. Design of a direct current generator and switchboard for a farm power plant. erence 13 110=17x8 = inches. The 25 2 width of a segment including insulation is then -^{3 = 0,213 inch. Allowing inch for insulation leaves the width of the segment proper to be inch. Since all of the current must pas3 in two sets of brushes and out at the other two, the amperes per set of90 9 brushes are —= Allowing 40 amperes per sq. in. of 45 .45 brush contact, this gives a brush contact of ——- • sq. in. per set of brushes. The brush should cover from 2 to 3 0,5 _ segments. Making the
. Design of a direct current generator and switchboard for a farm power plant. erence 13 110=17x8 = inches. The 25 2 width of a segment including insulation is then -^{3 = 0,213 inch. Allowing inch for insulation leaves the width of the segment proper to be inch. Since all of the current must pas3 in two sets of brushes and out at the other two, the amperes per set of90 9 brushes are —= Allowing 40 amperes per sq. in. of 45 .45 brush contact, this gives a brush contact of ——- • sq. in. per set of brushes. The brush should cover from 2 to 3 0,5 _ segments. Making them in. wide they will cover 0 #213 * segments. The length to give the required contact area must be 1*1^5 ? = inches for each set of brushes. Two0 .5 brushes were used, each 1 1/8 in. long, which makes the length of the two brushes together in. Then the area of contact is x 0,5 = sq. in. and the amperes per sq. in. of brush contact are 4^= Allowing 1/8 in. between the brushes, 3/8 in, next to armature and in, outside of. brushes gives the total length of commutator to + 1/8 +3/8 +1/2 = 3,25 in. Assuming a pressure of sq. in. upon the brushes, the volts drop due to the brusheswas read off from a set of curves for various pressuresplotted between volts drop and amperes per sq. in. of value was found to be about 2 for amperes per contact. The watts lost then were EI = 2 x 90 .9 =182 watts. The watts lost due to friction between the com-mutator and brushes was determined from the following formulaattS lostg x x Bx-ZgL *™,0t00a67 6Dn> 33,000 where, is the coefficient of friction bewteen brushesand commutator,8 is the area of contact, D is the diameter ofthe commutator,n is the revolutions per minute, and is thepressure on the brushes in lb. per sq. in. Substituting thevalues already determined, Watts lost. x x 8 x 1200= 58Therefore the total watts lost
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