A transformer doesn't rotate and has no friction, so where does the lost energy go? To just two places: the iron core and the copper windings. Distinguishing between the two losses and understanding the behavior of each is key to understanding efficiency, heat, and operating bills.
Map of the Three Losses
| Loss | Location | Cause | Design Remedy |
|---|---|---|---|
| Magnetic hysteresis | Core | Re-magnetizing the core every half cycle (120 times/second at 60Hz) | A magnetic material with a narrow hysteresis loop (silicon steel) |
| Eddy currents | Core | Induced circulating currents within the iron | Thin, insulated laminations |
| Copper loss | Windings | Conductor resistance: P = I²R | Conductors with appropriate cross-section and reduced resistance |
The Fundamental Behavioral Difference
- The core loss is approximately constant: it depends on the voltage and frequency, and exists at its full value as long as the transformer is energized — even with no load. Hence it's called the "no-load loss."
- The copper loss is variable: it's proportional to the square of the current; at half load it drops to a quarter of its full value. Hence it's called the "load loss."
A transformer's catalog states: no-load loss 1.1kW and load loss 10.5kW. At half load: the core loss remains 1.1kW, while the copper loss = 10.5 × (0.5)² ≈ 2.6kW. The total is 3.7kW instead of 11.6kW at full load.
These two figures in the catalog (No-Load Loss and Load Loss) are exactly what's measured practically with the open-circuit and short-circuit tests.
Sample answer: The sources are: hysteresis loss in the core (re-magnetization every half cycle), eddy current loss in the core (addressed with insulated laminations), and copper loss in the windings (I²R). The two core losses are approximately constant because they follow the voltage and frequency and exist even with no load, while the copper loss changes with the square of the load current, dropping to a quarter at half load.
Assuming an unloaded transformer consumes nothing. The core loss is continuous around the clock as long as the transformer is energized, which is why distribution companies are concerned with reducing the no-load losses of their transformers, deployed in the thousands.
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