Transformers
Transformers are based upon mutual induction which transform an alternating voltage from one to another of greater or smaller value.
Transformer Types
Commonly used transformer type, depending upon voltage they are classified as:
Step-up Transformer: They are used between the power generator and the power grid. The secondary output voltage is higher than the input voltage.
Step down Transformer: These transformers are used to convert high voltage primary supply to low voltage secondary output.
Construction:
A transformer consists of two coils wound on a soft iron core, called primary and secondary coils. Let number of turns in these coils are Np and Ns respectively. The input A.C. voltage is applied across primary coil whereas output A.C. voltage is across secondary coil.
We consider an ideal transformer in which the primary has negligible resistance and all the flux in the core links both the primary and secondary windings. Let Φ be the flux linkage through each of primary and secondary coils. Then.
Induced emf across the primary coil,
`epsilon_{p}=-N_{p}\frac{d\phi}{dt}` ...(i)
Similarly induced emf across secondary,
`epsilon_{s}=-N_{s}\frac{d\phi}{dt}` ...(ii)
From these equations,
AC voltage obtained across secondary / AC voltage applied across primary
`r=\frac{V_s}{V_p}=\frac{\epsilon_s}{\epsilon_p}=\frac{N_s}{N_p}`
r is called the voltage transformation ratio, which is a constant.
In a transformer, some energy is always lost. The efficiency of a well designed transformer may be upto 95%. If the transformer is assumed to be 100% efficient
`p=I_{p}V_{p}=I_{s}V_{s}`
`\frac{I_p}{I_s}=\frac{V_s}{V_p}\frac{N_s}{N_p}`
In actual transformers, small energy losses occur due to following reasons.
Flux leakage: There is always some flux leakage. Not all the flux due to primary passes through the secondary.
Resistance of the windings: Some energy is lost in the form of heat dissipation. It can be minimised using thick wire in case of high current, low voltage windings.
Eddy currents: The alternating magnetic flux induces eddy currents in the iron core and causes heating. The loss can be minimized using laminated iron core.
Hysteresis: The magnetisation of core is repeatedly reversed by alternating magnetic field. The resulting expenditure of energy in the core appears as heat and is kept to a minimum by using a material when has a low magnetic hysteresis loss.
Use of Transformers in Transmission
In electric power transmission, transformers allow transmission of electric power at high voltages, which reduces the loss due to heating of the wires.
In many electronic devices, a transformer is used to convert voltage from the distribution wiring to convenient values for the circuit requirements.
Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphones and record players to the input of amplifiers.
Audio transformers allowed telephone circuits to carry on a two-way conversation over a single pair of wires.
Resonant transformers are used for coupling between stages of radio receivers, or in high-voltage Tesla coils.