the presence of a VFD change how you test and interpret it?
Read the worked answer
Interpreting 5 MΩ: a single spot reading above 1 MΩ is generally not a failure indication for a 480 V motor, so it does not explain the trips - but one number proves very little. What matters is:
- Temperature and moisture: readings must be corrected for winding temperature; a cold, damp motor reads low and a hot one reads high.
- Trend: compare to the previous readings for this motor. A drop from 500 MΩ to 5 MΩ is a warning; a steady 5 MΩ for years is not.
- Dielectric absorption / polarization index: a 10-minute-to-1-minute ratio helps distinguish absorbed moisture from genuine insulation degradation.
- Phase-to-phase readings as well as phase-to-ground.
With a VFD in the circuit:
- Disconnect and isolate the drive before megging. Never apply a megger to the drive output terminals - the test voltage can destroy the power electronics.
- Megger the motor with the drive disconnected and the cable isolated, then test the cable separately so a bad cable is not blamed on the motor.
- If the motor is inverter-duty and runs long cable runs, check for reflected-wave damage and look at the drive's fault history - overcurrent and ground-fault trips in the drive log may tell a different story than the overload.
Actions: clean and dry the windings if moisture is suspected, re-test, and log the reading with the temperature and test voltage. Also confirm the overload is correctly sized to the motor nameplate and that the Class 20 setting is appropriate for the actual acceleration time of the conveyor.