TRANSFORMERS
LEARNING OBJECTIVES
Upon completion of this chapter you will be able to:
State the meaning of "transformer action."
State the physical characteristics of a transformer, including the basic parts, common core materials, and main core types.
State the names given to the source and load windings of a transformer.
State the difference in construction between a high- and a low-voltage transformer.
Identify transformer symbols as to the type of transformer each symbol represents and the method used to denote transformer phasing.
State the meaning of a "no-load condition" and "exciting current" relative to a transformer.
State what causes voltage to be developed across the secondary of a transformer and the effect of cemf in a transformer.
State the meaning of leakage flux and its effect on the coefficient of coupling.
Identify a transformer as step up or step down and state the current ratio of a transformer when given the turns ratio.
Solve for primary voltage, secondary voltage, primary current and number of turns in the secondary given various transformer values.
State the mathematical relationship between the power in the primary and the power in the secondary of a transformer and compute efficiency of a transformer.
State the three power losses in a transformer.
State the reason a transformer should not be operated at a lower frequency than that specified for the transformer.
List five different types of transformers according to their applications.
State the standard color coding for a power transformer.
State the general safety precautions you should observe when working with transformers and other electrical components.
TRANSFORMERS
The information in this chapter is on the construction, theory, operation, and the various uses of transformers. Safety precautions to be observed by a person working with transformers are also discussed.
A TRANSFORMER is a device that transfers electrical energy from one circuit to another by electromagnetic induction (transformer action). The electrical energy is always transferred without a change in frequency, but may involve changes in magnitudes of voltage and current. Because a transformer works on the principle of electromagnetic induction, it must be used with an input source voltage that varies in amplitude. There are many types of power that fit this description; for ease of explanation and understanding, transformer action will be explained using an ac voltage as the input source.
In a preceding chapter you learned that alternating current has certain advantages over direct current. One important advantage is that when ac is used, the voltage and current levels can be increased or decreased by means of a transformer.
As you know, the amount of power used by the load of an electrical circuit is equal to the current in the load times the voltage across the load, or P = EI. If, for example, the load in an electrical circuit requires an input of 2 amperes at 10 volts (20 watts) and the source is capable of delivering only 1 ampere at 20 volts, the circuit could not normally be used with this particular source. However, if a transformer is connected between the source and the load, the voltage can be decreased (stepped down) to 10 volts and the current increased (stepped up) to 2 amperes. Notice in the above case that the power remains the same. That is, 20 volts times 1 ampere equals the same power as 10 volts times 2 amperes.
Q.1 What is meant by "transformer action?"
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Basic Operation of a Transformer
BASIC OPERATION OF A TRANSFORMER
In its most basic form a transformer consists of:
A primary coil or winding.
A secondary coil or winding.
A core that supports the coils or windings.
Refer to the transformer circuit in figure 5-1 as you read the following explanation: The primary winding is connected to a 60 hertz ac voltage source. The magnetic field (flux) builds up (expands) and collapses (contracts) about the primary winding. The expanding and contracting magnetic field around the primary winding cuts the secondary winding and induces an alternating voltage into the winding. This voltage causes alternating current to flow through the load. The voltage may be stepped up or down depending on the design of the primary and secondary windings.
Figure 5-1. – Basic transformer action.
Q.2 What are, the three basic parts of a transformer?
THE COMPONENTS OF A TRANSFORMER
Two coils of wire (called windings) are wound on some type of core material. In some cases the coils of wire are wound on a cylindrical or rectangular cardboard form. In effect, the core material is air and the transformer is called an AIR-CORE
TRANSFORMER. Transformers used at low frequencies, such as 60 hertz and 400 hertz, require a core of low-reluctance magnetic material, usually iron. This type of transformer is called an IRON-CORE TRANSFORMER. Most power transformers are of the iron-core type. The principle parts of a transformer and their functions are:
The CORE, which provides a path for the magnetic lines of flux.
The PRIMARY WINDING, which receives energy from the ac source.
The SECONDARY WINDING, which receives energy from the primary winding and delivers it to the load.
The ENCLOSURE, which protects the above components from dirt, moisture, and mechanical damage.
CORE CHARACTERISTICS
The composition of a transformer core depends on such factors as voltage, current, and frequency. Size limitations and construction costs are also factors to be considered. Commonly used core materials are air, soft iron, and steel. Each of these materials is suitable for particular applications and unsuitable for others. Generally, air-core transformers are used when the voltage source has a high frequency (above 20 kHz). Iron-core transformers are usually used when the source frequency is low (below 20 kHz). A soft-iron-core transformer is very useful where the transformer must be physically small, yet efficient. The iron-core transformer provides better power transfer than does the air-core transformer. A transformer whose core is constructed of laminated sheets of steel dissipates heat readily; thus it provides for the efficient transfer of power. The majority of transformers you will encounter in Navy equipment contain laminated-steel cores. These steel laminations (see figure 5-2) are insulated with a nonconducting material, such as varnish, and then formed into a core. It takes about 50 such laminations to make a core an inch thick. The purpose of the laminations is to reduce certain losses which will be discussed later in this chapter. An important point to remember is that the most efficient transformer core is one that offers the best path for the most lines of flux with the least loss in magnetic and electrical energy.
Figure 5-2. – Hollow-core construction.
Q.3 What are three materials commonly used as the core of a transformer?
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