In theprevious article, we learned about the basic concepts of a transformer. A transformer is simply a pair of电感器magnetically coupled to enable electromagnetic induction between them. With the help of transformers, AC voltages can be stepped up or down at low costs without any hassle. Stepping up or down DC voltages requires complex and expensive circuitry. That is why AC is used for the distribution of electric power even though most of the electronic appliances use DC for their operation. The electronic appliances convert the AC mains to DC for their functioning.

变压器具有多种形状，尺寸和结构。变压器可以通过核心材料，几何形状和构造，电压水平和用法进行分类。

Core classifications are as follows:

• Laminated Iron Core
• 铁氧体芯
• 粉状铁芯
• Air Core

Geometry classifications are as follows:

• Utility
• Solenoidal Core
• 环形核心
• Pot Core

Voltage levels classifications are as follows:

• Step-Up
• Step-Down
• Isolation

Usage classifications are as follows:

• 力量
• Measurement
• Distribution
• Pulse
• 声音的
• IF
• RF

Transformer cores
在任何变压器的构建中，制造商都试图在两个电感器之间具有最大的磁耦合。通过使用铁磁材料或粉状铁作为芯，可以增加磁耦合。与空中核变压器相比，在铁磁芯上缠绕的一对电感器具有更好的耦合系数。但是，使用铁磁芯有其自身的局限性。铁磁岩心由于滞后和涡流带来了一些能量损失，并且也受到电流携带能力的限制。除这些局限性外，核心材料的选择还限制了变压器的频率范围。根据所使用的核心类型，变压器分类如下

Laminated Iron Transformers– These transformers use silicon steel as core material. Silicon steel is also called transformer iron or simply iron. The silicon steel is laminated into layers to avoid losses due to eddy currents and hysteresis. Eddy currents are circular currents that flow in magnetic material on magnetization. The eddy currents lead to loss of energy by the magnetic core in the form of heat. Hysteresis is the tendency of a magnetic core to be slow in accepting fluctuating magnetic flux. Due to losses by hysteresis and eddy currents, these transformers are only suitable for 60 Hz frequency and other low frequencies in the audio range. As the frequency increases above a few kilohertz, internal losses of the core increase beyond feasible limits.

铁氧体芯– Ferrite cores have high渗透性and require fewer coil turns. However, at frequencies above a few megahertz, such cores start showing significant energy losses due to eddy current and hysteresis. That is why these transformers are suitable for frequencies above audio frequencies up to a few megahertz.

粉状铁芯– Powdered iron also has high permeability and lower losses due to hysteresis and eddy current compared to ferrite cores. As the frequency increases, the need for high permeability decreases. The transformers using powdered iron core are suitable for very high frequencies up to 100 MHz. As there is no need for high permeability at very high frequencies above 100 MHz, air-core transformers are more suitable due to their energy efficiency.

Air Core Transformers– In air-core transformers, both primary and secondary coils are wound on a diamagnetic material. The magnetic coupling in such transformers happens through the air. In such transformers, not only is the inductance of both coils low, but the mutual inductance is also very low so there is very little magnetic coupling between the coils. These transformers have no loss of energy due to hysteresis or eddy currents and are also capable of moderating high currents. Such transformers are suitable for high voltage applications where energy efficiency is a prime concern, such as distribution transformers. These are also suitable for very high RF applications above 100 megahertz. At high radio frequencies, the value of required inductance is low, which can be easily achieved by air-core inductors, and energy efficiency is the primary concern of the VHF circuits.

It should be noted that the following symbol represents air-core transformers:

The transformers having a magnetic core are represented by a symbol in which two parallel lines are added between the coil symbols as follows:

Transformer geometry and construction
Transformers can also be classified by their shape and geometry. The shape of a transformer depends on the type of inductor used in its construction and the shape of its core. Any transformer is essentially a pair of inductors wound on the same core. The classifications are as follows:

Utility Transformers– Utility transformers are power transformers in which laminated iron is used as the core material. These iron core transformers come with a variety of core shapes like E, L, U, I, etc. and are bulky and heavy. The most common core shape used in these transformers is E core or E-I core because the laminated core has the shape of the letter ‘E’ with a bar placed at the open end of ‘E’ to complete the construction. The coils are wound on the core either by shell method or core method. In the shell method, both coils are wound on the middle bar of the ‘E’ atop each other. This ensures maximum magnetic coupling between the coils, but at the cost of high coil-to-coil capacitance. The shell method also limits the current carrying capacity of the transformer. In the core method, one coil is wound on the top bar of the ‘E’ and the other on the bottom. The magnetic coupling between the coils occurs only due to magnetic flux through the core. The core method reduces the coil-to-coil capacitance to a great extent and makes it possible to handle high voltages. The utility transformers with E-I core having a shell or core winding are most commonly used as 60 Hz transformers and other audio frequency transformers.

Solenoidal Coil Transformers– Solenoidal core transformers are generally used as a loopstick antennas for radio frequency circuits. These transformers have primary and secondary windings on a cylindrical core (ferrite or powdered iron). The coils are either wound atop of each other or separate. In such transformers, the primary captures the radio signals, and the secondary provides impedance matching to the first amplifier stage of the radio circuit. Such transformers have been quite common in portable radio communication equipment.

环形核心Transformers- 环形核心变压器在环形芯上具有原发性和继发伤口，并且线圈可能彼此或分离。环形核心是射频电路中螺线管芯的更好替代方法。它们在芯内包含磁通量，因此，只要线圈被绝缘，这些变压器就可以直接安装而无需任何其他屏蔽。除了没有电磁干扰外，环形核心还提供更高的每个线圈转弯电感。由于磁通量保持在芯内，因此环形芯变压器在线圈之间具有更好的磁耦合。

Pot Core Transformers– Pot core transformers have the primary and secondary wound on one of the halves either atop of each other or next to each other. Pot cores provide the highest possible inductance with the obvious advantage of self-shielding. One of the major disadvantages of pot core transformers is coil-to–coil capacitance. Due to coil-to-coil capacitance and unusually high inductance of both coils, pot core transformers are only suitable for low frequencies. At high frequencies, the required value of inductance is low, and capacitive reactance needs to be essentially minimized.

变压器电压水平
The most common application of a transformer is to moderate AC voltages. A transformer can step up, step down, or leave intact AC voltage levels. This is the easiest but most important classification of transformers. They are as follows:

Step Up Transformer– In a step-up transformer, the secondary has a higher number of turns than the primary. As the ratio of turns of primary-to-secondary is less than 1, the voltage applied to the primary is stepped up to a higher voltage in the secondary. This consequently comes at the cost of lower current levels at the secondary winding. Step-up transformers are used in stabilizers and inverters where lower AC voltages need to be converted to higher voltages. These are also used in power grids to step up AC voltage levels before distribution.

逐步变压器- 在降下的变压器中，主要的弯曲数量比次要的更高。由于初级绕组的转弯比大于1，因此次级电压低于原电压。逐渐下降的变压器通常用于电子应用中。电子电路通常需要5V，6V，9V，12V，18V或24V才能进行操作。降压变压器通常在整流器之前用于电源电路中，以降低120V或240V AC的电源至所需的低压水平。在电源分配中，逐步向下变压器用于降低高压，以向电线杆供应电源。这样可以确保电源分布的能源效率和成本效益。

Isolation Transformers- 隔离变压器在主要和次级中具有相同数量的转弯。由于初级与席位的转弯数的比例正好为1，因此两个绕组的电压水平保持不变。这些变压器用于在电子电路之间提供电隔离，或取消从一个电路到另一个电路的噪声传递。隔离变压器需要具有高电感耦合和最小电容耦合。这就是为什么这些变压器被设计为在高度磁性和自动固定的芯上的单独线圈上有最小数量的转弯。

The isolation transformers are also used for connecting balanced and unbalanced circuits. Balanced circuits are those that can be connected either way across a port. The unbalanced circuits are those that need to be connected in a specific way across a port. The balanced and unbalanced loads can be connected via isolation transformer by grounding the center tap on the balanced side. If the balanced and unbalanced loads have the same impedance, then the isolation transformer should have a turn ratio of 1. If a balanced and unbalanced load has a different impedance ratio, the turn ratio should be accordingly matched to the square of the impedance ratio. The isolation transformers are also used for coupling amplifier stages in radio frequency transmitters and receivers.

In thenext article,我们将继续tran的分类sformers by usage. By usage, transformers broadly belong to either electrical or electronic domain. In the electrical domain, transformers are generally classified by their respective applications. In the electronic domain, it is easy and obvious to classify transformers by the signal frequency of their operation.

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