WiremanWireman · Transformer loud or running hot: troubleshooting for electricians · 2023 NEC ·
Transformer loud or running hot
Normal hum and heat versus overload, high voltage, harmonics, mounting and ventilation.
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How it works
Transformers hum at twice the line frequency (120 Hz on a 60 Hz system) because the core steel changes size slightly with each half of the magnetic cycle. Heat comes from current in the windings and magnetism in the core. Dry-types are built to run hot, with temperature rise ratings like 80, 115 or 150 °C above ambient, so a hot case alone isn’t a problem. Heat or noise that has increased, or that comes with high load, high voltage or harmonics, is.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What are you noticing?
- Louder hum or buzz than normalCheck how it’s mounted. What do you find?
- Shipping bolts in, hard-mounted or touching somethingMounting carries the hum
- Mounting looks fineTest: Measure the primary voltageExpect: At or near the tap voltage.
- Noticeably above the tap voltageVoltage too high for the tap
- NormalTest: Measure the secondary currentExpect: Each phase at or below full-load amps, and the neutral lower than the phases.
- Above full-load ampsOverloaded transformer
- Below rating, but neutral high or lots of electronicsHarmonic heating
- Below rating, normal neutralAre the vents clear and the room cool?
- Blocked, too close to a wall, or a hot roomPoor ventilation
- Clear and coolProbably normal operating temperature
- Running hotTest: Measure the secondary currentExpect: Each phase at or below full-load amps, and the neutral lower than the phases.
- Above full-load ampsOverloaded transformer
- Below rating, but neutral high or lots of electronicsHarmonic heating
- Below rating, normal neutralAre the vents clear and the room cool?
- Blocked, too close to a wall, or a hot roomPoor ventilation
- Clear and coolProbably normal operating temperature
- Crackling, sizzling, arcing or burning smellPossible arcing or insulation failure
- Louder hum or buzz than normalCheck how it’s mounted. What do you find?