I read in sci.electronics.design that Danny <

[email protected]
irgin.net> wrote (in said:

Hi

I was told a while ago, 'the input current to a transformer is dependent

upon the output current taken by the load', first of all, is this true??

Yes, but a bit misleading. It is *dependent*, but the primary current

isn't zero when the secondary current is zero. This is called an

'affine' relationship, not 'proportional'.

If it is true, then what is the realtionship between the two? because

surely, when there is no load there is definitely a current through the

primary coil, that is what was puzzeling me.

Some current is needed in the primary coil to produce the magnetic field

in the core. Surprisingly, this is called 'magnetizing current'. It

actually has two parts; one part, Ifield, which is 90 degrees lagging

the applied voltage, actually makes the magnetic field, while a smaller

part (you hope), Iloss, is in phase with the applied voltage and is due

to the eddy-current and hysteresis losses in the core.

If we call the total primary current Ip, the secondary current Is, the

turns ratio n and the magnetizing current Im, then:

Ip = Im + nIs,

where the currents are phasors (vectors in old-speak). Note that if the

secondary current is MORE than the primary current, n is LESS than 1.

If we split Im into its parts:

Im = sqrt{(Ifield)^2 + (Iloss)^2)}.

Is is not in-phase with the secondary voltage unless the load is a pure

resistance.