Electrical Machine Design

Legend: : Formula | : Limits, Range | : Assumption, etc.

Item 36. Total equivalent IR Drop, per cent, %IR
%IR = (W_Cu x 100) / KVA * 1000

Item 37. Total reactive drop, per cent, %IX
IX_total = 2f(T_s)²I_L.T.l / (nhE_L.T. x 10⁵) x [ g + (1/3)( p + s) ]
Where: f = the frequency, usually 60 cps
T_s = T_L.T. = Number of secondary turns
I_s = I_L.T. = Secondary Current
n = number of high-low sections, usually 2
h = H = the height of the opening
E_s = E_L.T. = the Secondary voltage rating
g = the radial thickness of the insulation b/w the primary and secondary windings
p = the radial thickness of the primary windings and insulation
s = the radial thickness of the secondary winding and insulation.
Thus IX = E_H.T. x %IX volts.

Item 38. Regulation on unity power factor, per cent
Equal to the % IR drop. see item 36.

Item 39. Regulation on 0.8 power factor, per cent
Reg_0.8pf = %IR x 0.8 + %IX x 0.6 per cent.
Where 0.8 = cos 80°, and 0.6 = sin 80°

Tank Design & Temperature Rise

Total Outside Area of Tank, square inch, S_tank
S_tank = (W_core + W_copper) / ( TR_oil x c_n°C )
Where: TR_oil = 50° temperature rise
c_30°C = 0.0041 W/sq.in-°C for a 30° C rise in temperature
c_40°C = 0.0050 W/sq.in-°C for a 40° C rise in temperature
c_50°C = 0.0057 W/sq.in-°C for a 50° C rise in temperature

Item 40. Effective cooling surface of tank, square inch, S_eff
S_eff = H_tank(2W_tank + 2D_tank) +½(W_tank + D_tank)
Approximate values:
H_tank = H + M + 6 inch (allowance = 6 inch)
W_tank = 2(t + s + g + p) + D + 2L + 1.75 inch (allowance = 1.75 inch)
D_tank = 2(t + s + g + p) + S + 2.75 inch (allowance = 2.75 inch)

Item 41. Watts per square inch of tank surface, W/sq.in._tank
W/sq.in._tank = (W_Fe + W_Cu) / S_eff

Item 42. Approximate temperature rise of oil, degrees Centigrade, t_max
t_max = (W_Fe + W_Fe) / ( c_30°C x S_ef ) . ° C.
t_max = W/sq.in._tank / c_30°C . ° C.