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What are the reasons for abnormal overheating of low frequency transformers?
2026-07-01
It is a common phenomenon that low-frequency transformers generate heat during the power-on operation process. This is a natural physical loss that occurs during the conversion of electrical energy. However, in scenarios such as engineering debugging, equipment mass production, and after-sales maintenance, many technicians often easily fall into a dilemma: either mistakenly judge the normal temperature rise during operation as a fault and blindly replace the transformer or modify the structure; or regard the abnormal overheating caused by inter-turn short circuits as normal heat and allow the equipment to run with faults, ultimately resulting in serious accidents such as transformer burnout, equipment shutdown, and fire and short circuit.
Inter-turn short circuit is the most concealed and most harmful chronic fault in low-frequency transformers. Unlike obvious open circuits or grounding faults, a slight inter-turn short circuit will not cause immediate tripping or instant burnout. It only manifests as slight overheating, increased noise, and a slight voltage deviation, making it extremely difficult to detect. Normal temperature rise is a normal manifestation of a transformer's compliant operation and does not require any rectification. Once the load continues to operate, the short-circuit range will continuously expand, eventually causing complete burnout.
- Basic Understanding: Why does a transformer heat up? The core principle of normal temperature rise
To distinguish between genuine and false faults, one must first understand the source of normal transformer heating. During the operation of low-frequency transformers, there are two fixed losses that are the sole cause of normal temperature rise, and all compliant heating originates from these.
The first factor is copper loss. When current passes through the copper wires of the windings, it generates heat due to the resistance of the conductors. The greater the load current, the higher the copper loss and the more obvious the temperature rise. Under full-load conditions, copper loss is the main heat source, while under light-load conditions, copper loss significantly decreases.
The second factor is iron loss. The silicon steel sheets in the iron core generate hysteresis loss and eddy current loss under the alternating magnetic field. As long as the transformer is energized, whether it is no-load or full-load, iron loss will persist. It is the core reason for the slight heating of the transformer when it is no-load.
Standard national standard transformers have their temperature rise strictly controlled under rated conditions and in standard environments. Class B insulation transformers allow a maximum temperature rise of 80K, Class F insulation can reach 100K, and Class H insulation can reach 125K. In simple terms, in a normal temperature environment of 25℃, a qualified transformer operating under full load and in a steady state has a temperature controlled within 60℃ - 85℃, which is within the normal range. It feels warm and hot but not scorching, and the temperature stabilizes and does not continue to rise, indicating 100% compliance with normal temperature rise.
The core characteristics of normal temperature rise: gradual heating, an upper limit to temperature rise, stable steady-state temperature, stable parameters, and no additional abnormalities. In simple terms, once the temperature rises to a certain value, it does not continue to increase. There are no abnormalities in voltage, current, or noise, which is normal working heat and does not require any rectification.
The core characteristics of normal temperature rise: gradual heating, an upper limit to temperature rise, stable steady-state temperature, stable parameters, and no additional abnormalities. In simple terms, once the temperature rises to a certain value, it does not continue to increase. There are no abnormalities in voltage, current, or noise, which is normal working heat and does not require any rectification.
- Fatal Hazard: The Fault Principle and Hazards of Short Circuit in Transformer Windings
Short circuit in transformer windings refers to the damage of the insulation layer between adjacent enamel wires inside the winding, and the peeling of the paint film, causing multiple coils of copper wire to directly conduct and short circuit. This is pathological heating that is different from normal temperature rise. Unlike overheating caused by external overload or excessive voltage, short circuit in windings is a hidden internal fault of the transformer, with complex causes, concealed development, and irreversible deterioration.
Each coil of the normal winding bears the prescribed electromagnetic conversion function. The number of turns and the wire diameter are matched to the power requirements. Once a phase-to-phase short circuit occurs, the short-circuited coil will form a closed loop current, no longer participating in energy conversion, and will only continuously generate huge short-circuit copper losses. The more short-circuited turns, the larger the loop current, and the heating power increases exponentially. At the same time, a reduction in the effective working turns will cause the transformer to saturate prematurely, the no-load current to surge, and further exacerbate the overall temperature rise, forming a vicious cycle of "increased heating leads to insulation aging, and insulation aging aggravates short circuits".
In the early stage of a minor phase-to-phase short circuit, the transformer can still operate barely for a while, with a small voltage deviation and no obvious fault alerts, only showing slight abnormal heating and increased buzzing noise, which is easily overlooked. As the operating time increases, the short-circuit area continues to expand, the temperature rise sharply increases, and eventually leads to complete carbonization of the insulation, burning of the winding, and even fire of the coil, and even burning of the rear-end equipment and short circuit of the lines, causing serious safety accidents.
In the early stage of a minor phase-to-phase short circuit, the transformer can still operate barely for a while, with a small voltage deviation and no obvious fault alerts, only showing slight abnormal heating and increased buzzing noise, which is easily overlooked. As the operating time increases, the short-circuit area continues to expand, the temperature rise sharply increases, and eventually leads to complete carbonization of the insulation, burning of the winding, and even fire of the coil, and even burning of the rear-end equipment and short circuit of the lines, causing serious safety accidents.
- Core Distinction: Normal Temperature Rise VS Short Circuit Heating (Key Discrimination Criteria)
Most technicians are unable to distinguish the core cause of the fault. They only look at "whether it is hot" and do not consider "how it is hot, the trend of heating, and accompanying symptoms". Below, from the four dimensions of temperature rise trend, working conditions, electrical parameters, and noise and vibration, clear and implementable discrimination criteria are clarified to accurately distinguish the two types of heating.
1. Temperature Rise Trend Difference: Constant Temperature Stability vs Continuous Rise
Normal Temperature Rise: The temperature rises slowly during power-on, generally reaching a stable state within 15-30 minutes, and then the temperature remains permanently constant and does not rise further. The no-load temperature is low， and the full-load temperature is slightly higher, but it remains stable, with no significant changes in temperature during day and night operation, and the hand-held temperature is controllable, without the situation of getting hotter as time goes on.
Short-circuit faults between windings cause rapid heating: The temperature rises extremely quickly. After being powered on for 5-10 minutes, it becomes noticeably hot, and the temperature keeps climbing without a stable upper limit. Even in an unloaded state, the temperature keeps rising. The operating time is longer, and the temperature is higher. After long-term operation, there will be a situation where the body is hot, the casing is hot, and the temperature difference at local hotspots is extremely large. This is the most core and intuitive characteristic for judgment.
2. Performance under operating conditions: Load matching vs. Decline in load-bearing capacity
Normal temperature rise: Under a strictly matched load condition, the temperature is low at light load and rises at full load, and it will overheat only when overloaded. After removing the overloaded load, the temperature quickly drops. The output voltage is stable, the load-bearing capacity is normal, and there is no power attenuation problem.
Short-circuit faults between windings cause rapid heating: The load-bearing capacity significantly decreases. Even at light load or half-load, the temperature is abnormally high. Under the same load conditions, the temperature is much higher than that of a brand-new qualified transformer. The output voltage is slightly lower, and the load voltage drop increases. The equipment lacks power and operates abnormally. After disengaging the load, the temperature drops slowly, and the fault cannot be restored on its own.
3. Difference between noise and vibration: Smooth low noise vs. Abnormal noise and vibration
For transformers with normal temperature rise, the operating noise is uniform and weak. There is no obvious vibration on the body. The noise difference between no-load and full-load is small. There are no noises or howling sounds.
For transformer faults caused by inter-turn short circuits, due to the disorder of internal circulation and imbalance of magnetic flux, problems such as increased noise, noisy sound quality, and intensified vibration of the body will occur. The abnormal noise is slight when the transformer is unloaded, but it significantly intensifies when it is loaded. Some may even produce subtle whistling noises, which are significantly different from the stable noise of normal operation.
4. Appearance and Odor Differences: No Abnormality vs Aging and Foul Odor
The normal temperature rise is only physical heat generation. The body does not change color, has no odor, and the paint film does not age. The appearance remains intact after long-term operation.
Inter-turn short circuits can cause local high-temperature burning of the insulation layer. After running for a period of time, a slight plastic burning smell can be detected. The coil frame, insulation paper, and varnish layer will show slight yellowing and blackening carbonization marks. In severe cases, local bulging and paint cracking will occur, which are irreversible fault characteristics.
The normal temperature rise is only physical heat generation. The body does not change color, has no odor, and the paint film does not age. The appearance remains intact after long-term operation.
Inter-turn short circuits can cause local high-temperature burning of the insulation layer. After running for a period of time, a slight plastic burning smell can be detected. The coil frame, insulation paper, and varnish layer will show slight yellowing and blackening carbonization marks. In severe cases, local bulging and paint cracking will occur, which are irreversible fault characteristics.
- Precise Measurement and Inspection: Three Steps to Completely Identify Inter-Turn Short Circuit Faults
For minor inter-turn short circuits with unclear symptoms, it is impossible to determine with 100% certainty solely by visual inspection and touch. Three measurement methods can be used for precise detection to avoid misjudgment.
The first step: DC resistance comparison test
Use a multimeter to measure the DC resistance of the primary and secondary windings and compare it with the parameters of the same specification brand-new standard transformer. Inter-turn short circuits will cause the effective number of turns of the winding to decrease and the resistance to be significantly smaller. A deviation of more than 5%-10% can determine the existence of a short circuit fault. At the same time, the difference in resistance between the two phases of the winding can be compared. If the difference is excessive, it indicates an internal short circuit hidden danger.
The second step: No-load current and no-load temperature rise test
Connect the rated voltage for 30 minutes and power on the normal transformer. The no-load current is extremely small and the temperature rise is weak. For transformers with inter-turn short circuits, the no-load current is abnormally large, and the no-load temperature rises rapidly, which is the key to distinguishing overload heating from internal short circuits. Overload heating only causes high temperature when loaded, while the no-load temperature is normal. Inter-turn short circuits cause abnormal heating both during no-load and loaded conditions.
The third step: Withstand voltage and insulation test
Use a withstand voltage tester and multimeter to test the insulation resistance between the windings and to ground, and between windings. Inter-turn short circuit faults will be accompanied by insulation layer damage. The insulation resistance value will be significantly lower. The withstand voltage test is prone to problems such as creepage, breakdown, and excessive leakage current, which can accurately identify the hidden faults.
- High-Occurrence Causes and Prevention Schemes of Transformer Inter-Turn Short Circuit Faults
To prevent inter-turn short circuit heating faults, it is necessary to clearly identify the causes of the faults and avoid potential hazards at the source. Common causes can be divided into four categories:
First, production process defects, such as rough winding arrangement, paint film wear, incomplete varnish application, and loose coil friction causing insulation damage;
Second, long-term overload, voltage surges, and instantaneous impact currents, which can break the insulation layer;
Third, operation in a high-temperature and humid environment, where insulation materials age and leak electricity;
Fourth, transformer under-labeling and insufficient wire diameter, which accelerate insulation failure under long-term overcurrent operation.
Corresponding preventive measures: During the customization of procurement, strictly control the sufficient amount of pure copper, conduct vacuum impregnation, and adopt a regular winding process to completely eliminate inferior and deteriorated products; when selecting models, reserve sufficient power margin to avoid long-term full-load and overload operation; add surge protection and voltage stabilizing modules at the input end of the equipment to resist voltage fluctuations; for humid and high-temperature working conditions, use high-grade insulation transformers and conduct regular insulation status inspections; during the commissioning of new machines, strictly prohibit overpressure and overload tests to protect the integrity of the winding insulation layer.
In conclusion, a transformer getting hot does not necessarily indicate a fault. The steady-state warmth is a normal physical loss. Persistent overheating, getting hotter with use, accompanied by abnormal sounds and voltage drop, are the dangerous signals of inter-turn short circuit. Normal temperature rise is controllable at a constant level, with stable parameters and no additional abnormalities. In contrast, inter-turn short circuit causes a continuous increase in heat, abnormal no-load conditions, load-related decline, accompanied by vibration and burnt smell, which are fundamentally different from each other.
In actual operation, maintenance, and procurement processes, it is essential to abandon the subjective judgment that "anything hot is faulty". Instead, a multi-dimensional comprehensive assessment based on temperature rise trends, operational performance, and electrical measurements should be conducted. Precisely distinguish between normal temperature rise and inter-turn short circuit faults. This not only avoids unnecessary rework and replacement, as well as cost waste, but also enables the early detection of latent short circuit hazards, preventing transformer burnout, equipment failures, and safety accidents, thereby ensuring the long-term stable operation of the electrical system.
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