Question about Transformers

Thread Starter


Joined Dec 25, 2004
I have a few questions regarding transformers. Firstly, I really like the site's description as to how transformers work. I'm a checmial engineer that taught myself electronics and have only encountered texts(Horowitz Hill, Halliday Resnick Physics, and a Technicians Electric Circuits book) that mention basic transformer calculations but have never got a good description how they really work IE: magnetomotive force. It seems one can go far in an EE education knowing only the inductor(emv=di/dt) and primary secondary transformer relations (V, I, R) as a function of turns ration w/o knowing anything detailed about the magnetic phenomena

Most of the text that I've seen basically makes it look that when there is no current in the secondary - there's no current in the primary. This can't be true - an unloaded secondary will mean that the primary still looks like an inductor w/ reactive current. I would like to go beyond this simplification.

1. Can you recommend additional web text that describes tranformer action? Or can you recommend a good book that gives a good introduction to transformer action that goes beyond simple turns scaling? I basically would like to model ideal transformer action to fully understand counter mmf setup by a loaded secondary and it's impact on primary voltage/current/mmf and magnetic circuit.

2. I'm trying to visualize what happens when a resistor is placed across the secondary. Starting with the MMF created in the primary & unloaded secondary - when the secondary is resistively loaded, a load MMF is created in the second which means an additional anti-load MMF is created in the primary - altering the V-I in the primary? In other words, there are two mmfs in the primary and one in the secondary such that for the magnetic circuit: (1/2 coil location of mmf)

mmf1(due to voltage source) + mmf2(due to load) - mmf1(due to load) = mmf1(due to voltage source)

3. Suppose you had a piece of metal that's not magnetized. How to do an energy balance taking into account work required to permanently magnetize the metal.

Any text/web suggestions as to where I can get additional info - I would appreciate.




Joined Jan 18, 2005
Inductive current lags the applied voltage by 90-degrees. This lagging current is due to the counter emf that opposes the initial applied voltage.

"This lagging current is due to the counter emf that opposes the initial applied voltage"

That statement defines inductive reactance.

As far as current in the secondary/primary;

If you take a power transformer for instance and connect 120-VAC to the primary but leave the secondary open/unconnected, no current flows in the secondary. I think the text is basically stating that in the conventional sense, no current is flowing in the primary (with primary connected to source voltage) when no current is flowing in the secondary (secondary open/unconnected).

AAAHHH! :lol: but there is indeed a current flowing in the primary in the above scenario because the source connected accross the primary is a complete circuit and current does flow. however, this current in the primary is usually very small (on the order of 5-50-mA or so) and is due primarily to the inductive reactance of the primary coil.

Is this even remotely close to what your tying to figure out? :unsure:
current gives off magnetic field. that's a basic idea. If you warp a wire to a metal and apply current to the wire, there will be a uniform magnetic flex that passes through the metal on one side and wrip back to another side(left hand rule), that is how electromagnet works. Now in transform, this is exactly what happen in the primary coil. where the current will create magnetic flex to the secondary coil. But that doesn't completely answer the question yet. because if you just put a manget into a wire that is warp around you will not get any current out. However, if there is a change of flex, then there will be a change of current through the wire. Therefore, if you put the primary and the secondary coil together and you have a change of current in the primary coil, the secondary coil will "feel" the change of flex that caused by the primary coil and that will make the secondary coil to output current. this explain why only ac works and not dc.

I hope this is what u asking hehe..