A transformer is an apparatus for converting electrical power in an ac system at one voltage or current into electrical power at some other voltage or current without the use of rotating parts. Transformers have been an essential component in electrical as well as electronic circuits. Apart from stepping up or stepping down the voltages, they are used for providing isolation, for impedance mismatch and so on. Development of new technologies has reduced the usage of transformers, but still they are quite vital in many applications.
Transformers are of many types and are used as per specific requirements. In addition to the transformers used in power systems, in power transmission and distribution, a large number of special transformers are in use in applications like electronic supplies, furnaces, traction, audio applications, RF and microwave circuits, etc. Subsequent sections will present details about the transformers.
What is a TRANSFORMER ?
A transformer is an electrical device that transfers electrical energy at one voltage from one circuit to other at a different voltage merely by magnetic coupling; the transfer of energy doesn’t involve any kind of motion.
Transformers are analogous to gear box (used to convert torque and hence speed). Transformers step up or step down the voltage and therefore vary the current. As the product of speed and torque remains constant, product of voltage and current also remains constant.
Transformers utilises the principle of electromagnetism given by Faraday’s law. A conductor carrying changing current carrying sets up a changing magnetic field around it. When a second conductor is placed in this varying magnetic field, voltage will be induced into it. Get a clearer picture about transformers through exclusive images detailing about its internal structure and windings at the transformer insight page.
Fig. 1: A Simple Diagram Explaining Working of Transformer
When an AC voltage is applied to one (primary) coil, the varying magnetic field is set up around the coil. By virtue of mutual induction, it creates an AC voltage in the other (secondary) coil. A transformer can also be used with pulsating dc, but a pure dc voltage cannot be used, since only a varying voltage creates the varying magnetic field which is the basis of the mutual induction process.
An ideal transformer has infinite winding reactance, zero winding resistance, zero leakage inductance and zero winding capacitances. Voltage ratio is equal to the turns ratio under all loading conditions.
Transformers – Construction
TRANSFORMERS – CONSTRUCTION
A transformer primarily consists of three basic parts- a primary winding which receives the electrical energy from the applied voltage source, and a secondary winding which receives the induced electrical energy and a core which provides a circuit of low reluctance for magnetic lines of force.
Windings, primary as well as secondary, are the coils of conducting wires as a coil of conductors create a higher magnetic flux compared to the flux created by a single conductor.
Windings rated for higher voltages with more number of turns are designated as High Voltage (HV) winding. The windings for lower voltages are called Low Voltage (LV) winding. The HV winding is composed of many turns of relatively fine copper wire, while the LV winding is composed of relatively few turns of heavy copper wire. The current on the HV side will be lower as V-I product is a constant. Also the HV winding needs better insulation properties to withstand higher voltages across it. HV also needs more clearance to the core, yoke or the body.
The material used for the windings is application specific. Insulated solid copper wire is used for small power and signal transformers whereas copper or aluminium rectangular/strip conductors are used for larger power transformers. RF transformers use Litz so as to minimise losses due to skin effect.
Tappings (or external connections) may be provided from the intermediate points on the windings.
Double-wound transformers use separate primary and secondary windings, while autotransformers use single winding with tapping.
To ensure that the current travels around the core along the coiled conductor, and not through a turn-to-turn-short circuit, winding materials are enamelled thereby providing insulation. In addition, various other methods are used to provide insulation. The type of insulation has a definite bearing on the size and operating temperature of the unit.
Currently four classes of insulations are used
· Class 130 insulation-system transformers.
· Class 150 insulation-system transformers.
· Class 200 insulation-system transformers.
· Class 220 insulation-system transformers.
When properly loaded and installed in an ambient not over 40°C, Class 130, Class 150, Class 200 and Class 220 transformers will operate at not more than a 60°C, 80°C, 130°C and 150°C temperature rise on the winding respectively.
The insulation used for the electrical conductors in a transformer is varnish or enamel. In larger power transformers the conductors are insulated using un-impregnated paper / cloth and the assembly is immersed in a tank containing oil; the transformer oil acts as an insulator and also as a coolant.
Because of the resistance of its windings and the hysteresis and eddy currents in the iron core, a certain amount of the electrical energy delivered to a transformer is transformed into heat energy. The mechanism must be provided for removing the heat energy from the transformer and dissipating it into the surrounding air otherwise, excessively high temperatures may destroy the insulation of the transformer. To remove the heat generated in a transformer, coolant is used.
Various types of cooling mechanisms used are
· Self-air–cooled transformers (or dry-type transformers)
The windings are surrounded by air at atmospheric pressure. The heat is removed by natural convection and radiation. Self-air–cooled transformers are used in systems with 3000-kVA capacity and voltages up to 15,000 V.
· Air-blast–cooled transformers
In this type of transformers, the core and windings are enclosed in a metal enclosure through which air is circulated by means of a blower. These are used for large power transformers in ratings up to 15,000 kVA and voltages up to 35,000 V.
· Liquid-immersed, self-cooled transformers:
In liquid-immersed, self-cooled transformers, the core and windings are immersed in an insulating liquid and enclosed in a metal tank. Liquid conducts away the heat from the core to the tank surface and then, the heat is removed by natural convection and by radiation.
· Gas-vapor transformers
In Gas-vapor transformers, the transformer is insulated with a quantity of gas necessary for start-up, along with a vaporizable liquid which provides insulation and cooling during operation
To avoid any capacitive effect in the transformers (due to the proximity of primary and secondary windings), an electrostatic shield is used between the windings. Transformers may be shielded by magnetic or electrostatic shields, or both to prevent interference from other devices
Small transformers have leads brought out of the unit for circuit connections. Larger transformers may have bolted terminals, bus bars or high-voltage insulated bushings.
Any material inside a coil, used to serve as a form to support it, is called a core. Cores are made of different materials with permeability ranging from 1 to over 10000. The higher permeability aid in providing low reluctance path of the ?ux and the ?ux lines mostly con?ne themselves to the core. The permeability of air is 1 whereas the permeability of common “ferro-magnetic” materials is about 300 for ordinary steel, about 5,000 for 4% silicon transformer steel, and up to about 100,000 for some nickel-iron-molybdenum alloys. Because such materials concentrate magnetic flux, they greatly increase the inductance of a coil. Coil inductance is directly proportional to the square of the number of turns and also, direct proportional to the permeability of the core. Silicon steel, hot rolled grain oriented steel, Cold Rolled Grain Oriented (CRGO), etc. are some of the material used in the form of thin laminations for the core; the laminations (in the form of E & I, C & I or O) are coated with a layer of insulating varnish, oxide or phosphate.
Fig. 2: Various Types of Ferrite Cores
Ferrite cores are best suited for high frequency applications and steel laminations are best suited for low frequency applications. For lower frequencies, core material selection is governed by core saturation considerations. Eddy current losses are low so steel laminations can be considered. For higher frequencies, core material selection is governed by core loss considerations. Eddy currents can be significant. In such applications, ferrites are commonly used.
Numeric Codes representing the power handling ability have been assigned to the cores by the manufacturers; the assigned number is the product of its window area and the core cross-section area. The codes are available for laminations, C cores, pot cores, powder cores, and Toroidal tape-wound cores.
Transformers – Relevant Terms
TRANSFORMERS – RELEVANT TERMS
· Turns Ratio
Voltage induced into the secondary winding depends on the turns ratio of the transformer. The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. If the turns ratio of a transformer is 4:1, the induced voltage is lower compared to that of primary and the transformer is called step down transformer. On the contrary, if the turns ratio of a transformer is 1:4, the induced voltage is higher than that of primary and the transformer is called step up transformer.
If the turns ratio and the input voltage are known, the output voltage can be determined as follows
Fig. 3: Formula for Determining Output Voltage in a Transformer
Where, E1 & E2 are primary & secondary voltages, N1 & N2 are the number of turns in primary & secondary winding.
· Power Ratio
When a transformer steps up the voltage, it steps down the current by the same ratio, thereby input and output electrical power remains constant (neglecting losses). Transformer, being a passive component, cannot produce more power from the secondary winding than what is applied to the primary.
· Load Voltage
Load Voltage is same as the secondary voltage and is equal to the voltage delivered to the load.
· Line Voltage
Line Voltage is the primary voltage and the voltage available at the primary from the source.
· Coefficient of Coupling
In practice, no transformer is 100 percent efficient, i.e., output power is slightly less than that of input power. Since all the magnetic lines of force in the primary do not cut across the turns of the secondary coil, certain amount of flux leaks out of the magnetic circuit. The degree of how well the primary flux is coupled into the secondary is called the “coefficient of coupling”, this in turn, determines the efficiency of the transformer. Efficiency is the ratio of the output to the output plus the losses.
· Transformer Regulation
The regulation of a transformer is the change in secondary voltage from no load to full load. It is generally expressed as a percentage of the full-load secondary voltage.
The regulation depends upon the transformer design and the power factor of the load.
· Transformer ratings
Transformers are rated at their kilovolt-ampere (kVA) outputs. Transformers are generally rated on the kVA load which the transformer can safely carry at the ambient temperature, at rated load voltage and at rated frequency.
· Transformer Losses
· Copper Losses
These losses occur due to the finite resistance of the wire of the windings. When current flows through the windings, power = I2R dissipates in the form of heat and are treated as winding losses or I2R losses or copper losses.
· Hysteresis Losses
When alternating current reverses, once during each cycle, tiny magnetic domains are reversed and these physical changes in the core consume some amount of energy. These losses are referred to as hysteresis losses.
· Eddy Current Losses
With iron core, varying primary current sets up electromagnetic field in the secondary and also sets up EMF in the core, causing losses referred to as eddy current losses. These currents in the core oppose magnetic field changes in the core and hence must be kept very small.
Transformers – Types
TRANSFORMERS – TYPES
Transformers primarily can be divided into two types: voltage transformers and current transformers.
Constant-voltage transformer changes the voltage of a system and is designed to operate with its primary connected to a constant-voltage supply and to provide a secondary voltage which largely remains constant from no load to full load. The currents of both primary and secondary vary with the load supplied by the transformer.
Variable-voltage transformer also changes the voltage of a system but is so designed that when operated with its primary connected to a constant-voltage supply, the secondary voltage varies widely with the load.
Current transformer is designed for changing the current of a system. The primary winding is connected in series with the circuit of which it is required to change the current. The voltage of both primary and secondary will change with the current of the system.
Constant-current transformers are so designed that a constant value of secondary current is supplied to the load regardless of the load on the transformer. The primary is connected to a constant-voltage source but the secondary voltage varies proportionally with the load.
Some common type of transformers is: Power transformers, Audio transformers, RF transformers, Auto transformers, Poly-phase transformers.
1. Power Transformer
Power transformers are primarily used to step down or step up voltages (and currents). The size of the transformer varies depending upon the application; small sized transformers are used in power section of various equipments whereas very larger transformers are used for supply of high voltage grid electricity. Power transformers are often used to provide a system with a large number of AC supplies of different voltages and currents.
Typically, voltage transformers for the transformation of a large amount of power, more than 500 kVA, are called power transformers. Transformers for general power transformation, whose rating is 500 kVA or less, are called distribution transformers. All the methods of cooling are employed for power transformers.
Construction of a typical laminated core (Core of E and I shaped laminations) power transformer is shown in figure.
Fig. 4: Images Showing Construction of A Typical Laminated Core in Power Transformer
Another popular design of power transformer using toroidal core is shown in the following figure. It given excellent linkages as primary and secondary are wound on the same core, though this design proves more expensive
Fig. 5: Images Explaining Toroidal Cores used in Power Transformers
Issues like wide frequency response, harmonic distortion, etc. are not relevant for power transformers.
2. Audio Transformer
Though very similar with power transformers with respect to their construction, they are usually quite smaller in size compared to power transformers. Audio transformers are designed for audio range of frequencies, i.e., for frequencies ranging from 20 Hz to 20 KHz and are used primarily in audio circuits.
Fig. 6: Diagram Showing Insides of Audio Transformer
In addition to other transformer characteristics, important characteristics of an audio transformer are the frequency response, total harmonic distortion and insertion loss.
3. RF Transformers
RF transformers are widely used in low-power electronic circuits for impedance matching to achieve maximum power transfer, for voltage step-up or step-down, and for isolating dc from two circuits while maintaining ac continuity. They are also used for common mode rejection and as baluns.
RF transformers are designed to function in the radio range of frequencies. Typically, it uses an air core but more effective flux linkage is obtained with the use of a core of iron or ferromagnetic material with higher permeability than air. Functions and design principles are similar to that of audio transformer.
4. Auto Transformers
When the transformation ratio is not too large, an effective way of operating a transformer is an autotransformer. Normally used in power circuits, autotransformers use single winding and certain portions of the winding is used simultaneously as the primary and the secondary circuits. The autotransformer has only one coil, a certain portion of which is used for both the HV and LV windings. The number of turns of this coil is the same as would be required if it were used exclusively for the HV windings. Not only is the copper, core material requirement lesser in autotransformers. A comparison of a two winding transformer with an autotransformer is given in the following figure
Fig. 7: Diagrammatical Image Showing Comparison Between Two Winding Transformers
Depending upon whether the transformer is used as a step-up or step down transformer, a part or full winding is used as a primary; and a part or full of the same winding as the secondary.
5. Poly-phase Transformers
The term ‘poly’ is used in transformers used for AC supplies with more than one phase, for e.g., three phase transformers. A polyphase transformer generally consists of several one phase transformers with separate electric circuits but having certain magnetic circuits in common. A three-phase transformer is shown in the figure