Recurring Overload Trips on a 480 V Elevator Motor
A 40 hp, 480 V, three-phase geared elevator motor trips its thermal overload after about 20 minutes of normal passenger service. The motor is hot to the touch, and the machine room is at 95 F. You measure line-to-line voltages at the mainline disconnect: L1-L2 482 V, L2-L3 479 V, L1-L3 458 V. Running current with a clamp meter: L1 61 A, L2 59 A, L3 78 A. The motor nameplate full-load current is 52 A. The mainline disconnect, fuses, and motor starter contacts look discolored on one phase. The building electrician suggests simply raising the overload setting because the elevator runs fine when it is cool. The controller is older relay logic, and the brake picks and holds normally.
1. Calculate the voltage unbalance and current unbalance. What do the numbers indicate?
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Average voltage = (482 + 479 + 458) / 3 = 473.0 V. The greatest deviation from the average is 473 - 458 = 15 V. Voltage unbalance = 15 / 473 = 3.2 percent. Average current = (61 + 59 + 78) / 3 = 66 A. The greatest current deviation is 78 - 66 = 12 A. Current unbalance = 12 / 66 = 18.2 percent. The current unbalance is roughly 5.7 times the voltage unbalance, which is the pattern expected in an induction motor: a small voltage imbalance produces a much larger current imbalance and added heating. A 3.2 percent voltage unbalance is well beyond the 1 percent level normally considered acceptable for motor operation, and the 18 percent current imbalance explains the hot motor and recurring overload trips. The discolored disconnect and starter contacts on one phase point to a likely cause: a high-resistance connection or blown fuse in that phase. Raising the overload setting would remove the protective response and allow the motor to overheat until insulation fails.
2. What should you check and correct before replacing the motor?
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De-energize and lock out the mainline disconnect, then inspect and test the disconnect, fuses, starter contacts, and motor terminal connections phase by phase. Look for a blown fuse in one phase, pitted or burned contacts, loose lugs, and heat discoloration. Perform resistance checks across the disconnect and starter poles and megger the motor windings to phase and to ground. If the supply voltage is unbalanced at the utility side with the elevator disconnected, the problem is upstream and the electrician or utility must correct it; the elevator should not be returned to normal service on a 3.2 percent imbalance. If the imbalance appears only under load, repair or replace the damaged disconnect, fuse, or contactor. Correct the overload setting only to the motor nameplate full-load current and the manufacturer's selected heater or overload range, not to stop nuisance trips. Document the voltage and current readings before and after repair, and verify motor temperature and current balance under a loaded run.