Condenser Fan Motor Trips Overload on a Hot Afternoon
A 460-V, three-phase, 50-hp condenser fan motor trips its overload after about 15 minutes of running on a hot afternoon. Nameplate: 65 A FLA, service factor 1.15, 40°C ambient. Measured at the starter: line voltages 472, 481 and 465 V; running currents 62, 58 and 71 A; insulation resistance to ground 200 MΩ on all three phases; winding resistance line-to-line 0.42, 0.43 and 0.61 Ω. Local ambient at the motor frame is 118°F. The overload relay is a Class 20 thermal type set at 65 A. The motor is across-the-line started by a contactor, with no VFD. The motor was replaced two years ago and the overload setting was never reviewed.
1. Calculate the voltage imbalance, the current imbalance, and the winding resistance spread. What do the three numbers together tell you?
Read the worked answer
Voltage imbalance (NEMA definition, maximum deviation from average divided by average):
- Average = (472 + 481 + 465) / 3 = 472.7 V
- Maximum deviation = 481 − 472.7 = 8.3 V
- Imbalance = 8.3 / 472.7 ≈ 1.76%
Current imbalance:
- Average = (62 + 58 + 71) / 3 = 63.7 A
- Maximum deviation = 71 − 63.7 = 7.3 A
- Imbalance = 7.3 / 63.7 ≈ 11.5%
Winding resistance spread:
- Average = (0.42 + 0.43 + 0.61) / 3 = 0.487 Ω
- Maximum deviation = 0.61 − 0.487 = 0.123 Ω
- Spread = 0.123 / 0.487 ≈ 25%
Interpretation: NEMA guidance says a motor should be derated once voltage imbalance exceeds about 1%; roughly 2% imbalance costs on the order of 5% of capacity, and 3% considerably more. So 1.76% may trim the margin but cannot produce an 11.5% current imbalance. When current imbalance is several times the voltage imbalance, the cause is inside the motor circuit, not the supply. The 25% resistance spread points at the same thing: the high-resistance phase (0.61 Ω) is the high-current phase (71 A). That is the signature of a bad connection (loose lug, pitted contactor pole, failing splice) or a partially faulted winding. The 200 MΩ insulation reading only rules out a ground fault - it says nothing about turn-to-turn or connection resistance.
2. What do you check next, and how do you check it without damaging the motor?
Read the worked answer
Work from the supply toward the windings, de-energized and locked out.
- Thermal ambient, first, because it is free. The frame ambient is 118°F against a 40°C/104°F nameplate rating, and the overload is set at 65 A with a 1.15 service factor. The relay may be doing exactly what it was told: at 118°F a Class 20 thermal relay trips earlier than at 40°C. Ambient correction and the setpoint both deserve review - but heat alone does not explain an 11.5% current imbalance.
- Inspect and torque the line-side and motor-lead terminations with a torque wrench to the OEM values, and look for discoloration, heat damage, or arc pitting at the contactor poles and overload heater connections. A loose or pitted pole shows up as a hot spot and as a resistance mismatch.
- Measure contactor pole voltage drop under load with a meter rated for the voltage: place the probes across each pole (line to load) while the motor runs. A healthy closed pole should read well under about 1–2 V; a significantly higher drop on one pole identifies the bad contact.
- Re-measure the winding resistance with a low-resistance ohmmeter/micro-ohmmeter, with the motor leads disconnected from the starter, and compare all three line-to-line values. A repeatable 25% spread with clean leads and good contactor poles means the fault is in the motor, and the motor comes out for a shop test (surge comparison, turns-ratio, winding resistance).
Do not 'megger it again and call it good' - insulation resistance is not the test that finds this fault. Do not run it at full load to see whether the imbalance persists; that is how a turn-to-turn fault becomes a burnout.
3. What is your immediate decision about running this motor, and what temporary measures are acceptable?
Read the worked answer
Do not return it to service as-is.
Reasoning:
- An 11.5% current imbalance with a 25% winding resistance spread is a progressing internal fault. Current imbalance produces negative-sequence current, which heats the rotor and one phase disproportionately. On a condenser fan, losing the motor is a head-pressure event on the whole plant.
- The 71 A phase is already about 9% over the 65 A nameplate value, on a motor whose ambient is 14°F above its rating. Thermal margin is essentially gone.
Acceptable interim measures:
- Lock out the starter, tag it, and keep the fan off; carry the load with the remaining fans and monitor condensing pressure and approach continuously.
- If plant conditions require this fan, replace or repair the motor; do not simply raise the overload setpoint, which converts an overload trip into a burnout.
- Correct the overload setting only as part of the repair: size it per the applicable code and the motor nameplate/service factor, and apply the ambient correction for the 118°F location.
- Find the cause of the high ambient (recirculation, sun exposure, blocked airflow over the motor) - otherwise the replacement motor will fail the same way.
Document the phase resistances and currents now; they are the baseline that proves the repair fixed the root cause rather than the symptom.