Three-Phase Overload With 4.5% Voltage Imbalance
A 5-ton R-410A three-phase packaged unit has a nameplate rating of 208/230 V and a compressor RLA of 24.2 A. On a summer afternoon the compressor trips on internal overload after about 20 minutes. You measure line-to-line voltages of 208 V (L1-L2), 212 V (L2-L3), and 196 V (L1-L3). Compressor running amps are 18.9 A, 19.4 A, and 16.1 A. The dual run capacitor is rated 45/5 µF and measures 35/5 µF. The contactor points show heavy pitting, and the L3 set is discolored. The building service is 208Y/120 V. The condenser coil is clean and the crankcase heater is operating. The unit is three years old, and last year the building added several single-phase tenant loads to the same panel.
1. Calculate the percent voltage imbalance and interpret it against the accepted 2% limit.
Read the worked answer
Average voltage = (208 + 212 + 196) / 3 = 616 / 3 = 205.3 V.
Maximum deviation from average = |196 − 205.3| = 9.3 V (larger than the 6.7 V deviation on the 212 V leg).
Percent imbalance = (9.3 / 205.3) × 100 = 4.5%.
The accepted limit for a three-phase motor is 2%, and NEMA derating starts well before that. At 4.5%, the negative-sequence current produces rotor heating far out of proportion to the voltage error; a rule of thumb is that a 4.5% imbalance can raise motor heating by roughly 25% or more. That, combined with the low L3 current at 16.1 A, matches the discolored L3 contactor set and the 20-minute overload trip.
The imbalance source is upstream of the unit: the newly added single-phase tenant loads on a 208Y/120 V panel are the likely culprit, along with any loose lug or utility issue. An electrician should log voltages at the panel and at the unit disconnect, torque all terminations, and redistribute single-phase loads before the compressor is condemned.
2. Evaluate the capacitor reading and explain how it contributes to the symptoms.
Read the worked answer
The capacitor is rated 45 µF and measures 35 µF, a 22% loss. Motor-run capacitors are normally accepted only within about ±6% to ±10% of rating, so this one is well outside tolerance and must be replaced with an exact-rating part.
Why it matters: the run capacitor sets the phase angle of current in the start/auxiliary winding. A weak capacitor reduces starting and running torque, so the compressor draws more current for the same load, runs hotter, and is harder to start against the high head pressure of a summer afternoon. Combined with a 4.5% voltage imbalance, the motor is being squeezed from two directions at once, which is why the overload opens after 20 minutes rather than immediately.
A capacitor that is not bulging can still be failed; capacitance must be measured, not eyeballed. Check the replacement with a meter before installing and confirm the microfarad rating and voltage rating match the OEM specification.
3. The pitted, discolored contactor: what risk does it create, and in what order do you repair this system?
Read the worked answer
Pitted points and a discolored L3 set mean high resistance and arcing at that pole. The consequences are a voltage drop across the contacts (feeding the 196 V reading), erratic single-phasing on start, welded contacts that keep the compressor running on one phase, and eventually an open pole. A compressor single-phased at run will draw locked-rotor current and trip on overload within seconds to minutes.
Repair order:
- Lock out and tag out. Do not replace parts on an energized unit.
- Have an electrician correct the supply imbalance at the panel: torque terminations, redistribute single-phase tenant loads, and document voltages at the disconnect. Replacing parts without fixing the source guarantees a repeat failure.
- Replace the contactor with the correct rating and verify coil voltage.
- Replace the run capacitor with an exact-match 45/5 µF part.
- Check compressor winding resistance and insulation to ground before restart, to rule out damage from the prior trips.
- Restart and verify voltage imbalance is under 2%, running amps balanced within roughly 5%, and suction/superheat and subcooling on target.
Document the supply readings; this is a power-quality problem that happened to show up in the equipment.