Let's Talk Transformers

We use transformers for all types of jobs. There are step-up transformers, step-down transformers, potential transformers, current transformers, auto transformers, buck-boost transformers. Each is designed for a  specific use.
Basic design involves a magnetic core with primary windings and secondary windings. The number of primary windings to secondary windings (winding ratio) determines the voltage ratio. A 2-to-1 ratio would give us 120V and 240V, 240V and 480V, etc. Transformers have inherent losses built into them. Designers/engineers try to come up with new ways to reduce these losses. One type of loss is eddy current. Ferromagnetic cores are good conductors and constitute a single short-circuited turn throughout its entire length. Eddy currents flow along the core in a plane normal to the flux (magnetic field  developed from the applied voltage and current) and create resistive heating. Heat equals energy loss. Eddy current losses are a complex function of the square of supply frequency and inverse square of metal thickness. Most designers now utilize thinner, insulated metal plates laminated together to form the core in order to reduce the effects of eddy currents. By using this design, however, there is another phenomena created. Magnetostriction is the effect of magnetic flux in a ferromagnetic core that causes the expansion and contraction with each magnetic cycle. This is what causes transformers to hum.
Autotransformers utilize a single winding with two end terminals and one or more terminals at intermediate tap points. Primary voltage is applied across the two end terminations and the secondary is usually connected across one end terminal and an intermediate tap. These transformers are cheaper to manufacture but are not as safe as separate primary and secondary windings. Another style utilizes exposed windings in the intermediate area and uses a brush to make the secondary connection. The brush can be moved up or down the exposed section to increase or decrease voltage output. A rheostat is an example of an autotransformer.
Leakage transformers have a significantly higher leakage inductance. This loose coupling between primary and secondary provides an inherent current limiting effect. This makes it possible to create a short on the secondary side and not cause damage to the transformer. Doorbell transformers are an example of this design.
Current transformers and potential transformers, also known as instrument transformers, are used for measuring current and voltage in electrical power systems,  and for power system protection and control. These transformers are used where it is impractical or unsafe to use conventional meters to measure voltage or current due to the high values present. Current transformers measure current in a circuit without being electrically connected to that circuit. It consists of a core, usually circular or rectangular, with a single set of windings and two end terminations. The circuit to be measured is routed through the center of the core, and as current flows through the circuit it induces a current in the CT which sends this to either a meter or control device. A potential transformer is designed to be connected in parallel with the circuit to be monitored to provide a consistent value (proportional to circuit values) that can be measured or controlled. These transformers impart a very small load to the circuit, but provide a means to measure the voltage safely and accurately.