1. According to the actual needs of electricity consumption, the total apparent power PS of the transformer is obtained, and when the secondary side is multi-winding, the total apparent power is the sum of the apparent powers of the secondary windings:

PS=U2 I2+U3I3+...+UnIn

In the formula U2 U3...Un——the effective value of the winding voltage of the secondary side (V);

      I2 I3...In——the effective value (V) of each winding of the secondary side;

    2. Input the apparent power PS1 and the input current I1. When the transformer is loaded, due to the winding resistance heating loss and core loss, some of the input power is lost. Therefore, the relationship between the input power and the output power of the transformer is:

PS1=PS/η(W)

Where η - the efficiency of the transformer. η is always less than 1, and for a transformer having a power of 1 KW or less: η = 0.8 to 0.9.

After knowing the transformer input apparent power PS1, you can find the input current I1.

I1=PS1/U1×(1.1～1.2)(A)

In the formula, U1——the effective value (V) of the primary side is generally the applied power supply voltage;

      1.1 ~ 1.2 - Consider the empirical coefficient of the no-load excitation current of the transformer.

     3. Determine the core cross-sectional area S. The size of the E-type iron core commonly used in small single-phase transformers is shown in Figure 1.

Its mid-column cross-sectional area S is related to the total apparent power of the transformer, namely:

S=K0√PS(cm 2)

Where PS - total output power of the transformer (W)

Three-phase transformer capacity = root number 3 * rated voltage * rated current

Single-phase transformer capacity = rated voltage * rated current

 

example:

The capacity of the transformer is 1.6KVA

Three-phase 380V to three-phase 220V

Also on the nameplate: Y

            Primary current 2.4A

            Secondary current 4.2A

Q: How is 6KVA calculated?

According to the formula S=root 3*UI

Where S is the apparent power, unit KVA

U rated voltage in KV

I rated current, unit A

Three-phase 380V to three-phase 220V

Also on the nameplate: Y

Primary current 2.4A

Secondary current 4.2A

Use the primary side or the secondary side to do:

S=root 3*0.38*2.4=root 3*0.22*4.2=1.6KVA