Design of field winding for salient pole Alternator:
Design of the field winding is to obtain the following information.
(i) Cross sectional area of the conductor of field winding
(ii) Current in field winding
(iii) Number of turns in field winding
(iv) Arrangement of turns
(v) Resistance of the field winding
(vi) Copper loss in the field winding
Above informations can be obtained following the following steps
(i) Generally the exciter voltage will be in the range of 110 volts to 440 volts. 15-20 % of
voltage is kept as drop across the field controller.
Hence voltage per coil Vc = (0.8 to 0.85) exciter voltage / Number of field coils
(ii) Assume suitable value for the depth of the field coil
(iii) Mean length of the turn in field coil is estimated from the dimensions of the pole and the depth of the field windings. Mean length of the turn = 2( lp + bp) + π (df + 2ti) where ti is the thickness of insulation on the pole.
(iv) Sectional area of the conductor can be calculated as follows
Resistance of the field coil Rf = ζ x lmt x Tf / af = voltage across the coil/ field coil
Vc/ If = ζ x lmt x Tf / af
Hence af = ζ x lmt x If Tf / Vc
(v) Field current can be estimated by assuming a suitable value of current density in the field winding. Generally the value of current density may be taken as 3.5 to 4 amp/mm2.
Hence If = δf x af
(vi) Number of turns in the field winding Tf = Full load field ampere turns / field current = ATfl/ If
(vii) Height of the field winding hf = ATfl x 10-4/ √(sf df qf)
(viii) Resistance of the field winding Rf = ζ x lmt x Tf / af
(ix) Copper loss in the field winding = If2 x Rf
