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TIPS TO CUT DOWN THE NUMBER OF TROUBLE CALLS Here Is a Review of Maintenance Practices In Step With Growing Use of Dc in Industry By S SPENCE, Cheshire, Ireland F1NISHINO STANDS OENERATOR OPERATION. Where a compound gen the machine ammeter is in the generator lead that's erator (cede a motor load, diaconnect both electrically not connected to the series winding. Otherwise, with before bringing the units to rest. If both are left current flowing in the equaliser, the. ammeter would connected when prime mover is shut down, generator give.a false reading of load distribution between ma may come to rest before the motor, leaving the latter chines. to act os a generator. The motor would then pass current through generator series field reversing its polarity. You'd then run. into trouble on atart-up if fields were not flaohed first. Where a shunt generator Is one of several connected to the some bus, the field ean be energised from but by dosing main machine switch after raising the srmalure brushes. But hang a resistor across field terminals to prevent voltage build-up when field is opened. Series field of compound generator will be energized in correct direction if. equaliser and main switch are closed with armature brushes raised. MACHINE MAINTENANCE. Don't overlook the im portance of staggering brushes on commutator to prevent grooving. Dy staggering brush arms in pairs you'll also reduce the tendency toward "copper pick ing" at brush faces. I've found the following brush pressures work out best: carbon and graphite brushes, generally between 1.5 and 2 psi. Where motors must (see up to heavy overloads it may be necessary to go to 2.5 or 3 pa*. Still higher pressures are required for metat-grophile brushes; minimum about 3 psi. INSULATION. There are three practical methods of OENERATOR LOADING. If you run into trouble bal measuring resistance of electrical insulation with a ancing load between generators where field rheostat megohmmeter: (1) short-time or 'ipot reading (2) is hand operated, try slight shifting of brush arms;, time-resistance or dielectric absorption (3) 2-uoJiage forward movement increases, backward decreases. .. (Power, June 1950, pp 116, 117). The short-time Rut you're limited by the effect brush-shifting will test Is the most popular. But in the past some con have on commutation. fusion has existed shout when to stop the test sinee Remember, for atable operation of compound gen the value of insulation resistance would ordinarily erators in parallel an equalizer connection is neces keep climbing with time. So for purpoeet of stand sary between inner terminate of the aeries field. For ardisation, 500-v dc applied for 60 sec is recommended a true indication of load being carried, make sure where short-time single readings are to be made. Shooting Trouble SYMPTOM POSSIBE CAUSE OP TROUBLE: CURE General Overheating of Frame, Fiofd Coils 1 Machine ovsrloodod. Proper protective peer; lutes, relays, tsmpsrotare detectors generally keep tbit yaiptom Tn check 2 High ombient temperatures. May be limpte matter to Improve ventilation with ducts, font 3 Restricted ventllotion through machine because of dirt in vent duett or screens 4 If shunt field colls appear os heat source, check for Sorted hirni, toe-high voltage, port of abiding shorted-eut 5 Where Mriet field is overbading look for short-circolted turns; current density too high Indicating vlre site is inodequoto; machine overloaded Overheating of Armature Coils, Commutator 1 First moke sure machine it net overloaded 2 Mepohrometer insulation toil spots grounds; fry for minimum of .ooi mepokm for units to 600 * S Individual coils may bo shorted, open, Incorrectly connected to commutator lagmontt (odiocent leads to segment* moy bo accidentally svoppod in rewind) 4 Commutator overheating may indicota too greot brush friction, contact drop. Here ore soma point* to check: tarnishing of commutator (may.stem from presence of gas** from preduerien equipment), reogh surface, oscoisire brush pressure, wrong brush grade Excessive Sparking at Brushes I Once again, make a dose chock on machine loading to rule eat possibility ot overloading ron Dc Machines SYMPTOM POSSIBE CAUSE OF TROUBLE: CURE M POSSIBE CAUSE OF TROUBLE! CURE fcSi resistances, resulting in unequal current distribution l If brethai appear to vibrate when machine I* ran- in the arms. Thorough maintenance oa oil Joints ning, check for eccentric commutator, low ben or jp and terminals generally rules out this os o factor (lets, rough or grooved commutator. In tech ceu s* in brush sporklng tho care lies m turning, polishing commutate*, ' W II Remember, o partially shorted-or grounded main or preferably in its awn bearings > Interpol# field winding upset* the magnetic balance ) If commutator appear* distorted with some ben /.Vi high, the answer is to "re-season." This ataes ^ heating commutator uniformly to about 100 C roe- V ning full speed, then shutting down and tighteeiag commutator bolt*. Finish off with a fine itoabig to take off any high spots . .yj 4 Commutator moy bo oat of dynamic balance or vibrating at frequency ef some eslemol unit: namely, <jq transmission geors .jit, V. Sr. P'5 ;--A jj? HP & much o> would uneven oir gaps 12 Are interpolel reverted? Check this with compass. Commutating pole for meter should Hevu same polarity as tbe moin pole following, traveling in direction ef rotation. Far a generator It should be some polarity as moin pale preceding it, still coniJderinp fhe oonoo) direction of rotation. At fame time, check equaliser connections to assert that circulating currents are not at root of sparking 5 Do brushes slide easily ietheir holders? yX 6 Brushes may be off noufrol; shift brush bsses to (Motor Running ot Othor Than Normal Spaed Improve commutation .;u 7 Brushes may net be equally spaced around commato tor. Check this by actually measuring .dlitencti, & between all adjacent brashes; unequal spacing leads * to circulating currents being sst up '0 Incorrect brush pvtssere, brush type, unequal IJ. gaps can cause sparking. Unequal gaps can item from worn bearings *jj9 Open, short ot loose connection bi armature circuit- Open or shorted armature coifs con ataolly b*,$ spotted ot the commutator bar or bors ceenectini to tho faulty coil) copper or mica will be burned, v < There may be signs of solder throwing out of ik* 'td commutator Joints where o loose coneectioa h the foult 10 Brush arms and interconnection! moy have uecquef 1 Overspending of series motor generally indicates Iodd was accidentally removed. Wkere a compound meter speed it unstable, check that the series wind ing is net reversed 2 A shunt motor avenpeeds when field It too weak er a portion ef the winding ft shorted, grounded. Supply voltage moy be too high. If li*t running toe slew, check for overloading, law supply voltage, short In field rheostat 2 Sadden speed end current spurts moy moan o change in lood, defective transmission, such ot slipping belts er dotehes. Where a shunt meter "bunts" it moy point to commutator brushes that have shifted too far backward SYMPTOM POSSIBE CAUSE OF TROUBLE: CURE 4 Where o motor is to be reversed: Reverse either the field or armature leads foe a itrite motor, reverse field leedi enly In a shunt.motor, reverse both fields ie a compound motor Generator Fails to Build Up Voltage 1 No eseitatlon moy mean on open la field circeit, high resistance at joints or terminals---or Hie resi dual field may be lost. In tho latter cose, discon nect the field leads and separately esetta from o dc source 2 Individual field coils moy be reversed 3 Dirty commutator, law brush pressure, can build up contact resistance at commutator preventing voltage build-up 4 Is generator turning In proper direction? Generator Voltage Too High or low 1 Check speed oad eicitotion amperes 2 Where voltage Is law oad field cornet OK, check for shorted-portions ef field cells. Also look Info brush setting 3 If voltage is tee lew after tbe mechtoe has been repaired, impact wrong raceenoction of fields, too largo on air gap 4 If geesrotors foil to shore load evenly, machines may hove different voltage characteristic curves. Also check through the escitation circuit 5 When compound generators on unstable wbee paralleled, check cquolicor connections. PUNT OPERATION AND MAINTENANCE SECTION PLANT OPERATION AND MAINTENANCE SECTION 129b