Electrical M/C’s and Power Utilization 3(2+1)

LESSON 8. Transformers on load

      

Fig. 4.4  Model of practical transformer

Transformer model

• physical reasoning

• mathematic model of coupled circuits

• Winding resistance in series with leakage inductance

• Magnetizing inductance in parallel with core resistance

Referred equivalent circuits

• Practical transformer is equivalent to lumped parameters circuit and ideal transformer

 

Fig. 4.5  Equivalent circuit of ideal transformer

The ideal transformer can be shifted to either side as in Figure 4.6  below and the circuit parameters reduce to the appropriate values

Fig. 4.6  Reduction of parameter in equivalent circuit

E₁=E’₂= aE₂

V’₂=aV₂

I’₂=I₂/a

X’₁₂=a2x₁₂

R’₂=a2R₂

Approximate equivalent circuits

• I1R1 and I1Xl1 are small  _{Therefore, |E1| = |V1|}

• Shunt branch can be moved to supply terminal

• IΦ small (5% of rated current) Shunt branch removed

Fig. 4.7.  Approximate equivalent parameters

Determination of equivalent circuit parameters

• No-load test (rated voltage on one side whereas the other side is open)

Fig. 4.8. Equivalent circuit parameters under no load test