Transformer discussion

[damo]

Hi Guys,

My apologies if this is in the wrong place or if it has been discussed before.

Being a simple techie whom has not practiced commercially for a number of years and only having done 1st year engineering (needed the $$$) these things are quite the enigma to me and I would like to have a better understanding of them. I -think- I understand how they work, but not exactly. I'm a little bit sketchy with the details of how they work on inductive and reactive loads. So here goes... let me know if I got anything wrong/omitted anything. Oh, feel free to include magnetic circuits (which I am somewhat familiar with).

A transformer consists of two coils, a primary and a secondary, wound around a core of low reluctance. For the sake of this discussion, I will assume ideal conditions (0 resistance of coils) and core is a toroid.

When we connect the primary up to an AC supply, current flows through the primary which induces a magnetic field around the core (Faradays law?). This magnetic field will in turn induce a voltage in the secondary (mutual inductance?) as well as in the primary (self inductance & Lenz law?). The primaries self inductance will keep the current 90 degrees lagging out of phase with the voltage and the voltage in the secondary will be in phase with the primary voltage because the time of greatest flux change (when it crosses 0) is the same as the time of greatest current change, which is when the voltage in the primary is at its highest.

Now comes the tricky part. When we connect the secondary to a resistive load, current will flow through the secondary into the load, which will be in phase with the secondary voltage. This will in turn induce a magnetic field in the core (Faradays law?) which opposes the primary induced magnetic field. Now this flux will be 90 degrees leading the flux induced by the primary which, somehow (explanation please), has the effect of phase shifting the current of the primary more towards that appearing resistive (i.e. more towards leading). This makes the primary consume more real power and energy.

Questions:
Is the above description correct? What am I missing here?

How does the power factor of the primary affect the phase of the secondary voltage? I.e. as we increase the load, does the phase of the secondary voltage change? If it does, this would mean the phase of the secondary current will also change, thus changing the PF of the primary (the tangled web begins).

What happens when we connect an inductive load to the secondary?

What happens when we connect a reactive load to the secondary (I would assume it would approach/surpass resonance with the secondary, but what about the primary)?

Given a variable resistive load on the secondary, what kind of PF correction would be required (if any)? Would we use average load values PF for correction purposes or just correct for PF 0?

Thanks =)

[amspire]

1. Your description is correct. You explained the situation very well.

2. Don't start making it too complicated. The secondary output voltage is ALWAYS  in phase with the primary. OK in a real transformer, you don't get perfection, but you can usually just assume a close enough perfect phase match. The load does not really care about things like Power Factor - it only sees the transformed primary voltage.

3. Just think of the current being the sum of the transformer self inductance current, and the transformed current. The transformed current is exactly in phase with whatever current is flowing to the  load.  So the current into the output inductor will be in added to the transformers own self inductance current. This means the maximum inductive current out from the transformer has to be less then the resistive output current by the value of the self-inductance current (you have to still keep the core from saturating).

4. It doesn't matter what load is on the transformer. Whatever current flows in the secondary is instantly transformed into the primary. It does not matter what the phase.

5. No PF correction is needed for a resistive load. The self inductance current of a transformer is much lower then the maximum current into an output resistor, and so essentially the transformer does look resistive, and no power factor correction is needed. The exception is small transformers such as a 5W mains transformer. To save money, they are often designed with a relatively large self inductance current, but at that power level, no one cares.

Richard

[damo]

TY Richard. So it is a direct additive effect from the secondary to the primary caused by the action of the secondary flux =)

[amspire]

TY Richard. So it is a direct additive effect from the secondary to the primary caused by the action of the secondary flux =)

Yes