Magnetic lines of flux between two aligned coils of wire when one of the coils carries fluctuating or alternating current.
When two wires are near each other and one of them carries a fluctuating current, a fluctuating current is induced in the other wire. This effect is known as electromagnetic induction. All ac transformers work according to the principle of electromagnetic induction. If the first wire carries sine-wave ac of a certain frequency, then the induced current is sine-wave ac of the same frequency in the second wire. The closer the two wires are to each other, the greater is the induced current, for a given current in the first wire. If the wires are wound into coils and placed along a common axis (above figure), the induced current will be greater than if the wires are straight and parallel. Even more coupling, or efficiency of induced-current transfer, is obtained if the two coils are wound one atop the other.
primary and secondary
The two windings, along with the core on which they are wound, constitute a transformer. The first coil is called the primary winding, and the second coil is known as the secondary winding. These are often spoken of simply as the primary and the secondary. The induced current in the secondary creates a voltage between its end terminals. In a step-down transformer, the secondary voltage is less than the primary voltage. In a step-up transformer, the secondary voltage is greater than the primary voltage. The primary voltage is abbreviated Epri, and the secondary voltage is abbreviated Esec. Unless otherwise stated, effective (rms) voltages are always specified.
The windings of a transformer have inductance, because they are coils. The required inductances of the primary and secondary depend on the frequency of operation, and also on the resistive part of the impedance in the circuit. As the frequency increases, the needed inductance decreases. At high resistive impedances, more inductance is generally needed than at low resistive impedances.
The primary voltage (Epri) and secondary voltage (Esec) in a transformer depend on the number of turns in the primary winding (Tpri) versus the number of turns in the secondary winding (Tsec).The primary-to-secondary turns ratio in a transformer is the ratio of the number of turns in the primary, Tpri, to the number of turns in the secondary, Tsec. This ratio is written Tpri:Tsec or Tpri/Tsec. In a transformer with excellent primary-to-secondary coupling, the following relationship always holds:
Epri/Esec = Tpri/Tsec
That is, the primary-to-secondary voltage ratio is always equal to the primary-to-secondary turns ratio (above figure ).
Suppose a transformer has a primary-to-secondary turns ratio of exactly 9:1. The ac voltage at the primary is 117 V rms. Is this a step-up transformer or a step-down transformer? What is the voltage across the secondary?This is a step-down transformer. Simply plug in the numbers in the preceding equation and solve for Esec, as follows:
Epri/Esec = Tpri/Tsec
117/Esec = 9.00
1/Esec = 9.00/117
Esec = 117/9.00
= 13.0 V rms
Consider a transformer with a primary-to-secondary turns ratio of exactly 1:9. The voltage at the primary is 121.4 V rms. Is this a step-up transformer or a step-down transformer? What is the voltage at the secondary?
This is a step-up transformer. Plug in numbers and solve for Esec, as follows:
121.4/Esec = 1/9.000
Esec/121.4 = 9.000
Esec = 9.000 × 121.4
= 1093 V rms
Sometimes the secondary-to-primary turns ratio is given, rather than the primary-to-secondary turns ratio. This is written Tsec/Tpri. In a step-down unit, Tsec/Tpri is less than 1. In a step-up unit, Tsec/Tpri is greater than 1. When you hear someone say that such-and-such a transformer has a certain “turns ratio,” say 10:1, be sure of which ratio is meant, Tpri/Tsec or Tsec/Tpri! If you get it wrong, you’ll have the secondary voltage wrong by a factor of the square of the turns ratio.
Schematic symbols for transformers. At A, air core. At B, laminated iron core. At C, ferrite or powdered iron core.
If a ferromagnetic substance such as laminated iron or powdered iron is placed within the pair of coils, the extent of coupling is increased far above that possible with an air core. But this improvement in coupling is obtained at a price. Some energy is invariably lost as heat in the core. Also, ferromagnetic cores limit the maximum frequency at which a transformer will work well. The schematic symbol for an air-core transformer consists of two inductor symbols back-toback (above figure A). If a laminated iron core is used, two parallel lines are added to the schematic symbol (above figure B). If the core is made of powdered iron, the two parallel lines are broken or dashed (above figure C).
In transformers for 60-Hz utility ac, and also for low audio-frequency (AF) use, sheets of an alloy called silicon steel, glued together in layers, are often employed as transformer cores. The silicon steel is sometimes called transformer iron. The reason layering is used, rather than making the core from a single mass of metal, is that the magnetic fields from the coils cause currents to flow in a solid core. These eddy currents go in circles, heating up the core and wasting energy that would otherwise be transferred from the primary to the secondary. Eddy currents are choked off by breaking up the core into layers, so that currents cannot flow very well in circles.
A rather esoteric form of loss, called hysteresis loss, occurs in all ferromagnetic transformer cores, but especially laminated iron. Hysteresis is the tendency for a core material to be sluggish in accepting a fluctuating magnetic field. Laminated cores exhibit high hysteresis loss above the AF range, and are therefore not good above a few kilohertz.
At frequencies up to several tens of megahertz, powdered iron works well for RF transformers. This material has high magnetic permeability and concentrates the flux efficiently. High permeability cores minimize the number of turns needed in the coils, and this minimizes the loss that occurs in the wires.