Transformers

[edofbrighton]

Can anyone tell me why the ratio of I1/I2 approaches the ideal value as the load current is increased, and why the opposite is true for the voltage ratio V2/V1.

thanks,
ed
I1=primary current
I2=secondary current

v1=primary voltage
v2=secondary voltage

 

[Diver300]

There is some magnetising current needed to make the magnetic field oscillate at the mains frequency.

This current is fairly independent of the load current, and is a small fraction of the load current at large loads. However at small loads it can be a large proportion of the current. At no load, all the current taken by the primary is the magnetising current.

There are resistive losses in the windings. If you take those into account, the secondary voltage is:-

(v1 - I1*R1) * N - I2*R2
where R1 is the priamary resistance
R2 is the secondary resistance
N is the turns ratio.

As you increase the load, I2 and I1 increase, both of which reduce the secondary voltage from the ideal.

 

[edofbrighton]

Thank you, thats exactly what io wanted to know.

ed