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Field Magnet Coil

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Item 47.1. Pole Dimensions
The pole length parallel to the shaft is usually la - 1.5 d and the pole shoe is shaped to give an increasing air gap at the tips.

Item 47.2. Shunt- and Series-Field Ampere Turns, (TI)s_SE
(TI)s_SE = I_c x T/p.
Where T is the number of turns.

Mean-Length per Turn, m
m = 2(u + v) +pw.
Where:
u = inner length of the field coil
v = inner width of the field coil
w = number of layers
d = turns per layer

Item 47.3 Mean-Length per Turn m, CM(m)
CM(m) = m x TI x p / E

Shunt Field Current, I_SF

Output of machine. kW	Exciting current percentage of rated current
10	3.0
25	2.5
50	1.6
100	1.2
200	0.9
300	0.7
500	0.5
1,000 +	0.3 to 0.4

Refer to the table, right.

Use the next lower percentage of the output. Thus, if the given output is 300 kW and the corresponding exciting current as percentage of rated current is 0.7, use the next lower percentage, i.e., 0.9.

Wire Size of Shunt and Series-field, A_SE
A_SE = I_SE /D_SE.
Once computed, refer this to Table 1 Appendix 1 to get the available circular mils, CM, the cross section, CS, the resistance per 1,000 ft at 60°C, r_{1000ft @ 60C}, and the approximate number of turns per inch of dcc, T/inch
The current density, D_SE, in field windings, both for series and shunt, usually ranges from 800 to 1,600 amp per. sq. in..
If the current exceed 100 amp, the coils are made of flat copper strips wound edgewise. For small currents, round wires are rarely employed, unless the diameter is less than that of No. 8 B & S gage.

Wires per layer, wires/w
Use wires/w = (d - 2x¼) x T/inch
Where: T/inch is the approximate number of turns per inch from the Wire Table.
¼ is the allowance at the ends for insulation.

Total Number of Layers, layers
Use layers = T/inch x (w - thickness of insulation).

Turns per coil, d/coil
Use: no. of wires X layers.

Length of wire per coil
Use: Turns per coil x m/12 feet.

Resistance per coil at 60°C, R₆₀
Use: R₆₀ = Length per coil x r_{1000ft @ 60}/1000.

Exciting Current, I_E
Use: I_E = E x (1 - %RH) / (p x R₆₀)
Where: %RH is the percentage of voltage absorbed by the field rheostat, usually about 15%.

Ampere-Turns per pole., (TI)s
Use: (TI)s = Turns/ coil x I_E.
Compare this to the approximate ampere-turns per pole.

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