# Primary winding and secondary relationship

### BBC - GCSE Bitesize: Transformers - Higher tier

The difference in voltage between the primary and the secondary windings is the turns ratio expresses a very different transformer relationship and output. The primary and secondary windings of a transformer are usually made of low- resistance copper wire. The resistance of a given winding is a function of the. There is no electrical connection between the primary and the secondary coils. Transformers only work if AC is supplied to the primary coil. If DC was supplied.

Hence, the efficiency is approximately The voltage, current, and power-handling capabilities of the primary and secondary windings must also be considered. The maximum voltage that can safely be applied to any winding is determined by the type and thickness of the insulation used. When a better and thicker insulation is used between the windings, a higher maximum voltage can be applied to the windings.

## Transformers

The maximum current that can be carried by a transformer winding is determined by the diameter of the wire used for the winding. If current is excessive in a winding, a higher than ordinary amount of power will be dissipated by the winding in the form of heat. This heat may be sufficiently high to cause the insulation around the wire to break down.

If this happens, the transformer may be permanently damaged. The power-handling capacity of a transformer is dependent upon its ability to dissipate heat. If the heat can safely be removed, the power-handling capacity of the transformer can be increased.

This is sometimes accomplished by immersing the transformer in oil, or by the use of cooling fins. The power-handling capacity of a transformer is measured in either the volt-ampere unit or the watt unit.

### BBC - Intermediate 2 Bitesize Physics - Electromagnetism : Revision, Page 2

Two common power generator frequencies 50 hertz and hertz have been mentioned, but the effect of varying frequency has not been discussed. If the frequency applied to a transformer is increased, the inductive reactance of the windings is increased, causing a greater ac voltage drop across the windings and a lesser voltage drop across the load. However, an increase in the frequency applied to a transformer should not damage it.

But, if the frequency applied to the transformer is decreased, the reactance of the windings is decreased and the current through the transformer winding is increased. If the decrease in frequency is enough, the resulting increase in current will damage the transformer. For this reason a transformer may be used at frequencies above its normal operating frequency, but not below that frequency.

A brief discussion of some of these applications will help you recognize the importance of the transformer in electricity and electronics.

These transformers have two or more windings wound on a laminated iron core. The number of windings and the turns per winding depend upon the voltages that the transformer is to supply. Their coefficient of coupling is 0. You can usually distinguish between the high-voltage and low-voltage windings in a power transformer by measuring the resistance.

The low-voltage winding usually carries the higher current and therefore has the larger diameter wire. This means that its resistance is less than the resistance of the high-voltage winding, which normally carries less current and therefore may be constructed of smaller diameter wire.

So far you have learned about transformers that have but one secondary winding. The typical power transformer has several secondary windings, each providing a different voltage. The schematic symbol for a typical power-supply transformer is shown in figure For any given voltage across the primary, the voltage across each of the secondary windings is determined by the number of turns in each secondary.

A winding may be center-tapped like the secondary volt winding shown in the figure. To center tap a winding means to connect a wire to the center of the coil, so that between this center tap and either terminal of the winding there appears one-half of the voltage developed across the entire winding. Most power transformers have colored leads so that it is easy to distinguish between the various windings to which they are connected.

Carefully examine the figure which also illustrates the color code for a typical power transformer. Usually, red is used to indicate the high-voltage leads, but it is possible for a manufacturer to use some other colors. There are many types of power transformers. They range in size from the huge transformers weighing several tons-used in power substations of commercial power companies-to very small ones weighing as little as a few ounces-used in electronic equipment.

Note that a single coil of wire is "tapped" to produce what is electrically a primary and secondary winding. The voltage across the secondary winding has the same relationship to the voltage across the primary that it would have if they were two distinct windings.

The movable tap in the secondary is used to select a value of output voltage, either higher or lower than E p, within the range of the transformer. Audio-frequency transformers are designed to operate at frequencies in the audio frequency spectrum generally considered to be 15 Hz to 20kHz. They consist of a primary and a secondary winding wound on a laminated iron or steel core.

Because these transformers are subjected to higher frequencies than are power transformers, special grades of steel such as silicon steel or special alloys of iron that have a very low hysteresis loss must be used for core material. These transformers usually have a greater number of turns in the secondary than in the primary; common step-up ratios being 1 to 2 or 1 to 4.

### Power Relationship between primary and secondary windings

With audio transformers the impedance of the primary and secondary windings is as important as the ratio of turns, since the transformer selected should have its impedance match the circuits to which it is connected. The windings are wound on a tube of nonmagnetic material, have a special powdered-iron core, or contain only air as the core material. In standard broadcast radio receivers, they operate in a frequency range of from kHz to kHz.

In a short-wave receiver, rf transformers are subjected to frequencies up to about 20 MHz - in radar, up to and even above MHz. With less flux surrounding the primary, the counter emf is reduced and more current is drawn from the source. The additional current in the primary generates more lines of flux, nearly reestablishing the original number of total flux lines.

The ampere-turn I X N is a measure of magnetomotive force ; it is defined as the magnetomotive force developed by one ampere of current flowing in a coil of one turn. The flux which exists in the core of a transformer surrounds both the primary and secondary windings.

Since the flux is the same for both windings, the ampere-turns in both the primary and secondary windings must be the same. By dividing both sides of the equation by IpN s, you obtain: Notice the equations show the current ratio to be the inverse of the turns ratio and the voltage ratio.

This means, a transformer having less turns in the secondary than in the primary would step down the voltage, but would step up the current. A transformer has a 6: Find the current in the secondary if the current in the primary is milliamperes.

The above example points out that although the voltage across the secondary is one-sixth the voltage across the primary, the current in the secondary is six times the current in the primary. The above equations can be looked at from another point of view. Remember, the turns ratio indicates the amount by which the transformer increases or decreases the voltage applied to the primary.