Intermittent Overload Trips and a Voltage Imbalance

The voltages average about 203 V, and the 195 V leg is roughly a 4 percent imbalance. The currents average about 41 A, and the 47 A leg is roughly a 14 percent current imbalance. Current imbalance runs several times the voltage imbalance and produces extra rotor and stator heating, which explains the overload trips at peak load. The 47 A leg is also above the 46 A nameplate current, so the motor is at or over its rating on one leg before any margin is considered.

Next, localize the imbalance. Repeat the voltage and current measurements at the service entrance or main switchboard, and at the elevator disconnect, under the same loaded condition:

• If the imbalance is present upstream with little elevator load, it is supply-side (utility transformer, single-phase loading, or an open-delta condition), and it belongs to the owner and utility to correct.
• If it appears only downstream when the elevator is loaded, look for a high-resistance connection: disconnect blades, fuse clips, contactor tips, lugs, and motor terminal leads. A voltage-drop test across each contactor pole and fuse under load, a thermal scan, and torque checks on terminations will find it.

Also confirm the overload heaters match the nameplate current and ambient, and check for single-phasing symptoms, where current in the remaining legs rises sharply.

It is the wrong fix and creates a fire and motor-damage risk. Overload protection is sized to the motor's nameplate full-load current and the manufacturer's selection table, subject to the code's allowances; it exists to take the motor off line before the winding is damaged. Installing larger heaters raises the trip threshold so the motor can run hotter for longer, and the underlying cause, the current imbalance, keeps overheating one leg. The trips are the protection working, not a nuisance.

The correct path is to correct the imbalance at its source, verify terminations, and confirm the motor is not simply undersized or mechanically overloaded (check brake drag, guide friction, and actual car load). If the supply cannot be corrected and the motor must run on a persistently unbalanced system, the motor should be derated or replaced per the manufacturer and NEMA guidance, not protected less. After any electrical repair, re-measure voltages and currents under load and document the before-and-after values.

Treat this as live electrical work performed only by a qualified person. Perform de-energized work under lockout/tagout on the elevator disconnect and any upstream feed, verify absence of voltage at the point of work with a meter proven on a known live source before and after, and check for backfeeds from controls, capacitors, or building generators. For the live measurements needed to diagnose the imbalance, use a meter and leads rated for the available fault current at 208 V (CAT III or better for the panel and controller locations), inspect the leads, wear the PPE the shock and arc-flash risk assessment requires, and keep the meter and hands out of the enclosure as far as practical. Do not open the motor terminal box or tighten connections under load.

Keep the car under control while running it for measurements: confirm the car is empty or loaded as intended, the hoistway doors are closed, and communication with anyone at the car is established. Record the meter model and rating, the measurement points, and the load condition, because the diagnosis depends on where the imbalance first appears.