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In principle commutator insulation should be recessed below the surface of the segments, unless hard carbon brushes are used. Segment edges should be bevelled to ensure on the one hand proper communication and on the other to avoid high brush wear or brush breakage.

Commutators and slip rings should have a true round form otherwise contact will be interruped at high speeds and brush sparking may occur. Small irregularities can be removed by means of commutator hand stones pressed lightly on commutators or slip ring when running. In case of eccentricity, however, the commutator or slip ring must either be turned or ground, if possible at full speed in its own bearings.

Brush spacing should be as exact as possible particularly on high speed machines with difficult commutation. Brush spacing can be checked by means of a paper strip cut to length of the circumference of the commutator or slip ring. The paper strip is marked in equal spaces with the brush positions corresponding to the number of brush arms. It is then wrapped round the commutator and the position of the brush holders may be checked and adjusted if necessary (the leaving edges of the brushes are used for this adjustment). The brush on one brush arm must be exactly in line, except for any internal brush stagger.

Due to the fact that the cathode brushes (generator positive brushes and motor negative brushes) are responsible for most electrical attacks on the commutators or slip rings, care should be taken that these brushes are equally spaced (staggered) across the width of the collector. From the mechanical point of view it is recommended that the brushes of two adjacent spindles (one pole pair) are placed exactly one behind the other, while the brushes of next spindle pair, being in line with each other, are staggered with respect to the first pair.

While fitting brush holder the gap between the lower edge of the brush holder box and the surface of the commutator or slip ring should not exceed 2 mm to 2.5 mm, also, the brush holder edge and the collector surface should be parallel. Radial brush holders should be fitted in an exact radial position. In case of reaction brush holders, with brush inclined 30o, the brush holder must be fitted at the correct angle, with the commutator revolving towards the sharp angle of the face. In the case of trailing brush holders inclined at 8°, the commutator revolves away from the sharp angle of the brush.

While replacing brush sets it is most important that the brushes are bedded properly in order to ensure that the contact faces match the curve of the commutator or slip ring as exactly as possible, and that no electrical overload of individual brush parts occur. If only single brush is replaced, special brush bedding is unnecessary because other brushes in parallel cope with the additional load during the bedding-in period. The method of bedding brushes is to put a long strip of abrasive cloth under the brushes, covering the surface of the commutator or slip ring as far as possible. Then the abrasive cloth is drawn backward and forward. If the machine is uni-directional, the abrasive cloth between brushes and commutator surface should be pulled in the direction of rotation only, the brushes being lifted as the abrasive cloth is drawn back. Bedding should be done at normal brush pressure. The pressure must not be manually increased. A quicker bedding method is to use an artificial pumice stone, which is applied in front of the brushes when the machine is running so that the abrasive particles thereby released pass under the brush and shape its contact face very quickly. The running face of a well-bedded brush has the same curve ads the commutator or slip ring. After bedding the commutator or slip ring, the winding, brush holder and lifted brushes should be thoroughly clreaned.

One of the most important factors affecting brush performance is the pressure applied on the brushes, a matter which is often given too little attention. The pressure required is dependent on the prevailing conditions : i.e., type and use of the machine, grade and cross section of brushes. Generally, more the machine vibrates, higher the brush pressure required. If the pressure is too light , the wear of brushes as well as the commutator or slip ring may rise rapidly as a result of higher electrical losses at the running surface or sparking. This occurs mainly if the machine is subject to vibrations. If pressure is too high the wear of brushes and commutator or slip ring also rises but at a lower rate and here mechanical wear predominates. Excessive pressure, however, may lead to the danger of overheating of commutator or slip ring due to increasing friction. There is an optimum brush pressure which will give lowest possible wear. Brush grade and operating conditions affect the choice of brush pressure. Care must be taken that pressure are the same on brushes connected in parallel otherwise current may be shared unequally between brushes. This may lead to individual brushes being overloaded and their flexibles may overheat and even melt in extreme cases.

Brush Wear:
What is determined by electrical and mechanical load on the brush. Electrical wear is caused by longitudinal or active current as well as by cross current or short circuit current. In case of continuous sparking, the brush life may fall to a fraction of the normal rate due to burning and roughening of the contact faces. In order to improve commutation, it is often necessary to change to a better commutating and more elastic grade. One must, however, take into consideration that though such open textured and elastic brushes provide good commutation and reduce the electrical wear they have, on the other hand, a higher mechanical wear rate than dense and unelastic brushes due to their open texture. In order to achieve the longest possible brush life, one should always select a proper grade which is just sufficient for commutating requirements.

We have already explained that correct and even brush pressure is essential for satisfactory performance. In the case of bad brush performance the pressure should be checked although we feel that , the use of spring balance gives questionable result; it is however possible to check the uniformity of pressures of several brush holders by this method with sufficient accuracy.

It is of great importance for good brush performance that the brushes are located exactly in the neutral position. The makers of D.C. motors allow for adjusting the brush gear and mark its correct position. If the holder are fitted having a different distance between spindle center and center line of the brush running surface compared with the original holders then the brush gear must move accordingly. With the above, we wish to draw your attention to the most usual faults when fitting brushes and brush holders.