Basics of Transformers
Transformer is AC static electromagnetic machine (as it has no moving part) which steps up or downs the voltage level in power system keeping power at constant levels and frequency remains unchanged. In simplest form, it consist of two coils wound on an iron core. It works on law of electromagnetic induction.
It is based upon mutual induction between two coils. When current flows, there is magnetic field around the conductor. As magnetic current is due to changing current (AC) the magnetic field is also alternating. It magnetize the iron/steel core, voltage is induced in other coil. This is basics of transformer. Both sides are magnetically linked and no electrical connection between them so it is also isolates the primary circuits and secondary circuits.
The need of transformer in power system:
Transmission of electrical power over long distances occur a great loss in power. As the power losses is given by (I^2)R so when power system has to feed a big load of high current and it has to travel over a long distance, a handful amount of power will be lost. Also causes huge voltage drop to the receiving end. As voltage level can be increased using this device and as this keeps the power constant so with increase in voltage level, current reduces hence losses are reduced. This device replaces the whole DC transmission system to AC system.
Construction of Transformer:
One side of transformer is called as primary side (which is input side) and other is known as secondary side (the output side). Both sides are consist of just a coil wound on soft iron called as core of transformer. Both coils are magnetically linked and power is transmitted from primary to secondary side magnetically and has no electrical connection between them. Soft iron (ferromagnetic material) core helps the transmission of magnetic flux with less losses. Transformer is device which could be more than 98% efficient in its working.
It may be step up or step down transformer. In former, the voltage on input side is low and on other side it is raised and vice versa for later. As we know, electrical power is given by the product of current and voltage (P=VI). Transformer keeps the power same i.e. the product of voltage and current remains same on both sides. That’s why it is used in long distance electrical power transmissions. So, when it step ups or steps down the voltage level, the current decreases and increases accordingly in respective transformers.
This figure 1 shows the simplest transformer model. On the left side it is primary winding of Np number of turns having Vp voltage and on the right side secondary coil with Ns number of turns and Vs voltage. Both are isolated electrically and magnetic field is linked through the iron core. This core magnetize and demagnetize when AC current is given to the primary coil. The ratio of number of turn of primary to that of secondary is equal to the ratio of primary voltage to secondary voltage. The relationship between the number of turns and voltages is given by:
The coil made up of copper (or silver) wound wire with a specific numbers of turns. The core is made up of simple iron or silicon steel. When the primary coil is energized with AC signal, magnetic field is produced and it magnetize the iron core in one direction in first half cycle of AC signal. Due to this voltage is induced in secondary coil. In next negative half cycle, the core is magnetized in opposite direction inducing the voltage in secondary again but in reverse direction making the output AC. That’s why the frequency of input signal in transformer remains same.
Transformers are classified on basis of their power handling capability (kVA rating). There are international standards for transformer ratings and most of power transformers are one of that power ratings. According to R10 series of IEC transformer can be of rating: 10, 12.5, 16, 20, 25, 31.5, 40, 50, 63, 80, 100 and multiples of 10 of these basis.
Magnetization of Core and Losses in Transformer:
Let's see what happens when Alternating Current (as shown in figure 2) is applied to the windings on the core of transformer. Initially the flux in core is zero, when positive half cycle of AC signal starts for first time, flux starts to increase in core and reaches to peak of it. When it starts falling (going towards zero crossing and then to negative peak) it does not just retreat back on the same path it was rising rather it takes path of bcd infigure 3
Then it again rises from point d in figure after the negative peak of Alternating Current. And follows the path deb and continue to follow the path bcdeb for a complete cycle.
This path (as shown in figure 3) shows that it does not depends only on current applied but also to the previous history of flux in core. So, this dependence and not following the same path for both positive and negative cycle is called ashysteresis.The path bcdeb in figure is known as hysteresis loop.
The hysteresis occur due to reason that when the atoms of iron in domains inside the material (ferromagnetic material) are random in normal condition. When they are align in same direction it become magnet and when AC is in one direction (half cycle), they are aligned in one direction.
When all the domain align them in direction of magnetic field this is called as saturation of iron core. No further increase in flux after that point. With opposite polarity, they align themselves in opposite direction. But from peak to the zero, all the domains do not come to original position to align in opposite direction. Some of them are in aligned position. That’s why hysteresis loop occur. And it depends also on previous history.
Another loos in iron core is eddy current losses. When there is changing magnetic field in core, it causes the production of potential difference hence to the flow of electric current in the iron core. These currents are like ‘eddies’ on the bank of river i.e. almost circular form. The energy is dissipated by these current in form of heat.
How to Tackle these Losses?
- Hysteresis losses are tackled by using soft material core made up of ferromagnetic material. So that very little amount of energy require to magnetize and align the atoms of material.
- For eddy current losses, the core is divided into very thin strips and all the strips are laminated. That is because that the current path must be limited to that strip only and as the area will be small, resistance will be high. Hence eddy current will be very small. In fact, these loses are directly proportional to square of thickness of laminated core strip.