Back to Basics: Transformers

The Communicator Revisited - October 2018

From the Canadian Basic Question Bank

Back To Basics is a regular column in The Communicator Newsletter. Past issues are available at The Communicator Digital Edition: Amateur Radio Newsletter (ve7sar.blogspot.com)

B-005-11-1 If no load is attached to the secondary winding of a transformer, what is current in the primary winding called?

A.    Magnetizing current

B.    Direct current

C.    Excitation current

D.    Stabilizing current

A transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. A varying current in one coil of the transformer produces a varying magnetic field, which in turn induces a varying electromotive force (emf) or "voltage" in a second coil. Power can be transferred between the two coils, without a metallic connection between the two circuits. Faraday's law of induction discovered in 1831 described this effect (See story Page 4). Transformers are used to increase or decrease the alternating voltages (AC) in electric power applications.

An ideal transformer is theoretical… lossless and perfectly coupled. There exists no lossless transformer though. Transformer energy losses are dominated by winding and core losses.  Magnetic permeability of the core results in the most loss, often felt as heat.

One of the main reasons that we use alternating AC voltages and currents in our homes and workplace’s is that AC supplies can be easily generated at a convenient voltage, transformed (hence the name transformer) into much higher voltages and then distributed around the country using a national grid of pylons and cables over very long distances.

A varying current in the transformer's primary winding creates a varying magnetic flux in the transformer core and a varying magnetic field impinging on the secondary winding. This varying magnetic field at the secondary winding induces a varying EMF or voltage in the secondary winding due to electromagnetic induction. The primary and secondary windings are wrapped around a core of high magnetic permeability so that all of the magnetic flux passes through both the primary and secondary windings. With an AC voltage source connected to the primary winding and load connected to the secondary winding, the transformer currents flow in the direction indicated in the diagram below.

According to Faraday's law, since the same magnetic flux passes through both the primary and secondary windings in an ideal transformer, a voltage is induced in each winding proportional to its number of windings. This is determined by the equation:

The ratio of the transformers primary and secondary windings with respect to each other produces either a step-up voltage transformer or a step-down voltage transformer with the ratio between the number of primary turns to the number of secondary turns being called the “turns ratio” or “transformer ratio”. The transformer winding voltage ratio is thus shown to be directly proportional to the winding turns.

When connected to a source of AC power, current flows through the primary winding of a power transformer even when no loads are connected to the secondary winding. The primary winding remains an inductor and lets some AC current through despite its reactance. This minimal current is called "Magnetizing Current" Also known as the “Exciting Current”. This current establishes the magnetic field in the core and furnishes energy for the no-load power losses in the core. 

Therefore, the answer to our question is: 

A. Magnetizing Current.

~ 73, John VE7TI