Introduction to Electrical Transformers Part 1

This video is Part 1 in the Introduction to Transformers training series, which covers the basic types of transformers and some of the theoretical aspects of working with transformers, including some laws of energy and inductive reactance.

A transformer is a device that transfers electrical energy from one circuit to another through induction. In many cases, the transformer will take a higher voltage and step it down to a lower voltage, though some transformers step up a lower voltage to a higher voltage or don't change the voltage at all (these are isolation transformers). Two sets of wire, which we call "windings," wrap around an iron core; one of these is the primary, where power comes in, and the other is the secondary, where power leaves. These windings don't directly touch.

Transformers may come in single-phase or three-phase varieties. Single-phase transformers have one set of primary and secondary windings, and three-phase transformers have three sets of primary and secondary windings. It is possible to arrange three single-phase transformers to make a three-phase "bank," and three-phase transformers may come in a few different configurations, including delta-delta and delta-wye. Delta-delta configurations are common in industrial facilities that don't have much need for line-to-neutral (L-N) 120v loads; transformers in these configurations can still operate if they lose one leg of power, though at a reduced capacity. Delta-wye configurations are more common in commercial facilities with L-N 120v loads, as they support single-phase L-N 120v loads with a grounded neutral. If you attempt to take a 120v center tap on a delta-delta configuration, you will get a "high" or "wild" leg; you will not get a high leg if you derive the neutral from the center of a delta-wye configuration.

Buck-boost transformers are completely different from other types; they connect the primary and secondary coils to decrease (buck) or increase (boost) the voltage directly, such as to correct a voltage drop.

Lenz's and Faraday's laws help describe the way transformers behave. Lenz's law states that the magnetic field created by an induced current opposes the initial changing magnetic field, kind of like if there is a person pushing against a door you're trying to open. Faraday's law asserts that an electromotive force (EMF) is generated in a conductor within a changing magnetic field, and current will flow through that conductor if it is in a closed-loop circuit; the magnitude of the EMF is proportional to the rate at which the magnetic flux changes (linked with the coil). We can see Faraday's law in action if we move a wire back and forth near a magnet; the force will be stronger if we have more coils of wire and faster movement.

Invisible lines of force emanate from a magnet, and flux refers to the strength of that magnetic field in terms of magnitude and area. In an alternating current, the voltage reaches its peak, falls to zero, reverses direction, and repeats that cycle 60 times per second, which generates a changing magnetic field. The invisible lines of force generated by this cycle cut through conductors and excite the electrons inside, inducing the voltage. There isn't a direct connection; voltage on the secondary is induced by the iron core's magnetic field, which is generated when voltage on the primary magnetizes the coil. Counter-EMF induced on the iron core creates magnetic resistance, which we call inductive reactance. The currents on the primary and secondary remain proportional based on the primary EMF, counter EMF produced on the iron core, and counter EMF produced on the secondary winding.

Alternating current (AC) voltage, which transformers use, is derived from a generator. There are permanent magnets on a rotor, which spins inside a stator. These magnets have lines of flux, which pass through the stator's windings. A voltage will be induced, and its magnitude will rise and fall. Sine waves measure the rise and fall of that magnitude (y-axis) over time (x-axis) during that rotation.

We can figure out a lot about a transformer if we know the turns ratio, which is the ratio of wire wraps on the primary to the secondary. The turns ratio is the same as the voltage ratio and determines the factor by which voltage is stepped up or down from the primary to the secondary winding. The current ratio is the ratio of current on the primary to the secondary and is the inverse of the voltage ratio; the load on the secondary dictates the current flow in the circuit.

VA and kVA ratings tell us how many watts the transformer is capable of delivering, including how many amps it can handle at a given line voltage. Three-phase transformers have a phase angle, and it will produce a different current calculation than a single-phase transformer. A single-phase leg of power can be split by a neutral and may have two sides (L1 and L2), but it still has one set of windings and is still single-phase, not two-phase.