BASIC OPERATING PRINCIPLES
In Figure 19–13, one winding of the transformer has been connected to an alternating current sup- ply, and the other winding has been connected to a load. As current increases from zero to its peak positive point, a magnetic field expands outward around the coil. When the current decreases from its peak positive point toward zero, the magnetic field collapses. When the current increases toward its negative peak, the magnetic field again expands, but with an opposite polarity of that previously. The field again collapses when the current decreases from its negative peak toward zero. This continually expanding and collapsing magnetic field cuts the windings of the primary and induces a voltage into it. This induced voltage opposes the applied voltage and limits the current flow of the primary. When a coil induces a voltage into itself, it is known as self-induction. It is this induced voltage, inductive reactance, that limits the flow of current in the primary winding. If the resistance of the primary winding is measured with an ohmmeter, it will indicate only the resistance of the wire used to construct the winding and will not give an indication of the actual current limiting effect of the winding. Most transformers with a large kVA rating will appear to be almost a short circuit when measured with an ohmmeter. When connected to power, however, the actual no load current is generally relatively small.
EXCITATION CURRENT
There will always be some amount of current flow in the primary of a transformer even if there is no load connected to the secondary. This is called the excitation current of the transformer. The excitation current is the amount of current required to magnetize the core of the transformer. The excitation cur- rent remains constant from no load to full load. As a general rule, the excitation current is such a small part of the full load current, it is often omitted when making calculations.