What is Transformer Rating and Its Operating Characteristics in Detail

1. Rated voltage 【U1N】【U2N】.

Primary side rated voltage [U1N]: is based on the insulation strength and allowable heat specified in the normal operating voltage effective value.

Secondary rated voltage [U2N]: in the power system, when the primary side of the rated voltage, the secondary side of the no-load voltage effective value; in the instrumentation, when the primary side of the rated voltage, the secondary side of the rated load voltage effective value.

2. Rated current [I1N] [I2N]

Rated current 【I1N】【I2N】: refers to the maximum rms value of current allowed to pass through the primary and secondary windings during continuous operation.

3. Rated capacity 【SN】

Rated capacity 【SN】 = 【U2N】 × 【I2N】.

Refers to the first and second side of the rated voltage and rated current product, that is, their rated apparent power. Rated capacity reflects the transformer’s ability to transmit electric power, but the actual output power and rated capacity should not be confused.

For example:

For a transformer with rated capacity [SN] = 1000 kVA:

When the load power factor is equal to 1, the maximum active power output is 1000 kW.

When the load power factor is 0.7, the maximum active power P = 1000 × 0.7 = 700 kW.

The actual output power of the transformer depends on the size and nature of the load on the secondary side.

4. Rated frequency [fN]

Rated frequency [fN]: refers to the transformer should be connected to the power frequency. The standard frequency of our power system is 50 Hz.

Ideally, in order to analyze the working principle of the transformer and highlight the role of the main physical quantities, we ignore the primary and secondary windings in the resistance and leakage electromotive force on the actual work.

In practice, the transformer operates under load, and as the load increases, the resistance voltage drop and leakage electromotive force on the primary and secondary windings increase, resulting in a decrease in the terminal voltage [U2] of the secondary winding. When the transformer primary winding voltage [U1] and load power factor [cosψ] must be, the secondary winding voltage [U2] with the load current change curve is called the external characteristics of the transformer.

Transformer resistive and inductive external characteristics curve diagram

For resistive and inductive loads, the external characteristic curve is downward sloping, and the lower the power factor of the inductive load, the faster the voltage [U2] drops. The degree of change in the external characteristics of the transformer can be expressed by the rate of change of voltage [△U%].

Voltage change rate refers to the transformer from no-load to full load, the secondary side voltage [U2] of the amount of change and no-load secondary side voltage [U20] percentage. In general, the larger capacity transformer voltage change rate is smaller, the power transformer voltage change rate is generally about 5%, while the voltage change rate of small transformers can be up to 20%.

The voltage at household power outlets comes from the power transformer, which is why the voltage is not a constant 220V but fluctuates when measured. Voltage variation rate is an important technical indicator that affects the quality of power supply.

5. Transformer efficiency and loss

In the process of power transmission, the primary and secondary windings and the iron core will consume part of the energy, i.e., the copper loss [△PCu] on the windings and the iron loss [△PFe] in the iron core, so the output power will be slightly less than the input power.

Transformer efficiency [η

The efficiency of a transformer is defined as the ratio of the output power [P2] to the input power [P1], and is usually expressed as a percentage.

When no load, although [P2] = 0, but [△PCu] ≠ 0 and [△PFe] ≠ 0, so [η] = 0. As the load increases, the efficiency of [η] will first increase, but due to the copper loss with the current square proportional to the growth, the efficiency of the load will be reached before the maximum value and then begin to decline.

Transformer efficiency and load current [I2] relationship curve

In the rated load, small transformer efficiency of about 60% to 90%, the efficiency of large power transformers up to 96% to 99%. However, the efficiency of light load is lower, so the capacity of the transformer should be reasonably selected to avoid long-term light load or no-load operation.