1. electromagnetic field
In order to understand the working principle of a transformer, it is necessary to understand the principle of electromagnetic induction: when current flows through a conductor, an electromagnetic field is generated around the conductor. Figure 1 shows the magnetic field around a live conductor. When the current disappears, the electromagnetic field also disappears. This magnetic field generates force, and when the current changes direction, the force of the magnetic field also changes direction. In Figure 1, when current flows from A to B, a counterclockwise force is generated. However, if current flows from B to A, the magnetic field will create a clockwise force around the wire. This force is applied in motors, relays and transducers.
Although the magnetic field cannot be seen, its effect can be observed. For example, if a magnetic compass is placed near a wire, the magnetic force will deflect the pointer in one direction, and if the direction of the current is reversed, the pointer will deflect in the opposite direction. If the current increases, the wire is wound into a coil, the number of turns of the coil increases, or the coil is wound on the core, the intensity of the electromagnetic field will increase. Figure 2 shows the energized coil and the surrounding magnetic field.
2. Transformer principle
The AC voltage can be stepped up or down by the transformer. The transformer contains a primary coil and a secondary coil. The primary coil is the input end of the transformer, and the secondary coil is the output end. The primary and secondary coils are not physically connected together, but are wound at close distances to establish an electromagnetic field connection from the primary side to the secondary side.
Figure 3 shows a basic transformer. The current flowing from the primary coil establishes an electromagnetic field, which couples the secondary side, so that a magnetic field connection is established between the coils. When the amplitude and polarity of the input alternating current change, the amplitude and polarity of the magnetic field will also change. Because the primary magnetic field is coupled to the secondary side, the secondary side will be driven by the input voltage.
Another important electrical theory is that when a magnetic field cuts a conductor, an induced voltage will be generated in the conductor. This happens when the wire is moving in a fixed magnetic field, or when the fixed wire is cut by a changing magnetic field. In a transformer, the effect of the primary side inducing voltage to the secondary side is called the principle of mutual inductance. Mutual inductance is a key condition for the operation of a transformer, and this phenomenon occurs when alternating current is input to the primary coil. Because of the magnetic field and the cutting action of the coil, the transformer can raise or lower the alternating current. Since the polarity and amplitude of the direct current will not change periodically, the transformer effect will not occur in a stable direct current magnetic field and coil.
Figure 4 shows the symbols used to calculate transformer voltage and current; Np = primary turns, Ns = secondary turns, Ep = primary voltage, Es = secondary voltage, Ip = primary current, Is = secondary current.
Es/Ep =Ns/Np, Es=Ep×Ns/Np
In this example, Ep=AC240V, Np=240 turns, Ns=60 turns, calculate Es.
This is a step-down transformer. The number of secondary turns of the step-down transformer is less than that of the primary side, and the number of secondary turns of the step-up transformer is more than that of the primary side.
The power transmission efficiency of the transformer from the input end to the output end is very high. In many calculations, it can be assumed that output power = input power. If you need an accurate value of power transmission, please refer to the relevant parameters of transformer efficiency.
If there is a 10Ω load on the secondary side of a transformer and the load voltage is 60V, calculate the load power and the input power of the transformer.
Load power=E² secondary side/R=(60×60)/10W=360W
Input power = output power = 360W
In Figure 5, Ep=AC120V, Np=300T, Ns=600T, RL=100Ω. Calculate Es, Is, Ip input power and output power.
(1) Es=Ep×Ns/Np=120×600/300V=AC240V (step-up transformer)
(2) Is=Es/RL=240/100A=2.4A (applying Ohm’s law)
(3) Ip=Is×Ns/Np=2.4×600/300V=4.8A (the primary current is greater than the secondary current)
(4) Input power=primary power=Ep×Ip=120×4.8W=576W
(5) Output power=secondary power=Es×Is=240×2.4W=576W
Note that the input power in this example is equal to the output power because it is assumed that the power transmission efficiency from the primary side to the secondary side is 100%. It should also be noted that the ratio of the secondary current reduction of the step-up transformer is the same as the ratio of the high secondary voltage.