Intermittent Stall on a 480 V Geared Traction Unit
A 12-year-old geared traction elevator in a 6-stop office building runs at 250 fpm with a 40 hp, 480 V, 3-phase motor. On hot afternoons the car intermittently stalls on acceleration, the controller logs an undervoltage fault, and one line fuse has blown twice in two weeks. At the main disconnect you measure 492 V line-to-line with the car idle and 456 V during a stalled start. The motor nameplate is 460 V, 48 A FLA. A helper suggests increasing the overload setting and installing a 60 A fuse “to stop nuisance trips.” The building engineer reports that the utility transformer serves both the elevator and a rooftop HVAC unit that starts around the same time each afternoon.
1. Evaluate the voltage readings. Do they prove the utility is at fault, and what should you check first?
Read the worked answer
The readings are a clue, not a diagnosis. Idle voltage of 492 V is about 2.5% above nominal 480 V, which is generally acceptable. The 456 V reading during a stalled start is 36 V below the idle value, or about 7.3% drop, but it is also only about 0.9% below the motor’s 460 V nameplate rating. That means the motor is not necessarily seeing an out-of-tolerance steady-state voltage; the more important question is why starting current is producing that much drop and why a fuse is blowing. First, verify the readings with a true-RMS meter and check all three line-to-line voltages under load, not just one pair. A single low phase suggests a bad connection, a failing breaker, or an open fuse clip rather than a utility problem. Inspect and torque all terminations at the disconnect, starter, and motor; check the breaker and fuse holders for heat damage; and use a thermal camera during a start. Only after ruling out loose connections, high-resistance joints, and a shared transformer loaded by the HVAC start should you ask the utility to investigate.
2. What is wrong with the helper’s proposal to increase the overload setting and install a 60 A fuse?
Read the worked answer
The proposal masks the symptom and defeats the motor’s protection. The overload setting is based on the motor’s nameplate full-load current and the applicable Code requirements; raising it without a documented engineering basis allows the motor to run hotter and can lead to winding failure. A 60 A fuse in a circuit designed for a 48 A motor is also not a valid fix: fuses and overloads protect different things, and oversizing the fuse can let a fault persist long enough to damage conductors or start a fire. The correct approach is to find the cause of the high starting current or voltage drop. Possible causes include a loose line-side connection, a failing contactor, a bad motor winding or rotor, mechanical drag from the brake or guide shoes, a shared transformer with the HVAC load, or a utility voltage sag. The mechanic should measure current on all three phases during the start, compare phase balance, check the brake pick-up and drop-out, inspect the motor windings, and review the utility transformer loading. Only after the cause is identified should any protective setting be changed, and then only to the correct value.
3. List a safe diagnostic sequence you would follow before ordering a repair.
Read the worked answer
A safe sequence is: (1) place the elevator on inspection operation and establish communication; (2) verify the complaint and record the exact fault code and ambient temperature; (3) measure all three line-to-line voltages at the disconnect, starter, and motor terminals under idle and starting conditions with a true-RMS meter; (4) measure current on all three phases during the start and compare balance; (5) de-energize, lock out, and inspect the disconnect, fuse clips, contactor, overloads, and motor terminations for heat, pitting, or loose torque; (6) check the brake for drag and verify pick-up and drop-out; (7) megger the motor and inspect the windings; (8) review the utility transformer and HVAC loading with the building engineer; (9) if all electrical values are normal, investigate mechanical drag in the guide shoes, sheave, and gearbox. Document every reading. Do not change overload or fuse ratings until the root cause is established. If the fault is a shared-transformer sag, the repair may be a utility or building-load issue rather than an elevator repair.