ELECTRICAL EQUIPMENT—Q.72.Fit the commutator end bracket over thecommutator and release the brushes.3.Hold back each of the brush springs and movethe brush by pulling gently on its flexibleconnector. If the movement is sluggish, removethe brush from its holder and ease the sides bylightly polishing on a smooth file. Always refitthe brushes in their original positions. If thebrushes are badly worn, new brushes must befitted and bedded to the commutator. Theminimum permissible length of brush is 9/32 in(7,14 mm), i.e. when the spring arm reaches thebrush box.4.Measure the brush spring pressures, using aspring balance held radially to the commutator.The tension on the springs should be 30 ozf(0,85 kgf), maximum, when exerted on a newbrush and 13 ozf (0,37 kgf), minimum, on abrush worn to 9/32 in (7,14 mm). Bothpressures should be measured and defectivesprings replaced.(d)Commutator1.A commutator in good condition will be smoothand free from pits and burned spots. Two typesof commutator, the moulded and the fabricated,will be found in service. Moulded commutatorscan be recognized by the exposed end beingquite smooth, unlike that of the fabricatedcommutator from which a metal roll-over and aninsulating cone protrude (see Fig. Q.3).2.A moulded commutator can be re-skimmedduring service but care must be exercised toensure that the finished diameter is not lessFig. Q.3.Comparison of Fabricated and Moulded Commutators.Fig. Q.4.Undercutting Insulators.than 1.450 in (36,83 mm). The process of re-skimming consists of rough turning (ifnecessary) followed by diamond turning.Whether or not rough turning is carried outdepends upon the severity and unevenness ofthe wear which has taken place. A mouldedcommutator requires no undercutting inservice, the production undercut being ofsufficient depth to obviate any further need forthis. The insulation slots, however, muss bekept clear of copper and carbon residue.3.To remedy a worn fabricated commutator,undercut the insulators between the segments toa depth of 1,32 in (0,79 mm) using a hacksawblade ground to the thickness of the insulator,then mount the armature, with or without thedrive end bracket, in a lathe, rotate at highspeed and take a light cut with a very sharp tool.Do not remove more metal than is necessary.Polish the commutator with very fine glasspaper.(e)ArmatureIndication of an open-circuited armature windingwill be given by burnt commutator segments. Ifarmature testing facilities are not available, anarmature can be checked by substitution.
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