Transformer Manufacturers

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About Transformers

Electricity supplied from high-tension power lines has a very high voltage, up to 765,000 volts. This cannot be used directly in home and industrial applications as it would break domestic appliances. A transformer is used to convert the high voltage into a smaller voltage.

Michael Faraday invented the transformer in 1831. However, Westinghouse developed a commercial transformer in 1881. The transformer has changed the way in which electricity is used and allows easy conversion of high voltage to low and low voltage to high.

Transformers are used for long distance transmission of power. They are used in large electric furnaces. Rotating transformers are used in: radio telescopes, machine tool heads, audio amplifiers and audio systems, toy train sets and many other applications.

How Transformers Work

A basic transformer has two adjacent coiled conductors, the primary and the secondary coil. Input AC current is sent to the primary coil which creates a varying magnetic field around it. Per the law of mutual conductance, the secondary coil develops a potential difference called the electromotive force (EM) and a current is induced in the secondary coil circuit. Current from the secondary is taken out and fed to appliances or into an electric grid.

The amount of voltage that flows in the secondary depends on the number of coils and the conductor resistance. It can be different from the voltage given to the primary. By carefully selecting the number of windings, a lower, or higher voltage than the primary is generated. This is the basic principle of a transformer.

Types of Transformers

Power transformers come in two basic configurations, single-phase and three-phase. A three-phase transformer is made of three sets of primary and secondary windings, each set wound around one leg of an iron core assembly. In essence, it resembles three single-phase transformers sharing a joined core. The different types of transformers are:

• Step down: The secondary coil has fewer turns than the primary coil. The voltage (V) is reduced and the current (A) is increased.
• Step up: The secondary coil has more turns than the primary. The voltage (V) is increased while the current (A) is decreased.
• Isolating: The number of turns in both the primary and the secondary are the same, and this type transforms one voltage to the same voltage rating.
• Variable: The number of turns in the primary and the secondary can be adjusted with a slider. This allows the transformer to work as a step up or step down.

The amount of power that a transformer can handle is also used to classify them. Some examples are given below:

• 1 watt (W) and below: Signal transformers used for inter-stage coupling
• 1 to 1,000 watts: Small power, filament transformers, audio output transformers
• 1 kilowatt (kW) to 1 megawatt (MW): Medium sized power transformers
• Above 1 megawatt: Large power transformers, substations, large networks, power plants and transmissions.

Specific types of transformers

• Autotransformers: It is used in power distribution systems or to match two high voltages for transmissions in the range of 120,000 V. It has a single winding and this is tapped along the winding at a point.
• Polyphase transformers: These are used for three phase power supplies. Three individual transformers for each phase or a single polyphase transformer with three phases connected to it can be used.
• Resonant transformer: It is used to generate very high voltages like those in Tesla coils and ignition coils in gas powered automobiles. It works at the resonant frequency of its coils. The secondary coil works as an inductor and it is connected to capacitors in a series. The primary coil is given a periodic saw tooth or square wave AC current. The current pulse sets up an oscillation in the secondary coil, causes a resonance and a very high voltage is created in the secondary coil.
• Instrument transformers: They are used to step down the current and voltage for measurement. Different types are:
  • Current Transformers: These are used in electricity meters and allow measuring and metering of large current. They have a single primary with an insulated cable that passes through a toroidal core.
  • Voltage transformers: These are used in electricity transmission to measure the supply voltage without changing the measured voltage.
• Pulse transformers: These are used in digital communication networks and power supply systems to interface with semi conductor devices like IGBT, thyristors, and mosfets. The transformer is used to transmit pulses that have quick rise and fall times at constant amplitude.
• RF Transformers: They are used for radio frequency applications and have very large bandwidth. Ferrite line wound cores in different configurations are used. Windings of bifilar or coaxial cables are used.
• Switch Mode Transformers: These are a type of high frequency transformers and used in DC-DC converters and switching power supplies.
• Signal Transformers: They transfer data between circuits through electromagnetic induction.
• Audio Transformers: They are used in audio devices like microphones, speakers, tape drives to transmit voice and sound signals. The transformers have wide frequency bandwidth, low distortion and noise suppression.

Types of Cores

Coil windings are done around a core. The core confines the magnetic field to the transformer and the efficiency is multiplied many times. High frequency transformers have an air core. Magnetic saturation of the core is avoided by placing gaps in the magnetic path. Major types of cores are:

• Steel Cores: These are used in audio and power appliances. Stamped pieces of thin silicon steel cores are used. They help channel the magnetic fields and keep the field concentrated around the conducting wires. Eddy currents in the core are reduced. They can be identified as 'E' or 'I' shaped laminated cores. They are simple and economical but they retain magnetic field due to magnetic hysteresis even when input current is stopped.
• Solid Cores: They are used in switch-mode power supply devices. The core is made of powdered iron and this gives the core a high material resistance and magnetic permeability. For radio frequencies, the core is made of nonconductive ferrites like ceramics. The core in these devices is adjustable and allows the coupling circuit to be fine-tuned.
• Air cores: These are used by high frequency transformers and hystersis loss is eliminated. The coupling efficiency is high and is achieved by overlapping the secondary windings and the primary windings.
• Toroidal cores: These cores are used in high frequency applications in the kilohertz range. The core is ring shaped, and made of strips of silicon steel or ferrite. The air gaps are eliminated. The core reluctance is reduced and the grain boundaries are placed in the same direction. The toroidal shaped cross section is square, rectangular or even circular and the primary and secondary coils are concentrically wound. Lower amounts of conducting wire are used and the electromagnetic interference is reduced. These types of cores are more expensive but they are much more efficient than others as they have fewer losses, weigh less and produce little hum.

Windings

Windings play a very important role in the transformer performance. Small capacity transformers have enameled insulated solid copper wire winding. Large power transformers have windings of copper, aluminum or strip conductors. High frequency transformers that operate in the gigahertz range have windings of Litz wire. Unifilar has a single winding and bifilar has two windings per coil. Insulation between layers of windings ensures that arcing does not occur.

Cooling

Transformers generate a lot of heat and this affects their efficiency. Excessive heat can also cause a short circuit and transformer burn. Smaller transformers are cooled by using natural air drafts or by using fans. Powerful transformers are cooled by immersing the assembly in transformer oil. These oil-filled transformers use silicone based oils, fluorinated hydrocarbons or canola oils that are non-toxic, non-flammable, and stable at high temperatures. Absolutely no water vapor should be allowed inside the transformer as it can cause breakdowns. Buchholz relay safety devices are used to sense any gas build up inside the unit.

Power losses in Transformers

Transformers have an efficiency rating up to 98% or more. Power losses do happen due to inherent properties of windings, core, poor maintenance and other reasons however. Some types of losses are: eddy currents, stray losses, hysteresis losses, mechanical losses, magnetostriction, improper cooling system, etc.

Considerations for Transformers

Consider the following factors when choosing transformers:

• Transformers do not handle DC current and this type of current is only useful for auxiliary operations like drying the transformer oil bath.
• They cannot change AC to DC or DC to AC and they cannot change the current cycle frequency. They follow the universal law of conservation of energy and the output power cannot be greater than the input power.
• If multiple nominal voltages are needed, the transformer should have more than one primary winding.
• Primary incoming voltage frequency is different in different continents. It is 50 Hz in Europe; 60 Hz North America and 400 Hz is used in aerospace applications. This is important for global companies that operate on different continents.

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