Basic knowledge of power transformers

First, I will introduce several terms commonly used in power transformers:

1. Primary Winding: Refers to the power input of the power transformer.

2. Seco ndary Winding: refers to the output of the power transformer.

3. Rated input voltage U1: refers to the voltage connected to the primary of the power transformer, that is, the operating voltage of the power transformer. For GS transformers, U1 = 230V; for BS transformers, U1 = 240V.

4. No-load current I0: refers to the current flowing through the primary when the primary of the power transformer is connected to the rated input voltage U1 and the secondary is not loaded (ie, open). I0 is related to the design of the transformer. Even if the power transformer of the same specification produced by two different manufacturers, the I0 may be different.

5. No-load voltage U0: refers to the voltage across the secondary when the transformer primary receives the rated input voltage U1 and the secondary does not have a load (ie, open circuit). U0 is related to the design of the transformer. Even the power transformer of the same specification produced by two different manufacturers may have different U0.

6. Load current I2: refers to the current flowing through the load when the transformer primary is connected to the rated input voltage U1 and the secondary is connected to the rated load.

7. Load voltage U2: refers to the voltage at both ends of the transformer when the transformer is connected to the rated input voltage U1 and the secondary is connected to the rated load.

8. The fixed output power P2: refers to the output power of the transformer at the rated input voltage U1, which indicates the amount of energy transmitted by the transformer. In general, at the same frequency, the larger the P2, the larger the size of the transformer; the same as P2, even if the output voltage U2 is different, the size of the transformer is the same, that is, the price of the transformer should be similar.

It can be seen from the formula P2=U2*I2 that if the output power P2 is constant, the higher the output voltage U2, the lower the output current I2. For example, for a transformer with output power P2=10VA, if the output voltage U2=24V, the output current I2=P2/U2=10VA/24V=0.416A; if U2=12V, the output current I2=0.833A.

Secondly, let's discuss some of the requirements for power transformers in EN61558-1 (equivalent to IEC61558-1):

1. The no-load voltage U0 has a maximum limit value corresponding to the output power P2 of the transformer as follows:

When P2≤10VA, (U0-U2)/U2≤100%;

When 10VA2≤25VA, (U0-U2)/U2≤50%;

When 25VA2≤63VA, (U0-U2)/U2≤20%;

When 63VA2≤250VA, (U0-U2)/U2≤15%;

When 250VA2≤630VA, (U0-U2)/U2≤10%;

630VA2, (U0-U2) / U2 ≤ 5%;

U2 in the above equations is the load voltage of the transformer.

2. The load voltage U2 should be within a certain range and be evaluated by the following formula:

|(U2-U2R)/U2R|≤5%;

If the transformer has a rectifier, |(U2-U2R)/U2R|≤10%

In the above formula, U2R is the rated load voltage.

For example, for a transformer with an output of 24V and 12VA, connect the primary of the transformer to the rated input voltage, connect the secondary to the load, and adjust the resistance of the load so that the current flowing through the load is 12VA/24V=0.5A. When measuring the voltage across the load, it is U2. In this example, U2R=24V, if 23.8V≤U2≤25.2V, the output parameters of this transformer are qualified.

3. Transformer temperature rise ΔT

When the ambient temperature is 25 °C, the primary input of the transformer is connected to the rated input voltage, and the secondary is connected to the rated load to make the transformer work normally. Then increase the input voltage to 1.06 times the rated input voltage. After the transformer reaches the thermal stability state, measure the temperature rise ΔT of the transformer and the case temperature.

For transformers with a heat rating of Class B (130 ° C), ΔT ≤ 95 K and the case temperature is not more than 80 ° C.

Note: Thermal stability means that the temperature does not change by more than 1 °C within 1 hour.

4. Dielectric strength

The primary and secondary of the transformer, between the primary and the housing should withstand an AC voltage of 4200 V for 1 min. without breakdown or flashover.