Transformers_part1

• The physical basis of transformer (T/F) is mutual induction between two circuits linked by a common magnetic field. It is a static device.


• Mutual inductance is the same irrespective of which circuit is primary and which circuit is secondary.


• The voltage applied to the primary is almost completely concerned in opposing the induced e.m.f.


• If the primary voltage is constant, the mutual flux remains approximately constant regardless of the load connected across the secondary coil.


• Important tasks performed by T/F are

• Changing voltage and current levels in power system.
• Matching source and load impedances for maximum power transfer
• Electrical isolation

• In core type, to avoid leakage flux, it is usual to have half the primary and half of the secondary winding side-by-side or concentrically on each limb; not primary on one limb and secondary on other limb.





• The three phase core type, the principle that the sum of the fluxes in each phase in a given direction along the cores is zero.


• In core type inspection of coils and core is easy.


• The shell type is more robust mechanically.


• In shell type core, the cooling is good.


• The e.m.f induced in transformer given by: E = 4.44fΦmT volts


• Volt/turn is same in primary winding and secondary winding provided only that it links the same flux in both windings.


• Both primary and secondary e.m.f.’s are in phase.


• The e.m.f’s lag by 900 in time on the flux.


• The applied voltage V1 opposes E1, while E2 provides the secondary output voltage V2.


• E1/E2 = I2/I1 = T1/T2




• A small magnetizing current is needed to maintain the magnetic circuit or core in magnetized state, when the secondary is open




• The m.m.f. of the primary on no load is of the order of 5 per cent of its m.m.f. on full load.


• The no load current, Io has two components, magnetizing component, I_m and loss component, I_r.




• Leakage between primary and secondary could be eliminated if the winding could be made to occupy the same space.



• Reductions in leakage flux can achieve by sectionalizing and interleaving the primary and secondary coils.



• An equivalent circuit is useful for calculations of regulation, efficiency, parallel operation, etc.



• I2R loss and per-unit reactance voltage in primary and secondary is same.



• Core loss due to the pulsation of the magnetic flux in the iron producing eddy current and hysteresis loss.


• I2R loss due to heating of the conductors by the passage of current.


• Stray loss due to stay magnetic field causing eddy currents in the conductors or in surrounding metal (tank).


• Dielectric loss in the insulating materials (oil and solid insulation of HT T/F).


• Efficiency of transformer is given by:


Where S is full load kVA, x is per-unit load, P_i is iron loss, P_c is full load copper loss.

• For maximum efficiency occurs when the variable loss is equal to the constant loss, i.e. x² P_c= P_i.


• Maximum efficiency occurs below full load.



• Maximum efficiency point independent of power factor.


• The regulation of a transformer refers to the change of secondary terminal voltage between no-load and load conditions: it is usually quoted as a percent value for full load at given power factor.


• Per unit regulation:


• Maximum regulation occurs when
  


• Zero regulation occurs when
  i.e. leading p.f.


• On account of the easier insulation facilities, the low-voltage winding is placed nearer to the core in the case of core type and on the outside positions in the case of shell type transformers.


• Under no-load conditions, the “hum” developed by energized power transformer originates in the core, where the laminations tend to vibrate by magnetic forces.


• The essential factors in noise production are magnetostriction, mechanical vibrations by the laminations.