Design of the field System: Salient pole Alternator

Dimension of the pole:

 (i)  Axial Length of the pole: Axial length of the pole may be assumed 1 to 1.5 cm less than that of the stator core.

(ii)  Width of the pole: Leakage factor for the pole is assumed varying between 1.1 to 1.15.

Thus the flux in the pole body = 1.1 to 1.15

Area of the pole = Flux in the pole body/ Flux density in the pole body. Flux density in the pole body is assumed between 1.4 to 1.6 wb/m2.

Area of the pole = width of the pole x net axial length of the pole. 

Net axial length of the pole = gross length x stacking factor Stacking factor may be assumed as 0.93 to 0.95.

Hence width of the pole = Area of the pole / net axial length of the pole.

(iii)Height of the pole:  Height of the pole is decided based on the mmf to be provided on the pole by the field winding at full load. Hence it is required to find out the mmf to be provided on the pole at full load before finding the height of the pole. Full load field ampere turns required for the pole can be calculated based on the armature ampere turns per pole.

 Hence full load field ampere turns per pole can be assumed 1.7 to 2.0 times the armature ampere turns per pole.

Armature ampere turns per pole ATa = 1.35 Iph Tph Kw /p And

ATfl =  (1.7 to 2.0) ATa

 Height of the pole is calculated based on the height of the filed coil required and the insulation. 

Height of the filed coil:

If = current in the field coil

af = area of the field conductor

 Tf = number of turns in the field coil Rf = resistance of the field coil

lmt  = length of the mean turn of the field coil

 sf = copper space factor hf = height of the field coil df = depth of the field coil

 pf = permissible loss per m2 of the cooling surface of the field coil ζ = specific resistance of copper

 Watts radiated from the field coil = External surface in cm2 x watts/cm2 = External periphery of the field coil x Height of the field coil x watts/cm2

 Total loss in the coil =  (If2 x Rf) = ( If2 x ζ x lmt  x Tf / af)

Total copper area in the field coil = af x Tf = sf hf df

Hence af = sf df hf / Tf

 Thus watts lost per coil = ( If2 x  ζ x lmt  x Tf ) Tf / sf hf df  =  (If Tf)2 ζ x lmt/ sf hf df

  Loss dissipated form the field coil = qf  x cooling surface of the field coil

 Normally inner and outer surface of the coils are effective in dissipating the heat. The heat dissipated from the top and bottom surfaces are negligible.

Cooling surface of the field coil = 2 x  lmt  x hf

 Hence loss dissipated from the field coil = 2 x  lmt  x hf x qf

 For the temperature rise to be with in limitations

 Watts lost per coil = watts radiated from the coil

 (If Tf)2 ζ x lmt/ sf hf df  = 2 x  lmt  x hf x qf

 Hence hf = (If Tf) / [ 104 x √(sf df qf)] = ATfl x 10-4/ √(sf df qf)

 Depth of the field coil is assumed from 3 to 5 cm,

Copper space factor may be assumed as 0.6 to 0.8,

Loss per m2 may be assumed as 700 to 750 w/m2

 Hence the height of the pole = hf + height of the pole shoe + height taken by insulation

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  V_c =  (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( l_p + b_p) + π (d_f + 2t_i) where t_i is the thickness of insulation on the pole.

 (iv) Sectional area of the conductor can be calculated as follows

 Resistance of the field coil  R_f = ζ x l_mt  x T_f / a_f = voltage across the coil/ field coil

 V_c/ I_f = ζ x l_mt  x T_f / a_f

 Hence a_f = ζ x l_mt  x I_f T_f / V_c

 (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 I_f = δ_f x a_f

(vi)           Number of turns in the field winding T_f = Full load field ampere turns / field current = AT_fl/ I_f

(vii)        Height of the field winding  h_f  = AT_fl x 10^-4/ √(s_f d_f q_f)

 (viii)      Resistance of the field winding  R_f = ζ x l_mt  x T_f / a_f

(ix)           Copper loss in the field winding = I_f² x R_f