Understanding Transformer Losses

A transformer is a static electrical machine that transfers electrical energy from one circuit to another through electromagnetic induction, usually changing the voltage level. Like all electrical devices, transformers are not perfectly efficient and experience energy losses.

These losses are primarily converted into heat, which reduces the overall efficiency of the transformer.

Main Types of Transformer Losses

Transformer losses can be broadly categorized into two main types:

• Copper Loss (or I²R Loss)
• Core Loss (or Iron Loss)

Copper Loss (I²R Loss)

Copper loss occurs in the windings of the transformer (both primary and secondary). It is due to the resistance of the copper wire used in the windings. This loss is proportional to the square of the current flowing through the windings and the resistance of the windings. Mathematically, Copper Loss \(\propto I^2 R\).

Since the current flowing in the windings depends directly on the load connected to the transformer, copper loss varies with the load. It is minimum (or theoretically zero, ignoring exciting current) at no load and maximum at full load.

Core Loss (Iron Loss)

Core loss occurs in the iron core of the transformer. It is caused by the alternating magnetic flux in the core. Core loss consists of two components:

1. Hysteresis Loss: This loss is due to the repeated magnetization and demagnetization of the core material as the alternating flux passes through it. It depends on the volume and grade of the core material, the maximum flux density, and the frequency of the supply voltage.
2. Eddy Current Loss: This loss is caused by induced circulating currents (eddy currents) within the core material itself. These currents are induced because the core is a conductor and is subjected to a changing magnetic flux. Eddy current loss is minimized by using thin laminations for the core, insulated from each other. This loss depends on the thickness of the laminations, the resistivity of the core material, the maximum flux density, and the frequency of the supply voltage.

Core loss depends primarily on the voltage applied to the primary winding and the frequency of the supply, both of which are typically constant under normal operation regardless of the load connected to the secondary side. Therefore, core loss is considered to be nearly constant from no load to full load.

Transformer Operation at No Load

When a transformer is not connected to any load, the secondary winding is open-circuited. In this condition, the primary winding draws a small current from the supply voltage. This small current is primarily the exciting current, which consists of two components:

• A small active component (responsible for core loss).
• A larger reactive component (responsible for producing the magnetic flux).

The magnitude of this no-load current is typically very small (about 2-5% of the full-load current).

Losses Observed at No Load

Let's analyze the losses at no load:

• Copper Loss: Since the current flowing in the windings at no load is very small, the copper loss (\(I^2 R\)) in both the primary and secondary windings is negligible. The secondary current is zero, so there is no copper loss in the secondary. The primary current is small, so the copper loss in the primary is minimal.
• Core Loss: The core is energized by the primary voltage, which creates the alternating magnetic flux necessary for transformer action. As explained earlier, core loss depends on the voltage and frequency, which are present even at no load. Since the voltage and frequency are constant, the core loss is present and is essentially the same as it would be under loaded conditions (it's a fixed loss).

Therefore, when a transformer is not connected to any load, the dominant loss observed is the core loss.

Other Mentioned Terms

The terms "Thermal Loss" and "Voltage Loss" are not standard classifications of transformer power losses in this context.

• Thermal Loss: This refers to heat dissipation, which is a consequence of the power losses (copper and core losses) converting electrical energy into thermal energy. It's the effect of the losses, not a separate type of primary energy loss.
• Voltage Loss: This typically refers to the voltage drop across the transformer's internal impedance under load, which affects the output voltage regulation. It's a voltage phenomenon, not a power loss that reduces efficiency in the way copper or core losses do.

Summary of Transformer Losses

Based on this analysis, the loss that can be observed in a transformer when it is NOT connected to any load is Core Loss.

Revision Table: Key Transformer Loss Concepts

Additional Information: No-Load Test for Core Loss

The no-load test is a standard procedure performed on a transformer to determine its core losses and magnetizing characteristics. In this test, the high-voltage (HV) side is left open-circuited, and the low-voltage (LV) side is connected to a rated voltage supply at the rated frequency. A wattmeter, voltmeter, and ammeter are connected on the LV side.

Since the secondary is open, the primary draws only the no-load current, which is small. The copper loss associated with this small current is negligible compared to the core loss. Therefore, the reading of the wattmeter in the no-load test primarily represents the total core losses (Hysteresis loss + Eddy current loss) of the transformer at the rated voltage and frequency.