Transformers are essential parts of most power systems. Their role is to convert electrical energy at one voltage to some other voltage. Transformers are built in an astonishing range of sizes from the tiny units used in communication systems to monsters used in high-voltage transmission systems, weighing hundreds of tons. A circuit model and type analysis of transformers are necessary for understanding many electronic and control systems and almost all power systems.
Types of electrical transformers
Transformers are categorized based on their structure, voltage, winding arrangement, usage, instrumentation, and the number of phases.
Types of transformers based on structure
Structurally, the transformers are categorized into two main types: air-core and iron-core transformers.
The air-core transformers are usually used in high-frequency circuits, such as in instrumentation, radio, and TV circuits. An air-core transformer does not have a physical core, so it can be obtained by placing the two coils L1 and L2 close to each other, or by winding both the coils L1 and L2 on a hollow cylindrical-shaped core with isolating material as illustrated below figure. The circuit of an air-core transformer is shown in the figure, R1 and R2 represent the primary and secondary winding resistors of the transformer.
The air-core transformer is also known as a linear transformer. When the core of the transformer is made by the insulating material with constant permeability, such as air, plastic, wood, etc., it is a liner transformer. The air-core transformer is of interest mainly in radio devices and in certain types of measuring and testing instruments.
Iron-core transformers are usually used in audio circuits and power systems. The coils of the iron-core transformer are wound on the ferromagnetic material that are laminated sheets insulated to each other, as illustrated in the below figure.
When two coils are wound on a common core, they will have higher cross-linking flux and lower leakage flux. The ferromagnetic materials can provide an easy path for the magnetic flux. Furthermore, if two coils are wound on a common core, the flux generated in the coil L1 will almost all link with the coil L2. This means that the coupling coefficient k is close to 1, and this is the reason that an iron-core transformer is usually considered the ideal transformer (k = 1).
Types of transformers based on voltage level
Based on voltage level, the transformers are categorized into three main types: the step-up, step-down, and one-to-one transformers.
A step-up transformer is a transformer that can increase its secondary voltage. Since a step-up transformer always has more secondary winding turns than the primary, the secondary voltage of a step-up transformer (Vs) is always higher than the primary voltage (Vp), i.e., Vs>Vp. The value of the secondary voltage depends on the turns ratio (n). The equation of n = Ns/Np = Vs/Vp indicates that to have a higher secondary voltage, the number of turns on the secondary winding must be greater than that of the primary, i.e., Ns>Np as illustrated in the below figure, meaning the turns ratio n = (NS/NP) >1. This is an important characteristic of a step-up transformer.
A step-down transformer is a transformer that can decrease its secondary voltage. Since a step-down transformer always has fewer turns on the secondary winding than the primary, the secondary voltage of a step-down transformer (Vs) is always lower than the primary voltage (Vp), i.e., Vs<Vp. The value of the secondary voltage depends on the turns ratio (n). The equation n = Ns/Np = Vs/Vp indicates that to have a voltage that is lower in secondary than primary, the number of turns on the secondary coil must be lesser than primary, i.e., Ns<Np as illustrated in the below figure, meaning the turns ratio n = (Ns/Np) < 1. This is an important characteristic of a step-down transformer, which is the opposite of a step-up transformer.
If primary and secondary voltages are equal, the transformer is said to have a one-to-one ratio. One-to-one transformers are used to electrically isolate two parts of a circuit. One-to-one transformers are also referred to as isolation transformers. They are useful to isolate sensitive equipment such as microprocessor-based controllers.
Types of transformers based on winding arrangement
Based on winding arrangement, the transformers are categorized into four main types: center-tapped, multiple-tapped, adjustable, and autotransformers.
Center-tapped transformers have a tap (connecting point) in the middle of the secondary winding, and they can provide two balanced output voltages with the same value.
Multiple-tapped transformers have multiple taps in the secondary winding, and they can provide several output voltages with different values.
Adjustable (or variable) transformers
The output voltage of adjustable (or variable) transformers across the secondary winding is adjustable. The secondary winding of the adjustable transformers can provide an output voltage that may be variable in a range of zero to the maximum values.
It is a transformer with only a single winding, which is a common coil for both the primary and the secondary coils, and a portion of the common coil acts as part of both the primary and secondary coils. An auto-transformer can be made smaller and lighter. Autotransformers are normally used for voltage-transformation ratios close to 1:1 and in variacs, which provide a variable secondary voltage.
Types of transformers based on usage
Based on usage, the transformers are categorized into two main types: electronic and power transformers.
Electronic transformer’s operating voltages are very low and are rated at low power levels. These are used in consumer electronic equipment like televisions, personal computers, CD/DVD players, and other devices.
The term power transformer is referred to the transformers with high power and voltage ratings. These are extensively used in power generation, transmission, distribution, and utility systems to increase or decrease voltage levels.
Types of transformers based on instrumentation
Transformers are often used in instrumentation applications to match the magnitude of a voltage or current to the range of a meter or other instrumentation. Based on instrumentation, the transformers are categorized into two main types: current and potential transformers.
Current transformers step down the current of the primary circuit to conventional standard levels (normally 5 amperes) for measuring and controlling purposes. Their primary is connected in series with the circuit whose current is to be measured. The current of the secondary winding is applied to current coils of meters and relays.
Potential transformers are used for stepping down voltages to nonhazardous and conventional voltage levels—normally 120 volts—for measuring and controlling purposes. Their primary winding is connected in parallel to the high-voltage circuit whose voltage is to be measured. The voltage across the secondary winding is applied to potential coils of meters, relays, and other instruments, depending on what is desired.
Types of transformers based on number of phases
Based on the number of phases, the transformers are categorized into two main types: single and three-phase transformers.
Single phase transformers
Single-phase transformers usually have one input and one output winding often referred to as the transformer’s primary and secondary windings. These windings are not electrically connected but are magnetically coupled. The primary winding draws energy from a voltage source, whereas the secondary winding delivers energy to a load.
Transformers are not power amplifiers. For all practical purposes, the apparent input power (| S |) to a transformer’s primary winding is equal to the apparent power delivered to the load by its secondary winding. In other words, the volt-amperes of the primary winding (V1, I1) are approximately equal to the volt-amperes of the secondary winding (V2, I2). Mathematically,
| S | = V1*I1 = V2*I2
Single-phase transformers are used to provide power for receptacles, residential lighting, AC & heating requirements.
Three phase transformers
Three-phase, two-winding transformers are used to interconnect two distribution systems of different voltages, such as a 25 kV transmission line and a 480 V plant. The majority of industrial transformers are of this type. They are called two-winding transformers because for each primary phase winding, there is only one secondary phase winding. The transformers actually have six coils—three for the incoming three-phase power and three for the output three-phase power.
Three-phase transformers can be either of the core type or of the shell type. In the core-type construction, the windings are on separate legs of the laminated core. In the shell-type construction, both windings are on the same leg.
Three-phase transformers are widely used in power distribution systems. Three-phase power is the most common way in which power is produced, transmitted, and used. Although widely used in commercial and industrial applications, three-phase power is generally not used in residential applications.
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